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Wastewater Treatment Plant

The New Plymouth Wastewater Treatment Plant was built in 1984 and is located in the eastern area of New Plymouth.

The plant takes raw sewage and trade wastes from New Plymouth City, Bell Block, Inglewood, Waitara and Ōākura. It uses a biological treatment process known as activated sludge aeration to produce a high quality effluent. The clean effluent is discharged to the Tasman Sea via a 480m ocean outfall.

The quality of the water leaving the plant is one of the cleanest in New Zealand and well within the New Zealand standard for safe swimming and seafood gathering.

Did you know?

  • New Plymouth District has 454km of sewer mains (pipes), with 37 pump stations and 180km of service connections.
  • Every day about 20 million litres of sewage passes through the network to the New Plymouth Wastewater Treatment plant. Sewage will go through up to five pump stations on its way from a house to the treatment plant.
  • The Wastewater Treatment Plant is water conscious, reusing 500,000 litres of treated effluent every day to run the plant instead of using regular drinking water. 

How does the wastewater process work?

  1. The sewerage network conveys raw sewage and trade waste to the wastewater treatment plant via gravity sewers and pump stations.
  2. Five Terminal (Major) pump stations pump all the flow to the wastewater treatment plant. These are Area Q and Mangati in Bell Block, Glen Avon, Waitara and Te Henui.
  3. The wastewater passes through bandscreens where sanitary products and other rubbish (over 5mm in size) is removed from the wastewater. 
  4. The wastewater passes through a grit trap where steady stirring causes grit to swirl to the centre and bottom of the grit trap. This grit is removed for land disposal. 
  5. The wastewater then passes into one of two bioreactor basins where micro-organisms, collectively called “activated sludge” feed on the organic waste in the water. Pathogens (disease-causing organisms) and contaminants from the wastewater stick to the activated sludge ready for removal. Four large turbo compressors supply the oxygen required for these micro-organisms to live, feed and multiply. 
  6. The activated sludge is separated from the water in one of three clarifiers. The sludge settles to the bottom of the clarifier while the clear water is on top.  Most of the sludge is returned to the bioreactor basins to maintain the biological levels, while surplus is diverted to the solid stream process. Here the sludge undergoes thickening, mechanical dewatering and finally thermal drying to turn it into a biosolid fertiliser called Bioboost. 
  7. The clean water from the clarifiers is disinfected with a small dose of sodium hypochlorite before entering the contact tank to kill any remaining pathogens.
  8. The clean water is then piped to the marine outfall located 1km north of the plant where the water flows through diffusers which mix the seawater and treated water together. This diffuser arrangement is located 450m offshore.
Waste water process

Monitoring plan and peer review

NPDC has a monitoring plan as a special condition of its resource consent to discharge treated municipal wastewater from the New Plymouth Wastewater Treatment Plant through a marine outfall into the Tasman Sea. Included in this plan is the monitoring NPDC undertakes at the plant, the monitoring TRC undertakes of the receiving environment and the peer review by an independent, suitably qualified expert to ensure that the monitoring programme is still appropriate once every five years, which is also a special condition on the consent.

New Plymouth Wastewater Treatment Plant Monitoring Plan - March 2022

Page last updated: 09:41am Tue 17 October 2023

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06 759 6060 enquiries@npdc.govt.nz
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84 Liardet Street
New Plymouth

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