What is Anaerobic Digestion?

Anaerobic Digestion Explained

Anaerobic digestion is not a new concept. In fact, the method has been around since the 1800s. The process involves harnessing the power of animal and food waste to create renewable fuel that has less of a negative impact on our planet than fossil fuels.

Today, anaerobic digestion is largely used by farmers to deal with farm waste but as demand for renewable energy increases, more and more plants have been created and it is now widely recognized as one of the best food waste recycling measures.

What is anaerobic digestion?

Anaerobic digestion is a process in which organic matter such as food or animal waste is broken down by micro-organisms to produce biogas, a versatile and renewable fuel. This fuel can go on to power vehicles, generate heat and the power can even be fed into the national gas grids. Furthermore, any leftover organic material – known as digestate – is used to create a nutrient-rich biofertilizer, so nothing goes to waste.

Process of anaerobic digestion

There are several stages to anaerobic digestion that must be carried out in order to produce a clean and quality fuel that can be used in a variety of applications.

1. Screening

Firstly, organic waste is screened to ensure it is free from contaminants.

2. Treatment

Following the screening stage, it is then treated to create a smooth consistency which ensures a continuous flow.

3. Loading

Once screened and treated, the waste is put inside a sealed, oxygen-free tank called an anaerobic digester.

4. Heating

The waste is then heated to an average temperature of between 37°C - 38°C, though sometimes it may be as high as 50°C. The ‘mixture’ is then stirred continuously.

5. Breaking down organic waste

After the organic matter is inside the anaerobic digestor for at least 20 days, micro-organisms (bacteria and archaea) get to work with breaking down the organic matter. This happens in four stages as the waste is broken down into smaller and smaller parts:

Hydrolysis: the complex matter such as carbohydrates and proteins are broken down into sugars and amino acids

Acidogenesis: sugars and amino acids are further broken down into ethanol and fatty acids. Carbon dioxide and hydrogen sulfide are created as by-products at this stage

Acetogenesis: the ethanol and fatty acids are then converted into hydrogen, more carbon dioxide and acetic acid

Methanogenesis: at the final stage the micro-organisms convert the remaining hydrogen and acetic acid into methane and further carbon dioxide

6. Collection and use

During the process of anaerobic digestion, the biogas – methane and carbon dioxide – rises to the top of the digester which can be collected and used to produce heat and electricity. It can either be used in combined heat and power gas engines or burned solely for heat. Alternatively, it can be cleaned to remove the carbon dioxide to create biomethane which is then sent to the gas grid as a renewable alternative to natural gas or it can even be used as fuel for vehicles.

What is left of the solid content falls to the bottom of the chamber where it is then extracted and used to create a nutritious, chemical-free, organic fertilizer or a soil conditioner.

One of the things to bear in mind with anaerobic digestion is that, in order for it to work correctly, a precise mix of nutrients is required to ensure the micro-bacteria is working efficiently. Manure and other trace elements will likely need to be added to the ‘mixture’ to help with maximum energy recovery from the waste matter.

Benefits of anaerobic digestion

There are many benefits to anaerobic digestion projects and the creation of biogas, both to the environment and daily life, some of which include:

  • It is a completely renewable resource
  • It reduces the amount of organic waste that ends up in landfill which, in turn, lessens the amount of methane produced and released into the atmosphere
  • It removes harmful pathogens from biological waste
  • The process leads to a smaller carbon footprint
  • The fuel created from biogas is much cleaner than fossil fuels
  • It is waste-free and leftover digestate is used to create natural, organic fertilizer which is chemical free

For further information on biogas and biomethane and how it works, learn about the environmental impact of biogas in our dedicated guide, next.