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The potential for PACl sludge use

As part of the Polyaluminium Chloride (PACl) trial currently underway several landowners approached the Te Mato Vai (TMV) Project Management Unit (PMU) about the possibility of reusing the PACl sludge generated on their land for their own purposes, such as use in their gardens.

Initially PACl sludge was being considered for fill or capping material. At their request, and with the support of the Government, the PMU conducted preliminary tests to understand the composition of the PACl sludge and whether it would be suitable for reuse on the land.

Dr Murray Wallis, an environmental scientist of over 30 years with a PhD in soil science, reviewed the preliminary test results and confirmed that it was possible to reuse the PACl sludge with some management.

“Most of the content of PACl sludge is the fine soil particles that enter the water infiltration galleries. This is natural soil. The treatment of the water adds aluminium and chloride. 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 our land. Our native plants are well adapted to the salt. We have a lot of rainfall, and this will wash and drain the salt from the soil just as it does naturally. The aluminium added to the water treatment system will react with other naturally occurring elements and compounds in the soil, rendering it harmless,” said Dr Wallis.

“In addition to this, the natural soils on Rarotonga that are formed from volcanic deposits are already very high in aluminium because this is a major component of our soil minerals. Aluminium is NOT toxic to plants when the soil pH is near neutral or mildly acidic (the pH values are above pH 6). Most Rarotonga soils are above pH 6, and the silts and clays have very high natural buffering capacity meaning that their pH doesn’t change much when something more acidic is added. In fact, the PACl sludge we have tested is not acidic so it won’t make the soil more acidic.”

“The only effect of higher aluminium content (in natural soils as well as PACl sludge) is to reduce the availability of the plant nutrient phosphorus and some other trace element nutrients. Adding phosphorus fertiliser and trace elements will readily improve plant growth. Adding fertiliser is already needed to meet the nutrient requirements of crops on the island.”

While the potential for PACl sludge reuse at specific sites is looking positive, further tests are being done to confirm the preliminary results.

To date the Government has not issued any official recommendations or advice to encourage PACl sludge reuse for horticulture purposes.

Further information about PACl sludge reuse will be released to landowners and the public at a later date.


Note: Dr Murray Wallis holds a Bachelor of Horticultural Science (First Class Honours) and PhD in Soil Science from Massey University, New Zealand. For over 30 years Dr Wallis has consulted on soil and agricultural matters with extensive experience in the Cook Islands, other countries across the Pacific, NZ, Australia and the USA. He has previously held roles as lecturer in soil science and was a research scientist for the Australian Turfgrass Research Centre.

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