Nelson Wastewater Treatment Plant

What is wastewater?
Wastewater is what goes down your drains at home and comes from the likes of toilets, bathtubs, showers, bathroom and kitchen sinks. Once the wastewater has been treated it’s called ‘effluent’.

What is wastewater?

Wastewater is what goes down your drains at home and comes from the likes of toilets, bathtubs, showers, bathroom and kitchen sinks. Once the wastewater has been treated it’s called ‘effluent’.

When wastewater goes down the drain

When wastewater goes down your drain it enters the Nelson sewerage system -- many kilometres of buried pipelines and pumping stations that bring wastewater to the treatment plant. In Nelson, treated effluent is discharged into the sea because it is the most practical solution and follows worldwide accepted practice.

Nelson has two wastewater treatment plants. The Bell Island plant (a regional facility shared with Tasman District) treats sewage from Stoke and Tahunanui, while the Nelson Wastewater Treatment Plant (NWWTP) in Nelson North treats sewage from the rest of the City. Wastewater is pumped to the NWWTP via a nine-kilometre-long reinforced concrete pipe which follows State Highway 6. This pipe is called a ‘rising main’ because the wastewater has to be pumped (as opposed to flowing under gravity) to the treatment plant.

Wastewater needs to be treated

Raw sewage contains matter that could be potentially poisonous to our ecosystem, such as bacteria and parasites. Untreated, it can be harmful to sea life and plants as well as cause illness to humans by spreading disease. The effluent flowing into Tasman Bay is treated to very strict government standards to protect our ecosystem and health.

How the NWWTP works

  1. Wastewater flows to the Neale Park pump station by a variety of gravity and pressure pipes. It is then pumped to the treatment facility at Nelson North via the underground Atawhai rising main alongside SH6.
  2. Flow Buffer – during periods of high rainfall, when the flow of stormwater into the sewer system may be high, the flow buffer at the front of the treatment plant keeps the system from being overwhelmed. It temporarily stores the excess and controls the flow to the next stage: screening and grit removal.
  3. Screening and Grit Removal -- the screening system removes non-organic material such as toilet paper, rags and plastics, from the waste stream. This rubbish is compressed to remove liquid,taken to the York Landfill and buried. The grit removal system removes sand and stones, which could have a harmful effect on the next stages of the pre-treatment works.
  4. Clarifier -- a large, circular concrete tank with a sloping floor. Organic solids (sludge) settling out of the wastewater are forced to the centre of the tank by scrapers on a revolving mechanical arm inside the tank.
  5. Sludge Tanks -- the organic sludge from the clarifier is thickened by mechanical removal of liquid wastewater and then stored for shipment to Bell Island, where it is treated and sprayed as fertiliser on Rabbit Island pine forests. The remaining wastewater is pumped to the trickling filter.
  6. Trickling Filter – a large circular concrete tank which contains plastic media (plastic longitudinal square-shaped grilles) over which the wastewater is distributed from rotating arms above the tank. Bacteria growing on the plastic media use the food from the sewage to break down the sewage further.
  7. Oxidation Pond -- the oxidation pond system is a natural biological process which further breaks down the sewage. The NWWTP has two ponds -- a facultative pond and a maturation pond. The facultative pond further aids in the breakdown of incoming sewage, while the maturation pond removes faecal coliforms -- a disease-causing intestinal bacteria.
  8. Outfall – the treated effluent flows from the oxidation ponds out to Tasman Bay through the outfall pipe. (See ‘Wetlands’ below.)
  9. Bio-Filter – Sometimes bad odours from sewage treatment plants can drift long distances with unpleasant results for nearby residents. A large ‘bio-filter’, using air, water and bark to neutralise odours, has been constructed so that bad smells don’t reach State Highway 6.

How the oxidation ponds work

The action of  an oxidation pond in treating sewage is to stabilise organic matter using bacteria. The organic matter is converted into inorganic matter. The bacteria produce acids under aerobic conditions and carbon dioxide under anaerobic conditions. The aerobic conditions are created from surface aeration (wind or mechanical) and algae that, through photosynthesis, produce oxygen. 

Wetlands built

As part of the upgrade  wetlands were established in 2010 through which the effluent from the oxidation ponds will flow before discharge to Tasman Bay from the outfall pipe. The wetlands make use of special plants that live in water to assist in the treatment process.

History – A time line of the NWWTP


first drain (sewer and stormwater) draining into Maitai River from Rutherford, Nile, Hardy and Bridge Streets


Stormwater and sewer separated


Untreated effluent discharged to Boat Harbour


Construction of pumping stations in preparation for pumping to Nelson North


Construction of Tasman Bay outfall. Work completed in 1970.


Water right secured allowing discharge to take place into Tasman Bay


Establishment of the current 26-hectare oxidation pond to treat sewage discharge


Fisheries discharge channelled through separate outfall, thereby diverting this flow away from the oxidation ponds


Existing oxidation ponds sub-divided into two interlinked ponds, to improve discharge quality into Tasman Bay


Existing treatment plant facility upgraded

More information

For more information please email or phone Technical Services on +64 3 546 0200.