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There's no Net Zero without Biogas: ending waste, delivering the circular economy, tackling the climate crisis.

Anaerobic Digestion Basics

Greenfinch Anaerobic Digestion onfarm small following the anaerobic digestion basics.

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 the other basic AD process concepts. If you scroll down further you will see additional background information about anaerobic digestion, which we thought our 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 50% to 75% methane, and that's 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
The Benefits of Anaerobic Digestion

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. In under 3 minutes you will know a lot more about what happens in a basic 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 anaerobic digestion system it is being enhanced by adding further processing,  providing additional products and income. Doing this can greatly increase the financial profitability of on-farm anaerobic digestion (biogas) plant installation.

A Comparison of Anaerobic Digestion with Composting

Part of understanding the anaerobic digestion basics, is an appreciation of the differences between anaerobic digestion and 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 at top of page: A Greenfinch Anaerobic Digestion Plant which was built during 2004/5 for use on a farm.

Anaerobic digestion vs Composting comparison table helping readers understand the anaerobic digestion basics.Why are we excited about Anaerobic Digestion (AD) just now?

Image is an infographic to answer: How many cows make how much power?


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. This is in some instances encouraging uptake of anaerobic digestion as an on-farm waste disposal method.
  2. Farms Must Now Store 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 organisation 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.
  6. The availability of Carbon Credits (in developing nations): 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.
  7. 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.
  8. 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.

The above 8 points are the anaerobic digestion basics driving biogas plant development.

8 Main Source Materials for Anaerobic Digestion (Temperate Climates)

  1. Food/ catering waste from private households
  2. Food residues from food processing
  3. Restaurant and canteen residues
  4. Farm manure (e.g. liquid manure, dung)
  5. Vegetable residues on-farm, and from commerce and trade
  6. Waste water from food production
  7. Grease trap fat – Fats Oils, and Greases (FOG)
  8. Specially grown organic materials – biofuel food crops (e.g. maize, silage, clover). See also our bigger list here.

Products of Anaerobic Digestion

  1. A gas: Largely 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. Where co-composting on farms the liquid and solid fraction may not be separated and the digestate is spread as a slurry.

If you know of more reasons for growth in biogas production, or would like to add to our list of “anaerobic digestion basics” source materials, we would like to hear from you. Please use our comments form below to add your reasons for the rising number of Anaerobic Digestion facilities.

24 Responses to Anaerobic Digestion Basics

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  4. Matthew Martinez April 1, 2017 at 5:44 am #

    This really answered my problem, thank you!

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

    Interesting and vital useful info on biogas.

    Still, to me in order to make a really interesting blog, you need not only to inform about something, but do it in an interesting way :).

    Some parts are good but others I find hard to understand the meaning. you should make more simple to read.

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

    I will be making time to learn about your anaerobic digestion subjects and keep track of new improvements inside the scientific research of dealing with carbon usage reductions.

  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 #

    I was thinking I don’t know what to do as I have a lot of of work to do next semester. Will be using quite a bit of your ideas from this website. Hopefully it will all go well. Wish me luck.

  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 #

    This pointless spammer got past me. In English this translates to:

    “mousse: it is the language of the universe, at least, which is dominated by physical laws that are described in mathematical form. Then I doubt Alpha Centauri’s aliens use our own notation for numbers and mathematical relationships, but the essence, the #######, must be the same. In this sense, math is universal.”

    Just pointless spam… Why do they do it?

  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.

  12. biogasman April 6, 2018 at 9:47 pm #

    Thanks GoogleDude for highlighting what was clearly an error. We have amended the definition to state 50% to 75% methane. A full biogas composition is available at…


  1. GoogleDude - March 30, 2018

    Query about definition. In your definition of biogas/ AD it says, “produces a gas called biogas which is just over 15% methane”. Is this a typo. It seems too low. Elsewhere, I agree with the things on here, but cannot see this is correct…

  2. Nicholas Dennis - April 23, 2018

    I’m no expert on biogas, but for me I would advise everyone to think about going green in terms of energy use in small steps, especially if your home cannot accommodate the solar panels or wind turbines necessary for a major infusion of green energy. Things like laptops, cell phones, iPods and other small gadgets can easily be powered up with smaller solar cells, and if you can also use biogas for night time power – you are made.

  3. Oliver Coles - August 22, 2018

    Biogas is awesome but did you know this tip about renewables?
    If you live in a sunny area, you could generate your own energy. Invest in PV cells and have a professional install them on your roof. You should have your needs in electricity assessed by a professional to make sure your solar installation will provide enough power for your home.

  4. Peter Pan - August 23, 2018

    Who knows the biogas basics and knows how to tell me what I need? Tell me. Reply quick.

  5. Ole Ludvigsen - September 11, 2018

    This is one of our sites of interest in the resources we have a link to. We are telling you because our website coder says to tell every site we are linking to them. I trust you have no objection, and is OK?

  6. Abysimia - September 11, 2018

    what is basics? Look like definition plus talk about? Is good. Have found it.

  7. Bennett - October 13, 2018

    Forgive my ignorance on the subject, but I must ask: the microorganisms produce CH4, then there is a bunch of hand-waiving saying it will generate electricity. How does the gas produce electricity, and more importantly, how is the production of CH4, which is a potent GHG, kept from entering the atmosphere during the process?

  8. J Graves - October 18, 2018

    Here is an excellent Web Page, that we encourage to visit. I need help to calculate what size digester i need for a food waste concept digester and also I would seek to know what quantity (maybe just rough estimate) of energy i could expect to get as output from the digester at a average output, could you help me out with this? Send reply to my email.

  9. Hassam - October 24, 2018

    very informative article. I am working at waste water plant it has made me understand the plant. Now I want to build the biogas plant.

  10. Ivan - February 16, 2019

    Thanks a lot for your efforts.

  11. VidaNader - February 18, 2019

    Does it work with all foods or just organic food? Good.