Smart Pressure Sewer and the Development of South West Christchurch Helen Barclay

The South West area of Christchurch is experiencing rapid residential growth and development, partially as a result of the Canterbury earthquakes. To enable the development of land in this area for an additional 5,000 houses the Christchurch City Council needed to provide suitable wastewater infrastructure. The area zoned for residential development borders a large flood management basin which has high ground water levels and poor ground conditions. A conventional gravity wastewater system with deep pipes and a below ground wet-well pump station was not suitable. This paper presents the solution and discusses some of the design and construction challenges, and the lessons learned along the way.

A smart pressure sewer system was chosen for the area because of its resilience, suitability for the ground conditions, and ability to be developed in stages. Iota’s OneBox smart system was selected as it provides the benefit of being able to manipulate the flows delivered to the receiving pump station. At the start of the development, coordination of the pumps provides a flushing flow through the pipeline. As development progresses the system can be operated in a peak shifting mode to reduce the peak flow from the catchment, reducing the size of rising main required for full development. The lower infiltration expected from a pressure sewer system when compared to a gravity system, and the reduced emergency storage requirement all resulted in a smaller pump station and sewer network.

Due to high ground water and the flood prone nature of the area the receiving pump station building was able to be situated well above the 200-year flood level and was designed with an above ground wet-well tank to provide a cost-effective design when compared to a traditional gravity sewer pump station.

Construction of the $12M scheme is now complete and included installing 6.3 km of wastewater rising main, upgrades to an existing pump station, and a new 120 l/s pump station. Construction of the scheme was not without its challenges. This paper outlines the South East Halswell Wastewater project and the smart pressure sewer technology. Focus is given to the key lessons learned on this project.