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The real science behind PACl sludge



Each upgraded water treatment intake around Rarotonga has now nearly completed the six-month polyaluminium chloride (PACl) trial.


Since the use of PACl started, locals have told us their water is much clearer, particularly during heavy rain compared with before, and people with water filters have needed to change their water filters much less often.


But what does the science say?

Well, following more than 10,000 tests taken, the data collected tells us that there has been a huge improvement in water quality.


Turbidity (dirty water) and E. Coli (bacteria) levels have greatly reduced, meaning there are less harmful contaminants in the water that put the community at risk of illnesses including gastro (upset stomachs), ear infections and skin diseases.


Results from the thousands of tests conducted have also indicated that no environmental harm has been recorded.


So is PACl sludge toxic?

The short and scientific answer is no. PACl sludge is safe and not a health risk for our people or the environment.


According to ecotoxicologist Victor Perez-Landa, who holds a PhD in marine science and a master’s in environmental science with over 12 years’ experience, PACl sludge is not toxic or hazardous.


“In New Zealand, the classification of hazardous waste is defined under the Hazardous Substances and New Organisms Act, Environmental Protection Authority (EPA) 2012,” said Mr Perez-Landa.


“The EPA defines hazardous waste as waste that exceeds the hazardous characteristics threshold of explosiveness, flammability, capability to oxidise, corrosiveness, toxicity and ecotoxicity.”


“When you analyse PACl sludge against these thresholds PACl sludge is not considered a hazardous waste or a health risk for people. In fact, PACl sludge is only as toxic as store-bought potting mix, just a little saltier.”


Does that mean we can use PACl sludge for growing crops?

Yes, with the addition of some fertiliser.


Whilst using PACl to treat raw stream water adds aluminium and chloride, most of the content of PACl sludge is the natural fine soil particles that are filtered out of the raw water.

Initially PACl sludge was being considered for fill or capping material, however at the request of landowners and with the support of the Government, we did tests to understand the make-up of the PACl sludge and whether it would be suitable for growing crops.


Large samples of natural soil and PACl sludge were taken from five intakes around the island and flown to Hill Laboratories in New Zealand to uncover the composition of both typical island soil (which we have investigated before) compared to PACl sludge. Tests included pH, calcium chloride, extractable aluminium, phosphorus, chloride, metals and sulphur.


Dr Murray Wallis, an environmental scientist of over 30 years with a horticulture degree and PhD in soil science, analysed the latest results released earlier this week and concluded with certainty that PACl sludge could be used successfully to fill land or agriculture.


“Most of the content of PACl sludge is the fine soil particles that enter the intakes. The treatment of the water adds a small amount of extra aluminium, chloride and sulphur,” explains Dr Wallis.

“Chloride is the part of salt that we use for our meals. It is also the same as the salt from the sea that falls on the land. Native plants are well adapted to the salt. Rarotonga has a lot of rainfall, and this will wash and drain the salt from the soil naturally. The aluminium added to the water treatment system will react with other naturally occurring elements and compounds in the soil, rendering it harmless. Sulphur is a plant nutrient and will help grow crops because we have found the soils lack sulphur.”


“In addition to this, the natural soils on Rarotonga that are formed from volcanic deposits are already high in aluminium because this is a major component of Rarotongan soil minerals.”


“Because the material does not pose a hazard for people or for crops, the amount of PACl sludge that could be incorporated into the soil for cropping is quite high. The only practical constraint is the need to add phosphorus fertiliser with trace elements. Adding fertiliser is already needed to meet the nutrient requirements of crops on the island. We intend to work with the Ministry of Agriculture to provide recommendations for fertiliser use, which can include organic forms such as compost and manure.”


Ultimately the Te Mato Vai project aims to deliver cleaner and a more reliable water supply to all people in Rarotonga. This system has been specially designed for Rarotonga, taking into consideration the unique environment, and building on decades of scientific knowledge in best practice water treatment from around the world and particularly the Pacific region.

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