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Farm focus: Making slurry pay with an anaerobic digester

Anaerobic digesters could prove a valuable diversification project and one British firm believes it can reduce the costs of setting up such a system.

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Slurry is pumped from the 600-cow dairy herd to a separate tank
Slurry is pumped from the 600-cow dairy herd to a separate tank

There are up to 25 anaerobic digesters currently working on UK farms but Fre-Energy believes its system could help the government achieve its target of 1,000 units by 2020.


The establishment costs and technology, is seen by many as being an inhibiting factor but organic dairy farmer, Richard Tomlinson, believes their anaerobic digester could simplify things.


He has set-up Fre-Energy together with his brother Jona-than who has an engineering busi-ness, and Chris Morris and James Murcott who have a wealth of experience with anaerobic digesters.


A key feature of the Fre-Energy system is its sweeping arm used to brush any grit and stones to a dirt trap, while seven pipes distribute methane gas from the top of the tank to the floor to create air bubbles for aeration and keeping the contents moving.


By being self-cleaning, a wider range of materials can be used, such as chicken muck, which has a high grit content and is very good for producing methane. But more importantly, there is no need to empty the tank every six years or so to remove sediment and then wait up to six weeks for anaerobic bugs to re-establish themselves.


The tank is also fitted with a fibreglass lid, which Mr Tomlinson adds has a guaranteed service life of 20 years and is less susceptible to high winds than flexible covers popular on European systems. The heating elements are also placed further inside the tank for improved heat exchange, as there are no stirrers to accommodate like other systems.


Mr Tomlinson is currently building the anaerobic digester at Lodge Farm, near Holt, Wrexham and will use 25 tonnes of slurry per day from his 600-head organic dairy herd to help fuel the digester. There are long-term plans to use food waste from the many food processing plants around Wrexham but initially, locally-grown crops such as sugar beet could be used.

Using the top layer from a silage clamp

“The anaerobic digester works very similar to a cow’s stomach,” explains Richard. “We will be using some silage and research has found that the waste on the shoulders of the silage clamp is ideal, in fact, it may be worth not covering the clamp and using the top layer for the digester.”


The anaerobic digester should be fully up and running by early June 2009 producing 120kW. A second digester will be added at a later date, boosting output to 500kW providing enough electricity to power the local village of Holt.


Chris Morris explains that a 300kW anaerobic digester will have a typical cost of £750,000 and farmers should be looking to make this pay for itself in three to four years. Interestingly, he says smaller systems take longer to pay back, as it does not cost that much more to make the components slightly bigger.


The sweeping arm is driven by a hydraulically-powered ratchet arrangement and will take 15 minutes or more to do a full revolution. An auger will be used to remove any material from the two stone traps, where it can then be disposed of.

Benefits of a solid roof

By opting for a solid roof, Mr Morris points out that they can increase the gap between methane off-take point and the protective foaming valve.


“If you add too much protein then there is a danger of foaming, just like a cow would become bloated,” he explains. “There is up to 30 per cent liquid in the foam, so we must prevent this from getting to the methane-powered engine and generator and causing damage.


"We have been able to insulate the tank walls and lid to keep the temperature as stable as possible and by having the heat exchangers within the tank, it takes less energy to heat the contents. Also it only takes 2kW to use aeration, whereas stirrers need up to 10 times as much energy.”


Fre-Energy has also developed a very small-scale anaerobic digester which they showed at last year’s Dairy Event – this allows them to test the ration and access methane-producing abilities before using it on the full-scale model.


Farm facts

  • Lower Park Farm, near Rossett, Wrexham.
  • 600 dairy cows producing organic milk.
  • 400 followers (contract reared off farm).
  • Crops grown over 1,000-acre unit: Red and white clover grass swards; Fodder beat; Oats and vetches for wholecrop.
  • Farming Connect are holding an Open Day at Lodge Farm on Wednesday April 7 to provide a useful insight into the workings of an anaerobic digester.
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