Water treatment process

The Central Coast has the third largest water supply system in New South Wales. Find out about how Council treats and disinfects its water to ensure you have a top quality supply straight from your tap.

Water treatment process

The Central Coast drinking water supply has three treatment plants that deliver quality water complying with Australian Drinking Water Guidelines, 2011.

A water treatment plant is where the water from our rivers, creeks, dams and bores is filtered and disinfected to make it safe to drink. Watch a behind the scenes video of the Central Coast process.

Mardi Water Treatment Plant

Mardi Water Treatment Plant was built in 1982 with expansion in 1992 to increase capacity. The plant treats water from Mardi Dam and has can supply up to 160 million litres – that’s about 64 Olympic swimming pools - every day of the year. The plant uses direct filtration treatment and includes:

  • flocculation
  • filtration
  • pH correction
  • disinfection
  • fluoridation

Somersby Water Treatment Plant

Somersby Water Treatment Plant was built in two stages in 1971 and 1986 and treats water from Mangrove Creek Dam or Mooney Mooney Dam. The plant has a capacity of 144 million litres - just over 57 Olympic swimming pools – every day. The plant uses conventional treatment, which includes:

  • flocculation
  • sedimentation
  • filtration
  • pH correction
  • disinfection
  • fluoridation

Water treatment

Here are the steps in the water treatment process:

Alkalinity adjustment

At the beginning of the process, lime is mixed into the water. This will minimise the risk of pipe corrosion, taste and odour problems, and of disinfection becoming ineffective as the water travels through the system. Carbon dioxide is also added to help the coagulation stage of the treatment process.

Manganese removal

When necessary, the raw water is dosed with potassium permanganate so that soluble manganese can be easily removed. This reduces the possibility of discoloured drinking water.

Coagulation / flocculation

Liquid aluminium sulfate (Alum) and polymers are added to stick to sediments in the water - this is coagulation. The water is then stirred vigorously so that particles of silt and mud form larger, heavier clumps (called floc) which are easier to remove - this is flocculation.


The water and floc then move slowly into large horizontal sedimentation tanks. As the flow of water decreases, the floc settles to the bottom of the tanks and becomes sludge. This is then pumped into sludge lagoons while the water moves on to the next stage.


The clarified water is dosed with chlorine so that any remaining soluble manganese particles can be removed by the sand filters.


Here water flows vertically into slow sand filters which sieve any remaining suspended organic material from the water before it enters the clear water storage tank. Any solids that become trapped in the sand are removed by backwashing the filter.


The water is then disinfected with chlorine to destroy harmful micro-organisms before it enters the water supply system. Chlorine is used because of its immediate and long-lasting anti-bacterial effect. Council carefully controls and monitors this process to ensure the residual chlorine concentration poses no risk to health.


The water is then fluoridated to increase the natural concentration to 1.0 part per million. Fluoride levels are continually monitored to ensure they remain within the optimal concentration range determined by the NSW Department of Health.

pH Correction

Finally, the water is set back to its neutral pH level so that it can travel safely to your tap.

Drinking water quality

Every day, we supply around 80 million litres of water to about 120,000 properties. Occasionally our customers have problems with drinking water quality. You can report issues to Council on 02 4306 7900.

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Last updated : Tue 18 Oct 2022