What is anaerobic digestion?

In simple terms, anaerobic digestion is the breakdown of biodegradable materials such as food or green waste into gases – methane and carbon dioxide, or biofertilisers. Methane is a flammable gas which can be used to produce heat and electricity, whereas the biofertiliser is rich in nutrients such as nitrogen, potassium and other elements making it perfect for healthy plant growth and fertile soil.

The organic matter is broken down by microorganisms which are intolerant to oxygen; as a result, the process takes place in large, airtight containers. The term aerobic indicates the need for oxygen, whereas if oxygen wasn’t required the process would be anaerobic.

The gases which are created are then used as fuel, either for grid resources such as heat and electricity generation, or as biofuels for transport – just like our we do with our fleet!

How does anaerobic digestion make electricity?

This is a question we get asked a lot, so we’ve put together a step by step guide to the process and how it all works…

Step 1 - Removing Contaminants: Firstly, the organic fuel needs to be screened for contaminants. The fuel used in anaerobic digestion can be any sort of biodegradable materials, such as food and plant waste, or even in some cases crops, slurries and sewage.

Step 2 - Treating the Fuel: Once screened, the fuel needs to be treated to ensure a smooth consistency - this is because many AD units are fed by continuous flow for cost effectiveness. Once screened and treated, the fuel is then fed into a digestion unit for breakdown by microorganisms.

Step 3 - Into the Plant: Digestion units are either Mesophilic or Thermophilic, which refers to the temperature range which the microorganisms work best within. We are 'mesophilic' by design, but we actually operate In between the two ranges. We found that this temperature gives us stable biology with excellent gas production

Mesophilic units operate at between 20-42°C, with an optimal temperature of 37°C. These are more common and are often easier to maintain because of their lower operational temperature.

Thermophilic units on the other hand operate at a higher temperature, typically 50-60°C, but sometimes even higher.

Step 4 - The Different Stages of Breakdown: The breakdown of the organic material begins here, and involves four stages of chemical processes which then gives us our useable biogas. These stages are:

1. Hydrolysis: The first stage sees the breakdown of complex matter, such as carbohydrates and proteins, broken down into sugars and amino acids. These are normally long-chain chemical compounds, but hydrolysis breaks them down into single molecules.

2. Acidogenesis: In the second stage, microorganisms break down the single molecules of sugar and amino acids even further into ethanol and fatty acids, as well as producing carbon dioxide and hydrogen sulphide as by-products.

3. Acetogenesis: In the third stage, the ethanol and fatty acids are converted into hydrogen, carbon dioxide and acetic acid.

4. Methanogenesis: In the fourth and final stage, microorganisms convert the remaining hydrogen and acetic acid into methane and more carbon dioxide.

At the end of the process, we have our methane biogas. As mentioned previously, as well as producing biogas which can be supplied to the grid or converted into biofuel, anaerobic digestion also provides digestate, a nutrient-rich fertiliser, as a by-product of the process.

How long does anaerobic digestion take?

There are many factors which determine the overall time it takes to complete the anaerobic process, such as the type of system and the amount/type of feed material.

For example, a typical two-stage mesophilic digestion can take anywhere between 15-40 days, whereas a single-stage thermophilic digestion is normally faster at around 14 days.

There is also an upflow anaerobic sludge digestion system (UASB), where hydraulic process times can be as short as 1 hour to 1 day, with solid retention times up to 90 days.

Here at Warrens we employ a two stage, mesophilic digestion process with a hydraulic retention time of 36-40 days. Thanks to smart engineering we can effectively convert this into a single stage system and free up space for digestate storage.

Is anaerobic digestion good for the environment?

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