Anaerobic digestion (AD) is the process whereby bacteria break down organic material in the absence of air, yielding a biogas containing methane.
About anaerobic digestion
The products of this process are:
Biogas (principally methane (CH4) and carbon dioxide (CO2))
A solid residue (fibre or digestate) that is similar, but not identical, to compost
A liquid liquor that can be used as a fertilizer.
NB The term whole digestate can be used to describe the unseparated fibre and liquor.
AD has been used to process sewage sludge since the 19th century.
It is also the natural process that can break down organic material in pools and marshes to produce marsh gas and in landfill to produce landfill gas.
AD is typically performed on biological material in an aqueous slurry. However there are an increasing number of 'dry' digesters.
Using the outputs from the anaerobic digestion of biomass material
The methane can be burned for heat or electricity generation.
The solid residue of the AD process can be used as a soil conditioner, however its properties:
Will depend on the AD feedstock used
May or may not contain useful levels of nitrate or phosphate
May be contaminated with heavy metals.
The solid residue can, alternatively, be burned as a fuel, or gasified.
Provided the requirements of the Anaerobic Digestate Protocol the digestate is no longer classified as a waste by the Environment Agency and so handling and storage regulations are eased.
Anaerobic digestion processes
There are two basic AD processes, which take place over different temperature ranges.
Mesophilic digestion takes place between 20ºC and 40ºC and can take a month or two to complete.
Thermophilic digestion takes place from 50-65ºC and is faster, but the bacteria are more sensitive.
Anaerobic Digestion Portal
A major gateway giving a wide range of information on anaerobic digestion, biogas and digestate. Run by the National Non-Food Crops Centre.
Anaerobic Digestate Protocol
Published by the Environment Agency.
This can allow digestate from an AD facility no longer to be classified as a waste, with the associated administrative burdens. It can therefore be used on the land as a soil improver and fertilizer substitute, treated in a similar way to manure or slurry.
The products of this process are:
