/ Water Safety Plan Guide
Treatment Processes
– Chlorine Dioxide Disinfection
Version 1, Ref P7.2
January 2014

Citation: Ministry of Health. 2014. Water Safety Plan Guide: Treatment Processes – Chlorine Dioxide Disinfection, Version 1, ref p7.2. Wellington: Ministry of Health.

Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand

ISBN: 978-0-478-42742-4 (print)
ISBN: 978-0-478-42743-1 (online)

Previously published in 2001 as Public Health Risk Management Plan Guide: Treatment Processes – Chlorine Dioxide Disinfection, Version 1, ref p7.2. This publication’s title and any reference within the text to ‘public health risk management plan’ were changed in January 2014 to reflect the December 2013 legislation change of the term ‘public health risk management plan’ to ‘water safety plan’. No other changes have been made to this document.

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Contents

Introduction

Risk Summary

Risk Information Table

Contingency Plans

PHRMP Performance Assessment

Ref P7.2Water Safety Plan Guide: 1

Version 1, January 2014Treatment Processes – Chlorine Dioxide Disinfection

Ref P7.2Water Safety Plan Guide: 1

Version 1, January 2014Treatment Processes – Chlorine Dioxide Disinfection

Introduction

Chlorine dioxide (ClO2) treatment is used to disinfect, or oxidise contaminants in, drinking-water. This Guide is concerned only with using chlorine dioxide as a disinfectant.

If an event occurs during chlorine dioxide treatment (ie, the treatment process doesn’t work properly), the following could happen:

  • If there is not enough chlorine dioxide, germs can cause sickness
  • If there is too much chlorine dioxide, sickness can come from either the high chlorine dioxide concentration or from chlorine dioxide by-products
  • High concentrations of by-products from chlorine dioxide treatment can cause sickness, even when the chlorine dioxide levels are acceptable.

The on-site generation and use of chlorine dioxide can present risks to the health of treatment plant staff. These are acknowledged, but are not discussed further as such risks are the subject of health and safety in employment legislation.

The chlorine dioxide treatment process and the risks associated with it cannot be viewed in isolation. This Guide only looks at the introduction of chlorine dioxide into the water. Chlorine dioxide’s value as a disinfectant is also affected by elements of the water supply system dealt with in other Guides.

Several factors influence how effective disinfection is:

  • whether the chlorine dioxide dose is large enough for other substances in the water to react with the chlorine dioxide and still leave enough chlorine dioxide to disinfect the water effectively (discussed in this document)
  • how long the chlorine dioxide is in contact with the water (see Guide D1)
  • water temperature
  • turbidity of the water with the chlorine dioxide is added to it; this can hinder the access of chlorine dioxide to germs (see the S1, P1, P5 and P6 series of Guides).

If disinfection with chlorine dioxide is going to work as well as possible, all these factors have to be taken into consideration.

Treating poor quality water with chlorine dioxide can lead to the formation of high concentrations of chlorite. For this reason, chlorite is always a Priority 2 determinand (see the DWSNZ:2000 for a definition of Priority 2 determinands) when chlorine dioxide treatment is used.

Risk Summary

The event creating the greatest risk involved in the chlorine dioxide treatment of drinking-water is not having enough chlorine dioxide in the water to kill the germs (see P7.2.1).

The most important preventive measures are:

  • monitor the process to be sure there is enough chlorine dioxide in the water, even when the quality of the incoming water changes (see P7.2.1.4)
  • put alarms on the supplies of chemicals used for chlorine dioxide generation to let you know when supplies are running low. Maintain records so you are aware of when this might happen; always have spare supplies on hand (see P7.2.1.6).

(References in parentheses are to the Risk Information Table.)

Risk Information Table

Reliable information about water quality is essential for the proper management of a water supply. Knowledgeable and skilled staff are also essential for minimising the public health risks associated with water supplies. Please read the staff training (Guide G1) and the monitoring guides (Guide G2). While we haven’t pointed out every detail of how these documents are linked with the present document, the links are many and are important.

NB: This water safety plan assumes chlorine dioxide generation through the chlorine-chlorite process.

Abbreviations: DWSNZ – Drinking-Water Standards for New Zealand; MAV – Maximum acceptable value – see DWSNZ:2000

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW
Possible hazards: Germs not killed, chlorite (if generator conversion efficiency of chlorite is low).
Level of risk: High1
P7.2.1.1
Dosing malfunction (see Guide P10). /
  • Routine controller and dosing pump maintenance.
  • Replacement of controller if suspect.
  • Alarm system to warn if ClO2 residual concentration is incorrect.
/
  • ClO2 residual concentration.
  • Microbiological quality.
  • Maintenance log.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Frequent repair needed.
  • Maintenance log not signed off.
/
  • Identify cause of fault and rectify.
  • Manually dose reservoir with chlorine until repaired.
  • Replace controller.

P7.2.1.2
Dosing controller’s sensor incorrectly calibrated. /
  • Regular manual checks on calibration of controller (see DWSNZ:2000 Section 3.3.4.8).
/
  • ClO2 residual concentration.
  • Microbiological quality.
  • Calibration schedule.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Calibration schedule not signed off.
/
  • Recalibrate controller sensor.
  • Increase ClO2 dose rate until recalibration undertaken.

1The consequences of the event, and therefore the level of risk, will be influenced by the quality of the source water and the effectiveness of treatment processes prior to chlorine dioxide treatment.

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d
P7.2.1.3
Dosing controller’s set-point incorrect or incorrect dose calculation. /
  • Periodic manual checks on ClO2 residual concentration, especially during periods of water quality variability.
  • Independent check on calculations (especially after a system change when expected dose rates are uncertain).
  • Install visual flow indicators to allow flow to be checked by eye.
/
  • ClO2 residual concentration.
  • Flow rates.
  • Dose flow rates.
  • Dose calculations.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Frequent calculation errors found.
  • Calculation checks not signed off.
  • Flow rates lower than expected.
/
  • Adjust controller set-point.
  • Recalculate dose rates and change settings.
  • Train staff in making dose calculations.

P7.2.1.4
High ClO2 demand2 coupled with poor dose control. /
  • Ensure that monitoring of the ClO2 residual is adequate so that ClO2 dosing can be adjusted to take account of changes in water quality, or use a controller that automatically adjusts the ClO2 dose to maintain a satisfactory residual.
  • Upstream processes removing substances contributing to the ClO2 demand from the water.
  • Ensure ClO2 generator can deliver the maximum required dose rate.
/
  • ClO2 residual concentration.
  • Total organic carbon (TOC) or colour.
/
  • Low ClO2 residual (see Appendix), or E.coli or coliforms detected in 100mL sample of water leaving the treatment plant, onlyduring periods of poor water quality.
  • TOC or colour highly variable.
  • Inadequate ClO2 residual concentration even when generator running at maximum.
/
  • Replace dose controller with more suitable unit.
  • Manual monitoring and manual ClO2 control during poor water quality episodes.
  • Optimise upstream processes to better reduce ClO2 demand.
  • Replace generator with one with sufficient capacity.

P7.2.1.5
Power failure. /
  • Stand-by generator or battery bank.
/
  • Electricity supply.
/
  • Poor continuity of power supply.
/
  • Refuel generator (if appropriate).

2Chlorine dioxide demand is the difference between the amount of chlorine dioxide added to the water and the chlorine dioxide residual remaining after the chlorine dioxide has reacted with other substances in the water. If the chlorine dioxide demand of the water increases without an increase in the dose, too little disinfectant will remain to disinfect the water properly.

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d
P7.2.1.6
ClO2 supply exhausted. /
  • Place alarm on supply of chemicals for ClO2 generation to indicate when a supply is close to running out.
  • Maintain records of ClO2 use to provide a guide to the length of time the chlorine and chlorite supplies are likely to last.
  • Ensure spare supplies of chemicals are always kept on site.
/
  • ClO2 residual concentration.
  • ClO2 usage.
  • Levels of generation chemicals.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Level of ClO2 generation chemicals too low to allow replacement before they run out.
/
  • Install alarm system.
  • Hand-dose chlorine until system can be brought back on line.

P7.2.1.7
ClO2 generator malfunction. /
  • Preventive maintenance programme.
  • Soften the water used to make up the sodium chlorite solution to avoid clogging by scale (if water is hard).
/
  • ClO2 residual concentration.
  • Maintenance log.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms is detected in 100mL sample of water leaving the treatment plant.
  • Frequent repairs needed.
/
  • Identify cause of fault and rectify.
  • Dose with chlorine until system can be brought back on line.
  • Clear scale from lines and valves.

P7.2.1.8
Chlorine flow not reaching generator. / (See Guide P7.1 for preventive measures, checks and corrective actions.)
P7.2.1.9
ClO2 generation chemistry not optimised (wrong chlorine and chlorite ratio, concentrations, or pH); chemicals of unsatisfactory quality; or wrong chemicals used. /
  • Carry out pilot tests to determine optimum chemical feed rates for ClO2 production.
  • Check chemical flows are correctly set; pH set point is correct.
  • Ensure chemicals are of adequate quality. Obtain manufacturer’s certification.
  • Ensure chemical storage containers are properly labelled and an operator is present to supervise delivery.
/
  • ClO2 residual concentration.
  • Supplier’s certificate of analysis for chemicals used.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Unsatisfactory chemical quality.
/
  • Modify chemical feed rates until ClO2 production maximised.
  • Change chemical supplier.
  • Require chemical supplier to only deliver when the operator is present.

Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d
P7.2.1.10
ClO2 supply adequate, but insufficient ClO2 reaching dosing point. /
  • Routine maintenance of pumps dosing ClO2 solution.
  • Pump capacity too low to meet maximum ClO2 demand.
/
  • ClO2 residual concentration.
/
  • Low ClO2 residual (see Appendix).
  • Injector blockages.
  • Pump failure.
  • Inadequate pump specifications.
/
  • Identify and rectify causes of pump failure.
  • Obtain a new pump.

P7.2.1.11
Exposure to sunlight resulting in photodecomposition of ClO2. /
  • Ensure ClO2 dosingsolution is protected from sunlight.
/
  • ClO2 residual concentration.
/
  • Low ClO2 residual (see Appendix).
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
/
  • Cover open channels or reticulate in pipes.

P7.2.1.12
ClO2 monitoring samples taken incorrectly or incorrectly recorded (see Guide D4). /
  • Provide staff training for sample analysis and record keeping.
  • Develop monitoring schedule and roster.
/
  • Analysis records.
/
  • Independent checks show monitoring inaccuracies.
/
  • Identify staff training needs and provide training.

P7.2.1.13
Method of ClO2 measurement incorrect, incorrectly calibrated, or reagents used in analysis have deteriorated. /
  • Provide staff training for sample analysis and record keeping.
/
  • Analysis records.
/
  • Independent checks show monitoring inaccuracies.
/
  • Identify staff training needs and provide training.

Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO HIGH
Possible hazards: Chlorite, chlorate, chlorine dioxide.
Level of risk: Low–moderate
P7.2.2.1
Dosing malfunction (see Guide P10). /
  • Routine controller and dosing pumps maintenance.
  • Replacement of controller if suspect.
  • Fit an alarm to indicated incorrect CIO2 concentration.
/
  • ClO2 residual concentration.
  • Maintenance log.
/
  • ClO2 residual concentration more than 50% of its MAV.
  • Frequent repair needed.
  • Maintenance log not signed off.
/
  • Identify cause of fault and rectify.
  • Replace controller with new unit.

P7.2.2.2
Dosing controller’s sensor incorrectly calibrated. /
  • Regular manual checks on calibration of controller (see DWSNZ:2000 Section 3.3.4.8).
/
  • ClO2 residual concentration.
/
  • ClO2 residual concentration more than 50% of its MAV.
  • Calibration schedule not signed off.
/
  • Recalibrate controller sensor.
  • Manually monitor and adjust ClO2 dose rate until recalibration undertaken.

Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO HIGH cont’d
P7.2.2.3
Dosing controller’s set-point incorrect or incorrect dose calculation. /
  • Periodic manual checks on ClO2 concentration, especially during periods of water quality variability.
  • Independent check on calculations (especially after a system change when expected dose rates are uncertain).
  • Install visual flow indicators to allow flow to be checked by eye.
/
  • ClO2 residual concentration.
  • Flow rates.
  • Dose calculations.
/
  • ClO2 residual concentration more than 50% of its MAV.
  • Frequent calculation errors found by checks.
  • Calculation checks not signed off.
  • Flow rates below expected values.
/
  • Adjust controller set-point.
  • Recalculate dose rates and change settings.
  • Train staff in making dose calculations.

P7.2.2.4
Low ClO2 demand coupled with poor ClO2 dose control. /
  • Use a control method that links dose control to ClO2 residual at appropriate location.
/
  • ClO2.
/
  • ClO2 residual concentration more than 50% of its MAV.
/
  • Replace dose controller with more suitable unit.
  • Manual monitoring and manual ClO2 control when water quality is variable.

P7.2.2.5
ClO2 solution strength from generator too high. /
  • Check that a satisfactory ClO2 concentration is produced when new supply of chemicals for generation is first brought into use.
/
  • ClO2.
  • ClO2 level in the ClO2 solution.
/
  • ClO2 residual concentration more than 50% of its MAV.
  • ClO2 concentration in dosing solution is too high.
/
  • Determine the cause of the high ClO2 concentration and rectify.
  • Provide training in the preparation of solutions (including calculations).

Event: EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT
Possible hazards: Chlorite, chlorate.
Level of risk: Moderate
P7.2.3.1
ClO2 generation chemistry not optimised (wrong chlorine and chlorite ratio, concentrations, or pH). /
  • Carry out pilot tests to determine optimum chemical feed rates for ClO2 production.
  • Checks to ensure chlorite solution concentration is correct.
  • Provide staff training in chlorite solution preparation.
/
  • Chlorite.
  • Chlorate.
/
  • Elevated levels of chlorite and chlorate.
/
  • Modify chemical feed rates until ClO2 production maximised, and by-products minimised.
  • Identify staff training needs and provide training.

Event: EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT cont’d
P7.2.3.2
Natural organic matter present in the water being chlorinated (reduction of ClO2 to chlorite). /
  • Provision of treatment processes upstream to reduce levels of organic matter in the water.
/
  • TOC/colour.
/
  • Elevated TOC/colour.
  • Elevated chlorite formation.
/
  • Optimise treatment parameters in upstream processes to maximise removal of organic matter.

Contingency Plans

If an event happens despite preventive and corrective actions you have taken, you may need to consult with the Medical Officer of Health to assess how serious a problem is.

Event – ClO2 concentration is lower than minimum effective level
Indicators: /
  • A detectable ClO2residual cannot be obtained in the water leaving the treatment plant.
  • In 100 ml samples of water leaving the treatment plant, E. coli or coliforms are continually detectable, or E. coli is present at elevated levels (more than 10 per 100 mL).
  • Widespread illness in the community.

Required actions: /
  • Follow the actions given in Figure 3.2 of the DWSNZ:2000.
  • Identify the reason for the failure and rectify.
  • Record cause of system failure and steps taken to correct.
  • Modify your water safety plan if necessary.

Responsibility: / Manager designated responsible for the water supply.
Event – ClO2concentration is higher than maximum acceptable value
Indicators: /
  • Knowledge of a major overdose of ClO2 into the water.
  • Inability to obtain pink colour from DPD indicator despite high ClO2 dose rates. (NB: This indicates ClO2 levels well in excess of the MAV – very high ClO2 levels bleach the pink colour that normally develops in the presence of ClO2.)
  • Change in the odour or taste of the water.
  • Widespread levels of taste and odour complaints, or illness, in the community.

Required actions: /
  • Close down the plant. Provide another source of potable water until water of acceptable quality can again be supplied.
  • Inform the MOH of the situation.
  • Identify the reason for the ClO2overdose and rectify.
  • Dump the reservoir water, or add chemicals to neutralise the ClO2 if more appropriate (neutralisation may be required as a condition of the Resource Consent, anyway).
  • Flush the distribution system, if excessive levels of ClO2 are also present in the distribution system, and monitor water quality until ClO2 concentrations are again back to normal operating levels.
  • Warn consumers to thoroughly flush their taps before drawing water for use (if they are likely to have been affected).
  • Record cause of system failure and steps taken to correct.
  • Modify your water safety plan if necessary.

Responsibility: / Manager designated responsible for the water supply.
Event – Chlorite or chlorate concentrations higher than maximum acceptable value (MAV)
Indicators: /
  • Chlorite concentration more than 0.3 mg/L.
  • Chlorate concentration more than 0.3 mg/L.

Required actions: /
  • Decrease the ClO2 dose, and monitor E. coli and coliforms to ensure that reduction in the ClO2 has not led to inadequate disinfection. The ClO2 dose should still meet the C.t requirements for Cryptosporidium inactivation (see Appendix P7.2).
  • Inform the MOH of the situation, and determine what options for treatment changes could be considered. Supplementary use of chlorine may provide satisfactory disinfection.
  • Record the steps taken to correct the problem.
  • Modify your water safety plan if necessary.

Responsibility: / Manager designated responsible for the water supply.

Water Safety Plan Performance Assessment

To ensure that your supply’s water safety plan (formerly known as a Public Health Risk Management Plan, PHRMP) is working properly, periodic checks are needed. The overview document outlines what needs to be done. The following table provides the detailed information for checking this particular supply element.