Anaerobic digestion (AD) produces renewable energy by breaking down organic materials to release biogas. An anaerobic digestor is essentially a large mechanical digestive system. It requires regular feeding, and the feedstock materials are digested using a variety of chemical and biological processes.
Feeding an anaerobic digestor
A wide variety of organic materials can be fed into a digestor. Many of the materials used are waste products, including domestic food waste collected from households, waste from the food processing sector, sewage sludge and agricultural wastes such as manure or surplus straw. Some AD feedstocks also include purpose grown crops such as miscanthus and maize. These often complement waste feedstocks to help optimise biogas production.
Preparing feedstock for use in AD
While some types of feedstock are ready to go straight into the digestor, others need some preparation to allow them to be effectively broken down once they enter the tank.
Food waste feedstocks are often passed through a de-packaging unit, to remove any indigestible materials like plastics, which can then be recycled or disposed of separately. For example, the de-packaging unit at Wardley Biogas, allows the AD plant to utilise packaged food waste from businesses in Tyne and Wear.
Agricultural wastes may also require some pre-treatment. For example, AD plants which use a large amount of straw based manure can run into problems with the straw floating to the top of the digestion tank, where it is not properly digested. To prevent this, it can undergo several processes to shred the straw, reduce its thickness and use a steam explosion technique to transform it into a digestible substrate which can be added into the digester. Read more on processing straw-based feedstocks here.
The process inside an AD tank
Once the feedstock materials have been through any pre-treatments, they enter the digestion tank, and the oxygen is removed to create an anaerobic environment. Like in an animal, the digestion processes in AD work best when conditions are stable, for example by maintaining a consistent temperature to allow the microorganisms and enzymes involved in the process to function optimally. The digestion process takes place in four stages.
In the first step of digestion, the long and complex molecules in biological waste materials are split into simple parts, such as sugars, fatty acids and amino acids.
In acidogenesis, the simple molecules are further broken down in a fermentation process, carried out by available bacteria. Volatile fatty acids, ammonia, carbon dioxide and hydrogen sulphide are formed during this stage.
The products from the acidogenesis process are further digested to produce acetic acid, plus some carbon dioxide and hydrogen.
Specialist bacteria called methanogens use the products from the previous steps to generate methane, carbon dioxide and water. The biogas released is mostly made up of this mix of products, plus some impurities.
How to use biogas for energy
The biogas mixture alone has limited use for providing renewable energy. There are two main options available to make use of biogas – upgrading it to methane, or using in a Combined Heat and Power (CHP) system.
Upgrading biogas to methane
The biogas mixture can be upgraded to biomethane, the renewable equivalent of natural gas, using a biogas purification and upgrading system. This removes less desirable products such as carbon dioxide and volatile organic compounds, leaving pure biomethane, which can then be injected into the national grid to be used in home boilers in exactly the same way as its fossil fuel equivalent.
Use of biogas in CHP systems
In CHP systems, the biogas produced in the anaerobic digestion process is burnt in a controlled manner, releasing thermal energy, which heats water to steam which powers a generator. The electricity from the generator can then be injected into the grid. The heat released during the CHP process can also be used by the AD facility and to heat other buildings in the local area.
How to use digestate from AD
Biogas is not the only product which comes from the anaerobic digestion process. There is also digestate, which is the left over solid and liquid materials once the biogas has been released and siphoned off. Digestate is sometimes referred to as a by-product, however it is a useful substance in its own right, as it can be spread on agricultural land to provide a nutrient rich organic fertiliser.
Although not as straight forward at first glance to other technologies that produce renewable energy, anaerobic digestion can play a key role in our energy systems. The process can generate renewable energy from waste materials, helping to reduce volumes of waste going to landfill and pave the way towards a more circular economy.
For more information see the spotlight on AD section in the 2020 progress update on our manifesto for renewable energy from waste.