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Treating water sourced from dams

The majority of Auckland's water comes from 10 dams in the Hunua and Waitakere ranges. It is treated at four water treatment plants: Ardmore, Huia, Huia Village, and Waitakere. Raw water, which contains contaminants ranging from large debris to micro-organisms, undergoes a series of treatment stages to ensure the requirements of the Drinking Water Standards for New Zealand 2005 (revised 2008) are satisfied. The result is safe water that you enjoy from your taps each day.  

Treatment stages include the following steps:

Step 1: Screening

Water first passes through ‘screens’ that remove any large debris such as leaves, branches, rubbish, and dead insects. 

Step 2: Coagulation

Although obvious debris has now been removed, the water still contains dirt. To remove the dirt, a compound known as aluminium sulphate (alum) is mixed into the water. This makes the tiny dirt particles stick together or ‘coagulate’. The water is then piped into settling tanks. As the alum and dirt particles stick together, they form large, heavier particles called ‘flocs’ which sink to the bottom of the tank.

Step 3: Clarification and filtration

At this stage, the floc is removed and clarified or settled water is fed through to sand filters to further remove suspended solids.

Step 4: Disinfection

The final stage of treatment involves disinfection to remove any micro-organisms (germs) left in the water. Chlorine or sodium hypochlorite is added to kill any pathogenic bacteria that might be in the water after filtration. Watercare produces its own sodium hypochlorite on site from electrolysis of common salt. Chlorine has been used effectively around the world for over one hundred years and the amount of chlorine added is nominal. Chlorine evaporates very quickly. For drinking, place the water in a container and let it stand for up to two hours (or put it in the fridge to be used as needed).

Finally, fluoride is added to Auckland's treated water supplies at the request of Auckland's legacy councils (prior to Auckland Council integration). Currently, Onehunga and Huia Village are the only Auckland metropolitan areas where fluoride is not added. Lime is also added to help prevent the water supply pipes from corroding.