It was OK Last Month: Results of a Chlorate Survey

Annual Conference

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In 2016, Water New Zealand published an “Interim Good Practice Guidance Note - Supply of Chlorine for Use in Drinking-Water Treatment”. It provides purchasers, manufacturers and suppliers with the minimum physical, chemical and testing requirements for chlorine to meet safe limits for drinking water supplies. The Guidance Note proposes a specific impurity limit (SIL) of 2000 mg/L for chlorate in liquid hypochlorite products, because hypochlorite in solution decomposes to chlorate: a determinand with a provisional maximum acceptable value (pMAV) of 0.8 mg/L. There were concerns over the practical difficulties in supplying liquid hypochlorite products that met the SIL and whether these difficulties might result in unacceptable chlorate concentrations in reticulated drinking-water.

This paper reports the results of a limited survey of chlorate in 25 New Zealand water supply zones undertaken in 2017-18, and funded by the Ministry of Health, to help inform development of the Guidance Note. In most zones, samples were taken from each reticulation on two occasions, and also from hypochlorite dosing solutions.

Chlorate concentrations were variable. The median chlorate concentration measured in reticulated waters was 0.14 mg/L, but 6 of 47 (13%) test results exceeded the pMAV. Only one supply contained chlorate concentrations that exceeded the pMAV on both sampling occasions (1.20 and 0.97 mg/L).

The study concluded that.

  • Potentially health-significant chlorate concentrations do arise in New Zealand’s reticulated waters as the result of the use of hypochlorite.
  • Chlorate concentrations in reticulated water and chlorinating solutions are variable. Chlorinating solutions containing chlorate concentrations less than the SIL of 2000 mg/L are less likely to be associated with chlorate concentrations exceeding 50% of the pMAV in the reticulation.
  • There is evidence that chlorinating solutions prepared by electrolysis, online and as a batch product at low hypochlorite concentration, are less likely to contain chlorate at concentrations exceeding the proposed SIL.
  • Manufacturers and water suppliers share responsibility for taking steps to manage the risk associated with chlorate in water supplies.
  • Several steps have been identified (by other studies) for reducing the chlorate concentration in chlorinating solutions. The most important is dilution of the hypochlorite solution.
  • There are implications for the Guidance Note. The frequency at which manufacturers are required to test chlorate in their product needs to be re-examined, and the survey’s findings do not support raising the proposed SIL.
  • Several factors can contribute to the presence of elevated chlorate (and other oxyhalides) concentrations in drinking water chlorinated using hypochlorite. Water suppliers using hypochlorite need to be aware of these factors and ensure that this risk is managed in their water safety plans. This should include adequate monitoring as a check that their preventive measures are effective.
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    2. It was OK last month.pdf

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    15 Oct 2019

    1430 Nokes_Chris_It was OK last month - results of a chlorate survey.pdf

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    1 MB
    15 Oct 2019