Anaerobic Digestion of Food Waste: What’s In The Mix?
Anaerobic digestion (AD), or the process of breaking down waste by microorganisms in the absence of air, is one of the best ways to manage food waste. It is not limited in its scope, as composting can be. And, it produces a sustainable biofuel as well as a biofertiliser in the process. The best part is that almost anything that is organic can be put in the mix. You’ll notice the ‘almost’.
So, what materials can undergo the anaerobic digestion process?
- Municipal wastewater and wastewater solids,
- Livestock waste/animal manure,
- Agricultural waste, and,
- Food waste
- Industrial food by-products,
- Solid and liquid waste from agriculture,
- Manufacturing, food and drink processing waste,
- Out-of-date food or unsellable food from supermarkets and shops,
- Packaged food waste
- Restaurant waste, including fats, oils, and greases, and
- Bakery waste
What type of food waste cannot undergo anaerobic digestion?
Whilst any food waste can technically be digested, we tend to avoid using large quantities of brassicas (vegetables like cabbage, cauliflower, broccoli, mustard, radish etc) in our feedstock.
That is because these vegetables are high in sulphur. When they are digested, the sulphur is processed to form a gas called hydrogen sulphide (H2S) alongside methane.
H2S can impregnate the lubricating oils used in the Combined Heat and Power (CHP) units, clogging them and causing damage.
We also limit the amount of food waste that comes under the allium family, like garlic and onions. These are also high in sulphur, and can affect the bacteria in the mix, which has an impact on the digestion process.
Of course, it is not possible to completely eliminate these components from food waste. However, we do need to ensure that they are only a small percentage of the total feedstock.
What are the regulations for the food waste being anaerobically digested?
- Carcasses of animals suspected of being infected with TSE (Transmissible Spongiform Encephalopathy),
- Carcasses of animals that were suspected of having a disease that could be transmitted to people or animals,
- Carcasses of animals that had been used in experiments,
- Carcasses that had been contaminated due to illegal treatments,
- International catering waste, and
- Specific body parts that are high risk.
- Carcasses of abattoir animals that were rejected because they were diseased,
- Carcasses that still contained residues from authorised treatment,
- Unhatched eggs with dead poultry,
- Animals that were killed in order to control the spread of disease,
- Carcasses of dead livestock,
- Manure, and
- Contents of the digestive tract.
- Animal body parts that are fit for human consumption,
- Foods of animal origin that are being discarded for commercial reasons instead of health reasons,
- Domestic catering waste,
- Shellfish shells with soft tissue,
- Egg and hatchery byproducts,
- Aquatic animals, aquatic and terrestrial invertebrates,
- Animal hides and skins from slaughterhouses,
- Hides, skins, feathers, wool, horn, and hair from animals with no signs of diseases at the time of death, and
- Processed animal proteins (PAPs)
What happens to the mix during the anaerobic digestion process?
Organic materials are composed of carbon, hydrogen, oxygen, and nitrogen. During the digestion process, the microorganisms feed on the materials and produce CH4 and CO2 as by-products.
This anaerobic digestion process takes place over four stages. These stages are:
- Enzymatic hydrolysis
Enzymatic hydrolysis is the stage where larger molecules are broken down into smaller ones. For example, complex carbohydrates break down into simple sugars.
Similarly, proteins are broken down into peptides and amino acids, whilst fats break down into glycerol and short fatty acids.
During the Acidogenesis (literally translated to ‘the birth of acids’) stage, these compounds are further transformed into volatile fatty acids. During this process, the available oxygen is used up, leaving an oxygen-free (or anaerobic) environment.
The Acetogenesis stage sees the fatty acids broken down into acetates and hydrogen. The problem is that the acetogenic bacteria cannot survive in a hydrogen-rich environment.
That means they rely on the hydrogen-loving methanogens to absorb the gas to produce methane in the methanogenesis stage.
The methanogenesis stage is the final phase where the acetates, primarily acetic acid, are combined with the hydrogen gas to produce methane and carbon dioxide.
Since gases are lighter than the liquid slurry in which they were produced, they bubble out and rise above it. As they rise up, the CO2 and CH4 are easily moved into a storage chamber, leaving the digested material in the digesting chamber.
This remaining digestate is high in nitrogen and makes an excellent biofertiliser.
How much methane is produced during the anaerobic digestion process?
The amount of CH4 produced during AD varies according to the composition of the materials that are being digested in the feedstock. Also, since the process produces both CO2 and CH4, the result is never pure CH4.
If the mix is high in carbohydrates (sugars, starches, and cellulose), the ratio of the two gases produced will be almost equal.
However, when the mix is high in proteins and fats, the amount of CH4 produced is higher than CO2. In the case where the mix is all fats, you can get a concentration of up to 70% methane.