Wastewater flows by gravity to pump stations, which pump the wastewater from low areas around the city, particularly near the Avon and Heathcote Rivers. Five terminal pump stations then pump all the flow to the treatment plant.
There are 239 pump stations, lift stations and vacuum stations located throughout Christchurch. All stations are connected by radio to a main control room and monitored by the team at the Christchurch Wastewater Treatment Plant.
The pump stations deliver all the wastewater to the treatment plant.
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.
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.
There are seven pond cells 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.
The cleaned water in the oxidation ponds is discharged through a long outfall pipe which discharges 3km off New Brighton beach. The outfall pipe length from the oxidation ponds is 5.2 kilometres and it is 1.8 metres in diameter. It is buried about 8 metres below the sea floor.
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 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.
The biogas produced during the digester treatment process is used for generating electricity and for heating. Engines run on the biogas to produce electricity for the treatment plant.
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.