A Pig-Waste Bio-digester

An innovative and award-winning solution to an effluent problem on a Taranaki pig farm has resulted in a system that generates around half the farm’s electricity needs and could have potential for application on dairy farms as well.

The management and disposal of solid and liquid waste is a major challenge on any large scale commercial piggery. Steve Lepper’s farrow-to-finish pig farm in Taranaki is owned by the Lepper Trust. It produces roughly 9,500 offspring a year. The piggery was built in the 1980s and sits in the centre of the family dairy farm.

In order to breed and finish those animals, around 2,000 tonnes of feed is brought into the farm each year, and once the pigs have digested it, what is left is an enormous pile of waste effluent. This used to be managed with traditional oxidation ponds but complaints from local residents about odour had started to increase and the farmers knew they needed to come up with a solution.

The solution was to add a covered anaerobic pond (also known as CAP, or passive digester pond) to the waste management system. NIWA engineer Stephan Heubeck was behind the design of the CAP system that was installed in 2009.

Taranaki Regional Council presented Steve’s business with an environment award in recognition of the work done to address the effluent issue. The Lepper Trust Biogas System was also the winner of the 2010 EECA awards in the SME category.

Effluent from Steve Lepper’s 400 sow farrow-to-finish piggery is now treated in a 60 x 20 meter covered anaerobic pond (CAP) with about 7200 cubic meters capacity.

In the CAP a diverse mix of dozens of groups of microorganisms convert the majority of effluent solids into biogas – primarily methane and carbon dioxide. Odorous compounds, like volatile fatty acids (i.e. butyric acid etc.) are converted to odourless biogas, and since the majority of waste solids are removed in the CAP, the resultant effluent has much less potential to subsequently create “new” malodourous compounds when stored in an uncovered pond or being applied on land. Hydrogen sulfide, another malodorous compound, is released and removed from the effluent during CAP treatment, again reducing the potential for subsequent release.

The resultant biogases (primarily methane and carbon dioxide) including trace impurities like water vapor and hydrogen sulphide are trapped under the heavy plastic cover.

On average around 300 cubic metres of biogas, containing around 200 cubic metres of methane can be collected each day. The cover can also store as much as 1200 cubic metres of gas, providing flexibility for biogas use.

The biogas is removed from under the cover with a gas blower then “cleaned”. Water vapour is removed through cooling and a number of condensate traps. Hydrogen sulphide is removed through absorption on an iron containing filter media, which can periodically be regenerated.

At the Lepper farm, the resulting biogas is used to fuel a generator with 40 kW electrical output, which also allows the recovery of usable waste heat from the radiator coolant and exhaust gases via heat exchangers. The waste heat recovered (in the form of hot water) can be temporarily stored in a tank and is used at the piggery for under-floor heating of weaner sheds, substituting electric heat lamps.

The system cost in the region of $120,000 in total, with $30,000 coming from an Energy Efficiency and Conservation Authority grant. Payback on the investment was a bit over 3 years.

The biogas system makes the farm more than 50% energy self-sufficient and allows the piggery to remain “operational” (running lights, effluent pumps and water supply) during a power outage.

NIWA engineer Stephan Heubeck says he had worked with piggery effluent ponds before – but only to control odour. The resulting biogas was simply “flared off” to destroy odorous compounds (like hydrogen sulphide).

He says the Lepper design was the first based on a covered anaerobic pond to incorporate energy utilization in New Zealand. Stephan says indoor piggeries are ideal sites for this kind of technology because of the high waste output and high demand for electricity and heat.

For the pork industry the big plus is the reduction of waste odour but Stephan points out that there’s an excellent opportunity to utilise biogas as an energy source and reduce agricultural greenhouse gasses (fugitive methane emissions).

Heated and mixed bio-digesters were investigated seriously during the 1980s oil shocks but when oil prices dropped again in the 1990s they stopped being economic. The alternative CAP system offers a new perspective on the economic side. It can produce equal amounts of biogas from a given manure resource, and is cheaper to build and much cheaper to operate than a heated mixed digester. These advantages have to be balanced against a larger foot-print of the plant, a slight seasonal change in biogas production and the fact that a CAP cannot digest all feedstocks – only fluid feedstocks such as flush manure, processing effluents, distillery waste, etc. are possible.

The system has been sufficiently successful to encourage Australian Pork Ltd to collaborate with NIWA to build anaerobic biogas systems in Australia.

Stephan says more stringent dairy effluent regulations will see more and more farmers building bigger storage ponds. He says it would make perfect sense to build a biogas recovery system when those new ponds go in.

Retrofitting to existing systems is technically feasible, but Stephan says economically it is almost always better to add a dedicated CAP (say 1/3 of the size of the storage pond) up front, rather than cover a huge area of an (often shallow) existing storage pond.

He also says the potential biogas resource on most NZ dairy farms could make them between 50 & 100 % energy self-sufficient, with farm size not always being the big driver. Often the presence or absence of a feed pad is more important factor for dairy farms.

The other benefit of covered ponds is that it reduces greenhouse gas emissions.