Why only collecting stream water when the water is running 'clear' is not an option

Updated: Nov 25, 2020

PACl dosing tank

There are good reasons why turbidity detection and diversion systems are not a viable alternative to PACl. The option to bypass the intake during wet weather events is impractical and detrimental for three reasons:

1. PACl dosing is still required for treating ‘clear’ water flows

Even when stream water looks clear, tests have proven it stil contains harmful bacteria. PACl helps remove harmful bacteria and protozoa, from both clear and ‘muddy’ water, as well as removing sediment. This is why PACL is a key step for treating the water.

2. Collecting water when the streams are in high flow (generally during heavy rain) will help prevent public water supply shortages

Rarotonga’s current public water supply system has very little water storage, but the new system will significantly increase storage. Not collecting water while it is raining would limit our ability to collect as much water as possible for the water supply, to minimise water shortages.

3. An effective bypass system would most likely need a permanent power supply at each intake site

A bypass system would likely require the following equipment at each of the 10 unmanned Te Mato Vai sites:

  • An automatic turbidity meter, to measure water clarity. These meters are expensive and require significant maintenance to remain operable.

  • A Programmable Logic Controller (PLC), or similar, to convert turbidity meter readings into action.

  • A power actuated valve, to open and close the system depending on turbidity.

  • A sample pump, to continuously supply the turbidity meter with sample water for reading.

All of the equipment above would require power, which is unlikely to be able to be provided through batteries and/or solar panel installations. Therefore we expect a permanent power would be required at every site.

The cost of establishing a power supply at every intake site has already been calculated in the millions of dollars. This excludes the cost of purchasing and maintaining the additional equipment listed above.

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