The power station draws water from the Waikato River and uses it to cool geothermal fluid extracted to generate electricity.
Hydrogen sulphide occurs naturally in geothermal fluid, but due to the power station's cooling process, some of it also gets into the cooling water.
The new bioreactor pumps the used cooling water through 378km of pipes under a field next to the power station at the rate of 13,000 litres per second. Inside the pipes, the bacteria gobble the hydrogen sulphide from the water, removing up to 80 per cent of the toxic compound. The cleaned water is then returned to the Waikato River.
The new bioreactor helps ensure Contact Energy, which operates the Wairakei Power Station, can meet strict new cooling water discharge resource consent conditions, which came into effect last month.
The underground pipes provide an environment where the naturally-occurring bacteria colonise and grow to form a thin whitish film to remove the hydrogen sulphide as the cooling water flows through the bioreactor. Water takes about four minutes to travel the 378km of pipes, which equates to over three million litres of treated water being returned to the river every four minutes.
The bioreactor is thought to be unique internationally because the Wairakei Power Station was one of the first geothermal power plants in the world, and used the-then acceptable solution of discharging the cooling water directly back into the river.
It took Contact three years of trials to find the ideal conditions for sulphur-oxidising bacteria to grow. Scientists tried growing the bacteria using strings and mesh, but eventually discovered that they were best grown on smooth surfaces in a dark environment with the right conditions.
Project manager Peter Hewett said the bacteria used in the bioreactor occur naturally in the Waikato River water, so once the right conditions were provided, the bacteria began growing in the pipes.
Contact is delighted with the success of the bioreactor, which is currently removing around 80 per cent of the hydrogen sulphide in the water, 10 per cent more than is required in the power station's resource consent.
"As far as we know, this is the first time it's been done anywhere in the world."
Contact Energy chief executive Dennis Barnes said after many years of trialling different mediums to grow and house the bacteria, the eureka moment came when they discovered the bacteria would flourish in pipes, but it was still a leap of faith for the company to invest so much money into something that was "absolutely unique" in the world.
