Wastewater treatment is the process of taking raw wastewater, and converting it through a variety of treatments into a final effluent that is without the solids and as many of the harmful characteristics of the original wastewater as possible.

There are a variety of different treatments used and available around the world and New Zealand, as there are in the wastewater treatment plants in Christchurch and Banks Peninsula.

They are all quite different in the way they treat but follow the general treatment practice of preliminary, primary, secondary and tertiary treatment. Biosolids are produced as a result of the treatment process and these are all taken to the Christchurch Wastewater Treatment Plant.

The treatment process

  • Preliminary treatment - removal of floating material, heavy settleable, and large suspended solids, using structures like screens, grit chambers and flow equalization.
  • Primary treatment - the removal of organic solids using sedimentation in primary sedimentation tanks.
  • Secondary treatment - aerobic, or anaerobic biological decomposition of organic matter using such things as intermittent sand filters, trickling filters, aeration tanks and imhoff tanks.
  • Tertiary treatment - removal of the remaining organic matter and killing pathogens (bacteria, viruses and protozoa) by disinfection with maturation ponds or ultra violet (UV) light.

Most of the treatment plants that the Council operates discharge to the ocean and harbours, but there are two that discharge to land (Wainui and Tikao Bay). The treatment plants are often being upgraded to improve the final effluent quality and minimise the environmental effects.

The treatment plants are monitored by the Council and by the Regional Council (ECan) to make sure that they meet the environmental measures set under their discharge consents.

There is a plan to remove the treated wastewater discharges from Lyttelton Harbour and instead to pump the wastewater from Lyttelton, Governors Bay and Diamond Harbour Christchurch Wastewater Treatment Plant. For more information about this please see the Lyttelton Harbour Wastewater Scheme webpage.