With the Polyaluminium chloride - or PACl – trials well underway there has been a lot of community concern about PACl and the risk it poses to human health and the environment. Below we set the record straight on some of the most frequently asked questions.
What is PACl?
Polyaluminium chloride, or PACl, is a coagulant used in the water treatment process to remove small particles of dirt and other contaminants. When mixed with untreated water, the PACl sticks to small particles of dirt and other contaminants and clumps them together so they sink to the bottom of the tank. This process is called coagulation. Coagulation also helps remove harmful protozoa (tiny organisms that can cause diseases such as Giardia).
Why do we need PACl?
If a water source is very clean, like groundwater, water treatment without a coagulant is possible. In Rarotonga stream water contains high levels of sediment and suspended solid waste, such as bird droppings. PACl is required to help clump these contaminants together so they can be removed as part of the water treatment process.
How much PACl will be used?
The amount used will depend on how much dirt is in the water and the specific needs of each intake site. A trial will be conducted to provide us with more information on this. We will use a monitoring system to adjust the PACl dosage to suit the water quality and ensure we only use the minimum amount needed.
Do we still have to boil our water?
While using PACL will help to remove some of the contaminants in the water, we are only commissioning the first three steps of the water treatment process. The forth step, disinfection, will not be implemented yet so we recommend you continue to boil your water prior to drinking.
Is PACl safe?
Yes, it is safe to use PACl as part of the water treatment process. New Zealand and Australian Drinking Water Standards both recognise the use of PACl in water treatment to deliver clean and safe drinking water.
The water treatment steps includes coagulation, flocculation, clarification and filtration. PACl is used in the coagulation step and part of the process that is recognised in the Guidelines for Drinking-water Quality Management for New Zealand as “well-proven technology for the significant removal of colour and particulate matter including protozoa (e.g. Cryptosporidium oocysts and Giardia cysts), viruses, bacteria, and other micro-organisms.”
There have been concerns from interest groups and unsubstantiated reports in technical literature about residual chemicals, such as aluminium, in the water affecting human health. The World Health Organisation concludes from a large number of studies and scientific evidence, conducted over many decades (e.g. by Srinivasan et al 1999), that adverse health risks have not been demonstrated in any credible studies.
We will be following industry practice and guidelines to ensure PACl is used safely and effectively.
What is PACl sludge?
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 settles to the bottom of the tank is called ‘sludge’.
Is sludge safe?
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. However, like any waste product, PACl sludge must be managed responsibly.
How much sludge will the Te Mato Vai system produce?
We can’t 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. For example, heavy rainfall that disturbs the soil and carries more sediment into the stream will result in more PACl needed to remove the contaminants in the water and more sludge produced.
This is one of the reasons for the PACl trials – the trials will help us understand how much PACl to use at each site, how much sludge will be produced and the sludge’s consistency.
What will happen to the sludge?
There are a number of ways to safely and responsibly dispose of PACl sludge. Once we have results from the PACl trials we will be better able to determine the best method for Rarotonga.
During the PACl trials the sludge produced in the sedimentation tanks will be transferred to the sludge ponds for further drying and on-site storage until the best disposal method is identified.