Water treatment

Before water can be considered safe and not unpleasant to drink, a number of steps are required.

Flocculation: The first stage in treating water is a process known as flocculation, where small suspended particles in the water join together to form larger particles that sink and settle due to their own weight. This is achieved by adding a small amount of alum (aluminium sulphate) and a source of hydroxide ions, lime (calcium hydroxide). A product of this reaction is the gelatinous precipitate aluminium hydroxide which entraps other particles, helping to remove any colour from water.

Settling: The water is then left to stand to allow the floc to settle as sludge at the bottom of the tank.

Filtering: Water from the settling tank is allowed to filter down through a bed of sand and gravel. This removes any remaining suspended mineral particles.

Chlorination: A small amount of gaseous chlorine is added to the water. Chlorine reacts with water to form hypochlorite ions (OCl^-), that kill bacteria and algae.

In Australia, many water supplies have fluoride added in the form of different compounds such as sodium fluoride (NaF), sodium silicofluoride (Na₂SiF₆) or calcium fluoride (CaF₂). This is because the fluoride ions are incorporated into the structure of the enamel of teeth, making them less susceptible to the chemical action of acids.

Some water supplies (such as Adelaide's) have hard water, with dissolved calcium or magnesium ions. This is not dangerous as the concentrations are still relatively low, but the cations in hard water react with the soaps and detergents, making to difficult to get a lather. Boiling the water can have some effect, as hydrogen carbonate ions will breakdown and cause solid carbonates to form. The solids do not react with the soap.

Dissolved minerals in a water supply can also lead to the eventual blocking of hot water pipes, as solid precipitates form.

Melbourne's water supply has the lowest amount of dissolved solids of any capital city in Australia and is also the "softest" water supply. This is due primarily to the closed, forest catchment areas. Other cities in Australia may need to treat the water more intensively to make safe to drink.