In Christchurch, sewage (which is almost all water) is carried through a network of underground pipes and pumps to the Christchurch Wastewater Treatment Plant where it is processed. Over 90 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.
Preliminary treatment
Sewage passes through four fine screens that remove debris from the flow. Sewage is then taken through grit removal tanks, where air is injected along the sides of the tank. The air creates a spiral current in the tank that assists in removing fat, grease and grit. The debris from the screens and grit from the tanks is washed and removed to landfill.
Primary treatment
Sewage then enters the primary sedimentation tanks. The suspended organic matter settles to the bottom of the tank where a slow moving scraper drags the raw sludge to a hopper at the inlet end of the tank.
Secondary treatment
From the primary sedimentation tanks the effluent flows in a channel to large pumps that push the flow to the top of two trickling filter towers. These are packed with a plastic media that the liquid trickles through. A slime of naturally occurring bacteria lives on the plastic media and feeds on dissolved pollution in the wastewater. This process converts the dissolved pollution into biological solids that can be settled.
The filter effluent then passes through fine bubble contact tanks. Air is blown into the bottom of the tanks and oxygen is transferred from the bubbles into the wastewater. This part of the process removes more of the pollution load and improves the settling of the biological solids in the final clarifiers.
Odour treatment
All major sources of foul air are enclosed and pumped through soil and bark filters where the odorous compounds are absorbed onto soil particles and destroyed by naturally occurring soil bacteria.
Solids treatment
Raw sludge from the primary sedimentation tanks and the biological solids from the final clarifiers are pumped into large enclosed heated tanks called digesters. Two of these digesters operate at 55°C and four operate at 38°C.
Bacteria that thrive under these conditions break down the organic matter over a period of days, releasing carbon dioxide and methane and changing the decaying solids into 'biosolids' that are applied to land as a fertiliser.
The methane produced in the digestion process is used as a fuel for engines, producing power for the plant and the national grid. Heat recovered from the engines is used to heat the digesters.
Oxidation ponds
After secondary treatment, the flow of cleaned wastewater passes through a series of seven ponds covering 220 hectares. The pond system removes a portion of the remaining pollution load and grazing zooplankton and the sun's ultra violet radiation provide an inhospitable environment for disease causing micro-organisms whose numbers are reduced by 99.99 per cent.
Oxygen concentration in the ponds is maintained by a combination of mechanical aerators, wind-induced surface aeration and the photosynthetic action of naturally occurring algae.
The oxidation ponds also encompass the Te Huinga Manu Wildlife Refuge, an important habitat for a variety of bird species.
Operating data (2009/2010)
- Average flow: 172,000 metres cubed per day
- Average BOD received: 40.5 tonnes per day
- Suspended solids received: 53.9 tonnes per day
- Faecal coliforms removal: 99.992 per cent
- Gas production: 17,979 metres cubed per day
- Annual electricity production: 10,376,994 Kilowatt hours
- Connected population: 357,901
