Contact Energy

The Wairakei bioreactor - a world first

Contact Energy commissioned the Wairakei Bioreactor in August 2012 – a world first – on time and under budget.

Located at the site of the company’s 55-year-old Wairakei geothermal power station, it is the culmination of more than 10 years of research, planning, testing and execution.

The bioreactor has already reduced hydrogen sulphide levels in cooling water discharged into the Waikato River by up to 85 per cent – reducing by 8,000 kilograms of hydrogen sulphide per week. It is on track to reach the target 95 per cent reduction by 2016 – conditions voluntarily agreed to by Contact in order to have the power station re-consented.

The project is just one part of Contact’s $750 million investment programme at Wairakei which includes the new Te Mihi power station, where the company is reducing its operational impact on the environment and ensuring the geothermal field’s future sustainability.

Environmental impact

Contact began investigating the on-going environmental impact of its Wairakei operations in early 2000. It determined that hydrogen sulphide levels in the Waikato River downstream of the power station exceeded acceptable water quality guidelines and were contributing to a decline in fish stocks. The hydrogen sulphide is present naturally in geothermal steam, and ends up in the station’s cooling water as a result of the design of the steam turbine condenser, which is then discharged to the Waikato River.

The company began investigating how it could utilise the naturally occurring sulphur oxidising bacteria that are endemic to the Waikato River to reduce the level of hydrogen sulphide being discharged from the Wairakei operations. And it did so without a blueprint – there was no similar project anywhere in the world that the concept or design could be based on.

Early studies focused on finding the optimum environment for growing the bacteria. Discoveries during those investigations included finding that bacteria grows better on smooth surfaces, that black pipes limit the growth of algae that could displace bacteria, that pipe length and water velocity impact bacteria growth and hence hydrogen sulphide removal rates, and that a thin film of bacteria produces optimal results over time. Based on these findings the detailed design for the bioreactor could proceed with some certainty.


During the design phase the scale of the full size bioreactor quickly became apparent. It was initially calculated that 2,700 pipes – with a total length of more than 500 kilometres – would be required to meet the August 2016 consent limit.

Optimisations during the design phase led to the total pipe length being reduced to 378 kilometres. An ‘over and under’ pipe field configuration was also developed to minimise the pumping power required. This also simplified construction and reduced the land area and excavations required.

Other innovations during the project included the development of ‘soilcrete’ – a mix of pumice and cement that was used to form a supporting matrix around the polyethylene pipes. This development improved the overall environmental credentials of the project and reduced costs.

The Energy Project of the Year category is sponsored by ITL.