Process — A farm biogas plant (two products)

The diagram illustrates how a biogas plant on a farm converts animal waste into two useful products: electricity and natural fertiliser.

Overall, the process has seven stages and is notably circular — a single stream of waste yields both power and fertiliser, and the fertiliser is used to grow crops which in turn feed the farm's animals.

The process begins with manure from the farm buildings, together with food waste, being collected and blended with water in a mixing tank. This mixture is then piped into a sealed, dome-shaped digester kept at around forty degrees Celsius. Inside, bacteria break the waste down over roughly three weeks in the absence of air, and the biogas this releases, which is mainly methane, collects at the top of the dome.

From there, the gas is piped to an engine that drives a generator, producing electricity for the farm; any surplus is sold to the national grid. Meanwhile, the leftover slurry is drained into a storage tank. Rich in nutrients, it is later spread on the fields as a natural fertiliser, helping to grow the very crops that feed the livestock — at which point the cycle begins again.