Over 100 pumping stations pump the sewage from low areas around the city, particularly near the Avon and Heathcote Rivers. Five terminal pumping stations then pump all the flow to the treatment plant.
There are more than 100 pump stations located throughout Christchurch. All stations are connected by radio to a main control room and monitored by the team at the Wastewater Treatment Plant. The pump stations delivery 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. This water includes heavy organic matter or sludge which 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 found at the end of the primary 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 bacteria grows and turns to slime on them. 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 last tanks 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 alga kill the harmful bacteria. The cleaned water in the oxidation ponds is discharged through a long pipe or “outfall” that flows into the ocean 3 km off New Brighton beach. The whole pipe is 5.2km long, 1.8m wide and is buried about 8m below the sea floor.
The sludge that was scraped from the bottom of the 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 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 dryer. The treated biosolids now look like course 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 power and heating. Engines run on the biogas to produce electricity for the treatment plant.
Any odour or ‘foul air’ produced at the Wastewater Treatment Plant is blown through large bark filters. The bark absorbs the odour and ‘helpful’ bacteria break down the odour to leave fresh air.