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PACl supply crucial for effective water treatment system

The Te Mato Vai Project Management Unit (PMU) has ordered a ‘coagulant’ chemical to enable a key step in the new four-step water treatment process that will provide reliably safe water to Rarotonga’s communities. Coagulant plays a key role in cleaning the water but is different from disinfection, which kills bacteria and viruses.


Most developed nations use coagulants as part of water treatment for water that comes from streams, rivers or lakes. There are only a few chemicals used for coagulation; aluminium sulfate (alum) or polyaluminium chloride (PACl) are the most common.


The PMU has selected PACl for Rarotonga as it is safer than alum, has a longer shelf life and performs better in water quality tests. PACl is essential to the water treatment process.

The four-step water treatment process involves:

  • Sedimentation

  • Coagulation/flocculation

  • Filtration

  • Disinfection

All four steps are necessary to achieve a ‘potable’ water supply – one that is safe for drinking, bathing and food preparation.

At the coagulation/flocculation stage, the coagulant is slowly mixed into the water. The coagulant sticks to small particles of dirt and suspended solids (for example soil and other organic matter), and clumps them together so they sink to the bottom of the tank. Coagulation, together with filtration, also removes protozoa. Protozoa are tiny organisms that can cause diseases such as Giardia.


The cleaned water left at the top of the tank is collected, and moved on to the next step of the treatment process. The filtration step removes any residual coagulant, dirt particles and protozoa from the water before the water is disinfected. Disinfection is important as it kills viruses and bacteria.


If a water source is very clean, like groundwater, water treatment without coagulant is possible. Coagulation is necessary in Rarotonga because stream water contains high levels of sediment and ‘suspended solids’.


If a coagulant step was not used, the AVG filters would likely get blocked, and the filters may not operate at all during storms. Importantly, the water treatment would not achieve ‘potable’ quality, leading to continued health risks.


The PMU will be transporting, storing and handling PACl according to all the necessary health and safety standards.

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