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A quick guide to how a water treatment plant processes our drinking water

Water is necessary for daily life, as well as for our health, hygiene and community living. Local water treatment plants process water to ensure a constant supply of high-quality, safe drinking water. The water treatment process may alter slightly based on the plant’s technology, but the core principles remain the same.


Coagulation-flocculation is the initial stage of the water treatment process. It is applied to untreated, raw water. During the coagulation phase, liquid aluminium sulphate (alum) and/or polymer is injected into the tanks, which initiates a series of reactions with other ions to form a precipitate commonly called floc. 

The floc allows for particulates, such as clay, silt and simple organic structures, to absorb onto the surface. Flocculation is the agitation – or stirring – of the water to encourage the particles to agglomerate into masses large enough to settle and be filtered from the solution.


As the water and floc particles progress through the treatment process, powdered activated carbon may be added to the water during the flocculation period or as the water passes through the sedimentation tank into the filters. This process removes taste, odour compounds and any potential toxins when there is a blue-green algal bloom from the water source. 

The water moves slowly through the sedimentation tank, which causes the heavy floc particles, which are about 3mm in diameter, to settle and sink to the bottom of the tank. The floc – also called sludge – is then piped to drying lagoons. Water treatment plants that use direct filtration will not have a sedimentation phase – the floc is removed by filtration only.

As the concentration of solids increases at the base of the sedimentation tank, the water is pushed up and over the sides of long, narrow weirs that traverse the top of the tanks. Depending on the water treatment process, the water may be sent to Activated Carbon Treatment bays. The flocculants are sent to nearby sludge storage dams.


Gravity forces the water through a filter that is designed to remove the remaining particles. The filters are made up of layers of sand and gravel and, in some cases, crushed anthracite. The filtration system collects the suspended impurities in the water, with the filters routinely cleaned by backwashing.

pH Correction

After the water passes through the filters it will undergo a series of analytical tests. If the pH levels are too acidic, hydrated lime is added to the filtered water to adjust the pH and to stabilise the naturally soft water. This minimises corrosion in plumbing and distribution systems. 


The water will then flow into two concrete tanks to be disinfected before it enters the distribution system. This will ensure that any disease-causing microorganisms are destroyed. Chlorine is used because it works effectively as a disinfectant and any residual concentrations can be managed against possible microbial contamination in the distribution system. The final stage of the water treatment process occurs in these tanks.

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