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Innovative use of waste gas is saving the Dunedin City Council more than $300,000 a year.
It is 12 months since the council switched on its new gas-powered electricity generator at the Green Island wastewater treatment plant.
In that time the council has used the generator to power its plant, and has sold excess electricity on the spot market.
The treatment plant used to cost the council an average of $285,000 in electricity each year. As of this week, electricity use for the plant was $34,000 ''in credit'', DCC wastewater treatment manager Chris Henderson said.
''[This] represents a $319,000 saving in electricity costs to council for the year to date. We are really pleased with how it has worked,'' he said.
The generator used gas collected at the nearby Green Island landfill, as well as methane from the wastewater treatment plant, which would otherwise be wasted.
It was capable of producing up to 625kW of electricity, but Mr Henderson said when it was operating at 100% capacity, the gas engine became vulnerable to small variations in the quality and quantity of gas produced.
''We also encountered problems with instrumentation failure. For this reason, we have found that operating the engine at around 85% to 90% capacity (500kW to 550kW) is more stable,'' he said.
Up to May 30, the generator had used 642,234cu m of gas from the treatment plant, and an additional 885,339cu m of gas from landfill.
It produced 2,623,383kWh of electricity, 1,656,280kWh of which was used by the wastewater treatment plant. A further 967,103kWh was sold on the spot market.
The treatment plant used a total of 1,880,259kWh.
Mr Henderson said the generator could save the council considerably more in future.
''When the Green Island landfill becomes liable for emission trading scheme charges, the gas consumed by the generator will reduce the council's liability by an estimated $1 million,'' he said.
Maintenance of the generator to date cost $67,300. It was part of a $2.2 million project.
When the generator was switched on last June, Cr Andrew Noone said the council would ''get its money back'' through the project within four years.
The council began collecting and burning gas from the Green Island landfill in 2009, as required by a legislation change.
Now the gas is piped to the generator and mixed with methane from the wastewater treatment digesters to produce electricity.
Mr Henderson said the generator's lifespan was indefinite, provided the machine was regularly maintained.
Dunedin Mayor Dave Cull said benefits from the gas generator were threefold.
It saved the council money, was good for the environment and was in line with the council's energy strategy.
''It's a great example of a relatively modest investment with a huge return. We will be endeavouring to repeat that kind of thing across the city, and encourage other businesses to do so, as well,'' Mr Cull said.