The Nelson Regional Sewerage Scheme (NRSS), commissioned in 1983, is a joint venture between Tasman District Council and Nelson City Council. The scheme serves the residential areas of Stoke, Tahunanui, Richmond, Mapua and the Waimea Basin. It also serves the ENZA foods site, Alliance Freezing works and Nelson Pine Industries.
The scheme comprises four major pump stations:
Pipelines range in diameter from 100mm to 800mm and are mostly pump station rising mains. The total length of pipes is just over 155km. The pipes are mainly made of HDPE due to environmental and construction conditions.
Electronic equipment is used throughout the system for constant monitoring and recording, as well as to alert the operator to any malfunctions. Energy efficient, variable speed drives (VSDs) help reduce pump power usage by minimising hydraulic friction when the system is working at less than full capacity. The larger pump stations – Richmond, Saxtons Road and Stoke – boast two alternating VSD pumps as well as a third, larger pump on standby for peak flows.
Occupying 53ha of Bell’s Island on the Waimea Inlet, the Bells Island Sewerage Treatment Plant serves the communities of Nelson South, Richmond and Mapua.
Treated effluent is discharged into the inlet on the outgoing tide. Stabilised sludge (biosolids) is beneficially applied to forests on Rabbit Island. The treatment plant consists of an aeration basin, clarifier, Dissolved Air Flotation System (DAF) and an autothermal thermophilic aerobic digestion (ATAD) plant that treats captured solids to produce biosolids. A system of pumps and pipework transfers biosolids to Rabbit Island.
The oxidation pond system of Bells Island consists of three facultative ponds in parallel and two maturation ponds in series. Effluent from the last maturation pond is discharged into the Waimea Inlet via an outfall pipeline and diffusers.
The treatment plant treats sewage equivalent to that generated by a domestic population of around 133,000 people. Peak flows and loads are highly variable due to the combined effects of stormwater infiltration and the seasonal nature of industrial food processing activities.