What is anaerobic digestion
Anaerobic digestion is a series of processes in which micro organisms break down biodegradable material in the absence of oxygen.
Anaerobic digestion is best suited to 'wet' waste with a solid content of up to 30% and uses the bacterial action to generate bio-gas.
Suitable feedstocks include food waste, sewage and effluent sludges, animal slurries and energy crops.
The digestion process in total comprises four stages;
- Hydrolysis, in which enzymes secreted by hydrolytic bacteria break down organic polymers (proteins, carbohydrates) into their monomer components (amino acids, sugars etc).
- Acidogenesis, in which acidogenic bacteria break down the amino acids ans sugars into volatile fatty acids (VFAs) and alcohols
- Acetogenesis, in which acetogenic bacteria convert the VFAs into acetic (and propionic) acid and some CO2 is liberated
- Methanogenesis, in which the acetic acids are converted to methane and CO2 by methanogenic bacteria.
The resulting bio-gas is generally burnt in a combined heat and power (CHP) plant to make electricity and heat. However trial plants are now operating that clean and compress the bio-gas to create bio-methane that can be injected into the gas grid or used as a vehicle fuel.
For waste streams that contain packaging an up-front 'de-packing' system can be installed to ensure the feedstock to the anaerobic digestor is free from contaminates. The seperated packaging can then be sent for recycling.
The digestate is rich in highly available Nitrogen and is a good source of Phosphate, Potash and other elements required for healthy crop growth and fertile soil. PAS 110 is the UK's standard for ensuring digestate meets market needs and protects the environment.
