Tauranga southern pipeline - cost minimisation using innovative hydraulic management

In 2005, Tauranga City Council embarked on the implementation of a 14.5 km 700mm to 1000 mm diameter interceptor sewer across the city (known as the Southern Pipeline). Conventional interceptor sewer design often uses short rising mains followed by longer gravity sewer systems, often in tunnels or using multiple barrel inverted siphons. Route options analysis indicated that the preferred Southern Pipeline route was to avoid unstable ground and flood plains, but this meant using main roads though residential areas with the consequential need to fully understand and manage foul air discharges. The selected design provides cost effective solutions that involve significant lengths of pressure pipelines (approximately 12.5 km total), removal of the need for multiple barrels siphons, comprehensive analysis of air management and the provision of daily flushing to minimise grit and slime build-up. Analysis of pressure transients resulted in selection of simple and robust controls using both air vessels and vacuum relief valves. The air vessels are uncommon for large sewer systems. Commissioning of the initial 5.7 km of the system was carried out successfully in early 2012.