Rotorua District Council (RDC) has completed an $8.5 million upgrade of its waste water treatment plant which includes the commissioning of new membrane bioreactor technology to further improve Rotorua’s lake water quality.
A key feature of the upgrade has been installation of state-of-the-art membrane filtration technology, the first time a New Zealand city has utilised the process for wastewater treatment on a major scale.
RDC utilities operations manager Eric Cawte said the membrane filtration process produces the highest practicable removal of nutrients from sewage.
“It results in such high water quality that it is actually commonly used around the world in drinking water treatment processes.
“Our newly upgraded treatment plant is now able to filter out very high levels of nutrients from sewage derived from Rotorua’s urban area and from lakeside communities more recently connected to the district’s main sewerage network.
He explained that nutrients such as phosphorus and nitrogen were major causes of algae bloom that had been affecting the water quality of New Zealand lakes and waterways for many years.
“With membrane filtration technology we’re able to remove very minute particles containing nitrogen and phosphorus, on top of the already successful biological removal process. About a third of the sewage is treated in this way, enabling the current process to operate to its maximum efficiency without flow fluctuations.
“It also provides us with additional capacity for treating extra sewage now being transferred from a number of lakeside communities, and which would otherwise be at risk of leaching nutrients into our lakes from septic tanks.
Mr Cawte said the upgraded treatment plant had been undergoing testing and was now fully operational. It was currently being fine-tuned to achieve the highest possible levels of nutrient removal.
He said it was expected to significantly reduce reliance on the existing forest irrigation system for removing nitrogen from wastewater.
Mr Cawte said the city’s wastewater plant had historically treated sewage effectively and produced very high quality effluent which was in turn used for irrigating parts of the Whakarewarewa Forest.
However achieving the desired limits for nutrient nitrogen entering Lake Rotorua via the Puarenga Stream had always been challenging due to a number of natural factors.
The district council was now in the process of preparing an application to the Bay of Plenty Regional Council to amend resource consent conditions. It would be acknowledgement of the new high level of treatment being achieved at the waste water treatment plant following the plant’s $8.5 million upgrade and the installation of new filtration technology.
“This means that the forest irrigation system would be substantially less at the mercy of weather impacts and other natural factors beyond anyone’s control.
“The council is also in the process of undertaking a full review of the forest irrigation system and assessing a number of alternatives that could potentially better meet the community’s future needs for discharging highly-treated process water,” said Mr Cawte.