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Industry trials biogas system

Frank SmithCountryman

As a greenhouse gas, methane is more than 20 times powerful as carbon dioxide, so just collecting methane and burning it will reduce the carbon footprint of a farm.

That is not possible with cattle and sheep, but it is with pigs, because most of the methane from pig production comes during effluent disposal.

The WA Pig Producers’ Association (WAPPA) is trialling a relatively cheap method of collecting and disposing of methane at the Department of Agriculture and Food’s (DAFWA) Medina research station.

Earlier this year, DAFWA senior technical officer Hugh Payne visited New Zealand to witness a methane (biogas) collection system in action.

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“At that time, carbon caps and trade were being proposed and it seemed that eventually it would be applied to agriculture, ” Mr Payne said. “We wanted to be ahead of the game with a low-cost technical solution for small producers.

“The New Zealand Pork Industry Board had a double driver. It wanted to reduce greenhouse gas emission and had issues of odour from piggeries.

“We wanted to demonstrate that this technology would work under WA conditions.”

DAFWA needed to reline the piggery effluent pond at Medina, because it had been damaged by a bushfire and was in need of repair.

The plan was to retrofit the existing effluent system. The effluent would be put directly in the secondary pond, while the primary pond was desludged and relined.

“We had a problem. The sludge was too thick to pump and too thin to shovel. Moreover, the original pond was designed for more pigs than are currently kept at the research station, ” Mr Payne said.

So a new, smaller pond was built. The pond is 20 metres by 20 metres and 4.5 metres deep. It is lined with 1mm thick high-density polyethylene and covered by a sheet of 0.8mm low-density polypropylene, anchored in a trench around the lining.

Gas is collected through a perimeter ring of perforated 100mm PVC pipe positioned under the cover. Simple, flexible drainpipes filled with water are used to prevent the pond cover billowing excessively.

Effluent from the piggery is first screened to remove solids including pig hairs and undigested feed particles such as seed coats, and then pumped into the covered pond below water level.

“The cover should last about 20 years and that is the time when we will probably need to desludge the pond, ” Mr Payne said.

From the covered pond, effluent travels to a secondary evaporation pond, which is lined and open to the air. It is possible to dispose of the treated effluent from this pond by spreading it on pastures, but this has not proved to be necessary at Medina.

“A perimeter pipe around the pond collects the methane, which is then ignited and flared off, ” Mr Payne said.

“We will measure methane production from the pond before assessing the cost-effectiveness of heat or electricity generation.”

In the trial, gas is collected and flared using an industrial flare ignited by a spark plug.

“It cost about $7000 and was imported from the US. More complex systems are available that cost up to $50,000.”

While the objective of eliminating methane emissions and reducing odour are achieved in the small-scale set-up at Medina, Mr Payne believes large producers will be able to use the methane.

“We do not produce enough gas to be worth using, but a 500 sow piggery could produce enough methane to meet all the energy needs of the business, ” he said.

“In New Zealand, a 400 sow piggery was reusing the gas, which had basically paid for the cover in four years, ” he said.

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