Rotorua wastewater treatment plant upgrade: Project drivers and design approach
Annual Conference
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In 2008 the Rotorua District Council (RDC) identified that the capacity of the final clarifiers in the secondary treatment process at the Rotorua wastewater treatment plant (RWWTP) were limiting treatment capacity. Treatment of additional flows from the soon to be reticulated Eastern Area Catchments and solids loss due to overloading needed to be addressed.
A project was implemented for the planning of a new (third) clarifier and associated infrastructure. In addition, the capacity of the whole plant (including all solids and liquids streams) was assessed to determine which processes would require upgrade/expansion to meet the projected population to 2050.
The capacity study identified (assuming the existing Bardenpho process was retained for current and future flows) that to provide redundancy and process capacity that the Bardenpho would need to be extended and a fourth clarifier would need to be added before 2050.
The RWWTP is built on an area that is influenced by geothermal activity and by the water level of Lake Rotorua. The ground conditions are therefore challenging for the founding of large water retaining structures such as clarifiers (the additional clarifiers would be in the order of 3000m3 each). To overcome these conditions preloading and/or piling of the structures would be required. This presented both a construction/design risk and a program risk in terms of the time required for pre load (greater than 12 months) for a new clarifier.
A refined investigation was undertaken to find the most appropriate solution for the plant expansion taking into account geotechnical conditions, space restrictions, existing infrastructure, continuity of operations, whole of life and capital costs. The solution selected was to retrofit one of the three existing, disused, reactor clarifier tanks on site into a membrane bioreactor plant to be operated in a side stream to the existing Bardenpho reactor.
This solution will deliver an overall increase in capacity from a current average flow of 17ML/d to 24ML/d, providing sufficient capacity to allow for the projected growth of the catchment for the next 30 years as well as providing immediate redundancy for operations staff to conduct maintenance on the existing clarifiers. This MBR will be the first in New Zealand to use GE hollow fibre membranes (for an MBR application) and will be New Zealand’s largest municipal MBR, with a peak capacity of 11ML/d.
This paper presents the methodology of the capacity study, the selection of the membrane type and the configuration of the membrane bioreactor.