A1.8 Chlorination as an example of a critical control point
Disinfection is designed to kill pathogenic microorganisms, thereby preventing waterborne diseases. Chlorination is the most commonly used process for disinfection; it is effective in killing bacteria and can be reasonably effective in inactivating viruses (depending on type) and most protozoa, including Giardia. Cryptosporidium is not inactivated by the concentrations of chlorine that can be safely used in drinking water.
Although the microbial quality of drinking water is of primary importance and must never be compromised, chlorine levels and the formation of chlorination byproducts should be controlled to prevent any adverse health effects that may eventually be found to be attributable to disinfection byproducts.
The effectiveness of chlorination depends on several factors, including:
chlorine dose
contact time between chlorine and the water
chlorine demand
pH
temperature
turbidity.
Chlorine demand is important because it is the chlorine residual in the water and not the chlorine dose that determines the efficacy of chlorination. Natural water contains inorganic and organic compounds that react with chlorine. Reactions with naturally occurring organic matter produce chlorination byproducts, the most well known being the trihalomethanes. Chlorine may also react with compounds such as phenols to impart a taste and odour to water.
A sufficient chlorine dose must therefore be added to the water to allow for the chlorine demand reactions to occur, and to ensure that there is an adequate free chlorine residual available to disinfect the water effectively. Turbidity should be reduced as much as possible before the addition of the disinfectant in order to decrease the chlorine demand, limit shielding of microorganisms in particles and reduce the formation potential of chlorination byproducts.
Chlorination fulfils the requirements of a critical control point. The effectiveness of eliminating potentially harmful microorganisms is validated by extensive research and technical literature (e.g. see USEPA 1999). In addition, process control measures are readily available. Chlorination must be functional and effective at all times, as even short periods of suboptimal performance can represent a serious risk to public health.
Table A1.11 and the following text provide a summary of the chlorination process as a critical control point.
Table A1.11 Chlorination as a critical control point
Process controls
Effective operation of chlorination requires consideration of several associated process control measures. These include:
Chlorine dosing system, ideally with flow-proportional automatic dosing and feedback loops to achieve target chlorine residual and provide rapid responses to any changes in flow and water quality. Flow meters and alarms should be provided on the chlorine feed system to warn of disinfectant loss.
Plant flow-rate control and the design and operation of the clear-well or post-treatment reservoir (whichever is used to provide an adequate contact time). The infrastructure for chlorination should be of sufficient capacity to handle maximum flow rates and should not be hydraulically overloaded or subjected to rapid changes in hydraulic loading, as these conditions will compromise its effectiveness.
pH adjustment for supplies where sudden large changes of pH are known to occur (e.g. due to problems arising from chemical dosing with lime, permanganate, caustic soda etc).
Provision of an alarm system on the chlorine supply, to indicate when the supply is running low, and of a spare or surplus chlorine supply. Chemical suppliers should be evaluated and selected on their ability to supply product in accordance with required specifications.
Inspection, calibration and maintenance of equipment to ensure continuing process capability and accuracy of monitoring results.
Emergency measures such as backup generators, alarms and duplicate facilities (e.g. chlorinator, disinfectant feed system, pumps, monitoring equipment etc) to avoid loss of disinfection if failure occurs.
Operational monitoring
Operational parameters
It is essential to monitor residual chlorine concentration, flow rate (contact time), chlorine dose, pH, temperature and turbidity to determine whether water is being disinfected properly. Total coliforms and heterotrophic bacteria can also be used.
For processes such as disinfection, where failure can result in a rapid change in water quality and pose a significant health risk, monitoring should be online and continuous to provide an immediate indication of performance. Flow measurement and chlorine residual can be monitored online and continuously with feedback loops to ensure correct conditions are met. For supplies where sudden changes of pH are known to occur, continuous monitoring of this parameter should also be considered. Alarm systems that are monitored 24 hours a day should be installed to indicate when operational criteria have not been met.
Critical limits and target criteria
Operational criteria for chlorination are normally determined by calculating the C.t values required to attain target levels of pathogen inactivation at specified temperatures and pH. C.t is the product of residual chlorine concentration in mg/L and the contact time in minutes.
Free chlorine residuals and C.t values should be validated for individual water supplies. Tables of C.t values for various temperatures and pHs for the inactivation of Giardia and viruses by free chlorine and other disinfectants have been published (e.g. see Table A1.12).
Ongoing compliance with minimum C.t values should be confirmed.
Table A1.12 C.t values for inactivation by free chlorine (mg.min/L)
Giardia
7.0
75
37
150
74
8.0
108
41
216
108
Viruses
7.0
4
2
6
3
Source: USEPA (1999); Keegan et al. 2012; WaterVal 2017
Corrective action
Corrective action taken in response to target criteria or critical limits not being met could include:
examination of the chlorination process (investigate equipment)
adjustment of flow rate to increase detention time
adjustment of pH
recalculation of C.t values
adjustment of disinfectant dose rates
variation of the disinfection application point
verification of chlorine dose solution
increased sampling, verification of operational monitoring
inspection and calibration of equipment
engagement of backup chlorination equipment
secondary disinfection, spot dose or booster disinfection
water diversion or reliance on alternate supply (storage)
shutdown of plant, automatic immediate shutdown
implementation of an emergency response plan (e.g. issuing advice to boil water).
Verification
The chlorination process should be verified by supplementing with:
regular calibration and maintenance of the chlorine dose and monitoring equipment to ensure continuing process capability and accuracy of monitoring results. Procedures, schedules, responsibilities and records (maintenance logs) for the calibration and maintenance of equipment should be documented
routine sampling and testing of E. coli (or thermotolerant coliforms) in the distribution system and as supplied to consumers
monitoring of consumer comments and complaints regarding chlorine taste and odour
performance evaluation and operational audit to confirm that objectives are being met. This entails the periodic review of operational monitoring, drinking water quality monitoring data and consumer satisfaction, logbook records of planned and unplanned maintenance and calibration, and operating procedures, including microbial health-based targets.