Dr Rupert Craggs and Stephan Heubeck, of NIWA’s Hamilton office, have developed a covered anaerobic pond that captures the methane released from effluent, so it can be used to generate heat and power.
Their work shows that the system can work well across the spectrum of New Zealand climate conditions and with farm intensification and higher power prices the economic return is rapidly becoming more attractive.
NIWA’s work on methane harvesting began 10 years ago with pilot-scale systems at piggeries and dairy farms.
That was followed by full-scale demonstrations, with the first dairy system installed at Taupiri, in Waikato.
The Taupiri project is installing a boiler system that will use methane to heat farm dairy water systems.
Craggs said the move to low-application-rate irrigation from effluent storage ponds meant it was beneficial to remove effluent solids before storage and the digester pond was cost-effective compared to other means of solids removal, such as solids separators and weeping walls.
The NIWA system is designed for NZ conditions and is capable of producing usable methane quantities despite the relatively low level of solids in NZ farm dairy waste.
NIWA’s trials had provided assurance that the digester-conversion process could work even in the cooler southern climate, with bacterial action slowing but not ceasing over winter, he said.
“The pond essentially works like a battery, storing the energy of the effluent solids and releasing it as biogas, particularly during the warmer months.
“However, gas production does not stop even in winter, as long as there is some loading of the pond.”
The covered anaerobic pond is most suited to herds of more than 600 cows, but smaller herds can work if waste from feed or wintering pads is incorporated.
NIWA estimates payback periods of 3-6 years, depending on herd size and the degree of feed and wintering pad use.
