Managing wet weather sewage spills in a challenging environment

Sewage spills into the Whangarei harbour during wet weather cause a public health risk to those using the harbour for contact recreation and shellfish collection. Solving the cause of the spills, inflow and infiltration of stormwater into the sewer network, is made difficult for a number of reasons including: the inability of the community to fund large capital projects, Whangarei’s high rainfall rate, the private and public sewer network is aging, and the soil type exacerbates inflow and infiltration.

Whangarei District Council developed a range of innovative solutions to manage the effects of the spills. The benefits of these solutions were assessed using sophisticated hydrodynamic dispersion and risk models to determine the impacts on public health.

These innovative solutions included:

• An award winning upgrade of an existing pump station;
• Design of a 1,000 cubic metre overflow storage system that acts as a treatment process in extreme wet weather events;
• Disinfection of highly dilute wastewater using ultraviolet light, with a treatment level based on public health benefits;

The works programme is into year 3 of 7 with $13.5M committed to date. Further projects, totaling an estimated $23M, will be developed in line with the Councils 2012/2022 long term plan.