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PACl trial update

As of December 2020


The polyaluminium chloride, or PACl, trial is now well underway at each of the 10 intakes situated around the island since commencing firstly at Ngatoe in late September.

The use of PACl is the second step of the water treatment process and aims to remove small particles from raw water, including harmful protozoa that can cause diseases such as Giardia and some bacteria. The process sees PACl (a coagulant) slowly added to the water causing small particles to stick together and settle at the bottom of the tank. It is commonly used in water treatment systems around the world.

Trial results so far have been positive, with the water treatment plant performing as expected and delivering a significant increase in water quality overall.

All intakes have passed their pre-network connected trial (i.e. offline trial) and have been approved by To Tatou Vai to be connected to the network (i.e online trial).

The Aronga Mana, intake landowners, landowners’ guests and Government officials present to witness the start of the trials have provided positive feedback overall about the opportunity to learn more about the water infrastructure and see the difference in the water quality following the treatment process.

Results summary

Environmental testing and ongoing monitoring of the PACl trials at each of the intakes has been conducted on a regular basis by the Te Mato Vai project team.

Key findings:

  • Results collected and shared with landowners to date show a significant increase in the water quality overall

  • The turbidity, or the muddiness of the water, has been reduced through the PACl treatment process

  • There has been a good reduction of E.coli in the water through the settling tank and AVG filter (however there is still E.coli in the water as a disinfection has not yet been implemented)

  • Almost all dissolved aluminium concentrations in the treated water are below 0.1 mg/L, and none were measured above our target value of 0.2 mg/L

  • Our sampling in the streams suggests that the occasional discharges of AVG backwash water from the ponds to the stream do not raise the dissolved aluminium concentrations above the naturally occurring background level. See further details regarding these readings below.

  • As part of the desludging process we have observed a significant reduction in the volume of PACl sludge in the ponds once the residual water has evaporated and been absorbed by the ground

  • The remaining solid sludge volumes noted to date suggest the total volume of sludge that will be generated at each intake per year is far below what has been estimated by an independent reviewer.

Aluminium concentrations in the streams

On occasions where dissolved aluminium readings in the streams have been measured as above 0.055 mg/L (our target value for the streams), this is due to existing elevated readings of dissolved aluminium that are naturally occurring in the water upstream following heavy rains and/or storm events.

Our project goal is to ensure that the occasional discharges from the treatment plants to the streams do not cause harm to the environment. As a result, our monitoring has alternatively been focused on ensuring that the discharge does not increase the level of dissolved aluminium above the natural upstream level, and this has been achieved.

It is important to note that the September 2020 Te Mato Vai Ecology Baseline Report concludes that dissolved aluminium levels of up to 0.290 mg/L or greater (potentially up to 0.630 mg/L) would not result in environmental harm. Natural occurring dissolved aluminum levels have been measured to date up to 0.170 mg/L following rainfall events.

Next steps

The PACl trials at each of the intakes will continue into early-mid 2021.

Further results from environmental testing and monitoring will be shared with landowners, the Government and the wider community as we progress, with a final report to be developed and submitted to the Government at the conclusion of the trials.

The disinfection method is still to be determined and no chlorine has been added to the drinking water supply during this stage. Community members are reminded to continue to boil water prior to drinking.

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