Step 1. Waste removal / collection
The first step in the process is collecting ‘fuel’ for our plant. This comes in the form of food waste from industry, council collections or local businesses.
Food Waste from Food Manufacturers and Waste Contractors
The first step in the process is receiving the feedstock for our plants. This comes in a variety of forms and can include anything from pasties to fish heads. Our clients vary from big national organisations through to local dairies, cider mills and local authorities. Last year we processed around 67,000 tonnes of waste through our Holsworthy plant alone.
The Logistics of Food Waste
The waste is delivered to our sites from all over the country and we endeavour to keep the food miles as low as possible by managing the waste at our nearest plant to the source of production. Some of the waste is collected through our own transport or we can offer waste collections through our partnerships with several national haulage companies.
Step 2. Food processing using anaerobic digestion
Now that we have our ‘fuel’ we need to refine it so that we can power our engines.
The start of the process
After the waste has been delivered to site it enters the process in a variety of ways. Packaged waste is fed into a machine that separates the packaging from the food. The packaging is then recycled if possible and the food is mixed together with other waste streams in a tank.
Making the feedstock
The waste is then mixed together in our mixing tank to turn it into a pumpable soup. This process also helps to blend the varieties of substances found in the food waste into a more homogenous material.
Ensuring Quality Feedstock
This important part of the process allows us to accept animal by-products such as fish and abattoir waste. The macerator ensures any that food particles in the ‘soup’ are reduced to below 12mm ensuring that it can be fully heated in the next stage of the process.
Killing Unwanted Bacteria
The food waste is heated to 70°C for one hour to ensure all pathogens including as E-Coli and Salmonella are killed and can’t spread throughout the material. This process also helps us to heat up the feedstock so that the digesters are at the right temperature, 38°C – 40°C, to produce the biogas.
Production of Biogas
Following pasteurisation, the food waste is pumped into one of three 4,000tonne tanks called digesters. The digesters contain cultures of anaerobic bacteria that digest the waste over a period of 55days. The digestion process results in the production of biogas which is around 60% methane and 40% carbon dioxide. Roughly 10% of material that goes into a digester is turned into gas the rest is pumped onto the next stage of the process.
Storing Digestate – A Liquid Fertiliser
After 55 days the digested material is pumped into a storage tank ready to be used as a liquid fertiliser. The digestate is passed through a screening machine that removes any residual inorganic material such as plastic or grit which can’t be digested. The digestate is totally free from pathogens and contains high quantities of nitrogen, potassium and phosphate; all vital nutrients for plant growth.
Step 3. Energy production
Storing Renewable Energy
The biogas that is produced through anaerobic digestion process is stored in a 5,000m3 biodome. This storage system allows us to use the gas when we want meaning we can produce power when people need it. This is a huge advantage over other renewable systems that can’t efficiently store energy and can only produce power when the elements determine so.
The energy stored in the biogas is converted into electricity and heat by using it to run a combined heat and power (CHP) internal combustion engine. Some of the electricity produced is used to run the plant but most of it (>88%) is exported to the National Grid. The heat is used to run processes such as pasteurisation and heats our sites’ hot water. Last year we produced around 24,000 MWh/year of electricity and have the capacity to generate 5.2MW at any time from our Holsworthy plant alone.
Step 4. Digestate spreading
Digestate – A liquid Fertiliser
Digestate is nitrogen rich bio-fertiliser which is spread to land as a replacement for mineral fertilisers. Its use ensures that vital nutrients in the food waste are recycled and returned to to the ground.