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Wastewater collection and treatment

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

Where does wastewater come from?

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. It is important to treat wastewater before it is returned to waterways.

What used to happen to all the waste?

In the 1800s, before toilets were common, people would use a bucket placed in an outhouse, which was a small shed in the garden. In the evening, a night soil collector would come along in a horse and cart. They would empty the bucket and leave a clean one. The person who owned the outhouse had to pay for this service. People didn’t have a bathroom. Instead, children would be bathed on Saturday nights in a tin bath in front of the open coal fire in the kitchen.

Where did all the waste go in the past?

The night soil collectors worked between 10pm and 5am emptying buckets of human waste into their carts. They took the waste to a manure depot on the edge of town. The waste was buried there. Many people ignored this and continued to bury their waste on their own property, which caused the outbreak of diseases such as cholera and typhoid.

Where does your wastewater go now?

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. These pipes connect to the public wastewater network, which carry your household wastewater to be treated at a wastewater treatment plant.

If your house is connected to a public wastewater network, water that goes down the drain will be treated at a wastewater treatment plant. Image: LEARNZ.

People who live in smaller towns and away from larger cities are often not connected to the public wastewater network. Instead, they have a septic tank. A septic tank is usually underground, and takes wastewater from toilets, sinks, and basins. The septic tank contains bacteria that break down the wastewater. The wastewater then flows out into a soil disposal field on the property.

How to look after your wastewater system

People can help protect their wastewater pipes and save money on plumbing bills by only flushing the "3 P's" down the loo:

  • pee
  • poo
  • paper (toilet paper that is)!

Not so flushable

Disposable wipes (or any other material, other than toilet paper) should never be flushed down the toilet. They can block up wastewater pipes - yours and your city's - and it can cost thousands of dollars to get the blockages cleared.

Wet wipes flushed down toilets can block sewer pipes. Image: Watercare.

Treating wastewater at the Māngere wastewater treatment plant

A wastewater treatment plant is a place where wastewater is cleaned so that it is not harmful to people or the environment. New Zealand's largest wastewater treatment plant is in Māngere, Auckland. This plant uses a combination of physical, mechanical, and biological processes to clean waste out of the water. Treatment takes about 12 hours from start to finish.

  1. Primary treatment
    Raw wastewater first passes through screens which remove larger waste materials. Solid waste material separated during the screening process is taken to a landfill. Grit tanks then remove grit and sand. Primary tanks separate the bulk of the solids from the liquid waste. These solids are treated and become biosolids (see below). The liquid continues onto the secondary treatment.
  2. Secondary treatment
    Reactor clarifiers further separate solids and remove nutrients. One of these nutrients is nitrogen, which can be harmful to the environment.
  3. Tertiary treatment
    The wastewater is filtered and disinfected using ultraviolet (UV) light, which destroys bacteria and viruses. It then goes to the intertidal storage basin.


Biosolids are the solid organic materials that are produced during the wastewater treatment process. The solids are nutrient-rich and treatment has removed dangerous microorganisms and odours, so they can be used for farming, growing trees, and land restoration.

Aerial view of the Māngere wastewater treatment plant. Image: Watercare.

What happens after the water is treated?

The treated wastewater is tested in a laboratory to make sure it meets standards that protect public health and the local environment. It is then safely discharged out to sea.

Try the wastewater collection and treatment quiz.

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