Research & Development

Factsheet 29

May 2009

Use of anaerobic digestion

for food waste treatment

in Orkney

Summary

Shellfish waste is a major financial and operational burden to the seafood processing industry. Current estimates show that around 63,000 tonnes of shellfish waste is sent for disposal each year at a cost of almost £3,000,000. In theory there are many uses for shell, but to date there is no one solution to treating and subsequently using these materials as by-products. Most processors rely on disposal outlets costing up to £150 per tonne of waste. Shellfish processors want simple, local, cost-effective solutions for managing shellfish waste. Anaerobic digestion (AD) has been identified as a potential solution. The process produces methane which can be used to generate electricity, and create digestate (fertiliser) which has applications in agriculture and horticulture.

What is anaerobic digestion?

AD is a natural biological process in which

biodegradable waste is broken down by bacteria,

in the absence of oxygen. The process uses a

gastight reactor vessel (the digester). Waste

materials (feedstock) are fed in daily and biogas

and fertiliser taken out.

The project team pictured in front of a digester

AD is very flexible in that it can use a wide range

of organic feedstocks from agricultural waste

through to food processing wastes. Biogas is

made up of methane (about 55-70%), carbon

dioxide (about 30-45%) and traces of hydrogen

sulphide and water vapour.

During digestion, the feedstock material goes

through a number of different biochemical

processes. The amount of biogas produced varies

with the amount and type of material fed into the

vessel, as does the rate of decomposition. The

rate of decomposition can depend on various

conditions, such as the temperature inside the

vessel. It is important that conditions inside the

vessel are properly balanced to enable the

different types of bacteria to work effectively. The

most important parameters are temperature (the

AD vessel is generally kept between 35 to 55oC

using recycled heat from utilising biogas) and pH.

2 FS29 – use of anaerobic digestion for food waste treatment in Orkney

The four key biological and chemical stages of AD

Stage Activity

1. Hydrolysis A chemical reaction in which large

complex organic polymers

(cellulose, carbohydrates and

proteins) in the feedstock are

broken into smaller parts through

reaction with water. The smaller

molecules are simple sugars,

amino acids and fatty acids.

2. Acidogenesis Further simple molecules are

created ready for the third stage.

3. Acetogenesis Results in the production of acetic

acid, as well as carbon dioxide and

hydrogen.

4. Methanogenesis Bacteria convert the acids into

methane, carbon dioxide and

water. It is the methane and

carbon dioxide which form the

main components of biogas, which

can be extracted from the AD

vessel.

It is the feedstock that remains in the vessel which

the bacteria cannot feed upon, along with the

dead bacterial remains, which makes up the

digestate biofertiliser (this occurs after one to six

weeks depending on the feedstock).

One of the benefits of AD is the reduction in both

mass and volume of the input waste, thus

reducing storage requirements. Between 40-60%

of the organic matter in the AD process is

converted to biogas, the rest is left as odour-free

digestate. When a batch AD process has

completed, the vessel is emptied leaving about

10-15% behind which acts as a seed culture for

the next batch. The biogas is used as a biofuel to

generate electricity.

The AD process has been applied to many

agricultural and processing wastes, as well as

being an important element in sewerage treatment

plants. The nutrient-rich digestate can have a

variety of uses, such as spreading on land for

agricultural purposes and horticultural uses, as

well as soil cover for landfill sites.

The project

Heat and Power Ltd. were part funded by Seafish

to evaluate the potential of AD as a solution to the

shellfish waste disposal problem for Orkney based

shellfish and food processors. The main aims of

the project were to consider whether AD could

reduce commercial waste disposal costs, and if it

could be a sustainable waste management

solution for the island. It included a small-scale

batch digester trial on crab shells.

Food waste in Orkney

Food wastes from eleven sources were assessed

during the project, which included; meat, dairy,

salmon and crab processors. The quantities of

solid and liquid (effluent) waste were estimated for

each. In total, approximately 52,170 tonnes of

solid and liquid wastes are produced each year,

costing approximately £1,931,000 in disposal

costs. Only one company makes a profit from their

waste. Currently much of the food waste from

Orkney is sent to mainland Scotland.

The types of food wastes available are all

considered suitable for AD but they would have to

be mixed to achieve the correct carbon to nitrogen

ratio, lipid and dry matter content and to maximise

methane production.

3 FS29 – use of anaerobic digestion for food waste treatment in Orkney

Small-scale batch digester trial on crab

shell

Part of the project included a trial with batch

anaerobic digestion using crab shell. Common

problems with digesting wastes such as crab

shells is the potential for silting conventional

designs of anaerobic digester. The solids

remaining after digestion often settle out rather

than remaining in suspension in the digestate. A

new form of digester was designed and trialled as

part of the project. The first stage was to build a

scale model of the proposed digester to ascertain

to what degree the shells would digest. A small 25

litre digester was constructed and the digested

shells were sent for laboratory analysis. The

positive results from this experiment led to the

construction of a larger two cubic metre batch

digester.

Mobile batch digester arrives and is unloaded at Westray Processors to collect crab shells.

The shell was crushed and heated at the shellfish

processors. The pasteurised shells were kept at

over 90oC for an hour, before emptying into a two

cubic metre tank, where they were kept above

70oC for a further hour. The tank was fitted with a

12mm screen and digestate from a cattle slurry

digester was added to provide the bacteria

required to initiate digestion. Approximately four

times the volume of digestate was added to the

pasteurised shells. A chopper pump suction pipe

was connected to the bottom of the tank and

processed the material, delivering back via the lid

section over a mesh section.

The shell material after a first pass via the crusher and loaded into the digester.

The material was held in anaerobic conditions for

a period of three weeks and produced some

limited amounts of biogas in the gas holder.

The digester was mixed daily via a power take-off driven chopper pump and connected to a small gas holder.

Costs

After analysis of all the different feedstocks it was

estimated that from the 52,170 tonnes of solid and

liquid wastes produced in Orkney, the total value

of electricity that could be generated from the

biogas is £263,410. Additional revenue would be

realised if the digestate could be sold for use in

agriculture, however there is currently no market

for this material.

One of the envisaged outcomes of the project is to

develop a commercial scale, mobile facility which

would travel to the different sites around the

island. A mobile AD plant would be used, enabling

the waste to be treated during transport. The gate

4 FS29 – use of anaerobic digestion for food waste treatment in Orkney

Origin Way, Europarc, Grimsby, DN37 9TZ

t: 01472 252300 e: seafish@seafish.co.uk w: www.seafish.org SIN: http://sin.seafish.org

supporting the seafood industry for a sustainable, profitable future

fees for a proposed commercial scale facility would

be kept as low as possible. This will be achieved by

following a not-for-profit social enterprise model

that will provide savings for the food processing

companies compared to current disposal costs.

However it is impossible to provide estimates of the

potential costs or savings.

Potential set-up costs for a commercial scale unit

are in the region of £1500 per cubic metre of

capacity. The running costs would be covered by

the gate fees and sales of electricity. Again it has

not been possible to provide estimates of the

running costs. Heat and Power estimate that

transporting material from Orkney mainland to

Westray where it is planned to site the main

digester, would cost in the region of £15 per tonne

of m3.

Conclusions & recommendations

AD treatment of shellfish and other food

processing wastes in Orkney is principally

feasible. Financial profits may be created and

environmental benefits achieved through more

efficient resource utilisation, reduced pollution

and reduced waste transport compared with the

current situation. There may also be additional

brand value for food processors. Using a not-for-

profit social enterprise model will provide further

savings for the food processing companies.

Batch anaerobic digestion is suggested as a

solution for crab shells. There are certain

challenges for the AD process operation but the

batch plant design appears to have solved some

of the major operational restrictions.

A full investor-ready study for digestion capacity

to provide for the industry in the Islands is

recommended.

Further research and trials into the use of

digested crab shell for metal biosorbents is

required.

It is recommended that cleaning equipment for

those shells (crab) not currently cleaned to ‘free

of flesh’ standards is developed.

Follow-on work

As a result of this study, Heat and Power have

applied for a licence for the AD treatment of animal

by-products. Heat and Power is also going to

construct a larger ten cubic metre mobile digester

which is suited to the transport and treatment of

food processing wastes from the Orkney mainland.

Contacts & further information

Michaela Archer – Seafish –

m_archer@seafish.co.uk

Heat and Power Ltd. – www.heatandpower.ltd.uk

Full details of the project are available in the

Seafish Report – SR609 Use of waste as a

biofuel and fertiliser in Orkney, Heat and Power

Ltd, April 2009. To download a copy of this

report go to the Seafish website -

http://www.seafish.org/resources/publications.asp and

search for ‘Orkney’ on the publications page