Effectively managing the sewage, wastewater and stormwater flows in a modern city is essential to the health and welfare of the community and the environment. Unfortunately in so many cities around the world the underground networks have been allowed to deteriorate, and now the difficulties in upgrading them seem to be insurmountable. In the city of Wanganui, New Zealand, the city engineers utilised modern plastic pipeline materials and the latest installation technology to help them develop their new network, and now their citizens are benefiting from the improvements to the environment.
The Whanganui is New Zealand’s third longest river and originates high on the volcanic plateau of the North Island and drains into the Tasman Sea at the city of Wanganui, some 290 km downstream. As with virtually every city, the Wanganui sewerage system had developed in a piecemeal fashion as its community had grown. Many of the pipes were old and leaking, and there was no separation of storm water from sewage, which at times caused the system to overload. The old pipes were in bad condition, and there were many untreated industrial outflows discharging directly into the river, significantly reducing the river’s water quality.
By the end of the 1990s it was clear that the Wanganui sewerage system required a major upgrade to reduce leakage and infiltration, to separate the storm water from the sewage and to improve the quality of the river water. The development of a new system posed a number of major challenges but would create a better quality of life and a cleaner and healthier environment for the city’s inhabitants.
In 2002 the Wanganui city council developed a comprehensive plan to separate storm water and sewage, and to eliminate the discharges into the river by installing an 18 km sea outfall off nearby South Beach. This outfall would be served by a pumping station on the city side of the river. Allied to this plan was the design and construction of an interceptor main to convey screened sewage and industrial wastewater to a new treatment plant prior to discharge into the sea.
One of the major problems was that the new treatment plant was on the opposite side of the river to the existing pumping station. There was an existing large-diameter concrete pipe under the river, but this pipe could not withstand the design pressure. Therefore the council was faced with three options: build an additional pumping station, upgrade the current submarine pipe or build a new one.