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?
As we said, most organic waste can undergo AD. The feedstock (waste that undergoes anaerobic digestion) can include:
- Municipal wastewater and wastewater solids,
- Livestock waste/animal manure,
- Agricultural waste, and,
- Food waste
Most AD plants will focus on their own ‘recipe’ for feedstock. At BioteCH4, we primarily deal with food waste.
So, what types of food waste can undergo anaerobic digestion? We process a wide range of food waste, including:
- 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- Out-of-date food or unsellable food from supermarkets and shops,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?
As we mentioned earlier, the digested food waste that remains once it has been digested and the CH4 (methane) and CO2 (carbon dioxide) have been extracted can be used as a fertiliser for farmlands.
As a result, it must be free of any pathogens or chemicals that could affect people or livestock.
Therefore, if animal by-products (ABPs) are being used in the AD mix, they have to be treated as per regulations.
ABPs include both animal meat and carcasses as well as manure and digestive tract contents. Additionally, the term also includes any animal product that is not meant for human consumption.
All ABPs fall under three categories, based on how much risk they pose.
Category 1 ABPs are high-risk, and include:
- 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.
This category is considered unsuitable for anaerobic digestion. That is because the risk posed by these cannot be mitigated by heat treatment. These ABPs need to be incinerated to destroy the potential hazard.
Category 2 ABPs are also high-risk, and include:
- 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.
Category 2 ABPs can be used in the mix for anaerobic digestion provided they have undergone heat and pressure treatment (133 degrees C at 3 bar for 20 minutes).
However, certain category 2 ABPs, like manure or eggs and egg products, might not need to be sterilised before digestion.
Category 3 APBs are low-risk animal products, such as:
- 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)
Since these aren’t likely to spread diseases or chemical contamination, category 3 ABPs don’t require sterilisation before being used as feedstock for anaerobic digestion.
Of course, any site that uses ABPs for AD or composting must be approved and validated.
At BioteCH4, we are category 3 registered, but can also handle certain contaminated milk waste, which are category 2 ABPs.
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.
What do you need to consider to ensure that your waste can be reused?
Do you have food waste that you need to dispose of responsibly?
At BioteCH4, we have the knowledge and the means to help you with your food waste. Our facilities are equipped and certified to make sure that your waste is dealt with responsibly, and without any adverse impact on the environment.
What’s more, we are also happy to conduct a waste audit for your business and see how we can help you reduce waste.
If you would like to know how we can help you with your food waste, get in touch with us.