Wastewater collection and treatment

Wastewater, also known as sewage, is the used water that goes down sinks, washing machines, showers, baths and toilets. 

Most of it is water. The rest includes organic matter such as human waste, food scraps, cooking fats, oil and grease, and debris such as sand, grit and plastic.

Wastewater can also include household and business chemicals, paint and medicines. These can be harmful to our health, our harbours and waterways, so effective treatment is very important. 

Where does your wastewater go?

Each time you flush the toilet, pull the plug from a sink or have a shower, the water drains into a wastewater pipe on your property.

Our plants are always operating, treating wastewater so that it is safe to release back into the environment.  We never shut down and we always keep running, even during the worst of weather and power outages.

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.

About 12 hours after it enters the plant, the treated wastewater is safely discharged from the Māngere Wastewater Treatment Plant into the Manukau Harbour. This takes place on an outgoing tide to ensure the discharge is well mixed with the harbour water. At our Rosedale Treatment Plant, treated wastewater is held in two large ponds before being discharged 2km out to sea in the Waitemata Harbour. Click here to read about the Rosedale Treatment Plant Dam.
 
The final effluent meets standards that protect public health, the local environment, and Auckland’s coasts, estuaries and harbours.

Biosolids are a product of the nutrient-rich, organic sludge produced when we treat wastewater.

Our aim is to make the best use of this resource, so that we protect and enhance the environment.

At our Mangere plant we are using biosolids to restore a former quarry on nearby Puketutu Island.  At other plants, we dispose of biosolids in commercial landfills.

We’re also seeking ways to use biosolids in environmentally friendly ways. To explore other options, we are continually working with local iwi and businesses. As well as our own investigations, we draw on the experiences of wastewater utilities around the world.

Our initiatives include:  

• Considering how biosolids can be reused to benefit the environment.

• Taking part in an industry working group looking at national environmental standards for biosolids used on land.

• Finding ways to recycle wastewater at our sites and with external customers.

• Maximising the harvesting of biogas from wastewater to create energy.

• Changing technology to minimise our carbon footprint and greenhouse gas emissions.

• Exploring other opportunities including: extracting valuable end products from the biosolids, creating bio polymers, creating safe-to-use compost and producing phosphorus-rich fertilisers.