desk top study on digestate enhancement and treatment ... from anaerobic... · bod biological...

Desk top study on digestate enhancement and treatment

Enhancement and treatment of

digestates from anaerobic digestion

A review of enhancement techniques, processing options and novel digestate products

Project code: OMK006 - 002 Research date: Feb 2012 – May 2012 Date: Nov 2012

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Enhancement and treatment of digestates from anaerobic digestion

Executive Summary

Background Anaerobic digestion (AD) is a well established process for the treatment of organic wastes and the generation of renewable energy. Historically the digestate produced from the process has been applied to land as a fertiliser or soil conditioner. However with a planned increase in the number and capacity of AD plants to treat a variety of organic waste streams in the UK, digestate enhancement technologies are gaining more attention. Digestate enhancement technologies could be assessed by an AD operator looking to provide any of the following options for an AD plant:

increase the value of digestates;

secure use of digestates;

create new markets for digestate products; and

decrease the operating costs (OPEX) of the facility.

Objectives This study aims to identify digestate enhancement technologies and techniques, in order to raise awareness of them within the UK waste sector. The study has considered well established techniques, as well as novel or emerging processes currently under development. The project has reviewed technologies applicable to all digestates produced from the anaerobic digestion of a variety of feedstocks, whether or not they are compliant with PAS 110 or the Anaerobic Digestion Quality Protocol (ADQP). A key objective of the study is to raise awareness in the UK waste sector to the opportunities and challenges of digestate enhancement. The output of the study supports the delivery of a number of actions contained in the AD Strategy and Action Plan (June 2011) and the delivery of Scotland’s Zero Waste Plan. Methodology Data has been collected from an extensive desk based literature search, direct contact with technology providers, relevant industry focus groups, academic research, conference papers, policy documents, relevant industry texts and manufacturer’s literature and legislation. A web-based search was also undertaken. This information has been used to construct technical data sheets for each technology considered which form an appendix to the report. In addition a number of examples of applications of novel technologies have been included in the report.


Findings This study has found that there are a wide range of technologies available for digestate enhancement. Technologies are available to create a range of novel digestate products such as concentrated or balanced fertilisers, which have the potential to be marketed as “products”. However, no single technology has been found to be relevant for all applications so a range of solutions will be required to accommodate the increasing volumes of digestate generated within the UK. From an EU waste sector perspective, there are clearly similar challenges and goals to the UK waste sector; but AD investment in Europe has been driven by a series of different drivers and supported via different energy subsidy regimes. EU funded support of the research and development of digestate products and markets has assisted EU member states in making investment decisions over the last ten years. Research and development continues with a focus on the development of enhanced products. UK markets for waste derived digestates are immature. There is existing competition in land based markets, not least with conventional inorganic fertilisers. However, in the future as natural phosphorous resources decrease and the cost of inorganic fertilisers increase, farmers will look to find alternative and potentially cheaper sources of nutrients for their crops. The key challenge in the short term will be to manage increasing quantities of digestates seeking markets and secure outlets. Operational experiences should be sought from the EU, where systems have been installed and digestate products created to satisfy outlet demand.


Contents

1.0 Introduction ................................................................................................. 1

1.1 Objectives ................................................................................................. 2 1.2 Aims ......................................................................................................... 2

2.0 Methodology ................................................................................................. 3 3.0 Digestate Enhancement................................................................................ 4

3.1 What is Digestate? ..................................................................................... 4 3.2 Why Use Enhancement Techniques? ............................................................ 4

4.0 Digestate Enhancement................................................................................ 6 4.1 Pre-Digestion Enhancement Techniques ....................................................... 6

4.1.1 Thermal Hydrolysis........................................................................... 6 4.1.2 Autoclave Systems ........................................................................... 6 4.1.3 Enzymic Liquefaction ........................................................................ 7 4.1.4 In-Vessel Cleaning Systems .............................................................. 7

4.2 Post-Digestion Enhancement Techniques ..................................................... 9 4.2.1 Physical Enhancement Techniques................................................... 12 4.2.2 Thermal Enhancement Techniques .................................................. 14 4.2.3 Biological Enhancement Techniques ................................................ 16 4.2.4 Chemical Enhancement Techniques ................................................. 20

5.0 Digestate Enhancement Systems ............................................................... 23 5.1 Digestate Treatment Systems .................................................................... 23 5.2 Digestate Enhancement Systems ............................................................... 26 5.3 Current Barriers to Enhancement Systems .................................................. 27 5.4 Technology Example: Barkip Biogas Facility ................................................ 28

5.4.1 Background ................................................................................... 28 5.4.2 Process Description ........................................................................ 29

5.5 Technology Example: Lee Moor EFW ......................................................... 30 5.5.1 Background ................................................................................... 30 5.5.2 Process Description ........................................................................ 31

5.6 Technology Example: MINORGA ® Bio fertiliser, Norway .............................. 32 5.6.1 Background ................................................................................... 32 5.6.2 Process Description ........................................................................ 33

6.0 European Perspective ................................................................................. 35 6.1 Background ............................................................................................. 35 6.2 Survey Data ............................................................................................. 35 6.3 Transport Optimisation ............................................................................. 36 6.4 Summary of EU Waste Sector.................................................................... 37

7.0 Summary .................................................................................................... 38


Glossary

AD Anaerobic digestion. Process of controlled decomposition of organic matter under

anaerobic conditions.

Aerobic Molecular oxygen available.

Anaerobic No oxygen source..

Anaerobic Digestion

Quality Protocol (ADQP)

End of waste criteria for the production and use of quality outputs from anaerobic

digestion of source segregated biodegradable wastes.

Anoxic No available source of molecular oxygen.

Auto thermal Condition at which an exothermic reaction is self sustaining and no additional energy is

required from an external source.

Bio methane Methane generated by anaerobic digestion.

Biogas Gas generated by an anaerobic digestion process. Typically composed of 60% methane

and 40% carbon dioxide.

BOD Biological oxygen demand. Defined as the amount of oxygen required by aerobic

bacteria to oxidise the organic matter within the sample.

CAPEX Capital expenditure.

CH4 Methane.

CHP Combined heat and power. Cogeneration of heat and power from combustion of a

fuel(gas).

COD Chemical oxygen demand. Defined as the amount of oxygen required to chemically

oxidise the organic matter within the sample.

Digestate, Fibre Fibrous fraction of material derived by separating the coarse fibres from the whole

digestate.

Digestate, Liquor Liquid fraction of material remaining after separating coarse fibres from whole

digestate.

Digestate, Whole Material resulting from an anaerobic digestion process that has not undergone post-

digestion separation.

Dry solids (ds) Measure of solids content within the digestate. Defined as the % of mass remaining

after drying at 105°C.

Evapotranspiration The combined effect of evaporation and plant transpiration (normal water loss to the

atmosphere from plants).

H2S Hydrogen sulphide.

MBR Membrane bioreactor. The combination of a membrane process with a suspended

growth bioreactor.

MBT Mechanical Biological Treatment: waste processing facility that combines a mechanical

sorting facility with a form of biological treatment such as composting or anaerobic

digestion. MBT plants are typically designed to process mixed wastes and as such are

not capable of achieving PAS 110 or PAS 100.

Moisture content Measure of water content within the digestate. Defined as the % of mass lost after

drying at 105°C.

NH3 Ammonia.

OPEX Operating expenditure.

Pasteurisation Process step during which the number of pathogenic bacteria, viruses and other harmful

organisms in material are significantly reduced or eliminated by heating the material to

http://en.wikipedia.org/wiki/Membrane_technology

http://en.wikipedia.org/wiki/Bioreactor


a critical temperature for a specified period of time.

PAS 100 Publically Available Specification that controls the quality input to compost and the

process is managed and operated to generate composts that protects the environment

and meets market needs.

PAS 110 Publically Available Specification that controls the quality input to anaerobic digestion

and the process is managed and operated to generate digestate that protects the

environment and meets market needs.

Polyelectrolyte High molecular weight organic polymer used to assist flocculation in solid liquid

separation.

RHI Renewable Heat Incentive. Financial incentive for the use of renewable heat.

RO Reverse Osmosis: A membrane filtration technology that utilises a selective reverse

osmosis membrane to retain molecules and ions while allowing the solvent, ions and

small soluble molecules to permeate through.

ROC Renewable Obligation Certificate. The main financial support mechanism for large

renewable electricity projects in the UK.

Sanitisation Biological process that eradicate or reduce pathogens to acceptably low, sanitary levels.

Syngas Abbreviation of synthesis gas. Gas mixture that comprises of carbon monoxide, carbon

dioxide and hydrogen produced by the gasification of a carbon containing fuel.

UF Ultra Filtration: A membrane filtration technology that utilises a selective ultrafiltration

membrane to retain soluble macromolecules and larger contaminants while allowing the

solvent, ions and small soluble molecules to permeate through.

Acknowledgements

The authors would like to thank the various organisations who provided information and advice on digestate enhancement systems. Particular thanks to Tim Evans (Tim Evans Environmental Ltd), Nigel Horan (Aqua Enviro), Paul Bardos, Claire King and Ursula Kepp (r3 Environmental Ltd), Steve Wooler (HRS), Oddvar Tornes (IVAR IKS), Christian Toll (AeroThermal), Mike Weaver(Pyreg), Tobias Finsterwalder (FIMTEC GmbH) and Ian Crummack (DONG Energy).

Enhancement and treatment of digestates from anaerobic digestion 1

1.0 Introduction

The use of anaerobic digestion (AD) to recover value from organic wastes within the UK is emerging as an important treatment system and is forecast to increase significantly. There are currently 233 AD plants operating within the UK with a capacity to treat 5.4 million tonnes of material:

51 AD facilities operating on waste including food waste.

36 AD facilities operating on farm waste.

146 AD facilities operating on sludge generated by waste water treatment works.

There are currently planning applications to develop a further 222 facilities in the UK (1WRAP held data, Nov 2012) which will provide for significant increase in processing capacity. AD converts organic matter into biogas, a source of renewable energy, and a nutrient rich organic fraction known as digestate. Biogas can be used to generate electricity and heat to power the process. Excess power can be sold to the National Grid and excess heat can also be utilised, if the right infrastructure exists. The most commonly used digestion system is wet mesophilic digestion operating between 25°C and 40°C; the digestate produced from this process is an organic slurry, rich in nutrients such as nitrogen and phosphorus. Other less common systems include dry digestion, which uses a feedstock with very high dry solids content and thermophilic digestion, which operates at higher temperatures (50°C - 60°C). Currently the majority of AD facilities recycle the digestate to local agricultural land as an organic fertiliser (Fuchs et al., 2010). However the window for land application is limited to agricultural and crop requirements (Orr, 2011), and for large capacity AD plants, a substantial area of land is required to provide a secure and suitable market for the digestate. If application to agricultural land is not feasible, due to transport distances, legislative requirements or other restrictions, digestate can be used for land reclamation. This is particularly relevant for digestates from mechanical biological treatment (MBT) applications, as the use of digestates derived from mixed waste materials is currently restricted to use on land restoration projects only. As the use of AD increases the demand for agricultural land will also increase, potentially requiring plants to transport digestate further in search of suitable land. This is important for the increasing number of centralised AD facilities operating in urban areas. Digestate must therefore be carefully managed to ensure it is utilised as a resource and maximum benefit is achieved whilst avoiding excessive transportation costs.

1 Whilst the data is accurate to the best of WRAP’s knowledge, WRAP offers no warranty and accepts no liability relating to the completeness or accuracy of the information contained within. Information is compiled by various parties and recipients should make their own independent enquiries before relying on the information contained within the document.


1.1 Objectives

WRAP has acknowledged the need to raise awareness of digestate enhancement technologies to secure land application or develop novel products which is the key objective of this study and is a key action in the AD Action Plan. The study focuses on digestate produced from anaerobic digestion processes that cover:

both non-waste and waste-based digestates, whether or not they are compliant with

PAS 110 or the Anaerobic Digestion Quality Protocol (ADQP);

mixed-waste digestates as the output from Mechanical Biological Treatment (MBT);

amended sludge digestates or co-digestates, derived from feedstocks including

sewage sludge; and

sludge digestates (biosolids), solely derived from sewage sludge.

1.2 Aims

The aim of this study is to identify technology and techniques for the enhancement of digestate – from straightforward dewatering to the development of novel products. The project has reviewed technologies and enhancement techniques applicable to all digestates produced from the anaerobic digestion of a variety of both waste and non-waste feedstocks, whether or not they are compliant with PAS 110 or the Anaerobic Digestion Quality Protocol (ADQP). The output of the study is to support the delivery of a number of actions contained in the AD Strategy and Action Plan (June 2011) and the delivery of Scotland’s Zero Waste Plan. It is vital that digestate enhancement is seen in a holistic way as part of an overall materials processing and re-use system. It is important that the overall case for sustainable wastes and resource management is not negated by inappropriate digestate management choices, for example:

the use of downstream processing to treat digestate that consumes more energy

than is likely to be generated by the AD facility;

the generation of large volumes of effluent for treatment that create an unacceptable

overall carbon or water footprint for the AD facility;

the transmission of harmful impacts to soil and groundwater that are substantially

greater than using alternative materials such as composts or conventional fertilisers;

the reduction in carbon benefit of systems that generate high greenhouse gas (GHG)

emissions, for example the atmospheric release of methane (CH4 ) or nitrous oxide

(N2O);

the inappropriate development of AD facilities that have negative impacts on the

public perception and economic viability of digestion as an effective waste

management and energy recovery option; and

high capital and operating costs that limit the financial viability of AD and increase its

reliance on public subsidy.


2.0 Methodology

In order to identify and assess possible digestate enhancement techniques a detailed desk based literature search has been undertaken. Sources for the literature search included technical reports, academic research, conference papers, policy documents, relevant industry texts, manufacturer’s literature and legislation. A web-based search was also undertaken. In addition to the literature search, information was requested from a number of anaerobic digestion organisations and interest groups within the UK and EU. Over 30 organisations were approached to participate and provide information for this study. A full list of organisations contacted can be found in Appendix 1. Unfortunately a number of the organisations contacted were unable to participate and provide information for this study, partly due to commercial reasons or perceived conflict of interest. Data obtained from the research was compiled and used to construct technical data sheets for each enhancement technique identified, which can be found in Appendix 2. The aim of these data sheets is to provide a brief description of the operating principle of the technology/technique, operating conditions and associated benefits, challenges and opportunities. In addition to developing technical datasheets, a number of example enhancement projects have been included in the report. These focus on the application of emerging technologies which either significantly enhance digestate quality or support the development of novel digestate products.


3.0 Digestate Enhancement

3.1 What is Digestate?

Digestate refers to the material produced by the process of anaerobically digesting biodegradable materials. Digestate consists of a mix of microbial biomass (produced by the digestion process) and undigested material. The volume of digestate produced will be approximately the same as the feedstock volume, although the mass will typically be reduced by approximately 15%. Digestate contains all the nitrogen, phosphorus and potassium present in the original feedstock and as a consequence has value as an organic fertiliser. Typical nutrient values for digestate are given below, however the actual nutrient content will be highly dependent on the type of feedstock processed (Chambers, 2011).

Nitrogen: 2.3 - 4.2 kg/tonne.

Phosphorous: 0.2 - 1.5 kg/tonne.

Potassium: 1.3 - 5.2 kg/tonne.

(NNFCC, 2012)

Consideration must be given to the relationship between the quality of the feedstock and the quality of the digestate. The digestate will contain all material that has not biodegraded and converted into biogas within the process, therefore any contaminants in the feedstock will remain in the digestate. A good quality, well prepared feedstock will therefore produce a good quality digestate compared with poor quality feedstock which will produce a poor quality digestate.

3.2 Why Use Enhancement Techniques?

The majority of digestate produced in the UK is spread to agricultural land as fertiliser, either as whole digestate or as a separated fibre (Fuchs et al., 2010). Although this is a good use of the nutrients within the digestate, the value of the digestate to the producer is low (Horan, 2012). Once the costs of transportation and spreading are taken into account the digestate value can be close to zero, and may even be a cost to the producer (Lewens, 2011). The application of nitrogen in organic materials to agricultural land is regulated by the European Nitrates Directive (91/676/EEC.) (Fuchs et al., 2010). As a consequence the spreading of digestate to land is controlled (based on nitrogen content) and dependent on location and crop demand. This can result in digestate being transported greater distances to find suitable land-based markets and avoid over application; this will increase transport and operational costs. Furthermore land application is only appropriate during the growing season, requiring digestate to be stored for significant months of the year. More information on NVZs can be found on Defra’s website, http://www.defra.gov.uk/food-farm/land-manage/nitrates-watercourses/nitrates/. If the number of operating AD facilities increases, as currently predicted, local competition for land based markets will also increase, with a consequential impact on transportation and spreading costs.

http://www.defra.gov.uk/food-farm/land-manage/nitrates-watercourses/nitrates/

http://www.defra.gov.uk/food-farm/land-manage/nitrates-watercourses/nitrates/


The key aims of digestate enhancement techniques are to:

increase the value of the digestate;

create new markets for digestate products;

reduce the dependence on land application;

ensure more secure and sustainable outlets for digestate products; and

potentially reduce the operating cost of the facility.

Consideration has been given in this study to enhancement techniques and technologies that can be applied at three key stages:

pre-digestion;

within the digestion process ( i.e. in-vessel); and

post-digestion.

Each system considered is aimed at supporting the objective of enhancing the quality of the digestate or providing potential to develop new digestate products.


4.0 Digestate Enhancement

4.1 Pre-Digestion Enhancement Techniques

Pre-treatment systems employed upstream of anaerobic digestion can be used to enhance the digestion process, and as a consequence digestate quality. There are a number of techniques available to pre-treat the feedstock and improve the availability of organic constituents to enhance the digestion process. In addition the removal of contaminants and debris from the feedstock to the digestion process is key to maintain digestate quality and in the extreme secure stable operation of the digestion process. These systems are discussed below.

4.1.1 Thermal Hydrolysis

The thermal hydrolysis process (THP) is a high-pressure, high-temperature steam pre-treatment application for anaerobic digestion feedstocks. The feedstock is heated and pressurised by steam within a reaction tank before being rapidly depressurised (flashed). This results in the breakdown of cell structure within the biomass; as the organic matter is presented to the digester in a broken-down condition, the digestion process is more effective resulting in increased gas production and improved digestate quality. To ensure the process is thermally and economically efficient the system requires a dewatered feedstock at between 15-16% dry solids. As a consequence dewatering systems are an important pre-treatment stage. Details of dewatering systems are provided in Section 3.4.1. As the thermal hydrolysis process utilises a dewatered feedstock increased digester loading is achieved and therefore gas production is increased. The quality of the digestate is improved as the hydrolysed digestate is pasteurised, easier to dewater and achieve higher dry solids product, resulting in a product that is easier to store, handle and transport (CAMBI, 2011, Veolia, 2008). The process has widespread waste water treatment applications operating on sewage sludge. The process is being developed for organic and food waste applications in Europe, particularly Norway.

4.1.2 Autoclave Systems

An autoclave can be used to pre-treat digester feedstocks in a similar manner to thermal hydrolysis. The autoclave is a pressure vessel that steam treats its contents at a constant temperature and pressure, serving to pasteurise, clean and break-down organic matter within the feedstock. As the organic matter is presented to the digester in a broken-down condition the digestion process is more effective resulting in increased gas production and improved digestate quality. After processing inorganic material and contaminants can be easily removed via mechanical separation, providing a clean, pasteurised, organic rich feedstock for anaerobic digestion (AeroThermal Group, 2008).


4.1.3 Enzymic Liquefaction

An enzymic liquefaction system has been developed by DONG Energy A/S. for use on mixed waste streams prior to digestion. Called the REnescience process, the system comprises three process stages to breakdown and separate the organic matter from within the feedstock prior to digestion. Stage one is a non-pressurised thermal treatment utilising either hot water or steam, which “opens” the feedstock to make it accessible to enzymes. In the second stage, enzymes are added to liquefy and further breakdown the cell structure of the feedstock. The prepared feedstock is then digested in the third stage of treatment. Following digestion the component fractions are separated such that an organic rich liquid for land-based application can be easily separated from inorganic material and physical contaminants. The system appears to be suited to the pre-treatment of mixed waste streams (i.e. MBT). A pilot plant of the REnescience process is currently operational in Denmark where a range of waste materials, including municipal solid waste, source segregated food waste and sewage sludge has been processed. A schematic diagram of the REnescience Enzymic Liquefaction Process is shown in Figure 1.

Figure 1. Schematic of REnescience Enzymic Liquefaction Process

4.1.4 In-Vessel Cleaning Systems The inclusion of in vessel cleaning systems as an enhancement technique may not initially appear appropriate. However, detailed consideration must be given to the nature of the waste materials being feed into the digestion process i.e. waste containing varying quantities of:

plastic

timber

fibres (both natural and man-made textiles),

grit/sand/soil

metal fragments

solid fruit residues(pips/stones/stalks/peel)


Whilst the digestion process itself utilises significant mixing and agitation, the digestion vessel will act as a repository of all feedstocks. Heavy materials will tend to settle and lighter materials will tend to float to the top of the vessel and become entrained within a scum and foam layer. In-vessel cleaning systems can be used to good effect to remove contaminants from the digester, improving both digestate quality and preventing the build-up of inerts. In the extreme, hydraulic retention time in the digestion vessel can be severely reduced if these inerts are not removed. This can lead to impairment of the digestion performance and eventually potential process instability. Floating material can become dislodged, adversely affecting the quality of digestate, and in the extreme placing at risk the security of the land-based outlet and/or PAS 110 accreditation. Proprietary systems have been developed to overcome these problems with in-vessel cleaning techniques. Grit and heavy solids material accumulating at the bottom of the digester vessel can be directed by a rotating scraper system to the edge of the digester where it is removed and separated from the digestate. The separated digestate is returned to the digestion process. The separated grit/solids can be used as an aggregate amendment for construction or potential land remediation. However the land remediation operation will require a permit (Finsterwalder Umwelttechnik GmbH & Co. KG, 2012). A typical in-vessel system is shown in Figure 2. Floating material, such as plastics and rags can also be removed by a rotating skimmer. Material is forced to the edge of the digester where it is removed and separated from any entrained digestate. The separated digestate is returned to the digestion process and the separated solids disposed to landfill.

Figure 2. Typical scraper system installed within a digester


4.2 Post-Digestion Enhancement Techniques

The enhancement techniques identified by the research undertaken in this report are summarised in Table 1 below. Techniques have been divided into categories based on the type of process employed i.e. physical, chemical or biological. Where multiple technologies are available for the same enhancement principle (i.e. drying) these have been divided into sub categories.

Table 1. Digestate Enhancement Techniques

The listing of enhancement techniques in Table 1 does not contain all possible treatment types, and it is not an endorsement of those presented. However, the listing serves to illustrate potential options and provide information obtained in this study.

Physical Thermal

Thickening (Belt) Drying (Rotary Drying)

Thickening (Centrifuge) Drying (Belt drier)

Dewatering (Belt press) Drying (J-Vap)

Dewatering (Centrifuge) Drying (Solar)

Dewatering (Hydrocell) Evaporation (scraped surface heat exchangers)

Dewatering (Bucher press) Conversion (Incineration)

Dewatering (Electrokinetics) Conversion (Gasification)

Purification (Ultrafiltration and Reverse Osmosis) Conversion (Wet air oxidation)

Conversion (Pyrolysis)

Biological Chemical

Composting Struvite precipitation

Reed Beds Ammonia recovery (Stripping + Scrubbing)

Biological Oxidation Ammonia recovery (Membrane Contactor)

Biofuel Production (Algae) Ammonia recovery (Ion Exchange)

Biofuel Production (liquor as process water) Acidification

Biofuel Production (hydrolysis of fibre to Bioethanol) Alkaline Stabilisation

Microbial Fuel Cell


The following sections of the report provide a brief description of each of the treatment principles and how they may be employed. Further information on each of the techniques can be found in the Technical Data Sheets included in Appendix 2. Each Technical Data Sheet provides a schematic process flow diagram for the techniques, as well as a brief process description. The aim of the Technical Data Sheets is to provide an overview of the principles and objectives of each technology, as well as an indication of any particular challenges that may need to be considered in implementing the system. The flow sheet presented in Figure 3 provides an overview of the digestate enhancement techniques and how these can be combined into viable treatment systems. This is not an extensive list of treatment possibilities but highlights the principles available. The dependencies of some technologies on pre-treatments are also captured within the overview. For example, if thermal drying is to be employed the flowchart indicates that dewatering is likely to be required as a pre-treatment. Dewatering will produce a liquor stream which must also be treated, by membrane purification for example. Depending on local site conditions and requirements, the number of techniques required and the complexity of the treatment processes can vary considerably. This is discussed in more detail in Section 5. Given the dependencies between the technologies, digestate enhancement system design must be approached holistically. The available outlet must also be considered along with the demand for digestate products. For example, if nutrient recovery is to be employed a market for the recovered products must be secured. Once the desired outputs have been established a choice of process/technology can be made. It is likely that a number of different technologies will be available for selection and at this stage a detailed cost benefit analysis will be required in order to determine the preferred solution on a site specific basis.


Figure 3. Overview of Digestate Enhancement and Treatment Techniques

Digestate

Land

Application Acidification

Land

Application

Drying

Land

Application

Dewatering

Fibre

Drying

Land

application

Enzymatic

hydrolysis

(Biofuel)

Composting

Incineration Land

application Gasification

Reed Beds

Land

Application

Nutrient

recovery Algae Production

Direct

discharge to

watercourse

Discharge to

watercourse /

further

treatment

Microbial Fuel

Cell

Evaporation

Land

Application

Energy Recovery Energy Recovery

Ash Disposal* Residual carbon

disposal**

Energy Recovery

Wet Air

Oxidation

Energy Recovery

Further

treatment

Concentrated

Fertiliser

Land

Application

Concentrated

Fertiliser Biofuel

Discharge to

watercourse /

further

treatment Nutrient addition

Balanced fertiliser

Land

application

Purification

(UF + RO )

Notes

*Ash recovery and product development required

**Residual carbon product development required

Biological

Oxidation

Liquor

Alkaline

stabilisation

Pyrolysis

Char disposal

Land

Application

Land

Application

Land

Application

Enhancement / treatment process Products / benefits


4.2.1 Physical Enhancement Techniques

Physical enhancement techniques can be used to separate the solid and liquid fractions of the digestate. The separated liquid fraction is termed digestate liquor and the separated solid fraction is referred to as digestate fibre. This simple first step enables the separated fractions to be treated individually, providing a wider range of subsequent treatment options. The physical techniques considered in the study can be broadly split into three categories - thickening, dewatering and purification. These physical techniques are well established in waste water treatment; thickening and dewatering applications are the conventional approach to reducing the volume of digestate for subsequent storage, treatment processes or transport off site. The application of these physical techniques may be considered a natural progression into AD facilities and have potential for retro-fitting to existing plant.

Thickening

Thickening is a term used to describe the partial separation of the solid and liquid fractions to achieve a digestate of 5 – 10 % dry solids and a separated liquor. At this solids concentration the digestate is a thick liquid. Thickening is typically employed as an initial pre-treatment stage to reduce the volume of the digestate for subsequent storage. Increasing the solids concentration not only reduces the volume but can also improve downstream processing in terms of throughput capacity and associated electrical and chemical consumptions. Often polyelectrolyte can be added to digestate to improve coagulation and increase the overall solids capture and operability of the thickening system (Evans, 2008). Increasing solids capture is important to ensure the separated liquor does not impose a high biological treatment demand on waste water treatment systems.

Dewatering

Dewatering is a term used to describe the separation of the solid and liquid fractions of digestate to achieve a separated fibre content typically greater than 18% dry solids and a separated liquor. When whole digestate is dewatered, 80% of the mass is removed in the liquor fraction, leaving a dewatered cake of approximately 20%. The ammonium and potassium will be partitioned into the liquor whilst the phosphorus will be largely retained in the dewatered cake. (Fuchs et al., 2010). Dewatering is often employed as a first step in digestate processing. The digestate fibre is a semi-solid “cake” which is easier to store. This combined with reduced volume greatly simplifies handling and reduces subsequent transport costs. Dewatering is also an important treatment technique to improve the feasibility of land application. However as the nutrient content will be lower than in the original whole digestate, nutrient content will need to be considered in securing land-based outlets. Dewatering digestate and reducing the water content also enables a number of other technologies, such as energy recovery, to be economically employed (see Figure 3).


The liquor generated by the dewatering process will contain high levels of ammonium and potassium. Subject to site specific requirements it is likely that this liquor will require a form of treatment before it can be discharged to the public sewer. It may be possible to recycle a fraction of the liquor for feed processing, where the liquor can be used to dilute the feedstock to an acceptable solids concentration. However, the remainder of the liquor will require treatment, involving the removal of nutrients from the liquor, by either recovery or oxidation, to enable the liquor to be discharged. As with thickening, polyelectrolyte can be added to digestate to improve coagulation and increase the overall solids capture and operability of the dewatering system. Increasing solids capture is very important to ensure the separated liquor contains a minimal quantity of digestate solids to limit the biological treatment demand. A typical sample of dewatered digestate is shown in Figure 4.

Figure 4. Dewatered Digestate

Purification (Ultra Filtration and Reverse Osmosis)

Physical purification uses a membrane as a physical barrier which acts as a molecular sieve retaining contaminants, yet allowing water to permeate through. Subject to specific membrane selection, the permeable membrane separates contaminants from the digestate, at a molecular level; this produces a permeate stream potentially suitable for direct discharge to watercourse, and a concentrate which can be applied as a fertiliser (Chiumenti et al.). Depending on the type of membrane selected, different contaminants will be retained on the membranes. Ultra filtration (UF) membranes are capable of retaining soluble macromolecules and larger particles; reverse osmosis (RO) membranes are capable of retaining small molecules and ions. Due to the small pore size of the membranes (<5nm) membranes can be susceptible to fouling and can be damaged by larger particles. To prevent membrane fouling the process is typically used on the separated liquor rather than the whole digestate. The concentrated stream retained by the membrane can be used as a concentrated fertiliser (Fuchs et al., 2010).


4.2.2 Thermal Enhancement Techniques

Thermal techniques use thermal energy (heat) to either remove water from the digestate to increase solids and nutrient concentrations or to recover energy from the digestate (such as via combustion).

Thermal Drying

Thermal drying can be used to significantly reduce the remaining water within the digestate to produce a product of up to 98% ds (SEVAR, 2012, Siemens, 2011). As the thermal energy required to dry the digestate is directly proportional to the moisture content of the feedstock, dewatering is typically employed as a pre-treatment technique prior to thermal drying. The thermally dried product has a greatly reduced volume and, as it is a dry solid material, can be easily handled, stored and transported. If required the dry product can be pelletised to suit product use and aid both storage and transportation. The dried product has a number of potential uses but is normally applied to land as an organic fertiliser or used as fuel for energy recovery. At the elevated temperatures utilised within the thermal drying process, ammonia will come out of the solution and be contained within the evaporated fraction. This will need to be condensed to form condensate, a high strength liquor which will require treatment prior to discharge. It is possible for condensate treatment to be combined with treatment of dewatering liquors. In the case of solar drying no pre-treatment is required (Degremont, 2012, Thermo - Systems, Veolia, 2006b), although it may be beneficial; also no condensate treatment is required. Digestate is fed into a ventilated greenhouse where water is evaporated by thermal energy derived from the sun. The digestate is continuously turned to ensure consistent product quality. If an integrated system is developed, waste heat from a combined heat and power (CHP) system can also be utilised to supplement solar energy via underfloor hot water piping systems. A typical sample of dried digestate is shown in Figure 5.

Figure 5. Dried Digestate


Digestate Concentration (Evaporation)

To concentrate digestate or increase dry solids content, evaporation can be applied. Evaporation utilises thermal energy (heat) to release the moisture within the digestate and increase both nutrient and solids concentration. Unlike the drying techniques discussed above, evaporation aims to retain the nutrients and a proportion of the moisture contained within the digestate. Evaporation is typically utilised for liquor or whole digestate treatment. The final solids concentration will be dependent on the desired product, but concentrations of up to 20% ds can be achieved. As with thermal drying, high temperatures will cause ammonia to be released. This can be overcome by decreasing the pH of the digestate, typically with acid dosing, prior to evaporation (HRS Heat Exchangers, 2010). This approach allows the digestate liquor to be converted into a concentrated fertiliser.

Incineration

Incineration provides an alternative use of digestate to land-based application. Digestate is combusted in order to achieve destruction of organic matter (Perry, 1997). If the moisture content within the digestate is sufficiently low and the incinerator efficiency is high, the process can become auto thermal (the process generates sufficient heat to allow combustion to continue without the need for an external heat source or additional fuel) and energy recovery can be achieved (Envirotherm GMBH, Veolia, 2006a, Tchobanoglous et al., 2004). Autothermal operation will typically require a dry solids content of >40%. The incineration process is best suited to digestates with a high calorific value or where land-based application is not financially viable or practical. Ash from the process can be recovered and used as a construction material for roads or for concrete production. Phosphorus can also be recovered from the ash by acid leaching.

Gasification

Within the gasification process, the oxygen supply is limited to enable partial combustion of organic matter within the feed in order to produce a synthesis gas (syngas). Syngas is a mixture of mainly carbon monoxide and hydrogen, which can be burnt to produce energy (Perry, 1997). As with incineration, for the process to operate efficiently, the feed digestate must have a low moisture content and ideally be in a dry pelletised form (i.e. the product of a thermal drying process). Gasification provides another alternative use of digestate to land-based application. However the technology has yet to be fully developed for this application (Evans, 2008).

Wet Air Oxidation (WAO)

In the wet air oxidation (WAO) process organic material is oxidised within the liquid phase, rather than in the gaseous phase, in contrast to other combustion processes. WAO is achieved at elevated temperatures and high pressure to prevent evaporation. These conditions also enable chemical oxidation of mineral components within the feedstock (Chauzy et al., 2010). The products from WAO are a mineral sludge, a liquid effluent and off gasses (Siemens, 2006).


Full scale plants are operational in Europe - the largest installation is Brussels WWTW which treats digested sewage sludge from a population of 1.1 million. Heat recovery is possible under the right conditions making the process auto thermal (Veolia Water, 2010). The feed for the process is whole digestate, meaning that no pre-treatment is necessary to reduce the moisture content of the feedstock compared with other thermal destruction technologies. However, post-treatment of the by-products, mineral sludge and liquor, may be required.

Pyrolysis

Pyrolysis processes heat the digestate in an oxygen free atmosphere breaking down organics within the feedstock into char and syngas. The syngas typically contains mainly hydrogen, methane and carbon monoxide (Perry, 1997). For the pyrolysis process to operate efficiently the feed digestate must have a low moisture content, and similar to gasification, often requires digestate in a dry pelletised form. The pyrolysis process reduces the mass of the digestate by 70%, significantly reducing transport costs. The char produced by the process can be used as a soil amendment or as a partial replacement for peat in growing media production; both of these applications are undertaken in accordance with appropriate regulatory controls (PYREG, 2011). Pyrolysis process technology has been proven for this application however it is not yet well established.

4.2.3 Biological Enhancement Techniques

Biological enhancement techniques use naturally occurring micro-organisms to convert organic matter within the digestate in order to stabilise the digestate, reduce the organic load or produce novel products such as biofuels. Composting

The composting process aerobically breaks down organic matter in the digestate, resulting in the conversion of ammonia to nitrate which is more stable, and a highly mobile nitrogen source for plants (Tchobanoglous et al., 2004, Botheju, 2010). Temperatures within the compost process can reach 70°C or more due to the intensity of microbial activity, hence pasteurisation can be achieved. However the ability to achieve pasteurisation will be dependent on the composting process and the associated process control. If physically suitable, the digestate can either be composted on its own or it must be co-composted with a range of standard composting feedstocks, such as wood chip and green waste. As an additive to standard composting the digestate provides a source of nitrogen, phosphorus, magnesium and iron, as well as moisture. The standard composting feedstocks provide a bulking agent, improve the carbon (C):nitrogen (N) ratio and consistency of the final product (Evans, 2008). Co-composting is therefore beneficial for both waste streams. Compost quality is and its subsequent use is regulated by PAS100. Provided the required controls are in place, digestate from source segregated waste can be used as a compost feedstock in compliance with PAS 100.


Reed beds

Digestate reed beds can be used to dewater, sanitise and mineralise the digestate over a long period of time, typically 10-15 years (Nielsen and Willoughby, 2005). Whole digestate is fed into a sealed basin containing a bed of reeds. The digestate is treated by bacteria within the root systems of the reeds and evapotranspiration drives off water, typically dewatering the digestate to 30-40% dry solids. Liquor collected from the basins can be recycled as process water or used for irrigation. At the end of the treatment period the beds are dug out and the digestate applied to land (ARM Biosolids, 2012, Blumberg, 2012). The area required for treatment is dependent on the type of digestate but typical loading rates are between 20 and 60 kg dry solids/m²/yr. A typical view of digestate reed beds is shown in Figure 6.

Figure 6. Digestate Reed Bed

Biological Oxidation

Biological oxidation can be used to reduce the loading of biological oxygen demand (BOD) and ammonia in the digestate. The process is most commonly used to treat the digestate liquor prior to discharge either to sewer or watercourse, however it can also be used as a pre-treatment stage or used to treat the whole digestate (wet composting). Typically the digestate is aerated in the presence of bacteria which oxidise the BOD and ammonia. The treatment of liquors in this manner is well proven but can have high operating costs. The process produces a biological sludge as a by-product which can be returned as a feedstock to the digester. Examples of these processes include membrane bioreactors (MBR), sequencing batch reactors (SBR), moving bed bioreactors (MBBR) and the SHARON process.


Biofuel Production

Biomass within the digestate has the potential to be utilised as a feedstock for biofuel production. Several possible techniques are currently being developed. Digestate liquor can be used as a feedstock for the production of algae which in turn can be

converted to biofuel (Iyovo, 2010, Uttleu). Water separated from the algae can be used as

process water or for irrigation; waste algal biomass can be used as a digestion feedstock.

This process is currently operational at pilot scale in the Netherlands (Algae Food & Fuel,

2009).

A typical view of algal bioreactors is shown in Figure 7.

Figure 7. Bioreactors for the production of Algae from digestate liquors

The digestate fibre can also be converted into biofuel by a process of hydrolysis and biological fermentation (Yue, 2010). Ethanol yields from the process are reported to be comparable to some traditional energy crops (Teater, 2011). This process is only currently operational at laboratory scale. It has also been shown that freshwater and nutrients used for bio-ethanol production from traditional energy crops can be replaced with dewatered liquor (Gao, 2010). Using digestate liquor in this manner has been shown to significantly increase ethanol yields.


Microbial Fuel Cell

Microbial fuel cells (MFC) are a novel application of fuel cells that has potential to produce

bioelectricity from the biological oxidation of organic matter. The process utilizes the ability

of particular microorganisms to transfer electrons directly to an anode during respiration

(Aelterman, 2006). The reactions take place under anaerobic conditions. This process is

only currently operational at laboratory and pilot scales. Laboratory trials have shown the

process to be capable of removing 3.99kg COD/m³d (Peixoto, 2012).

A schematic of a microbial fuel cell is shown in Figure 8.

Figure 8. Schematic of a typical microbial fuel cell.

(Zeng et al., 2010)


4.2.4 Chemical Enhancement Techniques

Chemical enhancement techniques utilise chemical reactions and equilibria to recover nutrients from the digestate or modify its properties.

Struvite Precipitation

Struvite is the name commonly used for the chemical compound magnesium ammonium phosphate which can be used as an inorganic fertiliser (Evans, 2009). Under the correct conditions struvite can be precipitated, allowing ammonium and phosphorus to be extracted from the digestate. pH adjustment and magnesium ion addition are usually required (Nawa). Struvite is recovered as a solid material, well suited for export for use as either a fertiliser or as a base feedstock for fertiliser production. Phosphorus is a finite global resource and as a consequence struvite recovery is likely to become more important in the future (Driver, 1998). This process will not normally remove all of the ammonium from the digestate, as there are insufficient quantities of phosphorus present in the digestate.

Figure 9. Struvite products. Precipitated struvite crystals (left) and granular struvite pellets produced in a fluidised bed reactor (right)

Ammonia Recovery

Ammonia, in the form of ammonium, can be recovered from the digestate for use as a concentrated fertiliser or a chemical feedstock. A number of different techniques are commercially available (Maurer et al., 2001). The efficiency of all of these techniques can be improved by increasing the temperature and the pH of the digestate (Guštin, 2011). If waste heat from a combined heat and power (CHP) system is used to increase the temperature of the process, financial support from the Renewable Heat Incentive (RHI) can potentially be claimed.


Ammonia can be stripped from the digestate by contacting with air or steam. Ammonium can then be recovered by scrubbing the stripping gas in a second column (Colsen International, Ángeles De la Rubia et al., 2010). Depending on the scrubbing solution used, ammonium can be recovered in a number of forms including ammonium sulphate and ammonium nitrate, both of which have value as inorganic fertilisers (Evans, 2009). Membrane contactors can also be used to recover the ammonia (Liqui-Cel, 2009). Digestate and sulphuric acid are fed, counter currently, on opposite sides of a microporous hydrophobic membrane. Gaseous ammonia is removed across the air filled pores of the membrane where it reacts with the sulphuric acid to produce ammonium sulphate.

Figure 10. Schematic of a membrane contactor (Liqui-Cel, 2009).

Ion exchange processes recover ammonium by adsorption. Digestate is fed into a packed bed of adsorbent where the ammonium is selectively adsorbed by ion exchange. Once saturated the column is taken off-line and regenerated, recovering the ammonium. The form of the recovered ammonium is dependent on the regenerating solution used (Maurer et al., 2001). A wide range of adsorbents are available including zeolites, clays and resins (Cooney et al., 1999).


Acidification

Sulphuric or other acids can be added to the whole digestate prior to land application to decrease the pH and shift the ammonium/ammonia equilibrium towards ammonium. This reduces nitrogen loss from the digestate once applied to land. Careful consideration must be given to the soil type of the land bank as application of acidic digestate will not always be acceptable (Ministry of Economic Affairs Agriculture and Innovation of the Netherlands, 2010, Frandsen et al., 2011).

Alkaline Stabilisation

Alkaline stabilisation raises the pH of the digestate in order to achieve pathogen kill, neutralise odours (typically hydrogen sulphide) and prevent the digestate from becoming septic. However increasing the pH can cause ammonia to be released causing odour issues. Lime is typically used for the alkali stabilisation (Tchobanoglous et al., 2004). This technique is commonly used to treat dewatered sewage sludges.


5.0 Digestate Enhancement Systems

5.1 Digestate Treatment Systems

The number of enhancement techniques employed, and the complexity of the treatment system, will be highly dependent on the available land-based outlets, potential markets and the desired digestate products. Treatment systems are highly site specific and no single system will be optimal for all sites. For small sites, with readily available local agricultural land, it may be possible to spread the whole digestate to land, providing it is compliant with the relevant legislation (or end of waste status via PAS110 and the ADQP) and satisfies the seasonal spreading requirements. Under these conditions there may be no argument for any digestate enhancement. However, as the distance to the land-based outlet increases further enhancement may be required, such as dewatering or drying to optimise storage, handling and reduce transport costs. Investing in such enhancement technologies must also consider the implication of associated by-products as significant volumes of liquor will be produced, either as filtrate/centrate or condensate which will require treatment prior to discharge to sewer or watercourse. Where no land-based outlet is available thermal conversion may be the only economic option, potentially requiring dewatering, drying and associated liquor treatment. In recent years there has been increased focus on creating marketable products from digestate. Possible methods for achieving this include nutrient recovery and the addition of nutrients to create a more balanced fertiliser. Two possible techniques for enhancement are highlighted in the examples included in Section 5.4 and 5.6. The Scottish and Southern Energy (SSE) energy from waste plant at Barkip utilises HRS scraped surface heat exchangers to recover nutrients from the digestate. The IVAR IKS biofertiliser plant produces an organic fertiliser (MINORGA®) from food waste and sewage sludge in Norway following digestion and thermal drying. Technologies are also emerging to create novel products from digestate such as biofuels. These technologies are still at an early stage of development but have the potential to provide interesting possibilities in the future. The following diagrams provide examples of an integrated treatment system designed to recover nutrients from digestate, recycle dewatered digestate fibre “products” to land and convert digestate fibre to other product forms. These systems will not be applicable to all plants but the diagrams aim to highlight how technologies can be combined and the need to integrate the systems to create a wide range of digestate products. A schematic of a potential integrated digestate enhancement system for the liquor stream is shown in Figure 11 and the fibre stream is shown in Figure 12.


Figure 11. Flow Diagram of Potential Digestate Enhancement System (liquor)

Digester Dewatering Digestate

Liquor

Membrane Bio

Reactor

Magnesium Hydroxide

Struvite

Precipitation

Ammonia Stripping

Magnesium Ammonium Phosphate

(Struvite)

Sulphuric Acid Ammonium

sulphate

Effluent

Water reuse

Heat

Air

Biogas CHP

Power

Sludge

Feed

Potential “products” or resource recoveries

Process Stages

Whole

Digestate


Figure 12. Flow Diagram of Potential Digestate Enhancement Systems (Fibre)

Digester Dewatering

Whole Digestate

Digestate Fibre

Heat

Biogas CHP

Power

Feed Liquor to treatment (Figure 11)

Thermal Drying

Incineration Gasification

Ash recycling / disposal

Boiler

Steam Turbine

Heat

Alkaline Stabilisation

Land Based

Outlets

Syngas to CHP/ Gas turbine

Composting

Lime

Pyrolysis Biochar recycling

Power Power

Potential “products” or resource recoveries

Process Stages Enhancement

Option gate


5.2 Digestate Enhancement Systems

The following describes the combination of treatment technologies/techniques that have the potential to generate a number of complementary digestate “products” see Figure 12. Digestate is first dewatered to separate the liquid and solid fractions. The dewatered fibre can then be applied directly to land or utilised for energy recovery. The first stage of the liquor treatment process presented in Figure 11 is aerobic reduction of chemical oxygen demand (COD) within a membrane bioreactor (MBR). The process is configured such that no ammonia is oxidised. Sludge generated by the MBR is recycled back to the digester. Effluent from the MBR is dosed with magnesium hydroxide to increase the pH and magnesium ion concentration, enabling struvite precipitation. Heat from the CHP is also used to improve struvite removal and the use of heat in this manner may be eligible for financial support from the Renewable Heat Incentive (RHI). Struvite is precipitated and extracted for use as an organic fertiliser. As equimolar amounts of ammonia and phosphate are used in struvite production, and digestate is relatively rich in ammonia, there remain significant quantities of ammonium within the digestate liquor. In order to recover this ammonium the liquor is fed into an ammonia stripper; as the pH and temperature of the digestate have already been increased the conditions are more suitable for the stripping process. In addition, the risk of fouling within the ammonia stripping column is greatly reduced as the COD has already been removed by the MBR. The stripping process recovers the ammonia as ammonium sulphate. Sulphuric acid and heat are used within the process; again this application of heat from the CHP may also be eligible for financial support from the Renewable Heat Incentive (RHI). The treated liquor from the process can be reused as process water, used for irrigation or discharged to sewer. Alternatively, an additional treatment stage can be added in the form of reverse osmosis (RO) to produce higher quality process water or enable direct discharge to a watercourse. Combining the individual process units into the treatment system above provides an integrated and holistic treatment process capable of producing both solid and liquid fertiliser products from digestate. Waste heat from the CHP may also be utilised in order to generate additional income from the RHI and the effluent is suitable for reuse within the process. However this system incorporates several complex subsystems that require careful integration and operation.


5.3 Current Barriers to Enhancement Systems

The use of digestate enhancement technologies/techniques faces a number of barriers in the UK, preventing their widespread adoption. The most significant barrier is the current cost of installation, as well as the operational costs associated with the technologies. This barrier is directly linked to the cost of alternative disposal arrangements such as landfill or energy recovery facilities. However, the relatively low value of digestate products and the associated cost of developing outlets or markets for these products is also a significant barrier. It is imperative that the installation of a single digestate enhancement system does not frustrate, or in the extreme negate, subsequent treatment process additions. However the cost of installing a suite of totally integrated enhancement systems, as described in Section 5.1 and 5.2, presents a significant financial challenge that the sector is unlikely to be able to fund at this stage. A key step to overcoming these barriers is to raise awareness of the waste sector to enhancement technologies and techniques, to reduce costs and to emphasise the financial benefits of implementing enhancement systems to secure potential income from digestate. Whilst this research exercise has identified a range of potential techniques and methods for treating and enhancing digestates in the UK these options will not be adopted until the business models exist that ensure that the financial investment is worthwhile. There is a need to raise awareness of potential improvements to digestates and ensure that the industry is aware that there may be advantages in developing flexible sites where changes can be adopted as new technologies become available.


5.4 Technology Example: Barkip Biogas Facility

5.4.1 Background

HRS Heat Exchangers have been contracted by Scottish and Southern Energy (SSE) to install their Unicus scraped surface evaporators for digestate liquor treatment at the Barkip anaerobic digestion plant. Barkip biogas facility is the largest anaerobic digestion plant in Scotland. The site, located in a former landfill site in North Ayrshire, will process up to 75,000 tonnes of waste food, manure and organic effluent sludges. The plant is the first of its kind to incorporate the heat exchanger technology developed by HRS. Scraped surface heat exchangers use heat generated from the process to concentrate the liquid fraction of the digestate into a nutrient-rich fertiliser.

Scraped surface evaporator plants are designed to overcome fouling issues associated with the evaporation of organic digestate. The interior surface of the heat exchanger tubes is constantly cleaned by internal scrapers to reduce fouling and increase heat transfer efficiency. Although this is the first time the technology will be utilised for digestate processing, the heat exchangers are well proven for other applications, most relevant being the concentration of pig manures.

Key Facts

Technology Supplier: Xergi, HRS Heat Exchangers.

Client: Zebec Energy on behalf of Scottish and Southern

Electric.

Throughput: 75,000 tpa.

Feedstock: Food waste, animal manure, organic sludges.

Technologies employed: Two stage Thermophilic Anaerobic

Digestion, centrifuge, scraped surface heat exchangers.

Project stage: Operational, PAS110 certified.

Capital cost: £1.3 million (evaporation plant only).

Electrical generation: 2.5MW.


Figure 13. Scraped Surface Heat Exchanger Cut Away

5.4.2 Process Description Digestate from the process is fed into a centrifuge in order to separate the liquor and fibre fractions. The digestate liquor is then pre-treated with acid prior to evaporation within the scraped surface heat exchangers to prevent ammonia loss within the evaporator. The volume of acid dosed is dependent on the digestate and the desired retention. Within the evaporator the liquor is concentrated to approximately 20% Dry Solids (DS). The evaporator operates under vacuum at temperatures between 50°C and 70°C. Heat required for the process is provided by the combined heat and power plant (CHP). This application of heat may be eligible for financial support under the Renewable Heat Incentive (RHI). The evaporators at Barkip are capable of treating 10,800 kg/h of digestate liquor and producing 1,565 kg/h of concentrate. The concentrate can then be mixed with the separated digestate fibre to produce a nutrient rich solid fertiliser for export. Distilled condensate for the process is recycled as process water with any excess discharged to public sewer. For the Barkip application the heat exchanger tubes have been constructed from Duplex steel due to the high chloride content within the feedstock. Key Benefits

Nitrogen is retained within the digestate, improving fertiliser

potential.

Concentrated fertiliser reduces transport costs.

Additional income from Renewable Heat Incentive (RHI).

High levels of N/P/K retained within the digestate.


5.5 Technology Example: Lee Moor EFW

5.5.1 Background

British engineering company AeroThermal Group Ltd has been granted planning permission to develop a sustainable waste and resource treatment facility at the site of Imerys Minerals Ltd at Lee Moor in South Devon. AeroThermal’s autoclave is a pressure vessel that steam treats its contents at a constant temperature and pressure, serving to sterilise, clean, and break-down organic and lignin structures and reduce waste volume by approximately 60%. The Lee Moor facility will utilise an autoclave to pre-treat the digester feedstock. Pre-treatment by autoclave pasteurises, cleans and breaks down organic matter and lignin structures within the feedstock. This enables contaminants within the feedstock to be removed more effectively, greatly enhancing biogas generation and the quality of the digestate. Once operational the site will divert 58,000 tonnes of waste from landfill every year and generate 26 gigawatts of renewable electricity. Recyclable materials will also be recovered from the waste stream and the stable digestate, a by-product of the Advanced Anaerobic Digestion (AAD) process, will be used to help restore parts of the adjoining Lee Moor China Clay workings.

Figure 14. Autoclave Pressure Vessel


5.5.2 Process Description Feed material is loaded into a pair of autoclaves in weighed 10 tonne batches. The autoclaves are fed up to 10 times per day, each at a maximum of 10 tonnes per batch via a system of conveyors from the weighing hopper. The autoclaves operate in an alternating batch mode: residual steam is recycled from the duty unit that has completed its processing to the autoclave that has been loaded and is waiting to start the cycle. This procedure not only improves the steam utilisation efficiency but also significantly reduces the release of steam to atmosphere. Once loaded the duty autoclave is rotated. Flights within the autoclave lift the feed material towards the top of the chamber where it then falls back to the bottom of the vessel to create a continuous mixed flow. Steam is then injected until the autoclave internal pressure of 5.2 bar and a temperature of 160°C is achieved. These conditions are maintained for the duration of the treatment process. After treatment the autoclave is returned to atmospheric conditions, the bottom door is opened and the rotation of the vessel is reversed. This allows the flights within the vessel to act as a screw conveyor and force processed material out. After processing, inorganic material and contaminants can be easily removed via mechanical separation providing a clean, pasteurised, organic rich feedstock for anaerobic digestion. Digestate from the anaerobic digestion process is dewatered by a conventional centrifuge. The digestate fibre is used for land restoration. Liquors from the process are partially treated by dissolved air filtration before being recycled for feed preparation. Excess waste water which cannot be recycled will be treated by a membrane bioreactor (MBR) to enable direct discharge to an adjacent watercourse.

Key Facts

Technology Supplier: AeroThermal Group Ltd.

Throughput: 58,000 tpa.

Feedstock: Municipal waste.

Technologies employed: Autoclave, Anaerobic Digestion,

centrifuge dewatering.

Project stage: Pre construction.

Capital cost: £15 million.

Electricity generation: 3MW.


5.6 Technology Example: MINORGA ® Bio fertiliser, Norway

5.6.1 Background

Research has been undertaken in Norway at Stavanger Regional Wastewater Treatment Plant (RWTP) into developing a digestate-based organic fertiliser with a consistency, particle size and nutrient composition comparable to mineral fertilisers. Extensive field and product trials have concluded with an organic product called MINORGA®. During the period 2007-11 extensive trials and field experiments were undertaken into the development of an organic fertiliser based on thermally dried digestate. The research has been conducted at Stavanger Regional Wastewater Treatment Plant by the plant operator IVAR IKS in conjunction with the HØST Valuable Waste Company. The concept of the fertiliser product is based on supplementing the phosphorous within the thermally dried digestate produced at Stavanger with the addition of nitrogen and potassium. The resulting product, called MINORGA®, is a granular organic fertiliser with an N-P-K ratio of 10-2-5.

Figure 15. MINORGA® Pellets

Key Benefits

Pasteurisation.

Waste resource recovery.

Waste minimisation.

Digestate recycling.


The product is considered more environmentally friendly than similar mineral fertilisers leading to less run-off and a prolonged fertilising effect. In that context, the product also offers a better balanced phosphorus supply (Tornes et al, 2010). Agronomic trials showed no significant differences in nutrient uptake between MINORGA® and commercially available mineral fertilisers. Spreading tests performed by a conventional farm spreader showed distribution patterns very similar to that of commercially available mineral fertilisers. 5.6.2 Process Description

The development to date has been based on the manufacture of an organic fertiliser product from a mixture of digestate and mineral compounds such as urea and potassium chloride. The facility to produce the fertiliser product will be integrated into the existing sewage sludge treatment process at Stavanger (RWTP). The fertiliser facility comprises of storage silos for the addition of urea, potassium chloride, meat bone meal or other mineral salts/high quality organic by-products, a dosage and mixing system, pelletising plant and a big bag loading and packaging system. Thermally dried digestate produced by the Stavanger RWTP will be directed to a batch dosing and mixing system before the mixture is transported to the pelletising plant. The pellets will pass through an air cooler followed by a sieve to ensure uniform product quality before the product is stored in a product silo. Undersized pellets will be recycled back to the pelletising system. The final product will be packed in big bags containing 600 kg of MINORGA®, the registered name of the organic fertiliser. Pasteurisation of the digestate is achieved within the thermal drying plant; the presence of organic pollutants has been systematically investigated in surveillance studies and found to be either absent or at negligible and acceptably low levels. The process flow diagram for the system is schematically presented below.

Key Facts

Technology Supplier: Graintec, Denmark.

Throughput: 10,000 tpa of MINORGA®.

Feedstock: Sewage sludge and organic wastes including

domestic food waste and catering waste from hotels.

Technologies employed: Mesophilic anaerobic digestion,

thermal drying, nutrient addition, palletisation.

Project stage: Contract established.

Capital cost: NOK 40M (£4.3M).


Figure 16. MINORGA® Process Schematic

The total investment costs are approximately NOK 40 million. To reduce operating costs IVAR IKS continue to investigate alternative sources to urea and potassium chloride including nitrogen and potassium recovery from dewatering liquors generated at the regional wastewater treatment plant.

Key Benefits

Pasteurisation.

Waste minimisation.

Waste resource recovery.

Low transport volume of dried and pelletised product.

Production of a marketable organic fertiliser.


6.0 European Perspective

6.1 Background

Landfill rates for municipal waste have decreased steadily from 62% in 1995 to 37% in 2010 in the EU-27, with 38 % of municipal waste recycled or composted/digested in 2010 compared to 17 % in 1995. EU and national policies targeting municipal waste have been important drivers of this development (EEA 2012). The biomass categories used as substrates (feedstock) for anaerobic digestion in European biogas production are animal manures and slurries; agricultural residues and by-products; digestible organic wastes from food and agri industries; the organic fraction of municipal and catering wastes; sewage sludge; and dedicated energy crops such as maize, miscanthus, sorghum, and clover – particularly in Austria and Germany (Al Seadi et al. 2008). Digestate composition and qualities are a function of input materials and process approach, hence enhancement technologies need to be robust and capable of dealing with a range of inputs in order to achieve significant market penetration.

6.2 Survey Data

A 2011 survey of AD facilities across Europe (including the UK) identified several thousand specific facilities (excluding waste water treatment plants). Typically these are co digestion facilities accepting a variety of different inputs, constructed by a variety of different technology providers (Voss 2012). The survey indicated that EU countries can be grouped based on the number of AD facilities identified as follows:

>>1,000 facilities: Germany.

>>100 facilities: Austria, Belgium and Luxembourg combined, the Netherlands.

~ 100 facilities: Denmark, Italy, Czech and Slovak republics combined, UK.

< 100 facilities: Finland, France, Hungary, Poland, Portugal, Spain, Sweden.

~ 10 facilities: Bulgaria, Greece, Latvia, Romania, Slovenia.

The level of detail is variable but indicates that the majority of these digester facilities accept biomass crops in Austria and Germany; whereas facilities in the UK, Finland, France, Sweden, the Netherlands focus on waste derived from agricultural or urban sources. The number of digestion facilities reported in the survey is higher than a study quoted by the Technical Report for End-of-Waste Criteria (EC JRC 2011), which identified 166 anaerobic digestion facilities for biowaste and municipal solid waste across 15 EU countries. The difference between the total number of AD facilities is explained by the fact the End of Waste report did not include farm based systems. Typically solid/liquid separation is a precursor to any further product enhancement treatment, which is very similar to digestate management practice for wastewater treatment plants.


6.3 Transport Optimisation

There is growing interest in the development of more readily transportable products from digestion. Many European facilities operate solid/liquid separation, to reduce transport costs for the solids fraction, to facilitate storage (e.g. over the closed season) and to increase the potential radius of digestate use. Some facilities use evaporation to concentrate liquid fractions using waste heat, again to increase the effective radius of use. This is of particular interest in Germany where subsidies for electricity production provide the incentive for AD facilities, where there is no immediate CHP opportunity (Voss 2012). Hybrid systems utilising both AD and composting systems are also in use. Composting of digestate with additional feedstocks, is undertaken to reduce water content and overcome stability and odour problems; in addition hybrid systems have been found to improve materials handling and improve product value (e.g. Swedish Gas Association 2008). The IEA Bioenergy Task 37 group recommend that the solids fraction should be stored without disturbance, or even composted, in order to avoid methane emission (Lukehurst et al. 2010). The Netherlands view the development of solutions to digestate use as important in enabling the expansion of AD as a biogas resource, and has a range of enhancement approaches under consideration, including the extraction of substitute fertilisers from digestates (New Gas Platform, Green Gas work group 2010). The availability of opportunities for digestate use is a key factor in selecting the location of AD facilities in the Netherlands (New Gas Platform, Green Gas Work Group 2010, Energy Transition, New Gas Platform 2011). Example configurations include the following (IEA Bioenergy Task 37 2012, Swedish Gas Association 2008): Boden plant, Sweden: operating since 2003: thermophilic (55oC) co-digestion of sewage

sludge (960 tonnes dry solids per year) and household food wastes (1,200 tonnes per

year) producing biogas for transport vehicle use and waste heat which is used for district

heating. 1,600 tonnes of digestate is produced per year, some of which is used to

produce a soil conditioner. Digestate is de-watered by a centrifuge plant to

approximately 30% dry solids. The dewatered digestate is stored in silos and transported

by truck.

Helsingborg plant, Sweden: operating since 1997, feedstock: household food wastes,

food industry wastes and pig manure, approximately 45,000 tonnes per year. Digestate

is pumped to farm users via a 10 km pipeline (capacity 20,000 tonnes per year).

Karpalund plant, Sweden: operating since 1996, feedstock: household food wastes,

manure, slaughterhouse waste, approximately 60,000 tonnes per year. Digestate is

sieved to remove debris such as plastics and then dewatered before storage and use.

Inwil plant, Switzerland, operating since 2008, based on a thermophilic plug flow

digester treating source separated collection of municipal solid waste and two mesophilic

continuously stirred tank reactor digesters treating mainly pig manure. Industrial waste

is also accepted. The total waste volume treated is approximately 60,000 tonnes per


year. After dewatering the solid output (13,000 tonnes per annum) is matured under

aerobic conditions, and the liquid output is treated by ultrafiltration and reverse osmosis

to obtain a concentrated liquid fertiliser (10,000 tonnes per annum) and clean water.

The liquid fertiliser is transported to farmers. (Similar liquid fraction treatment

configurations have been tested at pilot scale in Sweden - Svenskt Gastekniskt Center

2010).

6.4 Summary of EU Waste Sector

There is widespread interest in the development of enhanced products from digestates. The work of the International Energy Authority (IEA Bioenergy Task 37) indicates research interests for processing digestate into value added products are present in Austria, Denmark, Finland, France, Germany, Netherlands, Norway and Sweden. The exact nature of the research being undertaken is not always specified in the IEA documents but is largely related to chemical, physical and thermal processes for solid liquid separation, and downstream conversion of solids or liquids into fertiliser products such as struvite. Research in Germany includes an investigation of the utilisation of CO2 and nutrients from digestate for micro-algae production and hydrothermal gasification of digestate for additional CO2 and CH4 production (IEA 2010, 2011, 2012). Over a number of years research has also been supported under the EC Framework Research Programmes, including several hundred projects related to anaerobic digestion through the Cooperation, Ideas, People and Capacities sub-programmes. These can be viewed using the CORDIS database at http://cordis.europa.eu/home_en.html. A small proportion of these projects are related to digestate enhancement. These include investigations of: struvite recovery, sulphur recovery (for high sulphur content wastes), ammonia stripping and recovery, ethanol production, algal production, and thermal conversion of digestate to energy. The EU funded Eco-Innovation programme supports the market replication of new environmental products and services across a range of categories, including projects related to anaerobic digestate products. Eco-Innovation project information is posted on http://eaci-projects.eu/eco/page/Page.jsp. Projects agreed for 2012 will be listed on the site. Several AD facility development projects are also being funded under the Intelligent Energy Europe Programme (http://ec.europa.eu/intelligentenergy).

http://cordis.europa.eu/home_en.html

http://eaci-projects.eu/eco/page/Page.jsp

http://ec.europa.eu/intelligentenergy


7.0 Summary

The technologies and enhancement techniques identified within this study represent a wide range of potential options for digestate enhancement which could support the development of digestate products. This wide range indicates that no one technology is applicable for all applications and a range of solutions will be required to support the planned increase in anaerobic digestion facilities in the UK with the consequential increase in volumes of digestate generated. The majority of digestate currently produced in the UK is recycled to land as either whole digestate or dewatered fibre. Digestate liquor is commonly treated by biological oxidation, particularly in the waste water industry. However, there is increased interest in creating improved fertiliser products from digestate, in order to increase its value, secure outlets and potentially generate an additional revenue stream for the plant. A number of the technologies identified in this report have proven potential to create these products. Currently there are a large range of options available for digestate treatment and recovery. However the most significant barrier in the UK is the current cost of installation and the operational costs associated with the technologies. Ultimately the type of treatment employed to provide the most economic recovery route will depend on a number of factors, including:

level of enhancement desired / required;

market for digestate products;

plant throughput;

available footprint;

feedstock;

availability of land-based markets;

distance to land-based markets; and

soil type of receiving land.

UK markets for waste derived digestates are immature. There is existing competition in land based markets, not least with conventional inorganic fertilisers. However, in the future as natural phosphorous resources decrease and the cost of inorganic fertilisers increase, farmers will look to find alternative and potentially cheaper sources of nutrients for their crops. It is anticipated that the practical demand for these products will increase as the financial and commercial value increases. The key challenge in the short term will be to manage the increase in digestate and secure outlets. From the EU waste sector perspective, there are clearly similar challenges and goals to the UK waste sector. A key difference is the cost of waste treatment and disposal which is significantly higher in the EU than in the UK. This economic differential has driven certain EU states to develop anaerobic digestion facilities with EU funded support for the development of digestate products and markets. This development work has been underway over the last ten years. Research and development continues with a focus on the development of enhanced products from digestates, most notably the International Energy Authority (IEA Bioenergy Task 37) and the Eco-Innovation project.


In conclusion, a wide range of technologies and techniques are available to create novel digestate products - such as concentrated nutrient streams for the production of standardised fertiliser products. However, operational experience of these technologies in the UK is currently limited and in many cases, direct land application is likely to remain the most economic option. Operational experiences should be sought from the EU, where systems have been installed and digestate products created to satisfy outlet demand. As the market for digestate products and competition for land-based markets increases, it is anticipated that these technologies will become more important in the near future.


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Enhancement and treatment of digestates from anaerobic digestion A1

Appendix 1

Organisation Contact List


Organisation Type Website

ADBA (Anaerobic Digestion and

Biogas Association)

Association http://www.adbiogas.co.uk/

Organics – Recycling (Association for Organics

Recycling)

Association http://www.organics-recycling.org.uk/

ESA UK (Environmental Services

Association)

Association http://www.esauk.org/

CIWM (Chartered Institution of

Wastes Management)

Association http://www.ciwm.co.uk/CIWM/CIWMHome.aspx

UK CPI (Centre for Process

Innovation)

Association http://www.uk-cpi.com/

Water UK Association http://www.water.org.uk/

Aqua Enviro Consultancy

http://www.aquaenviro.co.uk/

REA (Renewable Energy

Association)

Association http://www.r-e-a.net/

ADAS Consultancy http://www.adas.co.uk/

AWS Burdens Environmental Ltd

Joint Venture: Waste

Management Company and Consultancy

HRS Heat Exchangers Ltd Technology Supplier

http://www.hrs-heatexchangers.com/en/default.aspx

International Energy Agency Association http://www.iea.org/

Black and Veatch Technology Supplier

http://bv.com/

Pyreg UK Technology Supplier

http://www.pyreg.com/English.html

Leibniz Universität Hannover University www.isah.uni.hannover.de

University of Reading University www.reading.ac.uk/ges/

University of Brighton University http://www.brighton.ac.uk

University of Minho University http://bio4e.deb.uminho.pt

VE efficiency solution GmbH Consultant www.ve-gmbh.de

Biobench B.V. Business &

Service www.biobench.com

Environmental Institute Laboratory www.ei.sk

Cre-Composting Association of Ireland Teo

Stakeholder Association www.cre.ie

Simbiente RTD &

Consulting www. simbiente.com

http://www.isah.uni.hannover.de/

http://www.reading.ac.uk/ges/

http://www.brighton.ac.uk/

http://bio4e.deb.uminho.pt/

http://www.ve-gmbh.de/

http://www.biobench.com/

http://www.ei.sk/

http://www.cre.ie/


Organisation Type Website

C-CURE Technology

Supplier www.ccuresolutions.com

University of Debrecen University http://portal.agr.unideb.hu

ACR+ Association (www.acrplus.org)

Austrian Biomass Association Association http://www.biomasseverband.at/

COPA-COGECA, Association www.copa-cogeca.be/Main.aspx?page=HomePage&lang=en

ORBIT / European Composting Network

Association http://www.compostnetwork.info/

ISWA (international solid waste

association)

Association http://www.iswa.org/

WSSTP [sewage] Sludge Group

The Water Supply and Sanitation

Technology Platform

http://www.wsstp.eu/site/online/home

http://www.ccuresolutions.com/

http://portal.agr.unideb.hu/

http://www.acrplus.org/

http://www.copa-cogeca.be/Main.aspx?page=HomePage&lang=en


Appendix 2

Technical Data Sheets


APPENDIX 2 - Technical Data Sheets

This appendix includes the technical data sheets for the digestate enhancement technologies discussed in Section 4. The aim of these technical data sheets is to provide a brief description of the operating principle of the technology/technique, operating conditions and associated benefits, challenges and opportunities. The criteria within the data sheets are all scored using the same system of between 1 and 5 stars, where: 1 star represents a low value or poor performance against the criteria, and 5 stars represents a high value or excellent performance. The exception to the above is the stage of development which is ranked out of 6, as classified below.

Note that the stage of development refers to the technologies application to digestate. For example, a technology that is established in another field but is emerging as a digestate treatment will be scored as an emerging technology.

Stage of Development Rating

Established

Maturing

Emerging

Near commercial

Pilot

Research


Technical Data Sheets Page

Pre and in-digestion Enhancement Thermal Hydrolysis A4 Autoclave Systems A6 Enzymic Liquefaction A8 In vessel Grit removal A10 In-vessel scum removal A12 Physical Enhancement Thickening (Belt) A14 Thickening (Centrifuge) A16 Dewatering (Belt) A18 Dewatering (Centrifuge) A20 Dewatering (Hydrocell) A22 Dewatering (Bucher press) A24 Dewatering (Electrokinetics) A26 Purification (Ultrafiltration and Reverse Osmosis) A28 Thermal Enhancement Drying (Rotary Drying) A30 Drying (Belt drier) A32 Drying (J-Vap) A34 Drying (Solar) A36 Evaporation (scraped surface heat exchangers) A38 Conversion (Incineration) A40 Conversion (Gasification) A42 Conversion (Wet air oxidation) A44 Conversion (Pyrolysis) A46 Biological Enhancement Composting A48 Reed Beds A50 Biological Oxidation A52 Biofuel Production (Algae) A54 Biofuel Production (hydrolysis of fibre to Bioethanol) A56 Microbial Fuel Cell A58 Chemical enhancement Struvite precipitation A60 Ammonia recovery (Stripping + Scrubbing) A62 Ammonia recovery (Membrane Contactor) A64 Ammonia recovery (Ion Exchange) A66 Alkaline Stabilisation A69


Process Thermal Hydrolysis

Process Type Thermal (pre-treatment)

Objectives To break down the cell structure of organic matter to improve biogas production and ease of dewatering. Pasteurisation is also achieved.

Process Flow Diagram

Process Description

Within thermal hydrolysis processes the feed is heated and pressurised

by steam within a reaction tank before the tank is rapidly depressurised

(flashed). The process results in the breakdown of cell structures

within the biomass, improving digestibility and digestate quality. When

dewatered a higher dry solids cake can be obtained, making the

material easier to store and transport. The high temperature of the

process (150 – 180°C) pasteurises the biomass.

Reactors operate in batches, therefore several are usually combined to

provide continuous treatment. Energy recovery between the reactors

optimises energy efficiency.

If the anaerobic digestion process utilises a CHP plant, heat from the

CHP can be used to provide steam for the thermal hydrolysis process.

Several plants are operational in the UK treating sewage sludge

generated by waste water treatment works. Two full scale plants,

treating food waste, are also operational in Norway, with two more at

the construction / planning stage.

Benefits Challenges

Pasteurisation. Increase biogas yield. Increased dry solids if dewatered.

Improved solids destruction, reducing digestate mass.

High temperature and pressure. High energy requirement.

Heat Recovery

Digester

Steam

Feed Reactor Buffer


Operating Conditions Sustainability

Feed solids %ds <18% Power Usage

pH N/A Odour Potential

Temperature °C 150 - 180 Chemical usage None

Pressure 5-10 bar Water usage

Throughput (m³/d) Noise

Chemical Consumption N/A Hazard

Resistance to chemical attack

Temperature

Resistance to abrasives Pressure

Reliability Chemical N/A

By products N/A Carbon footprint

PAS 110 Achieves Pasteurisation

Product competition N/A

Ease of operation Product market security N/A

Safety of operation

Ease of commissioning

CAPEX High OPEX

Stage Of Development Established

Suppliers/Reference Plants CAMBI Lillehammer (Norway), Verdal (Norway) Cardiff WWTW (UK), Aberdeen WWTW(UK) Veolia (Biothelys) Saumur (France) Château-Gontier (France)

Availability of UK Support

Feasibility

There is extensive experience of this technology within the waste water industry and more recently it has been successfully applied to source segregated food wastes (Norway). The technology provides the pasteurisation required by legislation, as well as improving digestate quality and biogas production.

(CAMBI, 2011, Veolia, 2011) CAMBI. 2011. Biowaste Treatment [Online]. Available: http://www.cambi.no/wip4/detail.epl?cat=10645 [Accessed 11/05/2012]. VEOLIA. 2011. Biothelys [Online]. Available: http://www.veoliawaterst.com/biothelys/en/ [Accessed 11/05/2012].

http://www.cambi.no/wip4/detail.epl?cat=10645

http://www.veoliawaterst.com/biothelys/en/


Process Autoclave

Process Type Thermal (pre-treatment)

Objectives To pasteurise, clean and break-down the organic matter within the feedstock, rendering it more amenable to digestion


Process Description

An autoclave is a pressure vessel that steam-treats its contents at a constant temperature and pressure, serving to pasteurise, clean and break-down organic matter within the feedstock. After processing, inorganic material and contaminants can be removed via mechanical separation systems providing a pasteurised, organic rich feedstock for anaerobic digestion. The ability of this process to improve contaminant removal makes it particularly relevant for MBT and food waste applications.

Benefits Challenges

Pasteurisation. Easier removal of contaminants post

treatment. Improved solids destruction, reducing

digestate mass.

Increased biogas yield.

High temperature and pressure. High energy requirement.

Steam Heat Recovery

Digester Buffer

Steam

Feed

Autoclave 1

Autoclave 2



Feed solids %ds N/A Power Usage


Temperature °C 160 Chemical usage None

Pressure 5.2 bar Water usage (feed preparation)

Throughput Noise No data available



Temperature


Reliability Unknown Chemical N/A

By products Carbon footprint



Ease of operation No data available

Product market security N/A

Safety of operation No data available

Ease of commissioning No data available

CAPEX No data available

OPEX No data available

Stage Of Development Emerging

Suppliers/Reference Plants AeroThermal Limited


Feasibility

The first full scale plant to utilise this technology is currently being designed (see AeroThermal Lee Mill example). This technology has the potential to greatly improve the quality of digestate from MBT type processes and food waste applications. The odour potential associated with this process is caused by the requirement to open the vessel after each batch, however this can be contained with automated loading/emptying systems.

(AeroThermal Limited, AeroThermal Group, 2008) AEROTHERMAL GROUP. 2008. AeroThermal's Solution [Online]. Available:

http://www.aerothermalgroup.com/aerothermal/aerothermals-solution.aspx [Accessed 09/05/2012. AEROTHERMAL LIMITED. Municipal Solid Waste Processing [Online]. Aerothermal Limited. Available:

http://www.aerothermalgroup.com/aerothermal/municipal-solid-waste.aspx [Accessed 09/05/2012.

http://www.aerothermalgroup.com/aerothermal/aerothermals-solution.aspx

http://www.aerothermalgroup.com/aerothermal/municipal-solid-waste.aspx


Process Enzymic Liquefaction

Process Type Biological pre-treatment

Objectives To break-down the organic matter within the feedstock


Process Description

The REnescience process developed by DONG Energy utilises enzymes to

breakdown and separate the organic matter within the feedstock prior to

digestion. The enzymes convert the feedstock to an organic rich liquid

which can be easily separated from inorganic material and physical

contaminants. This system is therefore ideally suited for the pre-treatment

of mixed waste streams i.e. MBT.

The first stage in the process is a non-pressurised thermal treatment with

hot water or steam, which serves to open-up the biomass in order to make

it accessible to the enzymes. In the second stage enzymes are added to the

process to liquefy and further break down the biomass cell structure,

improving digestibility and separability. The third stage of the process

separates the bio-liquid from the remaining solid waste (which latter is then

split into a recyclable fraction, or fractions, and a potential RDF fraction).

A 1 tonne per hour pilot plant has been operating successfully in

Copenhagen (Denmark) since 2009, using untreated residual municipal

waste. Other C&I waste streams have also been processed.

Benefits Challenges

Improved digestate quality by removal of contaminants.

Very high recovery of the organic fraction of the residual waste into a bio-liquid which is easily handled for pumping, storage and transportation.

Increased digestibility and biogas yield.

Low volume of solid digestate results from the bio-liquid.

No operational facilities within the UK. Use of enzymes not yet widespread in the UK.

DONG Energy are searching for a partner or partners to establish a commercial demonstration plant in the UK.

Enzymatic treatment

Enzymes

Digester

Hot water / Steam

Feed Pre-treatment

Temperature

control



Feed solids %ds N/A Power Usage


Temperature °C <100oC Chemical usage

Pressure Atmospheric Water usage

Throughput (m³/d) Dependent on inputs

Noise

Chemical Consumption Use of enzymes Hazard


Temperature


Reliability Chemical


PAS 110 Not compliant on mixed waste

Product competition

Ease of operation Product market security to be established

Safety of operation


CAPEX Confidential, but competitive

OPEX Confidential, but competitive

Stage Of Development Pilot

Suppliers/Reference Plants Pilot plant operational in Copenhagen (Denmark)


Feasibility

This technology has the potential to greatly improve the quality of digestate from MBT type processes, however it is not yet established at full scale. The construction of a full scale plant is required to demonstrate the feasibility of this technology. DONG Energy are currently examining the feasibility for the design, build and operation of a commercial scale demonstration plant in the UK , and are seeking potential project partners.

Dong Energy,. 2012, RENESCIENCE [online] Available at http://www.dongenergy.com/REnescience/Pages/index.aspx. Accessed

13/06/2012

http://www.dongenergy.com/REnescience/Pages/index.aspx


Process FITEC Anaerobic digestion grit removal system

Process Type Mechanical separation

Objectives To remove grit and other solid contaminants from the digester/digestate


Process Description

No pre-treatment process will be able to remove physical contaminants

entirely, and as such these contaminants will be transported into the

digester. These contaminants accumulate within the digester causing

mechanical problems and reducing the operational volume of the

digester. Any material carried over into the final digestate will reduce its

quality. This is particularly relevant for food waste and MBT applications.

The FITEC grit removal system utilises a mechanical scraper to

continuously remove settled grit and sediment from the bottom of the

digester, preventing grit build-up and improving digestate quality.

Within the digester a centrally mounted scraper arm continuously

rotates, forcing settled material to the outside of the digester where it is

discharged to a hopper in the digester floor slab. Collected material is

pumped to an external gravity separator where the grit is removed and

the separated liquid is returned to the digester.

Benefits Challenges

Improves digestate quality. Continuous cleaning of the digester. Reduced requirement for feedstock grit

removal.

Improves lifetime of mechanical equipment within the digester.

Minimises digester volume loss due to grit accumulation.

Limits digester diameter to 18m. Requires a flat digester floor.

(Finsterwalder Umwelttechnik GmbH & Co. KG, 2012)

Separated grit and sediment

Digester Gravity separator

Digestate return



Feed solids %ds 0 - 7% Power Usage 0.75kW / d

pH 6-8 Odour Potential

Temperature °C 30-55 Chemical usage None

Pressure - Water usage

Throughput (m³/d) 1,5t/d Noise



Temperature



By products Grit Carbon footprint

PAS 110 Removal of physical contaminants



Safety of operation


CAPEX £25,000 (13m diameter)

OPEX

Stage Of Development Maturing

Suppliers/Reference Plants Finsterwalder Umwelttechnik GmbH & Co. KG: Langage Farm


Feasibility

This technology enables the continuous removal of settleable physical contaminants from the digester. This improves digestate quality and compliance with the parameters of PAS 110 by reducing the level of physical contaminants within the digestate. Grit and other contaminant build-up within the digester is also reduced. However a specific digester design/build is required.

FINSTERWALDER UMWELTTECHNIK GMBH & CO. KG. 2012. Digester Equipment [Online]. Available: http://www.fitec.com/user/endigequip.html [Accessed 16/05/2012].



Process FITEC Anaerobic digestion scum removal system

Process Type Mechanical separation

Objectives To remove floating contaminants from within the digester


Process Description

No pre-treatment process will be able to remove contraries entirely and

as such physical contaminants can be transported into the digester.

These contaminants then accumulate within the digester causing

mechanical problems and reducing the operational volume of the

digester. Any material carried over into the final digestate will impair its

quality. This is particularly relevant for food waste and MBT applications.

The FITEC scum removal system utilises a mechanical skimmer to

continuously remove floating contaminants and scum from the top of

the digester, preventing these materials from fouling mechanical

equipment and improving digestate quality.

Within the digester a centrally mounted skimmer continuously rotates

forcing settled material to the outside of the digester where it is pumped

to an external filter. The material is separated, contaminants discharged

to skip and the separated liquid is returned to the digester. The system

is self regulating and gas tight.

Benefits Challenges

Improves digestate quality. Continuous cleaning of floating

contaminants from the digester

No need to clean feedstock perfectly from plastics.

Reduces fouling of the mechanical equipment within the digester.

Maximises digester capacity .

Limits digester diameter to 18m. Requires a solid digester roof.

(Finsterwalder Umwelttechnik GmbH & Co. KG, 2012)

Separated material

Digester Digestate return

Filter



Feed solids %ds 0-7% Power Usage 2.5kW / h


Temperature °C 30-55 Chemical usage None

Pressure - Water usage

Throughput (m³/d) 5t/h Noise 30dB/A 10m



Temperature

Resistance to abrasives medium Pressure



PAS 110 Removal of physical contaminants



Safety of operation


CAPEX £70,000 OPEX


Suppliers/Reference Plants Finsterwalder Umwelttechnik GmbH & Co. KG: Langage Farm


Feasibility

This technology enables the continuous removal of floating physical contaminants from the digester. This improves digestate quality and compliance with the parameters of PAS 110, by reducing the concentration of physical contaminants within the digestate. Contaminant build-up within the digester is also reduced, minimising potential for fouling whilst maximising digester volume. However a specific digester design/build is required.

FINSTERWALDER UMWELTTECHNIK GMBH & CO. KG. 2012. Digester Equipment [Online]. Available: http://www.fitec.com/user/endigequip.html [Accessed 16/05/2012].



Process Gravity belt thickening

Process Type Thickening (physical)

Objectives To increase solids concentration


Process Description

Within a belt thickener the digestate is evenly distributed over a porous

cloth belt which is moved by a series of rollers. Water is removed by

gravity as the belt carries the digestate towards the discharge. Water

removal is improved by a series of plough blades along the belt that fold

the digestate allowing released water to drain through the belt.

Thickened sludge is removed from the belt by a scraper system, and the

belt then passes through a wash cycle.

Polymer addition is usually necessary in order to achieve good results.

Using this system digestates can be thickened to 5-7% dry solids with a

solid capture rate of up to 98%.

Benefits Challenges

Increased solids concentration. Reduced transport volume of solid

fraction.

Filtrate (liquor) disposal is still required.

Belt thickener Feed

Liquor

Thickened digestate



Feed solids %ds >0.5% Power Usage

pH >6.5, <7.5 Odour Potential

Temperature °C Ambient Chemical usage

Pressure Ambient Water usage Low(washing, polymer make-up and carrier)

Throughput (m³/d) Large range of unit sizes available

Noise

Chemical Consumption Polyelectrolyte

Hazard


Temperature

Resistance to abrasives Pressure (localised mechanical)


By products Liquor - volume dependent on degree of thickening

Carbon footprint

PAS 110 No effect on status

Product competition

Ease of operation Product market security

Safety of operation


CAPEX £50k + OPEX


Suppliers/Reference Plants Ashbrook Simon Hartley Numerous WWTW reference sites Ovivo Water


Feasibility

Belt thickening technology is widely implemented to good effect in the waste water industry to reduce the solids handling volume. Consideration must be given to the press liquor as further treatment may be required prior to reuse or disposal.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Ashbrook Simon- Hartley, 2012. Aquabelt [online] available at: http://www.as-h.com/uk/en-gb/aquabelt.aspx [Accessed 28/02/2012]

http://www.as-h.com/uk/en-gb/aquabelt.aspx


Process Centrifuge thickening

Process Type Thickening (physical)

Objectives To increase solids concentration


Process Description

Centrifugal thickening is a mechanical process that uses centrifugal

force, from the rapid rotation of a cylindrical bowl, to achieve solid -

liquid separation. Sludge is fed continuously into the rotating bowl of

the centrifuge. Solid particles are forced towards the bowl wall by

centrifugal force, where they are moved by an internal scroll towards

the solids discharge. Liquid effluent is discharged from the bowl via

adjustable weir plates.

Polymer addition is usually necessary in order to achieve good results.

Using this system digestates can be thickened to 5-7% dry solids with a

solid capture rate of up to 98%.

Benefits Challenges

Increased solids concentration. Reduced transport volume of solid

fraction.

Centrate (liquor) disposal is still required.

Centrifuge Feed

Liquor

Thickened digestate nedndeCake



Feed solids %ds >1% Power Usage


Temperature °C <80 Chemical usage

Pressure Ambient Water usage (washing, polymer make-up and carrier)

Throughput (m³/d) 20 m³/h + Noise


Hazard


Temperature



By products Liquor, volume dependent on degree of thickening

Carbon footprint


Product competition


Safety of operation


CAPEX £80k + OPEX


Suppliers/Reference Plants Alfa Laval

Ashbrook Simon Hartley Euroby Numerous WWTW reference sites GEA Westfalia MSE Hiller

Availability of UK Support High

Feasibility

Centrifuge technology is widely implemented to good effect in the waste water industry to reduce the solids handling volume. Consideration must be given to the centrate liquor as further treatment may be required prior to reuse or disposal.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Ashbrook Simon- Hartley, 2012. Decanter Centrifuge [online] available at: <http://www.as-h.com/uk/en-gb/centrifugedecanter.aspx> [Accessed 29/02/2012]


Process Belt Press

Process Type Dewatering (physical)

Objectives Separate solid and liquid phases of digestate


Process Description

Belt presses use the shear forces and mechanical pressure generated

between two opposing porous cloth belts to de-water digestate. The

process typically consists of three stages; gravity, low pressure and high

pressure. Digestate is continuously fed into the gravity section where

free water is removed via gravity. This section can include vacuum

assistance. Low pressure is then applied by two porous cloth belts

before the feed is subjected to high pressure and shear forces as the

belts pass through a series of rollers. Final de-watered “cake” is

removed from the belts by scraper blades. Liquor permeates through

the belts and is collected. Typically a “cake” of 18 – 25% dry solids can

be achieved.

Benefits Challenges

Increased solids and liquor concentration.

Reduced transport volume of solid fraction.

Filtrate (liquor) disposal is still required.

Press Feed

Liquor

Cake



Feed solids %ds >0.5% Power Usage


Temperature °C Ambient Chemical usage Medium

Pressure Ambient Water usage Low(washing, polymer make-up and carrier)

Throughput (m³/d) Large range of unit sizes available

Noise


Hazard


Temperature

Resistance to abrasives Pressure (localised mechanical)


By products Liquor - volume dependent on degree of thickening

Carbon footprint


Product competition


Safety of operation


CAPEX £100k + OPEX Low


Suppliers/Reference Plants Ashbrook Simon Hartley Ovivo Water Siemens Numerous WWTW Aquatreat reference sites Bioclere Technology International


Feasibility

Belt press technology is widely implemented to good effect in the waste water industry to reduce the solids handling volume. Consideration must be given to the press liquor as further treatment may be required prior to reuse or disposal. Ashbrook Simon- Hartley, 2012. Aquabelt [online] available at: http://www.as-h.com/uk/en-gb/aquabelt.aspx [Accessed 28/02/2012] Huntley DT. Improving waste treatment and minimisation with electrokinetic geosynthetic technology. Electrokinetic Ltd Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill

http://www.as-h.com/uk/en-gb/aquabelt.aspx


Process Centrifuge

Process Type Dewatering Physical

Objectives To separate solid and liquid fractions and reduce transport volumes


Process Description

Centrifugal thickening is a mechanical process that uses centrifugal

force, from the rapid rotation of a cylindrical bowl, to achieve solid -

liquid separation. Sludge is fed continuously into the rotating bowl of

the centrifuge. Solid partials are forced towards the bowl wall by

centrifugal force, where they are moved by an internal scroll towards

the solids discharge. Liquid effluent is discharged from the bowl via

adjustable weir plates.

A final dewatered “cake” of 18 – 20% dry solids can be achieved.

Benefits Challenges

Increased solids and liquor concentration.

Reduced transport volume of solid fraction.

Centrate (liquor) disposal is still required.

Centrifuge Feed

Liquor

Cake






Pressure Ambient Water usage (washing, polymer make-up and carrier)

Throughput (m³/d) 20 m³/h + Noise


Hazard


Temperature



By products Liquor, volume dependent on degree of thickening

Carbon footprint


Product competition


Safety of operation


CAPEX £80k + OPEX


Suppliers/Reference Plants Alfa Laval

Ashbrook Simon Hartley Euroby Numerous WWTW reference sites GEA Westfalia MSE Hiller


Feasibility

Centrifuge technology is widely implemented to good effect in the waste water industry to reduce the solids handling volume. Consideration must be given to the centrate liquor as further treatment may be required prior to reuse or disposal.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Ashbrook Simon- Hartley, 2012. Decanter Centrifuge [online] available at: <http://www.as-h.com/uk/en-gb/centrifugedecanter.aspx> [Accessed 29/02/2012]


Process Hydro Cell


Objectives To separate solids and liquid fractions, reduce transport volumes and create a potential fuel


Process Description

Within the HydroCell process the digestate is mixed with a high calorific

value additive in order to increase the energy content and improve ease

of dewatering. Typical additives include wood dust and coal dust.

Once the additive has been mixed with the digestate it is de-watered by

a hydraulic press. Dry solids contents of up to 80% have been achieved.

The improved energy content of the digestate additive mixture makes

this process particularly relevant as a pre-treatment for incineration.

Benefits Challenges

Separation of solid and liquid fractions. High solids content of cake. Improved energy content.

Liquor requires further treatment. Availability and cost of additive.

Digestate Fibre

Additive

Mixer

Liquor

Cake Press



Feed solids %ds 12 – 24% Power Usage No data available

pH Odour Potential No data available

Temperature °C Ambient Chemical usage No data available

Pressure Localised mechanical

Water usage No data available

Throughput (m³/d) Noise No data available

Chemical Consumption Hazard


Temperature


Reliability unknown Chemical

By products liquor Carbon footprint No data available

PAS 110 No effect Product competition No data available

Ease of operation Product market security No data available

Safety of operation




Stage Of Development Near Commercial

Suppliers/Reference Plants HydroCell Technologies, Ireland

Availability of UK Support Medium

Feasibility

If incineration is the final disposal route for the digestate fibre and a suitable additive is readily available, then this technology provides a simple method for turning the digestate into a valuable fuel for energy recovery.

Hydrocell Technologies Ireland, Enhanced Mechanical Sludge Dewatering. Hydrocell Technologies Ireland. O’Reilly, D., 2011. Hydrocell Technologies . Waste Treatment System. U.S. Pat 8,061,057 B2.


Process Bucher Press


Objectives To increase solids and liquor concentration and reduce transport volumes


Process Description

The Bucher press is a hydraulically driven cylinder-piston system. The press cylinder is filled by a central inlet. Once filled, the sludge is compressed by the piston forcing the liquid fraction through tubular filter cloths suspended between the piston and the cylinder. The piston then retracts while the cylinder rotates to loosen the cake. This process is repeated until the desired solids content is achieved. Once complete the casing is opened and the cake is discharged by the piston. Bucher report that dewatering up to 50% dry solids can be achieved.

Benefits Challenges

Separation of solid and liquid fraction. High solids content of cake.

Batch operation.

Bucher Press Feed

Liquor

Cake





Temperature °C Ambient Chemical usage No data available

Pressure Localised mechanical

Water usage No data available

Throughput (m³/d) No data available

Noise No data available



Temperature


Reliability unknown Chemical

By products liquor Carbon footprint


Product competition


Safety of operation


CAPEX High OPEX low


Suppliers/Reference Plants Bucher


Feasibility

The Bucher press is a well-established technology in the food processing industries. Although the technology is transferable, current experience of its application on digestate is limited. There is current interest in applying this technology within the UK water industry. These presses have a higher capital cost than other dewatering technologies, however offer potential to provide high dry solids content. A detailed cost benefit analysis would therefore be required to determine the feasibility and financial viability for a specific application.

http://www.bucherunipektin.com/html/en/5814.html


Process Electrokinetic Dewatering

Process Type (physical)

Objectives To remove water content from digestate to reduce solids handling volume

PFD

Process Description

In an Electrokinetic dewatering system digestate is fed into a cell

containing pairs of electrodes connected to a DC power supply. Water

within the digestate is removed by electro-osmosis and electrophoresis

in the electrical field applied between the electrodes. Once released the

water migrates to the cathode where it is collected.

Dry solids contents of up to 55 % have been produced in laboratory

scale systems. Electrokinetic systems have also been shown to reduce

pathogens, with trials achieving an 11 log reduction in Salmonella spp.

Trials have been undertaken at Long Reach STW (Thames Water) by

EKG and Ashbrook Simon-Hartley in the use of electrokinetic belts to

improve the dewatering efficiency of belt presses. Initial reports claim to

increase the product dry solids above 30%.

Benefits Challenges

Potential dewatering and disinfection in a single unit.

High solids content achieved.

Scale up from bench to industry. High power consumption. Removal of separated water.

Electrokinetic(EK) Cell Feed

Liquor

Cake

Applied voltage



Feed solids %ds Any Power Usage 128 kWh/m³

pH Any Odour Potential Unknown at full scale

Temperature °C Ambient Chemical usage None

Pressure Ambient Water usage


Chemical Consumption None Hazard


Temperature



By products Liquor Carbon footprint Unknown at full scale

PAS 110 Sanitisation Product competition Unknown at full scale

Ease of operation Unknown at full scale

Product market security Unknown at full scale

Safety of operation Unknown at full scale

Ease of commissioning Unknown at full scale



Stage Of Development Research

Suppliers/Reference Plants None

Availability of UK Support Low

Feasibility Low

This technology offers promising results, however it is too early to determine its feasibility at full scale.

Agnew, A., Et al. (2011) Electrokinetic remediation of plutonium-contaminated nuclear site wastes: Results from a pilot-scale on-site trial. Journal of Hazardous Materials: 186: 1405 – 1414 Huang, J., Elektorowicz, M. and Oleszkiewicz, J.A., Dewatering and disinfection of aerobic and anaerobic sludge using an elektrokinetic (EK) system

Lamont-Black, J., et al., 2005. THE DEVELOPMENT OF IN-SITU DEWATERING OF LAGOONED SEWAGE SLUDGE USING

ELECTROKINETIC GEOSYNTHETICS (EKG). 10th European Biosolids and Biowaste Conference. UK, November 2005. Aqua

Enviro Technology Transfer


Process Membrane Purification

Process Type Purification (physical)

Objectives To purify the liquid phase of digestate to enable direct discharge of purified water (permeate), and concentration of nutrients for further treatment or utilisation.


Process Description

Membrane purification uses a selectively permeable membrane as an

inter-phase in order to achieve separation between two liquid phases.

Separation is driven by a pressure gradient across the membrane.

Membrane purification is usually installed downstream of solid-liquid

separation (see dewatering) on the liquid phase. Typically a two stage

process is installed using ultrafiltration membranes followed by final

purification by reverse osmosis membranes. Ultrafiltration membranes

have a pore size of 0.1µm – 5nm and are installed to remove the

remaining particles and dispersed colloids. The final stage in the

purification process is reverse osmosis. The reverse osmosis membranes

have an effective pore size of <1nm and are used to remove dissolved

salts and any remaining organics. The products of the process are

purified water (permeate) and two separate nutrient rich concentrates.

Benefits Challenges

Purified water potentially acceptable for direct discharge.

Concentrated liquors lead to reduced volumes for further treatment (N stripping) or possible use as liquid fertilisers.

Concentrate disposal is still required, although reuse may be possible (as process water).

Membranes must be protected from particulates and large volumes of solids to prevent fouling and physical damage.

High energy requirement. Fouling.

Ultrafiltration

(UF) Feed

Reverse Osmosis

(RO)

UF Concentrate RO Concentrate

Permeate



Feed solids %ds <1% Power Usage High (16-25kWh/m³)



Pressure UF 200 kPa, RO 4000kPa

Water usage (washing, poly make-up and carrier)

Throughput (m³/d) Modular units available

Noise

Chemical Consumption Cleaning

Hazard


dependant on membrane material

Temperature



By products Concentrate Carbon footprint


Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants A3 Water Solutions GmbH Osmonics, Inc., Wehrle Wychwood Water Systems LTD Gea filtration Veolia Water


Feasibility

Membrane purification of digestate is not fully established within the UK. However there are several full scale plants within Europe. Careful consideration must be given to the nature of the digestate and the cleaning regime that will be required to prevent membrane fouling.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill

A3 Water Solutions gmbh, 2012. Digestate Treatement. [online] available at http://www.a3-

gmbh.com/_NewsBASE/content_a3/frame_english.php

http://www.a3-gmbh.com/_NewsBASE/content_a3/frame_english.php

http://www.a3-gmbh.com/_NewsBASE/content_a3/frame_english.php


Process Rotary Drying

Process Type Drying (Thermal)

Objectives To decrease moisture content, to reduce transport costs and increase marketability


Process Description

Within a rotary drier digestate fibre is contacted with hot gases in order

to dry the product by means of convection.

The rotary drier itself consists of a cylindrical drum which is rotated

about its axis. Flights within the drying drum pick up and cascade the

digestate. The drum is mounted on a slight slope from the horizontal to

facilitate transport of the dried product along its length. The feed to the

drier is blended with dried product to give a feed of approximately 65%

dry solids (DS) to improve movement within the drum. Waste gases are

passed through a cyclone to recover solids before further treatment.

Note that significant amounts of ammonia may be contained within the

exhaust gas stream.

Final product dry solids of up to 95% can be achieved. Screening can be

built in to the process to give a homogenous product pellet size.

Benefits Challenges

Reduced volume of digestate for transport and storage.

Improved marketability as a fertiliser/soil conditioner.

Effective pathogen kill.

High energy requirement. High temperature operation. Large capital investment. Reduced nutrient content of final

product.

Gas treatment required. Risk of explosive atmosphere within

drying plant.

Dried Product

Exhaust Gas

Treatment

Product recycle

Feed

Hot Gases Rotary Drier Cyclone



Feed solids %ds 18% Power Usage


Temperature °C ~400 Chemical usage

Pressure Water usage


Chemical Consumption Low Hazard


Temperature



By products Condensate, exhaust gases

Carbon footprint


Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Andritz AG – Tilbury and Glasgow WWTWs Swiss Combi GmbH – Ringsend WWTW, Dublin Vandenbroeck – Isle of Man WWTW Siemens


Feasibility

Rotary dryers are used to good effect within the waste water industry.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Siemens. 2012. Convective Thermal Drier Sub-systems. [Online] Available at: <http://www.water.siemens.com/en/products/sludge_biosolids_processing/sludge_dryers/Pages/Convective_Thermal_Dryer_Sub_SSystem.aspx> [Accessed 13/03/2012]


Process Belt Drying


Objectives To decrease moisture content to reduce transport costs and increase marketability


Process Description

Within a belt dryer digestate is contacted with hot gases in order to dry

the product by means of convection.

The (previously separated) digestate fibre is fed into the dryer where it

is evenly distributed over the drying belt by an extruder. The extruder

produces digestate strands in order to increase surface area and provide

a uniform size. The drier belt passes through a series of successive

chambers of increasing temperature. At the end of the belt digestate is

dropped onto a second belt which runs back through the drier,

underneath the first belt, to complete the drying process and cool the

product. Dried product is discharged from the drier at the end of the

second belt. Product is discharged at up to 90% DS and at a

temperature below 40°C.

As the digestate is not agitated during the drying process the risk of

creating an explosive dust atmosphere within the drier is significantly

reduced. This improves the safety and operability of the process.

Benefits Challenges

Reduced volume of digestate for transport and storage.

Improved marketability as a fertiliser/soil conditioner.

Effective pathogen kill.

High energy requirement. Large capital investment. Reduced nutrient content of final

product.

Belt Dryer

Hot Gases

Digestate Fibre

Dried Product



Feed solids %ds 18% Power Usage

pH Odour Potential

Temperature °C 400 Chemical usage

Pressure Water usage


Chemical Consumption Low Hazard


Temperature



By products Condensate, exhaust gases

Carbon footprint


Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Sevar Andritz AG


Feasibility

Belt dryers are used to good effect within the water and wood pulp industries. The feasibility of this technology will be dependent on the installation and the end use of the dried product.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Sevar. The SEVAR Belt Drier. [online] Available at: http://www.sevar.de/en/drying-plant/belt-dryer/ [Accessed 14/04/2021].



Process J-Vap

Process Type Dewatering and Drying

Objectives To separate the solid and liquid fractions contained within the digestate


Process Description

J-Vap is the trade name of a dewatering/drying system produced by

Siemens. The system is a two stage process combining a filter press and

a vacuum evaporation step to produce a dried product within a single

unit. It is claimed solids concentrations of up to 99% can be achieved.

Digestate is pumped into the filter chambers, and once full, the

diaphragms are inflated to mechanically de-water the digestate. Once

de-watered, heated pressurised water is pumped to the heat exchangers

contained within the filter plates. At the same time a vacuum is created

on the cake side of the filters. This vacuum lowers the boiling point of

the remaining water in the digestate. Once the cycle is completed the

filter plates are separated and the dried digestate is discharged.

A pasteurisation step can be included if required. After the drying cycle

the vacuum can be turned off and the temperature raised to meet the

time and temperature requirements for pathogen kill.

As the process operates in batches buffer storage will be required.

Benefits Challenges

Dewatering and drying in a single unit. High solids content leads to reduced

transport volumes.

Liquor treatment required. Reports of mechanical failures of plate

frames.

Batch operation.

J - Vap Feed

Liquor

Dried Fibre






Pressure Vacuum Water usage


Chemical Consumption Cleaning Hazard


Temperature



By products Liquor Carbon footprint

PAS 110 Pasteurisation can be included

Product competition


Safety of operation





Suppliers/Reference Plants Siemens – Installations at WWTW in America


Feasibility High

The J-Vap provides a single stage low temperature alternative to conventional dewatering and drying processes. A number of full scale plants are currently in operation in the USA.

8.0 Siemens.,2011. J-Vap® Dewatering and Drying System [Online}. Available at <www.siemens.com/jvap > [Accessed

12/03/2012]

http://www.siemens.com/jvap


Process Solar Drying


Objectives To increase the solids content of digestate


Process Description

Solar drying uses a combination of forced ventilation and solar energy to

de-water digestate. Waste heat from a CHP can also be utilised via

under floor heating. The feed to the process can be fed with either

whole digestate or de-watered fibre.

The digestate is fed into greenhouses where it is distributed across the

drying bed. Digestate is turned and ventilated to increase efficiency and

reduce odours. The greenhouses operate as a batch or semi-continuous

process. The final product is a dried digestate exceeding 50% dry solids.

Benefits Challenges

Increased concentration. Reduced transport volume. No liquor treatment required.

Large surface area required. UK Climate may restrict

application(Most current applications in Spain or Southern France).

Condensate (To atmosphere)

Solar drying bed Feed Dried digestate





Temperature °C Chemical usage



Chemical Consumption None Hazard


Temperature



By products None Carbon footprint


Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Thermo – System – Langage Farm AD (UK) Veolia – Solia TM - Multiple references in France Degremont - Heliantis TM - Multiple references in France


Feasibility

This technology provides a low Opex solution for dewatering digestate. However a large land area is required. There is one reference plant utilising this technology within the UK (Langage Farm). Most operational plants of this type are located in southern European countries with warm climates. It is not yet known how this technology will operate with the climate in the UK.

Thermo Systems http://www.thermo-system.com Veolia Water Technologies http://www.veoliawaterst.com/solia/en/

http://www.thermo-system.com/

http://www.veoliawaterst.com/solia/en/


Process Surface Scraped Heat Exchanger (HRS)

Process Type Concentration (Thermal)

Objectives To remove water from digestate liquor and increase the concentration of nutrients in the final product


Process Description

Evaporation utilises waste heat from the CHP to achieve concentration

of the digestate liquor.

Surface scraped evaporators are designed to overcome fouling issues

associated with the evaporation of digestate. The evaporators use a

shell and tube configuration. The interior surface of the heat exchanger

tubes is constantly cleaned by internal scrapers to reduce fouling and

increase heat transfer efficiency.

The digestate liquor is dosed with acid prior to evaporation to prevent

ammonia loss within the evaporator. The volume of acid dosed is

dependent on the digestate and the desired retention time. Within the

evaporator the liquor is concentrated to approximately 20% dry solids.

This concentrate can then be mixed with the (previously) separated

digestate fibre to produce a nutrient rich solid fertiliser.

Trials have indicated that the condensate from the process is suitable

for direct discharge to ground water, although further treatment may be

required for certain applications. Alternatively it can be recycled as

process water.

Benefits Challenges

Reduced transport volume. Potentially no further treatment of

condensate required.

Concentrated nutrient rich product. Use of heat eligible for RHI.

Acidic product may limit available land bank.

Surface scraped

evaporator

Separated digestate

liquor

condensate

Concentrated fertiliser

Acid



Feed solids %ds 1-2% Power Usage (350kWhtherm /ton evaporated)

pH Acidic Odour Potential

Temperature °C 50 - 70°C Chemical usage

Pressure Vacuum Water usage (cleaning)


Noise



Temperature



By products Condensate Carbon footprint


Product competition


Safety of operation


CAPEX OPEX Dependent on availability of waste heat


Suppliers/Reference Plants HRS Process Systems – SSE Barkip


Feasibility

Full scale plants utilising this technology for the concentration of pig slurry are operational and achieving good results in Europe. A newly commissioned plant in Scotland will use the technology for the concentration of digestate liquor (See Technology Example SSE Barkip). Should this technology prove successful at the Barkip plant it provides a very feasible method for treating digestate liquor and producing a balanced fertiliser product. Eligibility for the RHI also provides another potential income stream for the plant. Consideration will need to be given to the acidic nature of the final product and the affect this may have on the available land bank.

HRS.,2012. HRS Heat Exchangers: Providing the technology for heat transfer applications [Online] Available at

<http://www.hrs-heatexchangers.com> [accessed 15/03/2012]

http://www.hrs-heatexchangers.com/


Process Incineration

Process Type Thermal

Objectives The complete combustion of organic solids in order to minimise volume and recover energy


Process Description

Incineration is the complete conversion of all combustible elements

within the fuel to oxidised end products, primarily carbon dioxide, water

and ash. However complete combustion is very difficult to achieve and

therefore other chemical species, such as CO, NOx and pure carbon, are

likely to be produced. Flue gasses from the process will require

treatment before they can be discharged to atmosphere. Ash produced

from the process can be recycled for use within road construction, used

for the production of concrete, or sent to landfill. It may also be possible

to recover phosphorus from the ash by acid leaching.

It is beneficial to feed the incinerator with as dry a cake as possible.

This means that digestates must be dewatered prior to incineration in

order to reduce their moisture content. Additional fuel may be required

to dry the digestate before combustion can occur. The volume of

additional fuel required to initiate and sustain combustion will be

dependent on the calorific value of the digestate, moisture content and

the efficiency of the incinerator. Energy recovery from the process,

although desirable, is not always possible.

Several incinerator designs exist but the most widely employed system,

fluidised bed incineration, consists of a vertical cylindrical shell

containing a sand bed which is fluidised by air to enable combustion.

digestate is fed directly into the fluidised bed to provide rapid

evaporation and combustion. If operated correctly, and provided the

calorific value of the digestate is sufficiently high, there is no need for

additional fuel after start-up. Heat from the process can be used to

produce steam for electricity generation.

Co-incineration with municipal solid waste may provide a means of

increasing the heating value, such that additional fuel is not required

and energy recovery can be achieved. Thermal destruction may be the

only available disposal option for digestates which cannot be applied to

land due to non-compliance with legislation.

Benefits Challenges

Volume reduction. Destruction of pathogens and toxic

compounds.

Possible energy recovery.

High operating cost. Complex operation. Potential environmental impact of

residuals (exhaust air).

Loss of fertiliser potential Public perception.

Incineration

Off Gas

Ash Feed



Feed solids %ds High as possible Power Usage

pH Odour Potential


Pressure Atmospheric Water usage




Temperature



By products Flue gases, Ash Carbon footprint

PAS 110 N/A Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Veolia Water, Pyrofluid ® - Numerous references in France

Envirotherm GMBH ThyssenKruup – Shell Green (UK)


Feasibility

Incineration technology is well established. However capital and operating costs are high and as such it is usually only feasible for large plants or where land application of digestates is not possible due to digestate quality or land bank availability.

Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Veolia, 2011. Pyrofluid [online] available at < http://www.veoliawaterst.com/pyrofluid/en/> [accessed 07/04/2012]

http://www.veoliawaterst.com/pyrofluid/en/


Process Gasification


Objectives Thermal destruction of organics and production of syngas


Process Description

Gasification can be used to convert organic matter, such as coal, wood

or biomass, to a mixture of gases consisting mostly of carbon monoxide

and hydrogen, known as syngas. Reactions take place at high

temperatures with carefully controlled amounts of oxygen, air or steam.

The syngas can be burned in a gas engine to produce heat, and ash/

char from the process can be used for road construction, production of

concrete, or sent to landfill.

Gasification of traditional fuels such as wood and coal is well

established, and the process has also been used for municipal solid

waste. Full scale gasification of dried sewage sludge has also been

shown to be economic. A full scale plant is operational in Germany, with

another being commissioned and two more planned.

However the use of gasification for digestate is not well documented. As

most of the organics have already been released during digestion the

value of digestate as a fuel for gasification is relatively low. Digestate

must be dried and ideally pelletized before it can be gasified, placing an

additional energy demand on the process. Pilot studies on digestate

from pig manure have shown that energy recovery can be achieved but

that the additional gain over AD alone is low.

Thermal destruction may be the only available disposal option for

digestates which cannot be applied to land due to non-compliance with

legislation.

Benefits Challenges

Volume reduction. Destruction of pathogens and toxic

compounds.

Renewable heat and energy generation.

High operating cost. Complex operation. Ash disposal to landfill. Loss of fertiliser potential.

Gasification

Syngas

Ash Feed



Feed solids %ds 70% + Power Usage


Temperature °C 700- 1000 Chemical usage





Temperature



By products Ash Carbon footprint


Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Kopf SynGas GmbH & Co Veolia Water (technical advisors)


Feasibility

Gasification is a well-established technology, however research into its use for digestate treatment is still at the pilot stage. Further research is required to determine whether the additional energy gain outweighs the capital cost and additional processing requirements to prepare the feed.

Kuligowski, K and Luostarinen, S., 2011. Thermal Gasification of Manure. Baltic MANURE WP6 Energy Potentials.

Prapaspongsa, T., 2009. Energy production, nutrient recovery and greenhouse gas emission potentials from integrated pig

manure management systems. Waste Management and Research, 00:1-12


Process Wet Air Oxidation (WAO)


Objectives Destruction of organic solids


Process Description

Wet air oxidation (WAO) is a well-established technique for the

treatment of high strength industrial effluents. WAO is achieved at

elevated temperature and high pressure to prevent evaporation. The

oxygen source for the reaction can be air, oxygen-enriched air or pure

oxygen. Under these conditions the solubility of oxygen is increased

allowing the reactions to occur within the liquid phase (rather than in

the gaseous phase as with normal combustion processes). The

conditions also enable chemical oxidation of mineral components within

the feed.

The outputs from the process are an off-gas, a biodegradable liquid

phase and a mineral sludge (less than 5% organic content). Off-gases

consist mainly of CO2, O2 and NH3 and will require further treatment

before discharge to atmosphere. The mineral sludge can be dewatered

and used for land reclamation or potentially as a secondary raw material

for building and construction. The dewatering liquor and WAO liquid

effluent both require further treatment before reuse or discharge.

Solids reduction of up to 95% can be achieved. If heat recovery is

implemented it is possible to make the process auto thermal. A catalyst

(copper II ions) can be used to lower the operating temperature if

required.

Several full scale plants are currently operational in Europe treating

digested sewage sludge.

Thermal destruction may be the only available disposal option for

digestates which cannot be applied to land due to non-compliance with

legislation.

Benefits Challenges

Gaseous emissions free from toxins and particulates.

Greener image than incineration. Total oxidation achieved. Renewable heat and energy

generation.

Relatively high temperature and pressure.

WAO

Off Gas

Liquid effluent Mineral sludge

Feed



Feed solids %ds >2% Power Usage No data available

pH N/A Odour Potential No data available

Temperature °C 200 - 300 Chemical usage No data available

Pressure 70-150 bar Water usage No data available

Throughput (m³/d) Noise No data available

Chemical Consumption Catalyst if required, quantity unknown

Hazard


Temperature


Reliability No data available

Chemical

By products Mineral sludge/Liquid effluent/off gas

Carbon footprint No data available


Product competition No data available

Ease of operation No data available

Product market security No data available


Ease of commissioning No data available




Suppliers/Reference Plants Veolia (Athos) Installed at North Brussels WWTW Siemens (Zimpro)


Feasibility High

WAO presents a feasible alternative to incineration. Full scale plants are operational in Europe treating sewage sludge digestates. As with incineration this technology is likely to be best suited to large scale urban installations where no land bank is readily available.

Chauzy, J., 2010. Wet Air Oxidation of Municipal Sludge: Return Experience of the North Brussels Waste Water Treatment Plant. Water Practice Technology. 5 Siemens., Zimpro® Wet Oxidation: Innovative Technology for Difficult Waste Treatment Problems. [Online] Available at <http://www.water.siemens.com/SiteCollectionDocuments/Product_Lines/Zimpro/Brochures/ZP-WAO-BR-0906.pdf> [Accessed 16/03/2012]

Veolia., 2011. Athos. [Online] Available at < http://www.veoliawaterst.com/athos/en/> [Accessed 16/03/2012]

http://www.veoliawaterst.com/athos/en/


Process Pyreg

Process Type Slow pyrolysis

Objectives To convert digestate into syngas and biochar

Process flow diagram

Process Description

The Pyreg process is a modular pyrolysis process. Pyrolysis processes

heat the feed material in an oxygen-free atmosphere breaking down

organics within the feed into char and syngas. The term biochar refers

to char derived from the pyrolysis of organic matter.

The Pyreg process uses a twin screw pyrolysis reactor. Material is fed

into the reactor via a gas tight rotary valve. Material is moved along the

reactor by the twin screws ensuring uniform heating and residence time.

Syngas created by the process is burnt in a flameless burner (FLOX ®)

to reduce emissions. Exhaust gases from the burner are fed back to

provide heat for the process before discharge. Once operational the

process is thermally self-sufficient.

Approximately 70% of the feed mass is destroyed within the reactor

with the remainder recovered as a mix of carbon-rich char and ash. The

biochar has potential value as a soil amendment or as a feedstock for

growing media production.

The Pyreg process is contained within a standard shipping container.

The possibility of installing a turbine to recover electricity from the

exhaust gasses, and make the process entirely self-sufficient, is

currently under investigation. Research is also being undertaken into the

possibility of increasing biogas yield and ease of dewatering by blending

biochar with the digester feedstock.

Benefits Challenges

Carbon capture. Soil amendment - reduced application

of compound fertilisers. Biochar value as a growing media

constituent.

Renewable heat generation. Small footprint. Modular units.

Acceptance of the new technology. Securing a market for biochar.

Establishing a PAS and QP for biochar. Limited process experience. High feed solids content required.

Pyrolysis reactor

Syngas

Char + Ash

Feed Burner Exhaust gas

Heat



Feed solids %ds 55 to 70 Power Usage 3.5kW (self-sufficient operation possible)

pH NA Odour Potential None

Temperature °C 450 - 800 Chemical usage None

Pressure <0.5bar Water usage 30 litres/h

Throughput 1200 tds/annum (modular)

Noise



NA Temperature



Chemical NA

By products Biochar, Ash and exhaust gasses

Carbon footprint Negative, sequesters 1200t/a equivalent of CO2

PAS 110 N/A Product competition None

Ease of operation More operational data required

Product market security Emerging market

Safety of operation


CAPEX £300 - 450 OPEX GBP85k/a


Suppliers/Reference Plants Supplier : Pyreg Sonnenerde GmbH, Austria (paper sludge) Delinat Institute, Switzerland 2 plants (research and farm waste)


Feasibility

The Pyreg technology is proven for biomass feedstocks at full scale and digestate at pilot scale. The size and modular design of the process make it accessible to a range of plant sizes, however the digestate must be dried prior to processing. Ultimately the feasibility of this technology will be dependent on the market for the biochar produced.

GMBH, P. (2011). "PYREG." Retrieved 10/05/2012, from http://www.pyreg.com/English.html. PYREG (2011) Biomass to Biochar using Pyreg’s Slow Pyrolysis Technology. Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill

http://www.pyreg.com/English.html


Process Composting

Process Type Biological

Objectives To stabilise the solid fraction of the digestate to convert it into a soil improver or growing media constituent


Process Description

Composting of the solid (separated) fibre digestate fraction enables breakdown of the organic components and fixes the nitrogen to nitrates, although some may be lost as gaseous ammonia. As the organics decompose the compost heats to 50 - 70°C, due to the exothermic nature of the reaction, and sanitisation can be achieved. This results in a high quality product which can be marketed as compost. A number of different technologies exist including aerated static pile, windrow, and in-vessel composting (IVC). Each of these technologies has its own advantages and disadvantages but the process steps are essentially the same. 1) Pre-processing/addition of bulking material 2) high rate decomposition, aeration by addition of air, mechanical turning, or both 3) further curing and storage and 4) post-process screening. Processes accepting any kind of animal by-product (including most food wastes) need to be contained, in compliance with the Animal By-Products Regulations. The separated fibre digestate can either be composted on its own or co-composted with standard composting feedstocks e.g. wood chip, green waste etc. As an additive to standard composting the digestate provides a source of nitrogen, phosphorus, magnesium and iron, as well as moisture. The standard composting feedstocks provide a bulking agent, as well as improving the C:N ratio and consistency of the final product. Co-composting is therefore beneficial for both waste streams.

Compost typically has a lower fertiliser replacement value than

digestate, however it is a more readily accepted product for sale as a

bagged soil improver for gardeners or landscapers. Compost can also

have a longer land application season.

Benefits Challenges

Improved marketability as a soil improver/compost.

Increased solids content leads to reduced transportation costs.

Improved product stability, portability and materials handling.

Large footprint.

Potential ammonia emissions.

Composting Plant Digestate fibre Compost





Temperature °C 60 Chemical usage None





Temperature






Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Ecological Waste Apparatus SKM Enviros


Feasibility

Composting provides a simple way of improving the solid digestate fraction and adding potential value to the digestate. Composting of anaerobically digested sewage sludge is a well established and well understood technology. However depending on the quality of the de-watered fraction and the final outlet it may not be required.

Chartered Institute of Wastes Management (2002) Biological Techniques in Solid Waste Management and Land Remediation. Various authors. CIWM Report. Chartered Institution of Wastes Management, 9 Saxon Court, St Peter’s Gardens, Northampton, NN1 1SX, UK. Evans, TD., 2008. An independent review of sludge treatment processes and innovations. 4th Australian Water Association Biosolids Conference. Adelaide, 2008. Kayhanian, M. Lindenauer, K. Hardy, S. and Tchobanoglous, G. (1991) Two-stage process combines anaerobic and aerobic methods. BioCycle 32 (3) 48-53. Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill Von Felde, D. and Doedens, H. (1999) Full-scale experiences with mechanical-biological pretreatment of municipal solid waste and landfilling. Waste Manage. Res. 17(6), 520-526. Special Issue. Paper from Proceedings 3rd International Swedish Landfill Research Symposium, held Lulea, Sweden, 6-8 Oct. 1998. Diaz, L. (ed).


Process Reed Beds


Objectives Dewatering, pasteurisation and mineralisation of digestate

PFD

Process Description

Digestate is fed directly to a sealed basin containing a bed of aggregate on which the reeds grow. Drains are installed under the basin to collect the filtered liquor which can be used as process/irrigation water. The combined effect of this drainage and evapotranspiration de-waters the sludge to 30 – 40% DS. Further reduction is achieved through mineralization of the digestate. Reed beds provide treatment over a period of 10-15 years and during this period the beds must be cycled between loading and rest (treatment) periods. After the treatment period the beds are dug out, the sludge removed, and the process restarted. A number of reed beds are therefore usually operated cyclically to provide continuous treatment. During the treatment period sanitisation is achieved, up to 6 log pathogen kill. This increases the marketability of the digestate and potential application area.

Benefits Challenges

Low power and OPEX. Sanitisation. Stabilisation. Volume reduction.

Large land area required. Long operational time (10-15

years) to produce a stable product.

Reed Bed Digestate Feed

Conditioned digestate

Liquor



Feed solids %ds 0.5 – 5 % Power Usage

pH Odour Potential

Temperature °C Ambient Chemical usage None

Pressure Ambient Water usage None

Throughput 20-60 kg ds/m²/yr

Noise None



- Temperature None

Resistance to abrasives - Pressure None

Reliability High Chemical None

By products Liquor Carbon footprint (neutral)

PAS 110 Achieves sanitisation

Product competition


Safety of operation


CAPEX £1.5m /ha OPEX


Suppliers/Reference Plants ARM Biosolids


Feasibility

This technology is widely used for the treatment of digested sewage sludges in Denmark. If land is available to construct the reed beds this technology may provide a low OPEX method of producing a sanitised, mineralised and de-watered digestate product.

ARM Biosolids. Sludge Treatment in Reed Beds. 2012. [Online] Available at

<http://www.armbiosolids.co.uk/what_is_sludge_treatment_in_reed_beds.html> [Accessed 28/03/2012]

Nielsne, S. and Willoughby, N., 2005. Sludge Treatment in Reed Bed Systems and Recycling of Sludge and Environmental

Impact. In: Aqua Enviro Technology Transfer, 10th European Biosolids and Biowaste Conference.UK, November 2005, Aqua

Enviro Technology Transfer.

http://www.armbiosolids.co.uk/what_is_sludge_treatment_in_reed_beds.html


Process Biological oxidation


Objectives To biologically oxidise organic matter and ammonia (NH3) within the digestate to reduce liquor strength for discharge or further treatment.


Process Description

Biological oxidation can be used to remove the organic matter, which is

measured as biological oxygen demand (BOD), within the digestate and

converts ammonia to nitrate (nitrification). The process can be applied

to the whole digestate, but is most commonly used on the separated

liquor. The process can be managed to only remove BOD or to remove

both BOD and ammonia.

Within the process digestate is aerated in the presence of bacteria which

convert organic matter to CO2, H2O and new bacteria. Biological

nitrification utilises nitrifying bacteria to oxidise the ammonia within the

digestate to nitrate (NO3-). An anoxic stage can be included to convert

the nitrate to nitrogen if desired. This can reduce operating costs and

make the process more stable. However the nitrate is lost and hence the

value of the effluent as a fertiliser is lost. Process selection will

therefore be dependent on the desired product, a treated effluent for

discharge, a nitrified effluent for use as a fertiliser or a partially treated

effluent to simplify downstream processing.

Removing the BOD and ammonia will reduce the charges for discharging

the liquor to sewer if this is the chosen disposal route. Alternatively if

sufficient treatment is provided the liquor may be suitable for direct

discharge to groundwater. Nitrate is more stable than ammonia and

therefore can improve the digestate’s value as a fertiliser. The reaction

is sensitive to temperature and shock loading. Once the digestate has

been nitrified it can be concentrated by evaporation without the risk of

releasing large quantities of ammonia.

The most common reactor configurations are sequencing batch reactor

(SBR), membrane bioreactors (MBR) or aeration followed by dissolved

air flotation (DAF). Sludge from the process can be fed back into the

digester.

Benefits Challenges

Nitrified effluent can be better utilised as a fertiliser.

Effluent is suitable for concentration by evaporation.

Reduced disposal cost.

High power consumption.

Reactor

Air / oxygen

Feed Treated effluent

Sludge



Feed solids %ds N/A Power Usage (aeration)

pH 6.5 – 7.5 Odour Potential

Temperature °C 8 + Chemical usage


Throughput (m³/d) Noise (blowers)



Temperature



By products Sludge Carbon footprint




Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Wehrle Veolia Numerous WWTW and Leachate Treatment Plant SKM Enviros


Feasibility

Biological oxidation is one of the most commonly employed methods of wastewater treatment. The technology is well established and proven. However the operating costs can be high. If no outlet to land is available biological oxidation provides a proven method to reduce the cost of liquor disposal.

Fuchs W, Et al. 2010. Digestate treatment: comparison and assessment of existing technologies. Venice 2010, Third International Symposium on Energy from Biomass and Waste. Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill


Process Algal Pond/Photo bio reactor


Objectives To produce algal biomass to enable biofuel production


Process Description

The nutrients contained within the liquid fraction of the de-watered digestate can be used as a feedstock for algae. Lipids from the algae can be separated and converted into biofuel, and the remaining algal biomass can either be fed back to the digester or sold as animal feed. Algae are photosynthetic autotrophic organisms that fix carbon dioxide from the atmosphere. The biogas created from the AD process can therefore also be used as a carbon source capturing up to 95% of the CO2 within the biogas. Removing CO2 from the biogas by passing through the algal biomass also improves the quality of the biogas. Water from the process can be used as process water or for irrigation.

Algae can be cultivated in either simple ponds or in photo bioreactors

which achieve process intensification but at increased capital costs and

complexity. Waste heat from a CHP can also be used to improve process

performance.

Benefits Challenges

Improves quality of liquid fraction. Produces algae which can either be

sold or converted into biodiesel.

Removes CO2 from biogas.

Large surface area required for ponds. Careful control required for

bioreactors.

Algal Pond /

Photo bio reactor

Liquid digestate

Algal biomass

Irrigation water

Enriched Bio

Gas Bio Gas



Feed solids %ds Liquor Power Usage

pH Species dependent

Odour Potential

Temperature °C 15 – 30 Chemical usage





Temperature



By products Algal Biomass, process water

Carbon footprint Negative

PAS 110 No effect Product competition


Safety of operation




Stage Of Development Near commercial

Suppliers/Reference Plants Algae Cake (photo bioreactors) Advanced Algae (APAR, photo bioreactors) Algae Food and Fuel


Feasibility

A full scale pilot/demonstration plant is currently operational in the Netherlands. Further research is required to assess the feasibility of this technology and the market for biofuels and the algal biomass.

Algae Food and Fuels, 2009. Hallum [online] Available at: <http://www.algaefoodfuel.nl/english/projects/hallum/ >[Accessed 05/03/2012] Algecake Technologies Corporation ,2008, [online] Available at: < http://www.algaecake.com/> [Accessed 05/03/2012] AlgEn, algal technology centre. [online] Available at:: < http://www.algen.si/> [Accessed 05/03/2012] Iyovo GD, Du G, Chen J (2010) Poultry Manure Digestate Enhancement of Chlorella Vulgaris Biomass Under Mixotrophic Condition for Biofuel Production. J Microbial Biochem Technol 2: 051-057.

Iyovo GD, Du G, Chen J (2010) Sustainable Bioenergy Bioprocessing: Biomethane Production, Digestate as Biofertilizer and as

Supplemental Feed in Algae Cultivation to Promote Algae Biofuel Commercialization. Journal of Microbial & Biochemical

Technology 2: 100-106.

Rhodes, C.J. (2011). Making fuel from algae: identifying fact amid fiction. In: The Science of Algal Fuels: Phycology, Geology, Biophotonics, Genomics and Nanotechnology. Eds.: R. Gordon& J.Seckbach. Dordrecht, Springer Solarix, 2012. First harvest innovative Algae Project. [Press Release], Available at: http://www.solarix.eu/en/news/101 [Accessed 05/03/2012]

http://www.algaefoodfuel.nl/english/projects/hallum/

http://www.algaecake.com/

http://www.algen.si/

http://www.solarix.eu/en/news/101


Process Bioethanol production


Objectives To convert separated fibre into bioethanol


Process Description

Digestate fibres are first pre-treated with a dilute solution of either

sulphuric acid or sodium hydroxide. Fibres are then broken down into

simple sugars by enzymic hydrolysis. Ethanol can then be produced

from the sugars by fermentation.

Research has shown that digestate fibre (derived from animal manure)

is better than switch grass and comparable to corn stover as a feedstock

for bioethanol production. This shows promising results and further

research is required into the feasibility of other digestate feedstocks.

The lignin produced as a by-product is a valuable biomass that can also

be used to produce energy.

It has also been shown that freshwater and nutrients used for

bioethanol production from traditional energy crops can be replaced

with de-watered liquor. Using digestate liquor in this manner has been

shown to significantly increase ethanol yields.

Benefits Challenges

Conversion of digestate fibre into valuable fuel source.

Potential for reuse of digested liquor.

Early stage of development. Chemical consumption.

Feed

(digestate fibre)

Lignin / waste biomass

Ethanol Pre

Treatment

Sulphuric acid / Sodium

hydroxide

Enzymic

Hydrolysis

Fermentatio

n



Feed solids %ds >18% Power Usage No data available

pH Acidic Odour Potential No data available

Temperature °C 100-150 Chemical usage No data available

Pressure Ambient Water usage No data available

Throughput (m³/d) No data available

Noise No data available

Chemical Consumption No data available

Hazard


No data available

Temperature No data available

Resistance to abrasives No data available

Pressure No data available


Chemical No data available

By products Lignin Carbon footprint No data available

PAS 110 N/A Product competition No data available

Ease of operation Product market security No data available






Suppliers/Reference Plants None available


Feasibility Low

This technology shows promising results, however further research is required to determine its feasibility at full scale.

(Alkan-Ozkaynak, 2011, Ballesteros, 2010, Gao, 2010, Teater, 2011, Yue, 2010, Yue, 2011) ALKAN-OZKAYNAK, A. K., K, G. 2011. Anaerobic digestion of thin stillage for energy recovery and water reuse in corn-ethanol

plants. Bioresource Technology, 102, 9891 - 9896. BALLESTEROS, M. S., F. BALLESTEROS, I. MANZANARES, P. NEGRO, M, J. MARTÍNEZ, J, M. CASTAÑEDA, R. DOMINGUEZ, J, M,

O 2010. Ethanol Production from the Organic Fraction Obtained After Thermal Pretreatment of Municipal Solid Waste Applied Biochemistry and Biotechnology 161, 423 - 431.

GAO, T. L., X. 2010. Using thermophilic anaerobic digestate effluent to replace freshwater for bioethanol production. Bioresource Technology, 102, 2126 - 2129.

TEATER, C. Y., Z. MACLELLAN, J. LIU, Y. LIAO, W. 2011. Assessing solid digestate from anaerobic digestion as feedstock for ethanol production. Bioresource Technology, 102, 1856 - 1862.

YUE, Z. T., C. LIU, Y. MACLELLAN, J. LIAO, W. 2010. A sustainable pathway of cellulosic ethanol production integrating anaerobic digestion with biorefining. Biotechnology and Bioengineering, 105, 1031 - 1039.

YUE, Z. T., C. MACLELLAN, LIU, Y. J. LIAO, W. 2011. Development of a new bioethanol feedstock – Anaerobically digested fiber from confined dairy operations using different digestion configurations. Biomass and Bioenergy, 35, 1946 - 1953.


Process Microbial Fuel Cells (MFC)


Objectives Digestate polishing and power production


Process Description

Microbial Fuel Cells (MFC) are a novel technology that enables the

production of bioelectricity from the biological oxidation of organic

matter. The technology utilises the ability of particular microorganisms

to transfer electrons directly to an anode during respiration. The

reactions take place under anaerobic conditions.

The electrons are transferred from an anode (installed in the reaction

chamber) to a cathode via an external resistance.

Protons are transferred to the cathode chamber by either a salt bridge

or a proton exchange membrane (PEM) where they are combined with

oxygen to produce water.

Laboratory scale trials on digestate have shown that the system is

capable of removing 3.99kg COD/m³d with a power density of 42w/m³.

Benefits Challenges

Power production. COD removal.

Cathode stability. Membrane costs. Transition from laboratory scale to

industrial application.

Feed

(Whole digestate)

MFC

Power

Polished digestate



Feed solids %ds Power Usage Net generation

pH Further analysis required

Odour Potential Further analysis required

Temperature °C Further analysis required

Chemical usage Further analysis required

Pressure Further analysis required

Water usage Further analysis required

Throughput (m³/d) Further analysis required

Noise Further analysis required

Chemical Consumption Further analysis required

Hazard Further analysis required


Further analysis required

Temperature Further analysis required

Resistance to abrasives Further analysis required

Pressure Further analysis required

Reliability Further analysis required

Chemical Further analysis required

By products Water Carbon footprint

PAS 110 Product competition Unknown

Ease of operation Product market security Unknown

Safety of operation





Suppliers/Reference Plants None


Feasibility Low

This technology offers promising results at laboratory scale, however it is too early to determine its feasibility at full scale.

Aelterman P, et al. (2006) Microbial Fuel Cells for Wastewater Treatment. Water Science & Technology 54:9-15

Peixoto L. (2012) Microbial Fuel Cells for autonomous systems: kinetics and technological advances in wastewater treatment

and sensor applications. Ph.D. University of Minho


Process Struvite Precipitation

Process Type Nutrient recovery

Objectives To recover struvite (magnesium ammonium phosphate) for use as a product

PFD

Process Description

Magnesium, ammonium and phosphate can be combined to form a solid

commonly known as struvite (magnesium ammonium phosphate or

MAP). The reaction is highly dependent on pH, with the optimum range

for precipitation around pH10. However at values above pH9, ammonia

comes out of solution and is lost through evaporation. For most

digestates the reaction is also limited by the concentration of

magnesium ions.

It is therefore often necessary to artificially increase the pH of the

digestate and to add magnesium ions. pH can be increased by either

dosing with bases, in the form of sodium hydroxide or magnesium

hydroxide, or by aeration to de-gas the digestate. Dosing with

magnesium hydroxide has the advantage of increasing both the pH and

the magnesium ion concentration, but also prevents the pH and ion

concentration from being optimised independently. Magnesium ion

concentration can also be increased by the addition of magnesium

chloride.

Precipitation is typically carried out in fluidised bed reactors to produce

solid granules such as struvite, which has value as a chemical fertiliser.

Struvite contains equimolar amounts of ammonium and phosphate.

Since the digestate often contains more ammonium than phosphate the

treated liquor can be sent to a subsequent ammonia removal process to

recover the remaining ammonia. Otherwise additional treatment may

be required before discharge or further use. Under certain conditions

struvite can precipitate within the digester pipework causing fouling.

Benefits Challenges

Recovery of struvite which has value as a fertiliser.

Struvite is recovered as easily handled pellets.

Prevents fouling of pipe work and plant .

Contamination with solids.

Market security. Complexity.

Liquid Digestate

pH

Adjustment Magnesium

Dosing

Fluidised Bed

Reactor

Struvite

Treated Digestate



Feed solids %DS <1% Power Usage

pH 8-9 Odour Potential High (ammonia due to high pH)

Temperature °C 12-25 Chemical usage High





Temperature



By products Carbon footprint (chemical)

PAS 110 No effect Product competition


Saftey of operation


CAPEX No Data Available

OPEX No Data Available


Suppliers/Reference Plants Unitika Ltd. - Phosnix® - Reference sites installed in Japan DHV Water BV – Crystalactor® NuReSys Wehrle Full scale plant installed at Slough WWTW


Feasibility

The feasibility of this technology is dependent on the price that can be obtained for the struvite. This process may not be viable for all installations as the volume of Struvite that can be produced is determined by the composition of the digestate. However if a secure market for the struvite can be sourced this presents another possible source of income for anaerobic digestion plants.

(Parsons S, 2001, Giesen, 2010, Driver, 1998, Nawa, Evans, 2009) DRIVER, J. 1998. Phosphates recovery for recyling from sewage and animal wastes Phosphorus & Potassium, 17 - 21. EVANS, T. D. 2009. Recovering ammonium and struvite fertilisers from digested sludge dewatering liquors. Resource Recovery

Not Wastewater Treatment Conference. London: Aqua-Enviro. GIESEN, A. 2010. Crystallization Process Enables Environmental Friendly Phosphate Removal at Low Costs. NAWA, Y. P-recovery in Japan – the PHOSNIX process. Unitika Ltd. Environment & Engineering Div. PARSONS S, A. W., F. DOYLE, J. OLDRING, K. CHURCHLEY, J. 2001. ASSESING THE POTENTIAL FOR STRUVITE RECOVERY AT

SEWAGE TREATMENT WORKS. Environmental technology, 22, 1279 - 1286.


Process Ammonia Stripping

Process Type Nutrient recovery

Objectives To recover ammonium sulphate for use as product


Process Description

Ammonia is removed from digestate liquor by stripping with air at

increased temperature and pH. Air is contacted with the digestate to

absorb the ammonia which is less soluble at the operating conditions.

Ammonia is then recovered from the process air by acid scrubbing with

sulphuric or nitric acid to produce ammonium sulphate or nitrate

respectively. Once the ammonia has been recovered the air is returned

to the start of the process.

Heat for the process can be provided by a CHP. pH adjustment of the

digestate is usually by chemical addition.

Contact between the digestate and air is typically achieved by counter-

current flow in a packed column. This provides good transfer but is

susceptible to fouling. Promising results have been achieved recently by

diffusing air though the digestate in stirred tank reactors. This

eliminates fouling issues.

Biogas can be used in place of air to strip the ammonia. Recent research

suggests that if stripping is carried out within the digester it may be

possible to regulate the ammonia concentration within the digester.

The remaining liquor will probably require further treatment before it

can be discharged.

Benefits Challenges

Ammonia removed from digestate potentially removing restrictions on land application, due to nitrogen content.

Concentrated ammonium sulphate stream can be used as liquid fertiliser or feedstock for other processes.

High temperature and pH.

Precipitation of ammonium sulphate within washing column.

Fouling.

Digestate

Liquor

Treated

digestate

out

Sulphuric Acid In

Ammonium sulphate out

Air

Stripping

Column Acid

Scrubber





Temperature °C 35-80 Chemical usage





Temperature



By products Carbon footprint (chemicals)

PAS 110 Reduced N Load Product competition


Saftey of operation


CAPEX No Data Available



Suppliers/Reference Plants Colsen b.v. 3XR inc


Feasibility

Ammonia stripping can be used to produce a nitrogen rich fertiliser which can be sold as a product. This technology is well established within the chemical industry. The feasibility of this technology will be dependent on whether a secure outlet for the ammonium sulphate can be found.

Anasruron DFD, Bade O, Korner I. Nitrogen recovery from biogas plant digestates via solid-liquid separation and stripping. Technologies/systems for different manure and organic waste treatment options

Colsen b.v., AMFER [PDF] Available at: <http://www.colsen.nl/uk/brochure/index.html> Fuchs W, Et al. 2010. Digestate treatment: comparison and assessment of existing technologies. Venice 2010, Third International Symposium on Energy from Biomass and Waste.

Walker M, et al. 2011. Ammonia removal in anaerobic digestion by biogas stripping: An evaluation of process alternatives using

a first order rate model based on experimental findings. Chemical Engineering Journal 178: 138 - 145

http://www.sciencedirect.com/science?_ob=GatewayURL&_method=citationSearch&_eidkey=1-s2.0-S1385894711012666&_origin=SDEMFRHTML&_version=1&md5=b904da93cd99b3f88e1a571a74805e15

http://www.sciencedirect.com/science?_ob=GatewayURL&_method=citationSearch&_eidkey=1-s2.0-S1385894711012666&_origin=SDEMFRHTML&_version=1&md5=b904da93cd99b3f88e1a571a74805e15


Process Membrane Contactor

Process Type Nutrient recovery (Chemical)

Objectives To recover ammonia from digestate liquor


Process Description

By increasing the temperature and/or the pH of the digestate the solubility of ammonia can be reduced. The ammonia can then be removed by contacting the digestate with sulphuric acid, which reacts with the ammonia to form ammonium sulphate. Digestate and sulphuric acid are fed, counter currently, on opposite sides of a microporous hydrophobic membrane. Gaseous ammonia is removed across the air filled pores of the membrane. Mass transfer is driven by a concentration difference between the two liquid phases. The hyrophobicity and small pore size of the membrane prevent liquid flow across the membrane ensuring the two phases remain separate. The remaining liquor will probably require further treatment before it can be discharged.

Benefits Challenges

Single step process for recovering ammonia as ammonium sulphate.

Membrane fouling.

Membrane

Contactor Feed

Ammonium Sulphate

Treated digestate

Sulphuric Acid

Base




pH 10 Odour Potential

Temperature °C 40-55 Chemical usage



Noise



Temperature






Safety of operation





Suppliers/Reference Plants Membrana (Liqui-Cel)


Feasibility

Membrane contactors potentially allow for removal of ammonia in a single stage unit. Removal efficiencies of up to 95% have been achieved in lab scale systems. Further information on chemical requirement and operability at large scale is required.

Membrana. (2009) Using ‘TransMembraneChemiSorption’ (TMCS) for Ammonia Removal from Industrial Waste Waters


Process Ion Exchange

Process Type Nutrient recovery (chemical)

Objectives To recover an ammonium rich fraction which can be used as a fertiliser


Process Description

A number of materials can be used to selectively adsorb ammonia from

the digestate. These materials include zeolites, clays and resins. The

adsorption media can then be regenerated to recover the ammonia.

Regeneration can be achieved by a number of techniques including,

nitric acid washing, sodium chloride washing or biologically. The

technique used is dependent on the adsorption material and the desired

product.

Adsorption is carried out in a packed column, once the adsorption media

is saturated, the column is taken offline and regenerated. Adsorption

can therefore either be operated as a batch process using a single

column or a series of multiple columns can be sequenced to provide

continuous operation.

The remaining liquor will probably require further treatment before it

can be discharged.

Benefits Challenges

Recovery of concentrated ammonia. Fouling of adsorbent bed. Maintaining bed capacity after multiple

regeneration cycles.

Stage 1: Feed

Digestate

In

Treated

digestate

out

Stage 2:

Regeneration Regeneration

solution In

Ammonia

enriched solution

out



Feed solids %ds <0.5% Power Usage No data available

pH Neutral, effect dependant on media

Odour Potential

Temperature °C Ambient, effect dependant on media

Chemical usage



Chemical Consumption dependant on regeneration technique some can recycle

Hazard


Temperature



By products Carbon footprint (chemical)

PAS 110 Product competition


Safety of operation


CAPEX OPEX


Suppliers/Reference Plants Thermo Energy, ARP (resin with brine or sulphuric acid regeneration) Enpar Technologies Limited, AmmEL (zeolite with brine solution for regeneration, followed by electrochemical oxidation to produce N2) Carbtrol (resin) NanoChem


Feasibility

This technology is well established in other industries, although there is little experience in digestate applications. A pilot plant in the UK is operational at Didcot STW. Any issues of fouling within the column will need to be resolved to allow the technology to develop further. The feasibility of this technology will be dependent on whether a secure outlet for the ammonium sulphate can be located.

Cooney, L.E. et al., 1999. Ammonia Removal from Wastewaters Using Natural Australian Zeolite. I. Characterization of the

Zeolite. SEPARATION SCIENCE AND TECHNOLOGY, 34(12), pp. 2307 – 2327

Maurer, M. et al. Nitrogen Recovery and reuse

Seed, P.L. et al., a novel ion-exchange/electrochemical technology for the treatment of ammonia in wastewater. Enpar

Technologies Inc.

Thornton, A, et al., 2006. Ammonium removal from digested sludge liquors using ion exchange. School of water Sciences

Cranfield University.

EvTEC., 2000. Environmental technology verification report for ammonia recovery process. American Society of Civil Engineers.


Process Alkaline Stabilisation

Process Type Chemical

Objectives To raise the pH of the sludge in order to achieve pathogen kill


Process Description

Alkaline stabilisation raises the pH of the digestate in order to achieve

pathogen kill and neutralise odours (H2S). Lime is commonly used as the

alkali.

Lime can be added as liquid lime prior to dewatering (pre-treatment) or

as powdered lime (quicklime) after dewatering (post-treatment). In

both cases good mixing is required to ensure proper treatment.

Post-treatment is usually the preferred option as the quicklime reaction

is exothermic, the lime dose is lower, and no additional water is added

to the sludge. The final texture of the de-watered cake can also be

improved. Pre-treatment can also cause damage to dewatering

equipment through abrasive wear and scaling.

In areas of acid soil, lime-stabilised digestate can be very valuable to

farmers. However in areas of neutral soils lime addition can affect trace

element balances. Careful consideration must therefore be given to the

final outlet of the digestate.

In the extreme, the addition of lime can cause ammonia to be released

from digestate due to the increased pH, causing odour issues.

Benefits Challenges

Pathogen kill. Improved digestate quality. Simple process. Suppression of H2S.

Potential ammonia release (odour). Not suitable for application to all soil

types.

High chemical requirement.

Digestate Fibre

Mixer Stabilised product

Lime



Feed solids %ds Power Usage

pH 11 + Odour Potential

Temperature °C 50 (for post treatment)

Chemical usage

Pressure Ambient Water usage High (pre-treatment)


Chemical Consumption High (150 – 250kg lime / TDS)

Hazard


Temperature



By products None Carbon footprint

PAS 110 Pathogen kill (6 log)

Product competition

Ease of operation Product market security Area dependent

Safety of operation


CAPEX OPEX


Suppliers/Reference Plants SKE Solutions Numerous installations in UK WWTW Sodimate INC


Feasibility

Lime stabilisation is widely used in the water industry for stabilisation and enhancement of sewage sludges. The technology is well proven, simple and easy to operate. However consideration must be given to the final destination of the product and the potential for release of gaseous ammonia which must be managed.

Evans, TD., 2008. An independent review of sludge treatment processes and innovations. 4th Australian Water Association Biosolids Conference. Adelaide, 2008. http://www.britishlime.org/tech_sewage01.php Tchobanoglous, G.Burton, F.L. and Stensel, D.H. 2004. Waste Water Engineering Treatment and Reuse.4th Edition. New York:McGraw Hill

http://www.britishlime.org/tech_sewage01.php

www.wrap.org.uk