Small Scale On Farm Anerobic Digestion

In my previous working life, I had the pleasure of managing 2 demonstration Anerobic Digesters (AD). I became very passionate about the whole concept of using cow slurry to produce methane gas that could fuel a Combined Heat & Power unit (CHP) and also a gas boiler. Farmers who are committed to AD get a real buzz out of producing biogas as a renewable energy from on farm waste streams.

The whole concept of Small-Scale AD on livestock farms has yet to be properly promoted, the government should start to look at incentives to encourage 250 kW or less installations by providing funds for feasibility studies and low-cost loans. The digester (above right) is a low-cost concept designed by Marches Biogas which is focused on slurry-based enterprises called a Plug & Play, which is exactly what it is. It is constructed in the factory and delivered to the farm on the back of a lorry. It can be placed on a concrete pad or below ground level. The design concept is to provide a simple robust solution to small scale AD. The digester is a cylindrical glass reinforced plastic insulated tank with an internal heat exchanger and mixing is carried out by gas injection.

This digester has a volume of 100 m3 and would suit a 150 head dairy farm. If it is being fed 3 cubic meters of slurry over a 24 -hour period, there is a potential to produce around 65 cubic meters of biogas. This gas could be used to run a small CHP unit or feed a gas boiler to heat water for parlour washing, under floor heating in a pig or calf unit or household heating to name a few. Some milk buyers are focusing more on the carbon footprints of their producers. One avenue is purchasing these digesters and then hiring them to their producers. The beauty is they can be moved from farm to farm if a producer decides to move companies or ceases milk production.

Gas Storage
Although slurry has a low calorific value compared to other feeds it contains the right bugs needed in the production of biogas. Anaerobic digestion works on the same principles as a cow’s rumen so if you understand how to feed a cow you can feed a digester.

Typically, slurry with a high dry matter will produce better gas quality and quantity than lower dry matters, we recorded methane levels of 63% from slurry with a DM of 12% and from 1 cubic meter of slurry we were seeing 40 m3 of biogas.

The Liquid Digestate is lower in Phoshate and Potash as most of the nutrient will be a higher concentration in the solid fraction of the digestate.

The biogas industry is uniquely positioned to help achieve emissions reductions and mitigate many of the impacts of climate change through capturing organic wastes, producing renewable energy, and returning nutrients and organic content to the soil.

For further advice, or an independent assessment of the feasibility of your farm running an AD plant, please contact Mark Yearsley at or your local FCG Office.

Posted in Business Management, Dairy & Forage, Environment, Sherborne.