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Anaerobic Digestion - One Way Forward?

Posted Thursday, September 6, 2007

What is Anaerobic Digestion?

Anaerobic Digestion (AD) is a natural process which enables organic matter to be broken down by bacteria in the absence of air.

Organic matter is placed in a digester (a warmed sealed airless container) for between 10- 25 days. The materials ferment and produce methane (around 60%) and carbon dioxide (40%), both extremely harmful greenhouse gases. This ‘biogas’ can be burnt to produce electricity or heat, or used as a vehicle fuel. In the case of electricity it can be used to help power the AD process and, most obviously, sold to the national grid to heat domestic homes.

The process also produces a solid called the digestate, which in turn can be separated out into fibre and liquid. Once cleaned of contaminants, the biogas can be used in a furnace, gas engine or turbine or further refined for use in gas powered vehicles and Combined Heat and Power plants. The digestate can be returned to the farms for re-use as a soil conditioner and fertiliser.

AD can use organic matter such as waste from cattle, chickens and pigs, but also waste food and cardboard, garden cuttings and general green refuse, plus of course specially grown biomass crops (mycanthus, maize, clover, etc).

Why does it help farmers?

An AD plant can be part of an integrated waste management plan and can assist farmers in complying with legislation on the safe handling of waste: the process stabilises slurries, making them easier to handle and can reduce the odour by up to 80%. Improving the efficiency of waste management will also help to reduce the risk of land and water pollution by reducing and controlling residues. The remaining nutrients in the solid residue and liquor can be returned to the farms and used as a fertiliser, reducing the need for inorganic fertilisers.

What is an Anaerobic Digester?

The AD plant can operate on a small scale, recycling slurry products at an on-farm facility run by an individual farmer or – as Sustainable Youlgrave is envisaging – at a larger scale development at a centralised anaerobic digester, taking animal waste from local farmers and probably other organic matter from elsewhere. The size of the AD plant can vary from a small localised installation, which looks no different to standard farm sheds and tanks, to much larger plants. There is no odour, little noise and if it is sensitively sited the AD facility can blend in to existing farm developments without any problem.

Why does Anaerobic Digestion have a future?

~ Fossil fuels are running out and the price of these traditional fuels (such as oil and gas) is escalating rapidly
~ Farm waste requires better management, partly due to new EU Directives
~ Organic waste must be diverted from landfill
~ AD will help meet Government targets on renewable fuels and bioenergy
~ The demand for new sustainable technologies is growing all the time - AD on farms is already commonplace in Germany and elsewhere

So what does an Anaerobic Digestion plant look like?


This photo is of a demonstration digester built by a company called Greenfinch in Ludlow, Shropshire. It was part of a pilot project funded by the DTI in 1998 to recycle kitchen waste from 1,200 local households. Go to www.greenfinch.co.uk for more details.

Sustainable Youlgrave’s Fred Baker visited a German farm to see how AD works.

Above: The biogas digestion tank on the farm in Germany.

It has been a long, cold winter in Germany, and at the start of our short break visiting friends in the southern state of Baden-Württemberg, ice still covered the small ponds and wetlands which are feature of this landscape.

The winters have focused German minds on energy supplies where (as in the UK) prices are rocketing. In Germany, the price of natural gas is set at 15% above that of heating oil, the price of which has doubled in the last 12 months. The soaring energy cost has, however, started to encourage a new type of farmer – one that not only produces food but also energy.

Our host, Monica Steinhauser, is a farmer's daughter, and as well as a mother of three is also a local vet. Her father, Anton Dreher, is a leader in energy farming and four years ago was one of the first to install a biogas generator on his farm.

The principle of biogas generation is relatively simple. Organic waste when stored in the absence of air produces methane. If captured, the methane can be used to power a generator, which can then feed electricity into the national grid. Anton farms 34ha and has a herd of 200 dairy cattle. The beauty of the system is that the slurry from the dairy is fed into the biogas tank, then supplemented with maize which is grown on his arable land and compost waste from the village. The process generates enough energy not only to heat the farmhouse and seven holiday cottages, but also enough electricity to power 83 homes. At the end of the process the digested slurry can be used as fertilizer – just as it would normally have been.
Anton has been joined by a whole host of other conventional farmers who, faced with sinking profits, have seen the opportunity to diversify by growing their own energy; and it is not just biogas which is providing renewable energy. Almost every farm has at least one barn with an expanse of solar panels on the roof. I was struck by how little impact these had on this largely rural landscape which by UK standards would be worthy of national park status. And far from being a sandal-wearing ‘green’, Anton does not so much see himself as saving the planet but as using his farm to produce a high value product. Given the current global oil market it would appear that he is onto a good thing both financially and environmentally.

Is this a model of sustainability that rural communities such as Youlgrave could follow?

This article first appeared in The Bugle, Youlgrave’s monthly community magazine, in March 2006.


All material is copyright of Sustainable Youlgrave, 2007 onwards

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