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

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

Published in June 2001 by
Ministry of Health
PO Box 5013, Wellington, New Zealand

ISBN: 978-0-478-42740-0 (print)
ISBN: 978-0-478-42741-7 (online)

Previously published in 2001 as Public Health Risk Management Plan Guide: Treatment Processes – Chlorination Disinfection, Version 1, ref p7.1. 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

Water Safety Plan Performance Assessment

Appendix P7.1

Ref P7.1Water Safety Plan Guide: 1

Version 1, January 2014Treatment Processes – Chlorination Disinfection

Ref P7.1Water Safety Plan Guide: 1

Version 1, January 2014Treatment Processes – Chlorination Disinfection

Introduction

Chlorination is used to disinfect, or oxidise contaminants in, drinking-water. This Guide is concerned only with using chlorine as a disinfectant.

Chlorination is usually done at the treatment plant. If there is a problem in keeping the chlorine concentration high enough, possibly because the distribution system is large, the chlorine concentration can be boosted within the system. This process is also considered here.

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

  • If there is not enough free available chlorine (FAC), germs may cause sickness
  • If there is too much FAC, sickness can come from either the high chlorine concentration or from by-products of the chlorination process
  • High concentrations of chlorination by-products can cause sickness, even when FAC levels are acceptable.

Chlorine (either gas, liquid or solid) 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 chlorination process and the risks associated with it cannot be viewed in isolation. This Guide only looks at the chlorination of the water. Chlorine’s value as a disinfectant, and what happens when chlorine is added to water, are also affected by elements of the water supply system dealt with in other Guides.

Several factors influence how effective disinfection is:

  • whether the chlorine dose is large enough for other substances in the water to react with the chlorine and still leave enough FAC to disinfect the water effectively (discussed in this document)
  • how long the FAC is in contact with the water (see Guide D1)
  • pH of the water; this affects how much of the chlorine is in a form that is good at killing germs (see Guide P8.1 and the figure in the Appendix of this Guide)
  • water temperature
  • turbidity of the water when the chlorine is added to it; this can hinder the access of chlorine to target germs (see the S1, P1, P5 and P6 series of Guides).

If the whole chlorination process is going to work as well as possible, all these factors have to be taken into consideration.

Risk Summary

The event creating the greatest risk involved in chlorinating drinking-water is not having enough FAC to kill germs in the water, not only at the beginning of the process but all the way through it (see P7.1.1).

The most important preventive measures are:

  • monitor the process to be sure there is enough FAC in the water, regardless of how the quality of the incoming water might change (see P7.1.1.4)
  • put an alarm on the chlorine supply to let you know when the supply is running low. Maintain records so you are aware of when this might happen; always have a spare supply on hand (see P7.1.1.6)
  • monitor the pH of the treated water; use a properly calibrated pH probe (see P8.1.1.2 and P8.1.1.3).

(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.

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: NOT ENOUGH FREE AVAILABLE CHLORINE
Possible hazards: Germs not killed.
Level of risk: High1
P7.1.1.1
Dosing malfunction (see Guide P10). /
  • Routine maintenance of dose-controller and dosing pump.
/
  • FAC.
  • Microbiological quality.
  • Maintenance log.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
/
  • Identify cause of fault and rectify.
  • Manually dose reservoir with chlorine until controller repaired.

  • Replacement of controller if suspect.
  • Alarm system to warn if FAC concentration incorrect.
/
  • Frequent repairs recorded.
  • Maintenance log not signed off.
/
  • Replace controller.

P7.1.1.2
Dose controller’s sensor incorrectly calibrated. /
  • Regular manual checks on calibration of sensor (see DWSNZ:2000).
/
  • FAC.
  • Microbiological quality.
  • Calibration schedule.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Calibration schedule not signed off.
/
  • Recalibrate controller.
  • Increase chlorine 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 the chlorination process.

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.3
Dose controller’s set-point incorrect, or incorrect dose calculation. /
  • Periodic manual checks on FAC 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 meters to allow chlorine flow to be checked by eye.
/
  • FAC.
  • Microbiological quality.
  • Dose flow rates and dose calculations.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Frequent calculation errors found.
  • Flow rates lower than expected.
/
  • Adjust controller set-point.
  • Recalculate dose rates and change settings.
  • Train staff in making dose calculations.

P7.1.1.4
High chlorine demand2 coupled with poor dose control. /
  • Ensure that monitoring of the FAC residual is adequate so that chlorine dosing can be adjusted to take account of changes in water quality, or use a controller that automatically adjusts the chlorine dose to maintain a satisfactory residual.
  • Use upstream processes that remove substances contributing to the chlorine demand of the water.
  • Ensure chlorinator is designed to provide enough chlorine to meet the maximum required dose.
/
  • FAC.
  • Microbiological quality.
  • Total organic carbon (TOC) or colour level.
/
  • FAC concentration is less than 0.2mg/L or E. coli or coliforms detected in 100mL sample of water leaving the treatment plant during periods of poor water quality.
  • TOC/colour levels highly variable.
  • Inadequate FAC even when chlorinator running at maximum setting.
/
  • Replace dose controller with a more suitable unit.
  • Manual monitoring and manual chlorine control during poor water quality episodes.
  • Optimise upstream processes to reduce chlorine demand.
  • Replace chlorinator with one with sufficient capacity.

P7.1.1.5
Power failure. /
  • Stand-by generator or battery bank.
/
  • Electricity supply.
/
  • Poor continuity of power supply.
/
  • Refuel generator (if one is used).

2Chlorine demand is the difference between the amount of chlorine added to the water and the FAC residual remaining after the chlorine has reacted with other substances in the water. If the chlorine 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: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.6
Chlorine supply exhausted. /
  • Place an alarm on the chlorine supply to indicate supply is close to running out.
  • Maintain records of chlorine use to provide a guide to the length of time the supply is likely to last.
  • Ensure a spare chlorine container is always kept on site.
/
  • FAC.
  • Chlorine level in chlorine supply.
  • Chlorine usage.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Level of chlorine supply drops too low to allow replacement before the container in use runs out.
/
  • Install alarm system.
  • Hand-dose chlorine until system can be brought back on line.

P7.1.1.7
Chlorine concentration in the dosing solution is low because:
  • it has decomposed through the solution being old, or exposed to sunlight
  • the chemical used to prepare the solution is of poor quality.
/
  • Ensure chlorine solutions are kept dark and cool (hypochlorite chlorination).
  • Check that an adequate FAC level is produced when a new chlorine supply is first brought into use.
  • Check quality of salt (brine electrolysis), or chlorinated chemicals used (hypochlorite chlorination).
/
  • FAC.
  • Storage conditions of chlorine solution.
  • Chlorine concentration in solution generated from brine.
  • Supplier’s certificate of analysis of chemicals used to prepare chlorine dosing solution.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
  • Chlorine solution exposed to sunlight and/or allowed to warm.
  • No, or low, chlorine level produced in dosing solution.
  • No chlorine detectable when solution first prepared.
  • Quality of salt inadequate.
/
  • Increase the chlorine dose.
  • Obtain a fresh container of chlorine.
  • Replace chlorine dosing solution.
  • Increase the chlorine dose.
  • Prepare a new brine solution with salt of satisfactory quality.

Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.8
Chlorine supply adequate, but insufficient chlorine reaching dosing point. /
  • Install filters to avoid blockage of valves or venturi by particles in the water.
  • Routine maintenance of pumps (chlorine solution and venturi booster) (see Guide P10).
  • Check quality of salt; insoluble solids may cause blockages (brine electrolysis).
/
  • FAC.
  • Pump maintenance log.
  • Chlorine lines.
  • Gas flow meter readings.
  • Chlorine solution flow meter readings.
  • Supplier’s certificate of analysis of salt.
  • Min-max thermometer readings.
  • Chlorinator dose rate specifications.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
/
  • Check filter and unclog or replace.
  • Identify cause of pump fault and rectify.
  • Prepare fresh brine solution.

  • Ensure that all lines carrying chlorine are properly dried after opening for maintenance to avoid the development of corrosion products and their blocking the gas lines (gas chlorination).
  • Install visual flow meters to allow chlorine flow to be checked by eye.
/
  • Injector and chlorine line blockages.
  • Pump failure.
  • Gas flow readings insufficient to provide adequate dose.
  • Salt contains compounds likely to precipitate and cause blockages.
/
  • Replace chlorine lines and dry before resealing system.
  • Train staff in correct maintenance procedures.

  • Provide indirect heating for drum storage and chlorinator rooms to ensure chlorine is kept gaseous in supply lines (gas chlorination).
/
  • Minimum temperature less than 10C.
/
  • Establish reason for low temperature and rectify.

  • Ensure chlorinator is designed to provide enough chlorine to meet the maximum required dose.
  • Ensure adequate pressure for injector venturi by:
–good hydraulics design
–use of a booster pump. /
  • Maximum chlorine dose rate that can be delivered too low.
/
  • Replace chlorinator with one with sufficient capacity.
  • Establish reason for poor injector pressure and rectify.

Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.9
Inadequate output from chlorine booster stations (if used). /
  • Many of the causes and preventative measures noted in P7.1.1.1–P7.1.1.8 for chlorination at the treatment plant are also applicable to chlorine boosting in the distribution system. Refer to these.
  • Put in place either manual monitoring or in-line monitoring of the FAC residual to provide reliable control of the booster dose rate.
/
  • FAC.
  • Microbiological quality.
/
  • FAC concentration is less than 0.2mg/L.
  • E. coli or coliforms detected in 100mL sample of water leaving the treatment plant.
/
  • Take steps noted in P7.1.1.1–P7.1.1.8, as appropriate.
  • Adjust dose setting.

P7.1.1.10
FAC monitoring samples taken incorrectly or incorrectly recorded (see Guide D4). /
  • Provide staff training for sample analysis and record keeping.
  • Develop monitoring schedule and roster.
/
  • Records of FAC analysis results.
/
  • Monitoring inaccuracies.
/
  • Identify shortcomings in staff training and rectify.

P7.1.1.11
Method of FAC measurement incorrect, incorrectly calibrated, or analysis reagents have deteriorated. /
  • Provide staff training for sample analysis and record keeping.
/
  • Records of FAC analysis results.
/
  • Monitoring inaccuracies.
/
  • Identify shortcomings in staff training and rectify.

P7.1.1.12
pH too high (resulting in a lower percentage of the FAC existing in its more powerful disinfecting form). / For information on causes of pH being too high and preventive measures, see Guide P8.1. See the Appendix for a graph showing how the effectiveness of chlorine as a disinfectant decreases as the pH increases.
Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: TOO MUCH FREE AVAILABLE CHLORINE
Possible hazards: Chlorine; possibly high levels of trihalomethanes, haloacetic acids, chloral hydrate and chlorate (hypochlorite chlorination); possible heavy metals (from corroded fittings).
Level of risk: Low–moderate3
P7.1.2.1
Dosing malfunction (see Guide P10). /
  • Routine maintenance of dose controller and dosing pumps.
  • Replacement of controller if suspect.
  • Install an alarm system to indicate when FAC concentration is outside designated limits.
/
  • FAC.
  • Examination of maintenance log.
/
  • FAC concentration is more than 50% of its MAV.4
  • Maintenance log shows frequent maintenance needed.
  • Maintenance log not signed off.
/
  • Identify cause of fault and rectify.
  • Replace controller with new unit.

P7.1.2.2
Dose controller incorrectly calibrated. /
  • Regular manual checks on calibration of FAC controller.
/
  • FAC.
  • Calibration schedule.
/
  • FAC concentration is more than 50% of its MAV.
  • Calibration schedule not signed off.
/
  • Recalibrate controller.
  • Decrease chlorine dose rate until recalibration.

P7.1.2.3
Dose controller set-point incorrect, or incorrect dose calculation. /
  • Periodic manual checks on FAC 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.
/
  • FAC.
  • Dose flow rates and dose calculations.
/
  • FAC concentration is more than 50% of its MAV.
  • Frequent calculation errors found by checks.
  • Calculation checks not signed off.
  • Flow rates above expected values.
/
  • Adjust controller set-point.
  • Recalculate dose rates and change settings.
  • Train staff in making dose calculations.

P7.1.2.4
Low chlorine demand2 coupled with poor chlorine dose control. /
  • Use a control method that links dose control to FAC residual at appropriate location.
/
  • FAC.
/
  • FAC concentration is more than 50% of its MAV.
/
  • Replace dose controller with more suitable unit.
  • Manual monitoring and manual chlorine control when water quality is variable.

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

3The concentrations of disinfection by-products formed, and therefore the risk they present, will depend on the amount of natural organic matter in the water.

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: TOO MUCH FREE AVAILABLE CHLORINE cont’d
P7.1.2.5
Chlorine dose solution strength too high. /
  • Check that a satisfactory FAC level is produced when a new chlorine container is brought into use.
/
  • FAC.
  • Chlorine level in chlorine dosing solution.
/
  • FAC concentration is more than 50% of its MAV.
  • Chlorine levels in dosing solution too high.
/
  • Determine the cause of the high chlorine concentration and rectify.
  • Provide training in the preparation of chlorine solutions (including calculations).

P7.1.2.6
Spillage of chlorine compound or solution into dosing solution. /
  • Train staff in handling and labelling of chemicals.
/
  • FAC.
  • Chlorine level in chlorine dosing solution.
/
  • FAC concentration is more than 50% of its MAV.
  • Chlorine levels in dosing solution too high.
/
  • Identify the reasons for the spillage and rectify if possible.

P7.1.2.7
Chlorine overdose at a chlorine booster station. /
  • Many of the causes and preventative measures noted in P7.1.2.1–P7.1.2.6 for chlorination at the treatment plant are also applicable to chlorine boosting in the distribution system. Refer to these.
  • Put in place either manual monitoring or in-line monitoring of the FAC residual to provide reliable control of the booster dose rate.
/
  • FAC.
/
  • FAC concentration is more than 50% of its MAV.
/
  • Take steps noted in P7.1.1.1–P7.1.1.8, as appropriate.
  • Adjust dose setting.

Event: EXCESSIVE FORMATION OF CHLORINATION BY-PRODUCTS
Possible hazards: Trihalomethanes, haloacetic acids, chloral hydrate and chlorate (hypochlorite chlorination).
Level of risk: Low–moderate3
P7.1.3.1
Natural organic matter present in the water being chlorinated. /
  • Provision of treatment processes upstream to reduce levels of organic matter in the water.
/
  • TOC/colour.
/
  • Elevated TOC or colour.
  • Elevated disinfection by-product formation.
/
  • Optimise treatment parameters in upstream processes to maximise organic matter removal.

3The concentrations of disinfection by-products formed, and therefore the risk they present, will depend on the amount of natural organic matter in the water.