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PACl sludge is not ‘toxic waste’

Updated: Nov 24, 2020

We have received messages from members of the public concerned about the ‘PACl sludge’ that the new Te Mato Vai water treatment system will produce, and how it might be safely disposed of. Here we explain what ‘PACl sludge’ is, why it’s incorrect to label it ‘toxic waste’ and how we might manage this waste product in future.

PACl dosing tank
PACl dosing tank

What is PACl sludge?

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.

During the ‘coagulation’ treatment process, PACl is mixed into the water, where it attracts the contaminants. The PACl and contaminants clump together and sink to the bottom of the tank so they can be separated from the cleaner water above. 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’.

For more information on the coagulation process, see our post on PACl and video PACl demonstration.

How do we know the sludge is not toxic?

The well documented water treatment process science tells us that the PACl sludge is in many respects like a soil, and can be reused or disposed of in a number of ways without damaging the environment.

PACl sludge must of course be managed responsibly, but as a waste product safe to reuse for a range of purposes, it’s not considered “toxic waste”.

We are aware of claims that aluminium toxicity is an issue for soils in Rarotonga. Only two of Rarotonga’s 11 soil types (Pouara and Rutaki) are classified as having this problem. Avoiding disposal or recycling to areas with these two soil types are found will mitigate this concern.

How much sludge will the Te Mato Vai system produce?

It’s not possible to accurately predict how much sludge the Te Mato Vai system will produce, because it will depend on weather, its effect on stream flows, and the unique conditions of each intake site. This is one of the reasons for the PACl trials – the trials will enable us to better understand how much PACl needs to be used at each site, how much sludge is produced and the sludge’s consistency.

What will happen to the sludge?

There are a number of ways to safely dispose of PACl sludge, and the best method for Rarotonga will be determined during the trial period.

To Tatou Vai is currently preparing an Environmental Impact Assessment (EIA) for the new water treatment system’s operational use of PACl. The public will have the opportunity to comment on the EIA during the consultation period.

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