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 Reception and treatment Digestion Exhaust Air Purification Biogas treatment and Use Waste Water Purification Dewatering Digestate
FEEDSTOCK e.g Sewage sludge/distillery waste/food waste ANAEROBIC DIGESTION PROCESSES AND EMISSIONS E.g. ODOUR CONTAMINATED WASH WATERS/ DUST/ BIOAEROSOLS? EMISSIONS: Pre-Processing (size reduction/ feed mix prep/ pasteurisation* *pasteurisation is to meet ABP rules & can be before or after Buffer Storage? Carbohydrate/ Proteins/Fats DIGESTER Sugars/ Amino Acids/ Fatty acids Pressure relief valves Carbon based acids & Alcohols/ Ammonia/ CO2 and H2 EMISSIONS: Acetic acid Carbon dioxide Hydrogen OUTPUT OUTPUT ODOUR & BIOGAS BIOGAS = Methane CO2/H2S/ Condensate SOLIDS & LIQUID = DIGESTATE ODOUR/ CONDENSATE HIGH IN BOD/ COD EMISSIONS: Biogas clean up e.g Desulphurisation by treating with caustic? De-watering GAS ENGINES/ WASTE HEAT BOILER/ FLARE EMISSIONS: ODOUR from H2S or SOx/NOx/CO EMISSIONS: Liquid waste (High in Ammonia/ BOD/COD) Digestate Storage OUTPUT DEWATERED DIGESTATE PAS 110? HYDROLYSIS ACIDOGENESIS ACETOGENESIS METHANOGENESIS Bacterial conversion to Bacterial conversion to VFAs etc acetic acid H2S Formed in this stage Enzyme processing of solids to soluble matter 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. Bacterial conversion of acetic to methane + CO2 and Bacterial conversion of hydrogen and CO2 to methane EMISSIONS Odour/dust
Reception Tank Anaerobic Digester Plant Room Digestate Storage 80 m 3 Farm Biogas Plant
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 41 o C, ideal ~35 o C Thermophilic range 50 70 o C, ideal 60-65 o C 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 dioxide is 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 Sensitive process Replaces fertiliser Odour control Nutrients bio-available Corrosive environment Plenty potential feedstock in Orkney Reduce waste disposal costs for some industries Regulated process Cost to build