Bacteria in the slime feed on dissolved organic matter in the wastewater. When it has grown to a certain thickness, the slime breaks off to be washed into secondary sedimentation tanks. At this point 90-95 per cent of the settleable solids and 70 per cent of the original organic material has been removed. The treated wastewater is then released into the oxidation ponds, where it spends three weeks. From the ponds the treated wastewater is discharged into the Avon-Heathcote Estuary.
Discharge occurs twice a day for a few hours after high tide.
This timing ensures the treated wastewater is flushed more effectively out to sea. Other than immediately adjacent to the oxidation pond outfall, the bacteria and viruses remaining in the treated wastewater do not pose a serious threat to public health or the environment, and are quickly destroyed by seawater. Concentrations of nitrogen and phosphorus nutrient still remain in the discharged treated wastewater, some of which contributes to the ecosystem, feeding algae and photosynthetic micro-organisms such as plankton. Domestic customers pay for their use of the service through rates. The system takes an average of 600 litres of wastewater per day from each Christchurch domestic customer.