Christchurch wastewater treatment plant

The Christchurch wastewater treatment plant processes the wastewater from all of Christchurch city.

Wastewater treatment plant

Pump stations

There are 239 pump stations, lift stations and vacuum stations located throughout Christchurch. Five terminal pump stations pump all the flow to the treatment plant. All stations are connected by radio to a main control room and monitored by the team at the Christchurch Wastewater Treatment Plant.

Screens, grit removal and sedimentation tanks

As wastewater flows into the plant, screens catch the rags and tanks then trap the grit before the flow goes into the sedimentation tanks.

The rag and grit is removed and disposed of at the Kate Valley Landfill. About 2 tonnes of rag and 2 tonnes of grit are removed every day.

The wastewater then passes through primary sedimentation tanks. Heavy organic matter settles to the bottom of the primary sedimentation tanks and is scraped to one end and pumped to the digesters for treatment.

Trickling filters, aeration tanks and clarifiers

The clear liquid at the end of the primary sedimentation tanks is pumped up to the top of the trickling filters where it is evenly spread over the surface of the filters.

The filters are called 'fixed growth reactors' because a bacterial slime grows on the fixed media and consumes the nutrients in the wastewater. As more flow is pumped into the filters, the slime is washed off as a floating solid.

Air is injected into the bottom of the next tanks (aeration tanks) in fine bubbles. This air allows the fine slime solids to form larger solids which settle to the bottom of the next tanks which are called clarifiers.

The solids are sucked off the bottom of the clarifier tanks leaving a clear liquid which then flows out to the oxidation ponds.

Maturation ponds and ocean outfall

There are six maturation ponds and it takes about 16 days for the clear liquid from the clarifiers to flow through the oxidation ponds. Sunlight and natural processes kill the harmful bacteria and viruses without the need for additional chemicals or artificial UV light.

The treated wastewater from the maturation ponds is discharged through a long outfall pipe which discharges 3 kilometres off New Brighton beach. The total outfall pipe length from the maturation ponds is 5.2 kilometres and it is 1.8 metres in diameter.  It is buried about 8 metres below the sea floor.

Digesters and biosolids

The sludge that was scraped from the bottom of the primary sedimentation tanks and from the clarifiers is pumped to the digesters for treatment.

During this treatment process helpful bacteria break down the sludge to form biosolids. This process produces a mixture of methane gas and carbon dioxide which is called biogas.

The biosolids at this stage are very wet – 98 per cent water and only 2 per cent solids. To remove the water, the biosolids are passed through a machine that squeezes the water out, then they are pumped into a biosolids dryer. The biosolids dryer uses hot air to evaporate the water and kill pathogens (bacteria and viruses). The treated biosolids now look like coarse dry sand.

The dry biosolids are then stored ready to be transported to land remediation projects for beneficial reuse as a soil conditioner and fertiliser.

Electricity generation

The biogas produced during the digester treatment process is used to power our gas engines. The engines generate electricity for the treatment plant and hot water to warm the liquid in the digesters to the optimum temperature for sludge digestion.

Odour control

Any odour or ‘foul air’ produced at the treatment plant is passed through large bark biofilters. The bark absorbs the odour and ‘helpful’ bacteria break down the odorous compounds to leave fresh air.

Wastewater discharge consent monitoring

Monitoring the discharge of treated wastewater from the Treatment Plant Oxidation Ponds into the Pegasus Bay Coastal Marine Area via an ocean outfall.


During the warmer summer months people living near the oxidation ponds at Bromley may notice an increased number of midges.

Community liaison group

Promoting communication and providing access to community opinions, observations and activities regarding the Ocean Outfall operation.


Find out what we do with the organic wastewater solids from our treatment plants.