Much to digest
21 February 2008

As Ambridge discusses the proposal for an anaerobic digester at Home Farm, here's a quick background note on the technology.

A typical on-farm digester

Simply, anaerobic digestion (AD) is when organic material is broken down by micro-organisms in the absence of oxygen: the same process used in septic tanks.

The reason farmers are becoming interested in it is because not only does it treat their waste products (mainly manure and slurry) in a more efficient way, but it also produces valuable by-products.

As the organic matter degrades, it gives off biogas - a mixture mainly of methane and carbon dioxide (CO2). This is similar to natural gas and can be burned to generate heat and electricity, which can be fed to the national grid or used locally. There is also a liquid residue and a solid residue, both rich in nutrients, which can be used as a fertiliser and soil conditioner respectively.

Generates

So it generates heat and power without directly using fossil fuels, which should reduce CO2 emissions and allow farmers to become more self sufficient in energy. AD also greatly reduces the emissions of methane from slurry and other organic wastes. As a greenhouse gas, methane is about twenty times as potent as CO2.

Some large digesters serve small communities. For example, Britain's first centralised anaerobic digester - the Holsworthy digester in North Devon - collects farm slurries from 30 nearby farms, produces enough electricity for about 3,600 homes and provides heat for a health centre, hospital, swimming pool, school and council offices.

The Home Farm digester will be much smaller and might produce enough electricity for around 13 homes (although this will mainly be supplied to the grid). Some of the heat is recirculated to keep the digester tanks at the optimum operating temperature (about 35C), and the rest will be used to heat the polytunnels. Because of this, the digester will be located near to the polytunnel area.

Pat's objections

So far, so green. But it's what's mixed in with the manure which is causing concern in some quarters - including Bridge Farm. Like a compost heap, the system works best with a mixture of materials, so it's common for some crops - typically maize - to be grown specifically as a feedstock for the digester.

As with energy crops for transport fuel, there are arguments against taking land out of food production. And growing energy crops is not necessarily carbon neutral - especially in temperate climates - because of the fertilisers, chemicals and tractor fuel that are necessary for their production.

That's not to say that people like Pat are against AD in principle - simply that it's more suited to treating domestic and municipal waste without the addition of specially-grown feedstocks.

Detailed article

Pic: Greenfinch Ltd

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