Wastewater Treatment Plant
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.
The sewage passes through four fine screens that remove debris from the flow. The next stage takes place in the 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 composted in the Hot Rot machine, which converts the material to a high quality compost.
The sewage now enters the primary sedimentation tanks. The suspended organic matter settles to the bottom of the tank where a slow moving scraper drags this raw sludge to a hopper at the inlet end of the tank.
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.
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.
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 550C and four operate at 380C. 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.
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 99.999%.
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.
Discharge into the Estuary of the Avon and Heathcote Rivers occurs twice a day around high tide. The discharge to Estuary will be soon be replaced by a direct Ocean Outfall that will discharge the treated wastewater 3 km off shore.
- Average Flow: 181,000 m3/day
- Average BOD Received: 45.0 tonnes/day
- Suspended Solids Received: 56.8 tonnes/day
- Faecal Coliforms Removal: 99.999%
- Gas Production: 15,392 m3/day
- Annual Electricity Production: 11,250,000 kWh
- Connected Population: 323,019
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