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Aerial view of wastewater treatment plant

Rotorua Membrane Bioreactor Plant, New Zealand

Commissioning Rotorua’s membrane bioreactor plant, providing probably the best quality wastewater discharged in New Zealand, was the result of three years collaborative work between Rotorua District Council and our water specialists to optimise the existing plant and identify the best, most cost effective solution over a 30-year planning horizon.

Close up aerial view of MBR tank

Close up view of membranes within MBR plant

The side stream MBR process was a result of sound strategic investigations by the RDC/AWT team, well thought out and implemented procurement methods, followed up with a thorough detailed process/mechanical design customised to meet RDC’s drivers.

Rotorua District Council

Key facts

  • New Zealand’s largest membrane bioreactor (MBR) wastewater treatment plant and first to use hollow fibre membranes on a large scale
  • Meets the tightest nutrient effluent quality requirement in New Zealand
  • Treats a maximum daily flow of 11.3 ML/d
  • Finalist in the 2013 New Zealand Engineering Excellence Awards for innovation around infrastructure planning, options assessment and design


  • Rotorua’s sizeable wastewater treatment plant (WWTP) – which has the tightest nutrient effluent quality requirement in New Zealand – had to expand from a population equivalent of 65,000 to over 100,000.
  • Rotorua’s treatment plant is built on the edge of Lake Rotorua in an active geothermal area. Building large water retaining structures has significant risk. These were overcome by the use of existing structures.
  • Maximising the removal of nutrients (nitrogen and phosphorus) to the limits of technology was considered paramount by Rotorua District Council (RDC) calling for close team working between our team and RDC’s to find economic solutions to meet the stringent standards and increased demand.


RDC engaged us to consider the WWTP expansion and effluent quality improvement requirements needed to address the additional demands on the facility. Our first response was to initiate collaborative working with RDC’s operations and project delivery teams. Our skills in process optimisation, coupled with RDC’s understanding of the plant and future nutrient capacity requirements, identified several initiatives that were implemented over three years to enhance performance and drive down costs.

Once we had exhausted the potential from the existing assets the focus turned to the infrastructure expansion requirements to cope with the projected increase in demand over the next 30 years.

Through continued collaboration the team identified a side stream MBR as an economic option to keep within the stringent nutrient mass cap while providing for future expansion of the main treatment process for a 30 year period. Optioneering showed the MBR solution as the most viable, providing a capex saving of $3M compared to previous concepts. Our work in assisting RDC with procurement and detailed design included:

  • Capacity study identifying that a side stream process would minimise the need to build new structures on the existing site which has significant geotechnical issues
  • Preliminary design of MBR side stream including process design to fit within existing disused water retaining structures
  • Boundaries definition for the supply of membranes including design criteria, membrane performance measures and limits of supply
  • Tender documentation and evaluation of membrane supply under RDC’s procurement strategy and preliminary and process design for the side stream MBR
  • Detailed design and commissioning of new 7300m3/day MBR plant and detailed M&E design to retrofit the MBR to existing reactor tanks

Value and benefits

  • The collaborative approach between AWT and RDC built on our long-standing relationship of nearly ten years created an environment that stimulated innovation and helped identify solutions faster.
  • Through asking questions and planning in partnership with RDC, our team was able to develop an expansion concept that generated substantial savings for our client.
  • Using some of the existing WWTP water retaining structures minimised construction risks associated with poor ground conditions, and opting for a side stream MBR achieves high and consistent effluent quality output from a low footprint.
  • Our detailed knowledge of the plant enables us to continue providing optimisation support for RDC to keep improving effluent quality figures and thus maintain the lowest municipal nutrient limits in New Zealand.
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