Preventing The ‘Pong’, Whanganui’s Five Year

Whanganui has undertaken a long and interesting journey with wastewater treatment. From 1984, screened wastewater was discharged to a long ocean outfall; subsequently in 2007, the first Wastewater Treatment Plant (WWTP) was commissioned. This design was based on a deep aerated lagoon, which combined liquid stream treatment with in-pond storage and digestion of sludge, followed by the use of a smaller settling pond for additional solids removal, and finally UV disinfection. Significant odour events in 2013, along with ongoing resource consent compliance issues, initiated an investigation process culminating in the design and construction of a new treatment plant.

Cardno was engaged to provide an upgrade solution that would meet the outfall consent requirements, make best use of the existing invested capital assets, and mitigates the risk of future odour events. Through a collaborative investigation and design period followed by a rigorous peer review process, it was determined that a new treatment process would be required to consistently meet effluent compliance requirements and minimise the risk of odour as much as possible.

A significant factor in the Whanganui design is the presence of large wet industries, which comprise a substantial proportion of the wastewater load. At various points in the design and pre-construction phases, the necessity of continuing to treat separable trade waste was raised, based on the precedent set by other wet industries that discharge direct to ocean elsewhere in New Zealand. It was determined that a combined treatment plant for both industrial and domestic waste was the only option as this effectively manages risk related to resource consent compliance, solids disposal, and odour mitigation.

The liquid stream treatment is a covered primary pond, followed by a contact stabilisation process, clarification and finally UV disinfection. The solids stream treatment is co-settlement and stabilisation of sludge in the primary pond, followed by thickening, dewatering and finally drying. Odour is collected from the covered primary pond and treated via a gas flare; bark biofilters treat foul air collected from the inlet works, contact stabilisation, and sludge handling processes.

This paper outlines in further detail the treatment plant options that were considered, selection and development of the final design, design and construction issues encountered, and finally commissioning experiences.