Wastewater reclaimation for high quality reuse - A case study in Helensville

Resource consent requirements, cultural views, public perception, advances in technology and affordability are driving the treatment of wastewater to a higher effluent quality standards moving ever closer towards drinking water quality.

Helensville wastewater treatment plant is located in the Rodney District and is currently serviced by a two stage oxidation pond system prior to tidal discharge into the Kaipara River. The oxidation pond system has reached the design capacity and requires an upgrade. Moreover, the resource consent is lapsed and Rodney District Council is currently preparing an application to renew the consent.

Genesis Energy have proposed a new power station in the Kaipara area near the existing wastewater treatment plant and have held discussions with Council regarding the potential to use the treated effluent from the upgraded wastewater treatment plant as a water source for boiler water, which is typically of a very high quality.

This re-use of the treated wastewater not only allows a reduction in treated effluent discharge to the Kaipara river but also means that development of a new source from the Kaukapakapa River is not required. This re-use of the treated effluent provides a sustainable output in terms of reducing waste produced and conserving clean freshwater sources for other uses.

While the timeframe required for the Power Station to obtain the resource consent and the negotiation between Genesis Energy and Rodney District Council is being undertaken, a treatment plant upgrade strategy has been developed. This strategy needed to provide a flexible design to ensure the Helensville wastewater treatment plant meets the needs of the ratepayers in the short term whilst allowing the potential for the re-use of treated effluent in the medium to long term.

A two stage upgrade approach has been proposed. The first stage is a low-cost upgrade to improve BOD5 and TSS removal efficiency through implementing surface aeration and ultrafiltration (UF). The details of the second upgrade stage is subjected to future environmental monitoring and up to date population forecast. This may include the construction of a high rate activated sludge process to achieve biological nutrient removal and disinfection. The high quality effluent reuse requirement at the power station has also been taken into account in the plant upgrades.

This paper will cover the issues considered with developing the strategy, such as challenges with high quality water recovery and efficiency of design.