How does it work?
“If you feed the AD plant with only one input, that would be the same as you or I eating only custard creams. You need a balance.”
— Tom Chanter
Anaerobic digestion is the process by which bacteria break down organic matter, without any oxygen being present, to produce methane-rich gas and digested material. This process already occurs in nature; for example, your large intestine is an anaerobic environment, where special bacteria work to help convert everything you eat into fuel for your body. The AD plant is rather like a stomach in another respect, too – it’s important to ‘feed’ it at regular intervals, as this keeps the bacteria inside the chamber stable and provides a regular supply of gas and digestate.
Before the feedstock goes into the digestion chamber, we turn it into a sort of ‘soup’ by adding rainwater harvested from the roofs of our farm buildings - this makes it easier to mix, and also means it can be pumped directly into the chamber.
As the bacteria in the AD chamber get to work consuming the feedstock, they produce biogas, which is mostly methane. The gas rises naturally to the top of the digestion chamber, from where it is collected and piped to the nearby combined heat and power (CHP) cogeneration unit.
In the unit the gas is burned to generate electricity and heat for Greendale Business Park and Farm Shop; any leftover electricity is sold on to the National Grid, providing clean energy to the local community.
The solids (digestate) left behind simply fall to the bottom of the chamber, and we remove them to re-use either as part of the feedstock mixture, or as nutrient-rich organic fertiliser for our cattle-feed crops and for the maize we grow for the AD plant.
The whole process is surprisingly quick (feedstock added in the morning will typically produce biogas by the afternoon) and is reliant on maintaining a healthy microbial environment inside the digestion chamber – to this end, we carry out daily safety checks at Willowglen, including measuring the temperature and pH levels inside the chamber, as well as monitoring the status of the bacteria.