Anaerobic digestion: What is it and how does it work?

As the world tackles global warming and the problems created by climate change, solutions need to be found to stop our dependency on fossil fuels. One such way to make the switch to more renewable energy is by using biogas? But how is that made? In this article, we’ll look at the topic of anaerobic digestion and provide you with details of the process involved and what the end result is and can be used for.

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What is anaerobic digestion?

Anaerobic digestion is the process in which organic materials - such as food and animal waste - are broken down and turned into biogas. The process is done so without oxygen being present, and so takes place in sealed tanks or containers called anaerobic digesters (AD).

What is an anaerobic digester?An anaerobic digester is a sealed tank that is filled with either manure, slurry, food recycling or crops. Once inside the AD tank, microorganisms break down the materials and digest the biomass.

When multiple organic products are put into the AD tank, it is known as a process of co-digestion. Not all materials break down at the same speed, and co-digestion can actually improve the biogas production of low-yielding or difficult-to-digest organic waste.

What is the end product of anaerobic digestion?

Anaerobic digestion produces two main outputs, which can then be used for a variety of things: biogas and digestate.

Biogas production

Biogas is an example of a renewable energy and can be used just like natural gas to provide heat and electricity in your home - helping you reduce your bills and do you bit to lower carbon dioxide (CO2) emissions at the same time. However, it also has many other uses, such as:

• Compressed and used as vehicle fuel;
• Processed even further to generate alternative transportation fuel - something we are starting to see with the rise in hybrid cars;
• Energy products;
• Other advanced biochemicals and bioproducts.

Prince Charles and his biofuel cars. Did you know that Prince Charles, a long-time advocate of sustainability, drives a car powered by biofuel? Ahead of the COP26 climate summit in Glasgow in 2021, he revealed in an interview with the BBC that his famous Aston Martin was converted so that it runs on “surplus English white wine and whey from the cheese process”.

Digestate

Composed of both liquid and solid portions, digestate is the residual material left after the digestion process has taken place. Normally, the materials are separated and handled differently as they have varying uses.

The solid digestate can be used in products such as:

• Animal bedding;
• Fertiliser for gardens and crops farms;
• Organic-rich compost.

Liquid digestate can be used in materials such as:

• Crop irrigation;
• Horticultural products;
• Fertiliser.

Anaerobic fermentation and anaerobic digestion: Are they the same?While both processes are fairly similar, the aim of anaerobic digestion (AD) is to produce biogas which can be further upgraded to biomethane, while in fermentation, the aim to produce hydrogen.

What are the 4 stages involved in the anaerobic digestion process?

There are a number of steps involved in the anaerobic digestion process. In this section we’ll look at each one in more detail.

Step one: Removing contaminants

The first step is removing contaminants. The anaerobic digestion process only involves biodegradable material such as food waste, animal waste, crops, slurry and sewage. Anything else needs to be removed or the results of the process will not be achieved or will be less successful.

Step two: Treating the fuel

The material placed inside the anaerobic digester then needs to be treated to ensure a smooth consistency. Many ADs are fed by a continuous flow for cost effectiveness, so the smoother it is, the better the end result. Once inside the AD, the material can begin to be broken down by the microorganisms.

Step three: Into the AD

The anaerobic digestion process takes place inside the AD. The length of time it takes for material to be broken down depends on the type of anaerobic digester. There are two types of AD: Mesophilic or Thermophilic units.

• Mesophilic units are more common on farms around the UK as they are cheaper to build and operate. This is because they operate at a lower optimal temperature - around 37 °C.
• Thermophilic units do exactly the same job but operate at higher temperatures - usually between 50 and 60 °C - meaning the anaerobic digestion process takes place quicker. These types of AD do have higher running costs, however.

Step four: The stages of breakdown

Once the process of breaking down the organic material begins it actually undergoes four different stages until it is considered ready for use as biogas or digestate. The stages are as follows:

1. Hydrolysis: This is when complex matters such as carbohydrates and proteins are broken down into amino acids and sugars. Hydrolysis essentially breaks down long-chain chemical compounds into single molecules.
2. Acidogenesis: The microorganisms break down the single molecules into ethanol and fatty acids. Carbon dioxide and hydrogen sulphide are also produced as by-products.
3. Acetogenesis: The penultimate stage of the process is when the ethanol and fatty acids are broken down into hydrogen, carbon dioxide and acetic acid.
4. Methanogenesis: The microorganisms convert the hydrogen, carbon dioxide and acetic acid into methane biogas in the last stage of the process, ready to then be used for biofuel and other renewable solutions.

Pros and cons of anaerobic digestion: is it good for the environment?

Just like any other sources of renewable energy just as wind energy or solar energy, anaerobic digestion has advantages and disadvantages.

Advantages of anaerobic digestion

The pros of this type of organic waste recycling include:

• Renewable: Because it relies on using only organic waste such as food, animal and human waste, anaerobic digestion is a totally renewable solution and does not use any of the world’s limited natural resources;
• Consistent source of power: There will always be a continuous source of animal and human waste - so there is no need to worry about shortage of supplies for anaerobic digestion. According to Greencoast, a cow can produce around 2kwh – 3kwh of electricity daily, which can be fed into the national grid;
• Greenhouse gas reduction: Greenhouse gases (GHG) contribute to global warming, and farms are generally considered to be fairly large contributors of GHG. According to estimates, ADs reduce emissions by up to 66%;
• Improved water quality: The anaerobic digestion process helps remove phosphorous and other metals from the waste products, which, if left unattended or untreated, could end up in local water supplies.

Disadvantages of anaerobic digestion

Having looked up some of the benefits, it is important to look into the other side of the argument and highlight some of the disadvantages of anaerobic digestion. They include:

• Expensive to run: The set-up costs to install anaerobic digesters on a farm are high, as are the running costs, meaning that it is normally only large farms that can afford to run such a project, rather than smaller family-run farms;
• Time-consuming: Farmers need to dedicate time to maintaining the equipment and monitoring the anaerobic digestion process, which can take away from other tasks they need to do, such as tending to the animals;
• Land and space: Anaerobic digesters take up a lot of space, so farms will need to have enough land for them to be installed. In addition, the final product needs to be stored before it can be taken away to be made into other environmentally friendly products.

Anaerobic digestion plants in the UKAccording to a report by the European Biogas Association, there are just under 160 operational AD plants in the UK. 80 of those are farm fed, while 77 are waste fed. Combined, they have a capacity of 160 MWel.

Read more of our guides on recycling and find out how you can help the environment.