Anaerobic Digestion (AD)


Anaerobic digestion (AD) is a natural process in which microorganisms break down organic matter, in the absence of oxygen, into biogas (a mixture of carbon dioxide (CO2) and methane) and digestate (a nitrogen-rich fertiliser). AD is not a new technology, and has been widely applied in the UK for the treatment of sewage sludge for over 100 years. However, until quite recently it has not been used here for treating other waste or with purpose-grown crops.

There are two main outputs (by products) of the AD process primarily being biogas and digestate. Biogas has the traditional composition of 60% methane and 40% carbon dioxide with traces of other contaminate gases. The exact composition will depend on the input (feedstock) used along with technology and processes. The biogas is combusted to generate heat and/or power. The digestate produced is rich in nutrients such as Nitrogen and Potassium and can be used as an alternative to low carbon fertilisers as well as being further separated into liquid and fibre. There is a developing interest in AD contributing to the rural community, particularly in aiding farmers to manage the waste whilst contributing to a sustainable farming sector.


The sector is developing rapidly. In March 2013, it was reported that the AD industry in the UK had 106 AD plants operating outside the regulated water industry, with a reported processing capacity of up to 5.1 million tonnes of food and farm waste every year. A further dozen plants were reported to be under construction. In December 2014 it was reported that the number of operational AD plants in the UK is estimated at 159.

There has been a steady increase in the renewable power exported from AD corresponding to this increase in installed AD capacity. Preliminary data collected by the Green Investment Bank for 2012 indicated that the total power generation from AD (excluding sewage sludge digestion) had increased by 38% to 330 GWh in 2012, from 239 GWh in 2011. This is circa 10% of the potential identified in the governments “AD Strategy and Action Plan”.


Issues such as odours, health effects, location of any nearby specially protected sites and traffic movements may have to be addressed as part of any planning application. An Environmental Permit or a formal exemption will also be needed if the biogas is to be combusted. In addition, the AD operator will need to consider and establish the status of the feedstock and the outputs, including whether the feedstocks will be waste, non-waste or a mixture of both. The transportation, storage, handling and treatment of wastes are subject to a specific regulatory regime. The AD operator will also need to secure a reliable supply of feedstock – if feedstocks are to be bought from a third party it will be key to negotiate and obtain a long-term supply contract on acceptable terms.


AD is at the core of the Government’s waste policies. In 2011, the Department of Energy and Climate Change (DECC) and the Department for Environment, Food and Rural Affairs (DEFRA) jointly issued an “AD Strategy and Action Plan” which set out the opportunity for the AD sector (including sewage treatment) and indicated that AD could deliver between 3 and 5 TWh of electricity by 2020. Consistent with these policies, renewable energy produced by AD facilities is entitled to support under the Feed in Tariff (FiT) and the Renewable Heat Incentive (RHI).