Contact Energy

The Wairakei bioreactor - a world first

Contact Energy’s bioreactor at Wairakei is a world first.

The new facility is designed to improve the environmental management of cooling water discharges into the Waikato River. It does so by using naturally occurring sulphur oxidising bacteria to reduce hydrogen sulphide (H2S) levels in a large-scale, geothermal operation.

Thanks to its unique design, the bioreactor has reduced H2S levels in the cooling water by up to 85 per cent. Its commissioning in August 2012 was the culmination of 10 years of environmental and technical studies into the environmental impact of Contact’s 172 MW geothermal power station at Wairakei.

The project was initiated after a consent review in 2000 determined that H2S levels in the Waikato River downstream of the power station exceeded accepted water quality guidelines and were contributing to a decline in fish populations.

Contact volunteered to reduce the H2S levels from the power station, and was granted a new cooling water discharge consent that included this requirement and which came into effect in 2012 - previously there had been no consent requirement to restrict discharge rates of H2S.


The bioreactor is designed around a large network of pipes – 378 kilometres in total – in which bacteria thrive. Cooling water from the power station flows through the pipes and the bacteria removes the H2S before the water is discharged into the Waikato River.

 A small-scale pilot was built and tested between 2001 and 2004 and this informed both further trials in 2011 and the concept and detailed design of the full-size bioreactor. Discoveries during the investigation phase included finding that bacteria grows better on smooth surfaces, that pipe length and water velocity impact bacteria growth and hence H2S removal rates, and that a thin film of bacteria produces optimal results overtime.

A number of initiatives also improved the overall environmental credentials of the facility. They included using of recyclable polyethylene (PE) pipe, reuse of soil and pumice to make low-strength grout, extruding PE pipe onsite, and using a pump and siphon to reduce the electricity required.


The Environmental Excellence award category is sponsored by PEPANZ