Anaerobic digestion (AD) is a natural treatment process, similar in many ways to composting, that composts biodegradable material in the absence of oxygen, producing a biogas that can be used to generate electricity and heat and digestate that can be returned to the land.

The English Waste Strategy strongly supports the use of AD to treat food waste:

“AD has significant environmental benefits” and states "the government wishes to encourage more consideration of the use of AD both by local authorities and businesses".

As part of an integrated waste management system, anaerobic digestion both reduces the emission of landfill gas to the atmosphere and is a source of renewable energy (and therefore approved for Renewable Obligation Certificates or ROCs), helping replace fossil fuels.

Utilising anaerobic digestion technologies can help to reduce the emission of greenhouse gases in a number of key ways:

• Replacement of fossil fuels
• Reducing methane emission from landfills
• Displacing industrially-produced chemical fertilisers
• Reducing vehicle movements
• Reducing electrical grid transportation losses

WHAT GOES ON INSIDE?
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Methanogenic bacteria finally are able to convert these products to methane and carbon dioxide.  In one-stage systems this occurs in the same vessel, while in two stage systems, the different phases are physically separated.

MESOPHILIC OR THERMOPHILIC?
There are two conventional operational temperature levels for anaerobic digesters determined by the species of methanogens employed:

• Mesophilic which takes place optimally around 37^oC - 41^oC where mesophiles are the primary microorganism present
• Thermophilic which takes place optimally around 50^oC - 52^oC where thermophiles are the primary microorganisms present

There are a greater number of species of mesophiles than thermophiles. Mesophilic bacteria are more tolerant to environmental changes. Mesophilic digestion is therefore considered more stable than thermophilic systems.

An additional drawback to operating at thermophilic temperatures is that more heat energy is required to achieve and maintain correct operational temperatures, leaving less available energy for export.

BATCH OR CONTINUOUS?
A batch system is the simplest form of digestion. Biomass is added to the reactor at the start of the process in a batch and is sealed for the duration of the process. As the batch digestion is simple and requires less equipment and lower levels of design work it is typically a cheaper form of digestion.

In continuous digestion processes organic matter is constantly or added in stages to the reactor. Here the end products are constantly or periodically removed, resulting in constant production of biogas.

WET OR DRY?
Typically there are two different operational parameters associated with the solids content of the feedstock to the digesters:

• High-solids
• Low-solids

Digesters can either be designed to operate in a high solids content, with a total suspended solids (TSS) concentration greater than ~20%, or a low solids concentration less than ~15%.
Municipal waste is typically a high solids waste of between 20 – 40% solids.

PRODUCTS
There are three principal products of anaerobic digestion: biogas, digestate and water.
Biogas is the ultimate waste product of the bacteria feeding off the input feedstock, and is mostly methane and carbon dioxide, with a small amount hydrogen and trace hydrogen sulfide. The methane in biogas can be burned to produce both heat and electricity. Excess electricity can be sold to suppliers or put into the local grid.

Digestate is the solid remnants of the original input material to the digesters that the microbes cannot use. It also consists of the mineralised remains of the dead bacteria from within the digesters. Digestate can come in three forms; fibrous, liquor or a sludge-based combination of the two fractions.