accepted manuscript nutrient recovery from sludge using filtration systems conference: 17th european...

17th European Biosolids and Organic Resources Conference

www.european-biosolids.com

Organised by Aqua Enviro Technology Transfer

Nutrient recovery from sludge using filtration systems

Lovitt, R.W*. Zacharof M. P. and Gerardo M.L. CWater, MNC, College of Engineering, Swansea University, Swansea UK. SA2 8PP

*Corresponding Author Tel. 01792 295709 Email [email protected]

Abstract

The fluids remaining after the digestion are a significant and growing problem as they contain large amounts of ammonia and phosphate that have high environmental impact. However, these nutrients are of considerable value replacing expensive manufactured fertiliser which typically have large carbon footprints and whose cost is linked directly to that of natural gas. Typical disposal of these fluids is by land spreading, but the capacity for this is limited as it threatens to contaminate ground and surface water causing eutrophication. Alternative practices of disposal are problematic as they need to be either integrated with AD sites or the materials be transported from digester site to a point of use. Transport soon becomes prohibitively expensive as the solutions are too dilute and bulky. Concentrating and reformulation of the nutrients as fertiliser or growth nutrient can add considerable value to these fluids so allowing transportation and distribution. As a first step in this process the sludge fluids must therefore be separated and concentrated. The filtration of sludge obtained from digesters offers potentially low cost techniques by which solid materials can be separated from the fluids and thus represent the first step towards recovery concentration and reformulation of these fluids as fertilisers and nutrient. Conventional filtration is used to remove large particulates > 0.5 mm but there several process advantages in post processing if the clear fluids are produced. This report discusses experiments carried out on several sources of sludge using a ceramic cross-flow microfiltration. The aim of these studies were therefore to ascertain the suitability of cross flow systems for sludge filtration and develop and optimize techniques by which these systems could be used to obtain particulate free fluids conducive to further processing. We report the use of a pilot-scale filtration system that has processed 100L batches of agricultural and municipal sludge for nutrient extraction. We have investigated several strategies for nutrient recovery that allow the production balanced nutrient streams. We have been able to use process models that are able to predict processes performance and optimisation based upon the membrane flux using a membrane resistance model. We also discuss further process alternatives to produced concentrates as liquids and solids.

Keywords

membrane, cross flow, microfiltration, anaerobic digester sludge, Phosphate, Ammonia, recovery,

Introduction

Millions of tonnes of sewage are produced every day in Wales and England, and there has been much drive to reduce its environmental impact (Environment Agency, UK). Its treatment,




handling and reuse can be challenging and expensive due to the complex and dynamic nature of the wastewater. In addition, the disposal of sewage sludge must take into consideration economic, social and environmental aspects (Tchobanoglous and Burton, 1991). The composition of sewage sludges is complex, however potentially, these can be a valuable source of energy and materials such as nutrients and metals. The production of methane has been widely explored through AD of manure waste from dairy farms and municipal waste sludge (Borja, 2011, Borole et al., 2006, Tezel et al., 2011). The AD of sewage sludge also benefits from a conversion of organic nitrogen into plant available ammonia nitrogen, however phosphorous is usually bound to organic matter or tied up in insoluble materials. Phosphorous therefore, may not be easily available and require processing to be released (Cantrell et al., 2008, Honeycutt, 2004). Seemingly, nutrients from AD sludge can be a valuable resource in agricultural applications, production of microalgae (Cho et al., 2011, Christenson and Sims, 2011, Park et al., 2011) and in hydroponic systems (Mavrogianopoulos et al., 2002, Yang et al., 2008). The use of sludges as a source of nutrients is not simple due to their complex nature, high solids content, organically bound material and ultimately its potentially hazardous properties. Typically, in farming, the AD sludge undergoes a simple solid-liquid separation process which allows the application of the different fractions as fertilisers in either farm or grassland. This continuous and inadequate disposal of AD sludge can easily result in the eutrophication of the ecosystem due to loses of nutrients into the environment (Wilcock, 2004, Neal and Heathwaite, 2005). Due to the noticeable soil acidification and ecosystem degradation, policies for such disposal of AD sludge are therefore becoming more stringent (Giola et al., 2012, Mantovi et al., 2006). Membrane separation processes related to water treatment have been long established, however, only recently researchers have reported on the use of membrane filtration for the treatment of wastewater. Tay et al. (1995) demonstrated the feasibility of membrane filtration to decrease the level of contaminants in wastewater from the beverage industry by more than 90%. Other researchers have also achieved similar results in which chemical oxygen demand and total dissolved solids were reduced by more than 90% (Galambos et al., 2004). The are many advantages of membrane filtration and these include lower operating and maintenance costs, physical separations free of chemical additives, potential particle and pathogens free permeates (Chen et al., 2006). Typically, cross-flow filtration is used to avoid the problems encountered with dead-end filtration such as cake fouling and high energy consumption. (Wang et al., 2006). The recovery of nutrients from a multi-solute system in a particle and microbe free solution is not without challenges. MF membranes range from 0.1 µm to 1 µm and allow to obtain particle and microbe free solutions in which smaller particles such as aqueous salts, proteins, metal ions, and other inorganic and organic particles can be collected as solutions (Wang et al., 2006). Apart from separating certain solutes from a solution, membrane filtration can also be used to concentrate and purify the final product. DF is a membrane assisted process that allows the recovery, concentration and purification of materials from a multi-solute system (Fikar et al., 2010). Diafiltration (DF) consists of the addition of wash-water to the multi-solute system performed in batch mode, continuous mode or counter current mode. The application of a DF mode is case-specific and thus every multi-solute system will present itself as an unique challenge (Lipnizki et al., 2002). Herein we investigate the recovery nutrients, using filtration processes, from two different sources: manure sludge from a dairy farm and sludge waste from a trout aquaculture farm. This




work reports on the filtration characteristics for both types of sludges, nutrient recovery strategies using acidification and DF, and a model that relates filterability, costs and nutrient recovery is also presented.

Materials and methods

Sludge sampling and pre-treatment

AD manure sludge was sourced from a dairy farm located in North Wales and it consisted mainly on bovine manure mixed with the animal’s bedding: ash and straw. The sludge samples were collected from the AD fermenter before solid-liquid separation. Trout sludge waste was collected from a settling tank from a trout aquaculture farm located in England. The trout farm is operated in a flow-through mode and a tank is used as a catchment to allow solids to settle. The AD manure sludge was diluted on a 50 % basis to ease its processing. Pre-treatment consisted in allowing each sludge to settle for a minimum period of 24 hours, with subsequent screening using a 500 µm pore size mesh. All samples were stored at 4 oC to ensure sample stability. Sludge characterisation

The buffer capacity of the sludge was determined by acid titration with 0.1 M HCl and the pH was monitored using a standard pH meter. Particle size distribution was determined by dynamic light scattering using a Malvern Mastersizer, (%) dry mass and TSS were determined gravimetrically at 105 oC. MF processing unit

The filtration unit, represented in figure 1, was comprised of two 100 L vessels (one for the settling and other to hold the pre-treated sludge being filtered), two centrifugal pumps, a concentric tube heat exchanger, a pressure control valve, two stainless steel pressure gauges and a ceramic MF membrane fitted in stainless steel. The MF membrane was supplied by Membralox with 0.20 μm pore size and a membrane area of 0.22 m².

Figure 1: Diagram of the two-pump MF cross-flow filtration system. 1” nominal

pipe size P1 and P2 – pressure gauge; A and B – Centrifugal pumps.




Sludge processing

The suspended sludge samples collected after the pre-treatment were processed using the filtration system in figure 1. Initially all samples were filtered down to 50 % of the initial feed volume, which allowed to collect one permeate fraction and one retentate. This initial permeate (A) was used as a comparison for the following permeates collected through DF. Where applicable, the subsequent permeates collected, via acidic or non-acid DF, were labelled as B, C and D. DF was at all times performed in intermittent feed mode – after the initial concentration step wash water solution was added into the feed tank to set back the initial feed volume. Acidic treatment of the suspended sludge was performed at pH 3. Flux and cake resistance

The filtration characteristics of the process were assessed according to the general flux model. i.e.

� ��

�� equation 1

Where J is the flux (ms-1), Rm is the membrane resistance, Rc is the cake resistance and Π and µ are the osmotic pressure and the viscosity, both of which can be ignored when considering microfiltration. Using the equation above and the membrane resistance associated with pure water and the cake resistance can be determined when measuring the flux at a known transmembrane pressure (TMP). Chemical analysis

All permeate samples collected were analysed for nitrogen as ammonia (NH3-N) and phosphorous as phosphate (PO4-P). N and P were determined calorimetrically using the phenate method and the molybdate-vanadate method, respectively according to Standard Methods (APHA, 1998).

Results and Discussion

Sludge characterisation

The buffer capacity was determined on the suspended sludge samples using 0.1 M HCl starting at neutral pH. The amount of HCl added was related to the TSS in each suspended sludge. Typically, the initial pH was around 7.5 for both types of sludge and as HCl was added the pH declined. Initially, high buffer capacity was observed between pH 7 and pH 5 until a sharp decrease in pH was detected (transition zone). Strong buffer capacity was also observed after the transition zone, pH 3. Zeta potential of the sludge samples was also determined across a pH range which allowed the determination the isoelectric point of each sludge. Figure 4 show that for the AD manure sludge isoelectric point found at around pH 3. Previously published work has already established the negative nature of the surface charge of the suspended particles in the trout farm sludge across the entire pH range (Jung and Lovitt, 2011), however a transition zone between pH 4 and pH 3 was observed for this waste sludge (figure 3). Particle size distribution varied between 2.1-69.5 µm for the AD manure waste, and 6.9-104.7 µm for the trout farm sludge.




Figure 2: Determination of the buffer capacity of the AD manure sludge at 2.10 g TSS/L

using 0.1 M HCl.

Figure 3: Determination of the buffer capacity of the trout farm sludge at 1.88 g TSS/L

using 0.1 M HCl.

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Figure 4: Zeta potential determined based on the elctrophoretic mobility of the

suspended particles in the AD manure sludge at 2.10 g TSS/L.

Filtration studies

The filtration characteristics of each sludge were investigated. Membrane fouling and cake resistance, could be determined through the direct measurement of the permeate flow as a function of transmembrane pressures (TMP) and the concentration of solids (see equation 1). Membrane fouling was studied under constant hydrodynamic conditions at (25 ± 0.5) oC, inlet pressure of 200 kPa and TMP of 120.7 kPa. Under these conditions, the observed reduction in flux due to membrane fouling was related to the physic-chemical properties of the sludge suspension: particle size and charge. Figure 5 illustrates the decline in permeate flux at constant sludge concentration and constant hydrodynamic conditions. For both the AD manure sludge (A) and AD trout farm sludge (B), during the first few minutes of filtration the permeate flux drops drastically as a cake builds up and a steady-state was reached after 1500 seconds. The influence of TMP and sludge concentration on the cake resistance was investigated in terms of, whilst keeping constant all the remaining parameters temperature and inlet pressure associated to the filtration study. The influence of TMP and the solids concentration of sludge on the permeate flux is summarised in figures 6 and 7. Typically, for both types of sludge at the initial concentration, the flux obtained at a range of TMPs is dependent of the operating pressure. As the operating pressure is increased, the flux becomes more independent of the operating pressure and more dependent on the concentration of the sludge. For both types of sludge, at TMPs around 190 kPa the cake resistance associated with the filtration of the suspended sludge becomes the limiting factor, and thus highly dependent on their concentration.

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Figure 5: Permeate flux decline with time at constant hydrodynamic conditions for two

different sludge: (A) AD manure sludge at 1.49 gTSS/L sludge, and (B) AD trout

farm sludge at 1.88 g TSS/L sludge.

Figure 6: Permeate flux of AD manure sludge in relation to TMP at different concentration

of total solids in the sludge suspension

Strategies for the recovery of nutrients

• AD manure sludge

Preliminary work consisted on the assessment of the recovery of NH3-N and PO4-P from the AD manure sludge. The efficiency of the nutrient extraction was determined in terms NH3-N and PO4-P recovered in the filtrate. Chemical analyses showed that when the filtrate was pH adjusted, i.e. after the solid-liquid separation, NH3-N and PO4-P remained constant throughout the established pH range.

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Figure 7: Permeate flux of AD trout farm sludge in relation to TMP at different

concentration of TSS in the sludge suspension

The pH adjustment of the suspended sludge before the solid-liquid separation did, however, show an improvement on the recovery of PO4-P without compromising the concentration of NH3-N. Figure 8 shows the influence of pH on the nutrient composition, before (B) and after solid-liquid separation (A). The evidence is that the recovery of PO4-P is enhanced at low pH levels, which also allows a variable composition in terms of N:P in these permeates. The immediate pH manipulation prior to solid-liquid separation thus allowed sludge liquors with N:P ratios ranging from 3.7 to 9.7 at pH 7 and pH 3, respectively.

Figure 8: The influence of pH on the concentration of nutrients in AD manure sludge l

liquors: (A) pH adjusted after solid-liquid separation, and (B) pH adjusted prior solid-liquid

separation.

The effect of diafiltration(DF), was also investigated and was performed in three consecutive steps. Without pH manipulation, the concentration of nutrients in the filtrates obtained through the DF experiments, were typically half of that obtained in the previous permeates (data not shown). Moreover, the DF extraction steps with no acidic treatment, did not significantly change the N:P composition. The N:P ratios varied from 36.6, for the most concentrated fraction, to 22.8, for the most dilute fraction obtained. Nutrient extraction can be enhanced by both DF and acidification, however the acidic treatment was only effective on the release of PO4-P as a 3-fold

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increase was observed under these conditions. DF was effective on the extraction of NH3-N, however after a few extractive steps the permeates obtained were very dilute. The effect of both treatments, acidification and DF, was then investigated in a multi-step nutrient recovery process. An initial permeate (A) was obtained from direct filtration and was compared to the following permeates from treatments 1 and 2. Treatment 1 was performed via pre-acidification of the concentrate, obtained in the initial filtration, and subsequent DF. Treatment 2 consisted on a first non-acidic DF followed by an acidic DF step of the concentrate obtained, as a result permeates B2 and C2 were collected. Both treatments were performed separately using the same starting feed sludge. The data summarised in table 1 show that nutrient ratios ranged from 36.6 to 8.4. Permeate B2 confirmed that consecutive non-acidic DF, even though it further recovered NH3-N, it did not substantially change the N:P. Nutrient recovery by means of non-acidic DF yields around 50 % of that obtained in the previous extraction. Typically, acidic conditions allow for lower N:P ratios since it favours the release of P. Boyd (1982) has previously discussed the speciation of P at different pH values. At lower pH values, e.g. pH 2-3, the speciation of P is preferably in the form of orthophosphoric acid (H3PO4), therefore P is mainly found in its inorganic form (soluble). At alkali pH values the PO4

3- molar fraction dominates and possibly precipitation of organic forms of phosphorous takes place (Boyd, 1982). Table 1: The influence on nutrient composition of a multi-step acidic and non-acidic DF.

Both experiments 1 and 2 started with an initial filtration step (A) and are

independent of each other. Experiment 1 was a two step recovery process

with A1 being obtained under acidic conditions. In experiment 2 was a three

step recovery process with B2 obtained through non-acidic DF and C2 through

acidic DF.

Treatment Sample NH3-N

mM

Std Dev

(%)

PO4-P

mM

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(%) N:P

A (initial permeate) 48.99 7.8 1.34 15.3 36.6

1 B1 (acidic DF) 19.36 2.2 2.31 3.5 8.4

2 B2 (non-acidic DF) 26.00 7.0 0.83 5.6 31.5

C2 (acidic DF) 14.45 8.0 1.72 1.6 8.4

• AD trout farm sludge

A similar assessment was performed in terms of nutrient recovery from AD trout farm. An initial filtration step of the suspended sludge, allowed to investigate the quantity of NH3-N and PO4-P recovered (permeate A). A further fraction was then collected (permeate B) through DF of the resulting concentrate from the previous filtration step. Moreover, the concentrated sludge obtained was acidified and a further fraction was then collected via DF (permeate C). The residual sediment from the sludge settling pre-treatment stage (see Materials and Methods), was also acidified and a permeate (D) was collected through DF of the acidic AD trout farm sludge sediment. The nutrient composition of the different fractions is summarised in figure 9. AD trout farm sludge is seemingly very dilute and low on readily soluble nutrients. Low levels of NH3-N were typically observed, thus in this investigation the focus became the recovery of PO4-P. As previously observed, acid treatment benefited the release of P even when preceded by a DF step (permeates A-C). When compared to the first fraction obtained, a 100-fold increase of




the PO4-P concentration was observed in permeate C. A much more concentrated fraction was obtained from the acidic DF of the AD sludge residual sediment. A 1000-fold increase on the recovery of PO4-P was observed. These results give evidence that, AD trout farm sludge is very poor in N and rich in P, however P is mainly present in the solids. Atypically, the recovery of NH3-N was also enhanced via acid treatment. This might be due to effect of acid digestion of inorganic and organic substances present in the AD trout farm sludge.

Figure 9: The effect of DF and acidic treatment in the recovery of nutrients from AD trout

farm sludge. Permeate A was obtained through direct filtration, B was a non-

acidic DF and C an acidic DF step. Permeate D was obtained through and acidic

DF of the residual sediment.

Sludge processing – cost and modelling

The application of DF in intermittent feed of the wash-water enhanced the recovery of nutrients, whilst also concentrating the suspended sludge. Consecutive non-acidic DF steps favoured the continuous extraction of nutrients, however these fractions became increasingly dilute. The development of a model that relates to both extraction of nutrients and processing requirement can be used for a better understanding of the nutrient recovery process. This tool would allow development of strategies that simulate, optimise and control the recovery of nutrients from AD sludge. In effect, technical and economic considerations cannot be set apart from when developing strategies for the recovery of nutrients. The nature of an intermittent DF implies that very dilute fractions are produced which may, therefore, impair the economic feasibility of the nutrient recovery. Processing time and power consumption will increase with several extractive steps, causing increased capital and operating costs. Filterability based studies can certainly help to determine the viability of the process, in order to obtain fractions of a determined concentration based on determined physical and chemical characteristics of the feed sludge. Equation 1 shows a basic filtration model based on the prediction of cake resistance (Rc), in relation of the amount of solids in the suspended sludge. Figures 10 and 11 show the influence on the cake resistance as a function of sludge concentration from AD manure or Trout form sludge. Considering that Rc is the limiting factor in the filterability of sludge, the prediction of the permeate flow can be computed against the solids present in the suspended sludge. The total volume processed, time of processing and the quantification of the total nutrients recovered would allow the establishment of a relationship between cost and the production of nutrients. As the number and processing time of the of recovery steps increases, increasingly dilute fractions are obtained at a higher cost due to increased processing time. Due to the fact that the bulk of nutrients are generally extracted after one DF step.

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Figure 10: AD manure sludge cake resistance as a function of TSS.

Figure 11: Trout farm sludge cake resistance as a function of TSS.

Figure 12 illustrates the nutrient recovery operating costs model that relates these considerations. Typically, it is observed that the operating costs increased with the increased number of DF steps (dotted line in figure 12). The relation between the nutrients recovered, for both NH3-N and PO4-P, and operating costs also seem to have an almost linear relation. The increasingly dilute fractions collected as a result of further DF steps, translates into low quantities of nutrients recovered whilst the operating cost increases. There is therefore, a point at which the operating costs relate to both optimal number of DF steps and amount of nutrients recovered. Considering the AD manure sludge as a source of NH3-N and the trout farm sludge as a PO4-P, figure 12 gives evidence that one DF step is the optimal number of recovery steps.

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Figure 12: Diagram of the estimation of the operating costs for nutrient recovery in

relation to the number of DF steps with 50 L filtration. Nutrient recovery is

based on NH3-N extracted from AD manure sludge (on the left), whilst PO4-P is

extracted from AD trout farm sludge (right). The processing line (dotted) are in

relation to cost and number of DF steps, whereas the recovery of nutrients is

in relation to amount recovered and costs.

Considerations for optimal processing and reduced costs

Relevant improvements that maximise nutrient extraction whilst minimising costs by either power usage, improved system design, enhanced membrane performance and optimised processing parameters can be implemented. The implementation is based upon sound knowledge of the physic-chemical properties of the suspended sludge. Considerations such as temperature control can greatly influence the flux, since higher temperatures can reduce viscosity, and thus reduce the pumping requirement and ease the filtration. The reduction of power consumption can also be achieved by determining the pumping requirements related to the membrane area, i.e. relation between flux and pumping requirements. There is also a fundamental consideration in terms of the concentration of the sludge. Dilute sludge are easier to process (higher permeate fluxes), however these will require more processing time to yield the same level of extraction than more concentrated sludges. Due to their dynamic nature, each type of sludge will present itself as a unique challenge in which appropriate physical and chemical characterisation of the raw material is paramount for the development of a successful strategy focused on the maximisation of the nutrients recovered at a minimum cost. The cost estimation here presented are only intended to be an indicative estimation of the operational cost of the pilot scale sludge processing unit designed and operated by Swansea University research group. The operation of the unit has not been optimised neither the cost has been calculated to process volumes larger than 125L (maximum capacity of the system). At this moment, the cost of fabrication, construction and management of the unit has not been determined. A definite conclusion, considering the cost of total cost of the unit cannot, therefore, be given at this stage seeing that further research is required. Although the report outlines the potential of the approach it is realistic to expect that to produce nutrients at 25-50% of the cost shown here. As such the system can represent a cheaper substitute for fertilisers.

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Conclusion

This work has shown that is potential in recovery N and P using membrane filtration and in streams from anaerobic digesters. These streams are microbes and particle free which makes theme potential useful at fertiliser or be used in further treatment processes. Using an acidification strategy the ratio of N:P may be altered to produce Tailor-made nutrient streams. Further work is required to optimise these processed and determine the accurate costs of recovery nutrients using membranes

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accepted manuscript nutrient recovery from sludge using filtration systems conference: 17th european...

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