MUCK AND slurry should be viewed as a source of energy, rather than a waste product, say brothers Tom and Wayne Pugh.

At the 133ha (330-acre) Bank Farm, near Churchstoke, Powys, the Pugh family has been digesting slurry produced by the farm”s 110 cows for the past 15 years. The resulting methane has been of sufficient volume to heat two farmhouses, provide enough hot water for the milking parlour and keep the digester working at about 40C via water-filled heat exchangers.

 “We produce more than enough gas for our own needs from just 110 cows,” says Tom Pugh. “We flare-off about 20% of the gas because we have no additional use for it – yet.”

The anaerobic digester is at the bottom of the cow yard, so slurry is scraped directly to a filling hopper, where an auger automatically runs to keep the digester topped up with slurry.

The 270cu m circular vessel contains heat exchangers and a collection of 36 aeration pipes arranged around the floor of the tank.

 Dirty gas that first collects in the top of the vessel as it is filled is then pumped through the aeration pipes to mix the slurry and start the creation of methane. This gas is then continually circulated until its quality is sufficient to be drawn off the tank and used as a fuel.

A rotary valve on top of the tank diverts the flow of air to each aeration pipe in turn for about 15 seconds, so the tank is continually stirred by the passage of an air/methane mix through the slurry.

“Getting the process started requires a lot of heat to be put into the tank,” he says. “We produce about 300cu m of methane gas a day, but use half of it to keep the tank heated via hot water.”

 Operating temperature is from 38-42C, critical, says Mr Pugh, to maintain gas production and quality.

“It is not the cleanest of gas samples, but contains 65% methane, plus carbon and other deposits,” he says. “Using this system, we can extract about 75-80% of the methane content from slurry.”

As processed slurry is gradually drawn off the tank, it is passed through a steel press belt, which separates liquid from solids. Each product is analysed, stored and spread individually.

 Mr Pugh says savings are about 6000 a year on bought-in fertilisers as a result of the way the digester unlocks nutrients in the manure.

 “We have found that the two manure products have more nutrient availability for grass. Spreading liquid in the spring means we do not need to buy any bagged fertilisers for first-cut silage.”

Given the success of the farm”s first digester, the Pughs have invested heavily in a second plant, but one that is capable of taking in waste products from other sources, possibly food and green wastes. A waste management licence application is being processed.

 “The second plant is much more sophisticated and uses three 175cu m tanks,” he says. “And given the waste streams we would like to take in, the new plant has a pasteurising facility to ensure that what we eventually put back on the land is properly treated and safe to handle.”

Though the plant has not yet been completed, it should give the chance to eventually produce and sell electricity, making the farm self-sufficient for its energy requirements.

“Ultimately, we would like to move to CHP – combined heat and power – and use the methane to run gas engines with two 50kW generators and sell electricity back to the National Grid.”

fwmachinery@rbi.co.uk