Sewage sludge is loaded into airtight tanks called anaerobic digesters. Here it is kept at 35°C and mixed with bacteria to break it down. As the bacteria digest the sewage, methane (a natural gas) is produced, as well as a fertilizer by-product. The methane is piped into large engines which produce heat and electricity. Electricity can be used on site or exported to the National Grid, whilst the heat is used to keep the digesters at the right temperature. Re-using the heat makes this a very efficient process. We currently have 42 engines across 24 sites.

We’re always looking to refine the technologies involved in generating electricity from sewage and as part of this we're running a continuous programme of renewal and introducing new technologies like gas to grid. We’re also looking at ways to increase the amount of methane produced in the digesters, perhaps by adding fats, oils and greases which build up in the sewage treatment process. If these can be broken down, they will release large amounts of energy as gas which we can use to produce even more power.