More than 90 per cent of Auckland’s wastewater goes to our plants at Māngere and Rosedale. Here it is treated to standards that protect public health, the local environment and our coasts and harbours. We are continually seeking more environmentally friendly ways of doing this.
Screening
Before treatment, raw wastewater goes through screens to filter out any solids such as rags, debris and other objects. Rotating drum screens feed this material into a conveyor and deposit it in a large bin. Trucks then take it out to landfill.
Treatment stages
Treatment takes about 12 hours from start to finish, and we cannot stop it or slow it down.
1. Separating solids from liquid
The wastewater goes into sedimentation tanks where sand and grit is removed, and any solids (sludge) sink to the bottom.
The sludge is then thickened and pumped to anaerobic digesters, where it is stabilised. It is then dewatered and limed to become biosolids – ready for disposal or reuse.
The liquid then moves to the next treatment stage.
2. Biological nutrient removal
This natural, chemical-free process uses ‘activated sludge’, which contains an army of micro-organisms. They eat the carbon in the liquid wastewater, and reduce the amount of phosphorus and nitrogen. These nutrients can pollute waterways and be toxic to fish.
3. Settling, filtering and disinfecting
The microorganisms are removed as the wastewater passes through clarifiers and filter blocks containing anthracite, a very hard form of coal.
The liquid is then disinfected under powerful ultraviolet lamps. This results in a 10,000-fold reduction of harmful pathogens, bacteria and viruses. There are no chemical byproducts, and it is safe for the environment.