ANAEROBIC DIGESTION

Anaerobic digestion (AD) processes plant-derived materials, known as biomass, into gas to be used for heating and power; the resultant gas is called methane or biogas and is created by bacteria which digest biomass and produce methane as a by-product.

WHAT IS ANAEROBIC DIGESTION?

Anaerobic digestion (AD) processes plant materials, known as biomass, into gas to be used for heating and power; the resultant gas is called methane or biogas and is created by bacteria which digest biomass and produce methane as a by-product.

Biomass includes anything that is plant-derived; municipal solid waste, manure, crop residues, compost, food waste, paper and waste water. Crops can be grown specifically for use in AD, as a supplementary feedstock, or a stabilising material. Biogas has been used in the UK since 1895 when gas from sewage was used in street lamps by the city of Exeter.

Looking at this in more detail, anaerobic digestion is a natural process where plant and animal materials are broken down by micro-organisms in the absence of air as follows:

The AD process begins when biomass is put inside a sealed tank or digester
Naturally occurring micro-organisms digest the biomass which releases a methane-rich gas, known as biogas that’s used to generate renewable heat and power. The process helps cut fossil fuel use and reduce greenhouse gas emissions
The remaining material, known as digestate, is rich in nutrients and can be used as a fertiliser

Many forms of biomass are suitable for AD including food waste, slurry and manure, as well as crops and crop residues. However, woody biomass cannot be used because the micro-organisms can't breakdown the lignin - the compound that gives wood its strength.

AD is not a new technology, it has been used in the UK since the late 1800s, but an increasing number of AD plants are being built to generate clean renewable energy. AD is also used to treat waste produced in homes, farms, supermarkets and industry which helps divert waste from landfill.

The products of AD are referred to as biogas and digestate. Biogas is a mixture of 60% methane, 40% carbon dioxide and traces of other contaminant gases. The exact composition of biogas depends on the type of feedstock being processed.

Biogas can be combusted to provide heat and/or electricity. Alternatively, the biogas can be upgraded to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can then be injected into the mains gas grid or used as road fuel.

Digestate is made from left-over indigestible material and dead micro-organisms. It contains valuable plant nutrients such as nitrogen and potassium. It can be used as a fertiliser and soil conditioner.

Balmoral Tanks wishes to thank NNFCC, The Bioeconomy Consultants, for their kind assistance in preparing the copy for this page.

Different types of anaerobic digestion

The term AD covers several different processes; the most common are shown below and can be classified according to whether they are:

Mesophilic, 25-45°C; or thermophilic, 50-60°C
Wet, 5-15% dry matter in the digester; or dry, over 15% dry matter in the digester
Continuous flow or batch
Single, double or multiple digesters
Vertical tank or horizontal plug flow

Mesophilic or thermophilic

Wet or dry

Continuous or batch flow

Single or multiple digesters

Vertical or horizontal plug flow

Planning an AD project