Where there’s muck, there’s brass. An introductory look at Anaerobic Digestion for the LVC.

by Charles Paxton

I first heard of this technology from a BBC World TV program called Earth Report that I watched while living and working in Tokyo. Amongst other innovative projects, the show featured a Nepali lady running her gas ring for about an hour a day from household waste and the waste from a few cows. I showed the video to my students and we were all highly impressed and wondered what could be achieved with the combined sewage of Metropolitan Tokyo! So much waste could become so much resource. Amongst other things, we discovered that among advanced nations, it was the Germans that were leading the field in terms of municipal ‘energy from waste’ projects. An efficient lot, the Germans.

When I learned recently that the Lyvennet Valley Community were investigating Anaerobic Digestion locally, I pricked up my ears. This is an exciting development. When my brother-in-law told me that biodigestion of farm waste could be so valuable that a Dairy farm’s milk becomes a secondary byproduct of energy production, I was amazed and heartened. Economics can be pretty surprising! With the recent collapse of a major northern milk cooperative due to brutally competitive milk pricing, there have been fears for the viability of some Cumbrian dairy herds. Diversification may become a necessity for farm survival. Diversification into AD could greatly benefit some farms. Where there’s muck, there’s brass.

When the Common Agricultural Policy was reformed in 1992, it was recognised that farms could diversify and produce a variety of ‘public goods’ ranging from farm-stay tourism and paint ball recreation to energy production and reduction of Greenhouse gas emissions. With AD, farms have a role to play in assisting climatic stability and can make a tidy profit while doing so.

“How’s that?” you may well wonder. “By digesting agricultural waste in a controlled manner and tapping off the valuable gas and heat that results” would be the answer!

Methane gas (CH4), a digestive byproduct from cattle and their decomposing waste, is a surprisingly powerful warming gas – about 25 times more effective at warming our atmosphere than the more notorious CO2. It therefore makes more sense to burn it off usefully rather than to let it loose in the environment. But isn’t some carbon emitted in the burning process? Yes, surely, but this isn’t fossil carbon that is being re-released after about 300 million years of natural sequestration underground in coal or oil, this carbon was trapped in the past year or two and is part of the current carbon cycle.  This methane is, in effect, a valuable renewable energy resource that can ( and should if possible) be piped and bottled to enrich farmers and help power the country.

To quote Laurence Smith in his excellent article “It’s a gas” from Organic Farming magazine (Spring 2009) Anaerobic digestion (AD for short) also known as biodigestion, is

“a composting process without air. Wet organic material such as slurry, food left-overs or forage, is collected and placed in a sealed, airless container (the digester).

The absence of oxygen encourages growth and activity of certain microorganisms which then break down the organic matter and produce methane and a stable, low odour material called the ‘digestate’.”Smith states that “digesters can accept most forms of non-woody biodegradable matter, but on-farm units are mostly used for the digestion of slurry and energy crops, such as maize silage.”

Apparently the slurry provides the requisite moisture and bacteria and the energy crops (often referred to as feedstock) feed the culture and magnify the gas yield.

This gas can either be piped, or bottled for cooking or heating, or burned on site to generate steam-powered electricity. According to local Green Architect and AD advocate Mike Archer of Maulds Meaburn, Cumbria, the digestate is a high quality fertilizer, with a roughly 4-1-4 NPK ratio, that can be applied directly to farmland. In some cases the nitrogen component of the digestate can actually be a bit higher than that of the feedstock and it doesn’t ‘burn’ vegetation.

“For a major farm that can spend anything up to £70,000 on fertilisers annually – switching to home-made digestate could make all the difference between profit and loss for the year”, Archer explains.  He says “AD replicates the very same natural processes that occur in a cow. ”  This is controlled, synthetic flatulence, useful flatulence!

According to Laurence Smith, AD is also an ancient idea that is being revisited, he says the Assyrians were heating bath water with AD in 10 B.C! The practice is not just tried and tested, it is now widespread. The Germans have over 3,000 units currently working on farms and the Chinese are leading the field when it comes to numbers – they have 15 million active units.

This technology is particularly attractive to organic farmers as it helps to make a farm more of “a closed loop” in terms of energy and waste. It’s a great idea for our age of threatened energy shortages and it is far less controversial for application in Cumbria than on-shore wind farms. The energy generation is more consistent, provided that there is a regular supply of feedstock. Archer says that an average-sized project can cost about £3 million to set up, but you can expect it to pay for itself in about somewhere between five and eight years. Henceforth the profits will really stack up!

The digesters themselves are low-profile and no more visually intrusive than a typical modern farm slurry tank, silo or barn; if necessary they can be partially buried, but in most cases this isn’t necessary. Country folk don’t complain about ‘earthy’ fragrances on the whole. Odours are largely trapped and would be no more intrusive than regular slurry anyway. Options for filtration exist that can cut odours if necessary.

Smith says that in the UK the expense of grid connection has been a significant obstacle, as it wasn’t designed to accept electricity from small scale producers. He says that an additional 10-15% of set-up costs should be ear-marked for getting a project hooked-up to the grid. He therefore advocates the following ideas to more favourably leverage the economics in the farmers’ favour:

1) cooperative projects between multiple farms. Sharing the set-up costs and producing more inputs and hence outputs in relation to the invested capital.

2) using the hot water byproduct, i.e. in the farm houses, nearby homes or public buildings or to heat green houses depending upon the local logistics

3) adding more productive ingredients such as poultry manure

4) ensuring year-round inputs

5) charging “gate-fees” to the local council for accepting food wastes that would otherwise go to land-fill for disposal. Apparently WRAP, DEFRA and The Environment Agency are working to reduce the health & safety red tape that currently makes this a difficult proposition. The digestate emerges in an effectively pasteurised state.

Smith informs us that funding assistance for AD projects is available from various sources, including The Bioenergy Capital Grants Scheme and Axis 1 of The Rural Development Programme.

To read the full article It’s a gas” by Laurence Smith, from Organic Farming magazine, centenary edition (Spring 2009) as a PDF, please click HERE

For information about the LVC Anaerobic Digester Group, please contact Mike Archer, perhaps via the new LVCP Forum or Maureen Newrick of Crosby Ravensworth Parish Council. The equipment that they are currently considering for possible use in an LVC AD project is fifth generation German technology – the cutting edge, apparently.


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