Bacteria that dine out on toxic waste are being used to consume hydrogen sulphide from the once world leading Wairakei geothermal power station in a unique environmental project.

Contact Energy's new facility near Taupo has been named the ''bioreactor''.

Since the power plant was built in the late 1950s it has drawn water from the Waikato River to be used as a coolant before pumping the water back into the river.

The only catch was it picked up hydrogen sulphide from the geothermal steam.

Studies show that hydrogen sulphide damaged the aquatic environment and it was likely the Wairakei discharge was having an impact on the Waikato River ecosystem and nearby lakes.  

The Waikato Regional Council indicated in the early 2000s that the situation would soon change and the new consent conditions kicked in this August.

The bioreactor uses sulphur oxidising bacteria, that grow naturally in the river, to remove the contaminant from the power station's cooling water as it flows through five banks of piping.

Each pipe is 100-millimetres in diameter and 100 metres long with a combined length of 378 kilometres.

A three year trial study from 2001 showed the ideal growing conditions for the bacteria were on smooth surfaces in a dark environment, hence the pipes. 

The cooling water takes about four minutes to travel through the bioreactor at a speed of 13,000 litres per second - that's over three million litres of treated water every four minutes returned to the river. 

The new discharge consent requirements dictate that discharge levels must not exceed 2,800 kilograms of hydrogen sulphide, in total, per week.

The bioreactor has a lifetime of 20 years and by that time the power station should be decommissioned.

The bioreactor will be officially opened on September 18.

- © Fairfax NZ News