Wastewater is the name for the water that comes from your sink, toilet, laundry bath and shower. Wastewater goes down your drain into Metrowater's wastewater pipes. We transport it to the Mangere Wastewater Treatment Plant to be treated before it goes back into the environment.

Why is wastewater treated?

It's very important for our environment and for our health that we treat wastewater properly. In Auckland our wastewater goes into the Manukau Harbour after it is treated, so it needs to be cleaned to a high standard first. Otherwise we could harm the plants and animals that live in and around our rivers, lakes and harbours.

If it is not properly cleaned, water can carry diseases. In Auckland we live, work and play close to water so we need to make sure harmful bacteria are removed to make the water safe.

How is my wastewater treated?

Metrowater takes the wastewater from your toilets, bathrooms, kitchens, laundries and car washing, as well as the waste from industries, and transports it to Watercare's sewer pipes. Watercare then takes it to the Mangere Wastewater Treatment Plant to be treated.

Once the wastewater reaches the Wastewater Treatment Plant, it passes through mechanical screens. This is where large pieces of wood, gravel and plastic are removed. The screens also break up the wastewater and take out any substances like paper or fruit and vegetable pulp. All of the debris is collected in screening bins to be taken away and disposed of as landfill.

The wastewater then goes into grit tanks. Air is blown into these tanks and the grit and sand settle on the bottom. This grit is washed off and taken to landfill for disposal. On average four tonnes of sand, grit and fine gravel are removed each day.

The next step involves transporting the wastewater to primary sedimentation tanks. Here the wastewater flow is slowed down. This allows all of the sludge (organic material) to sink to the bottom. The wastewater stays here for two hours while scrapers move across the bottom of the tank and push the sludge into a deep container.

In the next part of the process, the liquid wastewater and sludge are treated using different methods. Lets look at the sludge first.

Sludge
The sludge from the bottom of the primary sedimentation tanks is taken to gravity thickening tanks where it is passed over gravity belt thickeners. This helps reduce the amount of water in the sludge. It also means there is less sludge to transport to the red brick tanks called digesters.

In the digesters, the sludge is heated up to blood temperature (37 degrees Celsius). Because there is no oxygen in these tanks special bacteria, called anaerobic bacteria, break down or 'digest' the organic material and fat. This process releases biogas - methane and carbon dioxide. The biogas is piped off and used to produce electricity to run the Wastewater Treatment Plant.

After 15 days in the digesters, the sludge goes to the dewatering plant. Here a polymer solution is added to help thicken the sludge before it is 'spun dry' in a centrifuge. After this it has a dry powdery consistency and is called biosolids.

Next, lime is added to the biosolids. This raises the temperature to 70 degrees Celsius to kill off any remaining pathogens and help reduce odours. Finally the biosolids travel on a conveyor to a storage building where they stay for up to 96 hours before being disposed of in a large on-site landfill. The Wastewater Treatment Plant produces about 270 tonnes of biosolids each day - about the same weight as 55 elephants.

Liquid
The liquid wastewater in the primary sedimentation tank makes its way to the end of the tank and flows over a weir (dam). From here it is pumped to nine very large reactor clarifiers. These are like large 'bug farms'. Here, natural micro-organisms found in the water are used to strip out the pollutants, making it safe to put it back into the Manukau Harbour. This process is called activated sludge and is the most widely used wastewater treatment in the world.

The process also removes the nitrogen from the liquid wastewater. If nitrogen enters our harbours and seas, it encourages algae and seaweed to grow and they compete with the marine animals for oxygen. In the reactors the 'mixed liquid'  (liquid wastewater and micro-organisms) passes through eight chambers where ammonia is converted to nitrate and nitrites and then into harmless nitrogen gas. This is released into the atmosphere.

Next the 'mixed liquid' goes into the clarifier sedimentation tank where it settles out and the bacteria and other micro-organisms sink to the bottom. Once it has settled, the bacterial sludge is collected and returned back to the first chamber of the reactor so the bacteria can go to work again.

The clarified liquid is then taken to the filtration facility where it passes through massive filters made of a very hard coal. The filters remove any suspended solids (very small particles). This means that in the next stage, ultraviolet (UV) disinfection, the light can penetrate the liquid properly. The UV disinfection kills any pathogens (viruses) left in the wastewater and has a 99.99% kill rate.

Finally the treated wastewater is released into the Manukau harbour.

If you would like to find out more about wastewater treatment or arrange a tour of the Treatment Plant, visit Watercare at www.watercare.co.nz.