Anaerobic Digestion
We use a specialised process called Anaerobic Digestion to recycle unwanted food waste.
Anaerobic Digestion is a natural process that biologically breaks down organic material to generate large amounts of biogas – a combination of methane and carbon dioxide. It works in a similar way to a compost heap, only on an industrial scale. And, because of the lack of oxygen, we're able to capture the biogas, rather than lose it to the atmosphere.
Our Eight-step Process:
1. Food waste pre-treatment
All food waste is placed into hoppers, which act as large mixing vessels, and is then assigned to depacking - a process that separates packaging from organic waste. Recovered plastics are cleaned and used to create refuse-derived fuels. Then, liquid is added to the remaining waste so it can be pumped.
2. Heating
All food waste is passed through a heat exchanger and then warmed to 70°C.
3. Pasteurisation
The waste is held at 70°C for one hour to comply with ABP regulations.
4. Cooling
The pasteurised waste is cooled naturally by channelling it to run alongside the cold, incoming material, which lowers its temperature to approximately 40°C.
5. Mixing
The organic waste is moved to a mixing tank and held for three days to ensure a smooth product gets transferred to the digestors.
6. Digesters
Each digester tank is filled with 3,500 tonnes of blended organic material. Over the next 30 to 35 days, it is broken down by bacteria to produce methane.
7. Gas collection
Gas is collected from the top of the digesters and passed through a biological scrubber to remove any hydrogen sulphide molecules before moving onto the final stages of production.
8. Screening, storage and dispatch
The biogas is kept in storage tanks until required. It is then pre-conditioned, cooled and pressurised before being sent for processing.
At our Doncaster plant, the gas is used to fuel large industrial-scale combined heat & power engines, the electricity generated is delivered directly to the National Grid – enough energy to power 10,000 homes per year. The heat produced by the engines is used to produce hot water and steam for the process and the pasteurisation of the raw food waste.
At our London and Widnes plants, the biogas undergoes a further refining process to remove carbon dioxide and volatile organic compounds. Once the gas has completed the refining process an odorant is added to the biomethane before it is pumped directly into the National Gas Grid. The process produces enough gas to supply up to 12,600 homes per year.
Once the biogas process has been completed the remaining liquid is transformed into our ReGrow bio-fertiliser for local farming and agricultural use.