Anaerobic Digestion in Orkney

Pros and Cons

What is Anaerobic Digestion?

• Anaerobic digestion (AD) is a managed

biological process in which biodegradable

waste is broken down by naturally occurring

micro-organisms in the absence of oxygen to

produce a stabilised residue

Process Description

• The anaerobic digestion process is broken

into various stages:

• Pre-treatment

• Digestion process

• Outputs and their treatment

• Biogas

• Digestate

Generic Process Flow

Exhaust AirPurification

Reception and treatmentDigestion

Biogas treatmentand Use

Waste WaterPurification

Dewatering

Digestate

ANAEROBIC DIGESTION PROCESSES AND

EMISSIONS

EMISSIONS:

Carbohydrate/

Proteins/Fats

Sugars/

Amino

Acids/

Fatty

acids

Carbon

based

acids &

Alcohols/

Ammonia/

CO2 and

H2

Acetic

acid

Carbon

dioxide

Hydrogen

BIOGAS =

Methane

CO2/H2S/

Condensate

HYDROLYSIS ACIDOGENESIS ACETOGENESIS METHANOGENESIS

Enzyme processing of

solids to soluble matter

Bacterial conversion to

VFAs etc

H2S Formed in this stage

Bacterial conversion to

acetic acidBacterial conversion of

acetic to methane + CO2

and

Bacterial conversion of

hydrogen and CO2 to

methane

Acidogenic bacteria multiply relatively quickly and

thrive in low pH environments.

Methanogens are slow to multiply and need stable

neutral pH and Temperature. Process interruptions

and changes are not well tolerated.

FEEDSTOCK

DIGESTER

SOLIDS &

LIQUID =

DIGESTATE

GAS

ENGINES/

WASTE

HEAT

BOILER/

FLARE

DEWATERED

DIGESTATE

PAS 110?

e.g Sewage

sludge/Distillery

waste/Food waste

Liquid waste

(High in

Ammonia/

BOD/COD)

Pre-Processing –

(size reduction/

feed mix prep/

pasteurisation*

*pasteurisation is to meet

ABP rules & can be

before or after

EM

ISS

ION

S:

E.g. ODOURCONTAMINATED

WASH WATERS/

DUST/

BIOAEROSOLS?

Buffer

Storage?

OUTPUT

OUTPUTDe-watering

Digestate

Storage

Biogas clean

up e.g De-

sulphurisation

by treating

with caustic?

EM

ISS

ION

S:

ODOUR from H2S or

SOx/NOx/CO

EM

ISS

ION

S:

ODOUR/

CONDENSATE

HIGH IN BOD/

COD

OUTPUT

EM

ISS

ION

S

Odour/dust

Pressure

relief

valvesEMISSIONS:

ODOUR &

BIOGAS

80 m3 Farm Biogas Plant

Reception

Tank

Plant

Room

Anaerobic

Digester

Digestate

Storage

Suitable Feedstocks

AD is suitable for treating many wastes

• Sewage sludge

• Organic farm wastes

• Municipal solid waste

• Green wastes & Food wastes

• Organic Industrial Wastes (inc liquids)

Reception and Treatment

• Mixes different feedstock, adds water or

removes undesirable materials such as large

items and inert materials (e.g. plastics, glass)

• Allows for a better digestate quality and a

more efficient digestion process

• Pre-conditioning – pasteurisation, sterilisation,

filtration, grit removal etc

Key AD Parameters

Feedstock Type & Quality

• Substrate Quality C:N ratio 30:1

• Particle size

• Moisture Content

Ensure suitable feed quality check systems

are in place and in use

Gas production can be affected by reactor

upsets – nitrification, digestate quality, solids

separation, clarification

Digestion Process: Design

Options

• Temperature

• Mesophilic range 29 – 41oC, ideal ~35oC

• Thermophilic range 50 – 70oC, ideal 60-65oC

• Moisture

• Wet (5-15% Total Solids)

• Dry (25-35% Total Solids)

• Process

• One-stage

• Two-stage

Key AD Parameters –

pH/Temperature

• pH (and alkalinity)

• pH control via feed addition rate and acid/base pH addition

Effective pH control must be in place – trends

• AD Reactor Temperature

• Mesophilic or Thermophilic

• Feeds require to be heated or cooled to give optimum reactor temperature control

• Too low or too high - Biogas production rate falls, microbiological activity suppressed/dies

• Thermophilic systems are less temperature stable

Effective temperature control must be in place

Outputs of AD

• Biogas

• Generation of

renewable energy

• Vehicle fuel

• Gas to grid

• Powers the AD

process

• Nutrient rich

digestate

• Good fertiliser

Biogas

• Biogas has a composition of 55-70% of methane, with the higher concentrations in the wet digestion systems

• Upgrading needed if it is to be used to create a source of income

• Hydrogen sulphide and water vapour need to be removed for boilers and combined heat and power units

• Removal of carbon dioxideis required if the gas is to be used as natural gas or vehicle fuel

Digestate

• Digestate has the ability to improve the humus

balance of the soil

• Contains beneficial nutrients for plant growth

• But the properties of digestate are different to

compost

• In order to obtain a high quality soil

amendment, AD is often combined with

composting, adding value to the digestate

PAS 110• Inputs

• Separately collected

biowastes inc ABP

• Sewage sludge not

allowed

• Process Controls

• Quality Management

System

• Pasteurization

• Outputs

• Quality requirements

• Information and record keeping

Licensing Options

• PPC Permit

• Waste Management Licence

• Exemption

Planning Stage Considerations

• Acceptability of the development as a land

use

• Location

• Design and layout including stack height

• Consentability

• General description of the process,

techniques and technology

• Impacts on sensitive receptors

• Heat and power plan

Sources of Guidance

• National Operations Waste Unit intranet site

• Odour Guidance: www.sepa.org.uk/air/odour.aspx

• Useful Websites:

• www.biogas-info.co.uk: “the official information portal” on all things AD

• www.adbiogas.co.uk: the website of the Anaerobic Digestion and Biogas Association

• www.biogas.org.uk: trade

association for the biogas

industry

Pros & Cons Summary

• Production of biogas

• Replaces fertiliser

• Nutrients bio-available

• Plenty potential

feedstock in Orkney

• Reduce waste disposal

costs for some

industries

• Sensitive process

• Odour control

• Corrosive environment

• Regulated process

• Cost to build