Water Treatment Plants

Watercare operates six water treatment plants in the Auckland region that supply high quality drinking water. The newest of these is the Waikato Water Treatment Plant at Tuakau in the Franklin District, which has a supply capacity of 50 million litres a day, around 14% of Auckland's daily water requirements. 

Raw water coming into the treatment plants from the Hunua and Waitakere dams is exceptionally clean, from only 2-5 ntu (ntu is a measurement of turbidity or presence of dirt particles). This is because there is almost no human, industrial or agricultural activity in those areas. Raw water from the Waikato River has a higher turbidity (20-40 ntu), however the sophisticated 4-stage treatment process means that the water, once treated, is as clean if not cleaner than water supplied from the other treatment plants.

Treatment Processes
To meet New Zealand Drinking Water Standards 2000 most raw waters need to be treated. The treatment process removes suspended solids, turbidity (dirt particles) and harmful microbiological organisms.

The standard 3-stage treatment process followed at all of Watercare's treatment plants, with the exception of Waikato, is as follows:

1. Coagulation
Coagulation destabilises the predominantly negatively charged particles suspended in the water. By lessening their repelling qualities the particles attach to each other for easier removal later on. The coagulant added to the raw water is aluminium sulphate.
The water then flows through mixing tanks where the particles increase in size, to create what's known as floc. This floc settles to the bottom of the tanks. Positively charged polyelectrolyte molecules and alkaline lime are also added to aid the coagulation process.

2. Clarification and filtration
The floc is removed at this stage in vertical flow clarifiers shaped like inverted pyramids. This process produces a blanket of floc, above which clarified or settled water decants into troughs at the top of the tanks. Floc, now called clarifier sludge, is drained off from the blankets through cones suspended at the tops of the clarifiers.
The settled water is then fed through to sand filters to further remove suspended solids. The filters are washed every 24-48 hours to clean out entrapped floc.

3. Disinfection
Either gaseous 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. The amount of chlorine in the water that leaves the stations is about 0.9 grams per cubic metre.