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How does polyaluminium chloride (PACl) work?

Updated: Nov 24, 2020

Polyaluminium chloride (PACl) is a ‘coagulant’ chemical added to water to help remove small particles of dirt and contaminants that can make people sick, and block the water system’s filters.

We need to use PACl in Rarotonga because stream water contains high levels of contaminants – everything from bird poo to leaves and soil. This is true for water taken from streams, rivers or lakes around the world, not just in Rarotonga. Even stream water that looks clear still contains harmful protozoa and bacteria too small for people to see.

We’ve filmed a small-scale demonstration of how PACl works, using water taken from the local water supply in Avatiu on an ‘average’ day. We took our samples the day after a regular rain, so the water we used was not particularly clear or dirty. You can view a video showing how we take samples for PACl testing.

As you can see in our demonstration video, when PACl is mixed into the water it attracts the contaminants and they clump together so they can sink to the bottom and be separated from the cleaner water.

In the intake site systems, the clean water left at the top of the tank is then directed into the next step of the treatment process.

The PACl and dirt that’s dropped to the bottom of the tank is called sludge. One of the reasons we are trialling PACl is that we don’t yet know how much sludge we will get at each intake site. This is because each intake site is different. The monitoring reports from the trials will be used to inform Government and landowners’ views on longer term PACl use, and also to inform sludge management.

Watch the video of our PACl demonstration.

You can also read more about the treatment process.

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