Contaminants in wastewater discharge to lake exceed consent levels

At 34 sq km, Lake Waikare is the largest of several shallow lakes in the upper floodplain of the Waikato River. Photo / Supplied

Waikato District Council is working urgently to upgrade its Te Kauwhata wastewater treatment plant as it struggles to keep nitrogen, phosphorus and E. coli discharge into Lake Waikare within permitted consent levels.

The main issue is the increase in the population of Te Kauwhata since the council's wastewater discharge consent was issued in July 2013. It expires in 2028.

"We have much more wastewater to process through the plant and yet we still have to keep within limits that were set when the population and the wastewater generated was much lower," said the council's special infrastructure projects manager, Ian Cathcart.

The council says the quality of the discharge varies throughout the year and although it will be compliant for much of the year, at times it is failing. The treatment plant upgrade will let it process the wastewater to a better quality so it can process higher amounts of wastewater but remain within consent levels.

Cathcart said the amount of treated wastewater from the plant entering the lake is incredibly small compared with the overall contribution of water from waterways entering the lake.

At 34 sq km, Lake Waikare is the largest of several shallow lakes in the upper floodplain of the Waikato River. It is east of Te Kauwhata and 40km north of Hamilton. The water in the lake is in poor condition due to its shallow nature (2m at its deepest), the heavy use of fertiliser in the surrounding district, the large population of koi carp, and, to a lesser extent, wastewater discharge from the local treatment plant.

"With urgency we are putting in place the upgrades of the wastewater treatment plant to reduce the overall impact on the lake and meet targets in the discharge consent," Cathcart said.

Council is rebuilding the treatment plant to include a new UV unit to be commissioned in December, and a membrane aerated biofilm reactor (MABR) plant, which is scheduled to be commissioned by April 2022. The UV plant is used to kill E. coli and viruses while the MABR plant will reduce suspended sediments, nitrogen and phosphorus.

The ultraviolet disinfection unit, which will be constructed by the end of the year, will cut E. coli levels in the treated wastewater to almost zero. This technology is expected to bring the treated wastewater discharge back into compliance with the consent.

The work is partly funded by a $16.2 million loan from the Government's Housing Infrastructure Fund, which was put in place to assist local authorities in areas of significant projected population growth.

"We have been working on the current upgrade of the plant since 2019 when it became apparent we were not able to keep within the current consents for nitrogen, phosphorus and E. coli," said Cathcart.

"But, since the new designs were agreed, and since early last year with the onset of Covid-19, we have run into international supply chain difficulties that have held up the parts that we have ordered," he said.

E. coli has many sources including animals and birds in the catchment. The net contribution of the treated wastewater to the overall lake E. coli concentrations is probably very low, the council says.

Aside from upgrading and maintaining the wastewater treatment processes collaboratively with mana whenua and key stakeholders, the council also works closely with partners, landowners and volunteers to contribute to the management of the lake through the Waikato Regional Council's catchment management plan for Lake Waikare and the Whangamarino wetland.