Analysis of the performance and efficiency of pressure wastewater collection systems in New Zealand

Pressure sewer systems are being installed throughout New Zealand with the first significant networks commissioned in June 2008 in Rotorua and Rodney. This paper analyses flow, pressure and rainfall data collected from Rodney District Council’s Point Wells pressure sewer network and provides a qua ntifiable review of pressure sewer design assumptions and operation. This system consists of approximately 7.5km of pressure sewers and will eventually service 510 dwellings. As of 28 June 2009 the network included 161 grinder pump u nits servicing 66% of the existing community.

The preliminary results from system performance monitoring on this pressure sewer system confirm that both the design and operational performance of the system is similar to historical experience in the USA and Australia. There is a strong correlation that the system does experience wet weather inflow, however Rodney District Council has identified the sources of inflow by tracking pump run hours. Therefore, while a wet weather peaking factor of approximately 1.2 is currently experienced, Rodney District Council have quickly identified the sources and are in the process of eliminating the inflow.

When compared to the costs related to installing an equivalent gravity sewer system, the pressure sewer system is considerably cheaper over the life of the system. Also, as wet weather flows can be identified and eliminated, the cost of treating wet weather flow volumes is considerably reduced. This study shows that the pressure sewer system has resulted in reliable and cost effective solution for the community of Point Wells