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Anaerobic Digestion Basics

Greenfinch Anaerobic Digestion onfarm small

An Early Greenfinch Anaerobic Digestion Plant Onfarm

When we titled this page “Anaerobic Digestion Basics” we decided to include all the basic information someone new to Anaerobic Digestion would be looking for. We start out with a definition of the AD process, provide a menu of links to other AD process concepts, and if you scroll down further you will see additional background information about anaerobic digestion which we thought readers would find useful to learn a basic understanding of the AD process.


Anaerobic digestion (AD) is a biological process similar in many ways to composting, but without air. It is a natural treatment process and, as in composting, bacteria break down organic matter and reduce its bulk or “mass”. Its huge advantage is that it produces a gas called biogas which is just over 15% methane, and a clean-burning gas with huge potential for reducing carbon dioxide emissions.

Anaerobic Digestion Basics: Contents

What is Anaerobic Digestion?
What is Digestate?

Small Scale Anaerobic Digestion for Homes and Communities
Anaerobic Sludge Digestion Explained
Anaerobic Digestion of Food Waste

Financial (UK Government Financial Assistance for Anaerobic Digestion):
UK Biogas Funding for Biofuels and CHP
UK Feed-in Tariffs for Small Scale Electricity Generation Including Small Biogas Plants

To gain an initial and rapid introduction into the basic concept of the modern anaerobic digestion plant (also known as a Biogas Plant, we recommend that you watch the video below this line. In just 3 minutes you will know a lot more about what happens in a biogas plant:

Watch the following video if you would like to see an introduction to “state of the art” Anaerobic Digestion (at a Dairy Farm), and how the basic system it is being enhanced by adding further processing,  providing additional products and income, and greatly increasing the financial profitability of on-farm anaerobic digestion (biogas plant installation).

A Comparison of Anaerobic Digestion with Composting

Unlike composting AD is carried out in an oxygen-free environment (known as anaerobic conditions) to allow the presence of bacteria adjusted to these conditions which then multiply and grow, and by so doing achieve the process aims of:

  • sanitisation of the feed material and of any liquid discharged;
  • a net positive surplus generation of energy as a biofuel to allow power production from methane gas (biogas) produced by the organisms.

Photo: A Greenfinch Anaerobic Digestion Plant which was built during 2004/5 for use on a farm.

Anaerobic digestion vs Composting comparison tableWhy are we excited about Anaerobic Digestion (AD) just now?

Here are 7 reasons for the current high rate of growth in anaerobic digestion facilities in particular (also known as “Biogas Plants”) in the industrialized nations, and the UK:

  1. Farm waste requires better management: As from 2006 the EU Waste Management rules have been applied to farming and agriculture. The historical exemption of farming from compliance with waste disposal regulations having been been removed no longer exists. This is in some instances encouraging uptake of anaerobic digestion as an on-farm waste disposal method.
  2. Farms Storing Slurry to Prevent Run-off Causing Pollution: New regulations which were implemented in 2012 and avoid its discharge, and this often requires a provision for 6 months of storage. Rather than invest in tanks of this size, farmers can treat their slurry by AD and gain an income from the biogas, while at the same time reducing their slurry lagoon (or tank) storage requirements.
  3. Organic waste must be diverted from landfill: This is the result of the EU landfill Directive and AD is an excellent technology for organic waste landfill diversion.
  4. There is a demand for new sustainable technologies investment opportunities often powered by individuals, but increasingly manifested in actions and investments by large companies and big investors. They are looking for “green” investments enabling their organization to show social responsibility, and are an outcome of the push toward large corporations becoming more sustainable. (May 2012: For example the UK has seen a number of the major supermarkets starting to use using renewable energy technologies in their stores, including operating anaerobic digestion plants to digest their own food waste and provide power to run their stores.)
  5. Rising demand for renewable fuels and bioenergy: This includes government targets and incentives to anaerobic digestion which the United Kingdom and other governments provide to help them meet their committed targets.
    the availability of Carbon Credits to those who avoid Carbon Dioxide emissions and apply Carbon Discharge Management (CDM): The developing world has been able to apply and obtain Carbon Emissions Reduction (CER) credits where they achieve carbon emissions reductions, and such reductions are achieved very effectively by installing an anaerobic digestion plant with biogas production and also using the waste heat to provide hot water to be used heating homes, or in factories.
  6. Easier planning permission: In the spring of 2012 the UK government also relaxed planning permission requirements for anaerobic digestion plants on farms, below a certain size.
  7. Rising oil prices: Suddenly traditional energy sources are so much more expensive. In the past AD was a much more expensive energy source, and to be honest it remains so when all capital spend up-front is considered, unless governments provide financial incentives, but even in nations where incentives are absent the gap is narrowing toward mainstream take-up of AD.

8 Main Source Materials for Anaerobic Digestion (Temperate Climates)

  1. Catering waste from private households
  2. Food residues
  3. Restaurant and canteen residues
  4. Farm manure (e.g. liquid manure, dung)
  5. Vegetable residues from commerce and trade
  6. Waste water from food production
  7. Grease trap fat
  8. Specially grown raw material – biofuel (e.g. clover) See also our bigger list here.

Products of Anaerobic Digestion

  1. A gas: Methane – a fuel.
  2. Solid fibrous material (“digestate”); which is spread without further treatment, or after post composting (maturation), to provide organic matter for improvement of soil quality and fertility (improves soil structure and reduces summer irrigation demand).
  3. The liquid fraction contains two thirds of the nutrients and can be spread as a fertiliser and sprayed on crops.
  4. Co-composting on farms – liquid and solid fraction is mostly not separated and is spread as a slurry.

If you know of more reasons for growth in AD, or would like to add to our list of source materials, we would like to hear of them, so please use our comments from below to add your reasons for growth in Anaerobic Digestion.

12 Responses to Anaerobic Digestion Basics

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  5. LstrStype August 28, 2017 at 4:24 pm #

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  6. David Lat August 29, 2017 at 5:04 am #

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  7. StevenLon August 29, 2017 at 10:17 am #

    Hello everyone, I am new to the biogas subject. All looks great for my study work, pleased to look at a lot, thank you!

  8. Ivan Sudarkin August 29, 2017 at 4:51 pm #

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  9. Penny Vame September 25, 2017 at 1:25 am #

    The most basic AD facilities are. small-scale biogas reactors which are typically designed to produce biogas at the household or community level in rural areas.

  10. biogasman September 25, 2017 at 5:17 pm #

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  11. Edward Drogis September 28, 2017 at 5:13 pm #

    Your pages I like. I am suggesting that at present, over a million biogas plants already exist in China. We made the finding that they meet more than 80% of the heat demand of the rural localities where they are located. Germany has over the last 20 years produced no more than the region of 5,000 biogas production plants. UK even less than Germany. Do you know what I am saying? You can go to the Chinese who do the best at having knowledge on the basis of biogas making. That is my thought.