Anaerobic digestion

Anaerobic digestion (AD) is the process of using decomposed organic matter to make biogas, which can then be used to provide heat or electricity.

What is AD?

AD simulates a process that occurs in nature – for example, in a cow’s stomach. Microorganisms digest and decompose biomass (organic matter) by feeding on it in the absence of oxygen. In the process, they produce biogas, a mixture of 60% methane, 40% carbon dioxide and traces of other gases.

Feedstocks for AD

Many forms of biomass are suitable as AD feedstocks, including food waste, slurry and manure, as well as crops and crop residues such as maize. Woody materials cannot be used because the microorganisms can’t break down the lignin, the compound that gives wood its strength.

Uses for biogas

The presence of methane means biogas can be combusted in a combined heat and power gas engine to produce electricity, heat or both. Alternatively, it can be ‘upgraded’ to pure methane (often called ‘biomethane’) by removing other gases. Biomethane can be injected into a mains gas grid or used as a road fuel.

Why we use 2-stage AD

AD involves four separate chemical processes. In hydrolysis, the chemical bonds of fats, carbohydrates and proteins are broken down to form sugars, fatty acids and amino acids. During acidogenesis the products of hydrolysis are broken down, degrading them into carbon dioxide, hydrogen, alcohols and organic acids including acetate. At the third stage, acetogenesis, alcohols and organic acids are degraded into acetates. At the final stage, methanogenesis, methanogenic bacteria use the products of acidogenesis and acetogenesis to make methane.

These four processes can take place at the same time if conditions are right. However, this method is relatively slow and inefficient. Instead, we deploy a two-stage AD process, using the innovative plants developed by Snow Leopard in Germany (see ‘Our partners’ section).

In this process, the hydrolysis stage is separated out, giving both the mesophilic and thermophilic bugs the temperature and conditions they each like best. Happier bacteria produce up to 25% more gas than in a single-stage digester, and retention time is markedly shorter too – just 19–20 days compared with 50 for single-stage.

Two-stage AD has other benefits too. Tanks can be smaller, which means less concrete for construction and lower capital expenditure. Plants are also more forgiving in terms of feedstock, giving more flexibility in terms of time and type.