1.2: Overview of disinfection

Disinfection of public water supplies commenced early in the 20th century and together with filtration is credited with substantial reductions in waterborne disease (Cutler and Miller 2005, CDC 1999). Most Australian public water supplies are disinfected and it is recognised as a fundamental barrier to microbial contamination.

Disinfection may be practiced alone in circumstances where the source water is of a high quality or as the final step in a water treatment process that utilises multiple barriers. The decision to use disinfection alone, or in combination with other water treatment processes, is determined by a system-specific risk assessment, which includes an assessment of the quality of the source water. In most cases disinfection will be used as part of a multiple barrier approach to the production of safe drinking water.

In all cases disinfection should be treated as a critical control point (CCP) (see section 3.3.2 for more information on CCPs). This means the failure of the disinfection process may lead to the water being unsafe to drink.

Common agents used to disinfect water include chlorine, chloramine, chlorine dioxide, ozone and ultra violet (UV) light. Information sheets 1.3 - 1.7 describe these disinfectants. This information sheet on disinfection should be read in conjunction with the relevant fact sheets for each disinfectant, which discuss the chemical by-products produced by each disinfectant.

Properties of an ideal disinfectant

An ideal disinfectant should:

• Effectively inactivate pathogenic micro-organisms over a wide range of physical and chemical conditions;

• Be able to have its efficacy continuously monitored;

• Provide ongoing protection from pathogenic micro-organisms that may be introduced into the distribution system;

• Provide a disinfectant residual that is stable and easily measured in the field;

• Produce minimal levels of by-products;

• Be readily available, safe to handle and suitable for widespread use;

• Not degrade in quality or strength during storage; and

• Be affordable (in terms of both capital and operating costs).

No single disinfectant currently meets all these criteria. Despite the limitations, disinfectants are an essential step and widely used in the production of safe drinking water. Choosing the appropriate disinfectant is an important decision for effective drinking water quality management in any water supply system. Factors which influence that decision include:

• The quality of the source water being treated including:

  • The type of micro-organisms present (e.g. pathogenic bacteria, viruses, and/or protozoa)

  • The type and concentration of organic matter present in the source water

  • Propensity of the source water to form disinfection by-products

  • Turbidity, colour, UV absorbance, pH, alkalinity and water temperature (i.e., factors that affect disinfectant effectiveness)

• The characteristics of the water supply system including:

  • Volume and dose rate of disinfectant required to maintain effective disinfection

  • The type and concentration of organic matter present in the treated water

  • Propensity of the treated water to form disinfection by-products

  • Retention time within the distribution system

  • Pipe condition

  • Size and complexity of the distribution system

• Specific local circumstances such as:

  • Personnel safety

  • Public safety

  • Supply logistics

  • Technical capacity of staff to operate the disinfection facility

  • Technical support for operators to operate and maintain the disinfection facility

  • Cost

Effectiveness of Disinfection

Determining the effectiveness of chemical disinfectants

The C.t concept describes the relative effectiveness of a specific aqueous disinfectant against different microorganisms under specified conditions. It is determined by multiplying the concentration or residual of the disinfectant (in mg/L) by the contact time (in minutes). The C.t concept is expressed mathematically as:

where:	k = Cn.t C = concentration of residual disinfectant n = constant (also called the coefficient of dilution) t = contact time required for a fixed per cent of inactivation k= constant for a specific microorganism exposed under set conditions.

Reported values for “n” range from 0.5 to 1.8 for most aqueous disinfectants. Generally, however, “n” approximates 1, and the equation is simplified to k = C.t.

C.t values for specific organisms exposed to particular disinfectants can be calculated. A low C.t value indicates a strong primary disinfectant.

Comparative effectiveness of disinfection, based on the C.t concept, for the four major disinfectants for a range of micro-organisms, are given in Information Sheets 1.3 to 1.6. The figures presented represent published C.t values that achieve 99% (or 2 log) inactivation of the target microorganism. In summary, the published C.t values show that ozone and chlorine dioxide are very effective at inactivating most microorganisms. Chlorine is effective at inactivating bacteria and viruses, but is less effective against Giardia, and not effective at inactivating Cryptosporidium at a C.t value that could be applied to a drinking water supply. Chloramine requires a considerably higher C.t value than chlorine to inactivate bacteria and viruses; it is ineffective against Cryptosporidium and Giardia at C.t values that could be applied to a drinking water supply.

The relative merits of various disinfectants, and ultraviolet light (UV), are summarised in Table IS1.2.1.

Table IS1.2.1 Applicability of disinfection techniques to different situations

Consideration	Chlorine	Chloramination	Ozone	Chlorine dioxide	Ultraviolet light
Relative complexity of technology	Simple to Moderate	Simple to Moderate	Complex	Moderate	Simple to Moderate
Safety Concerns	Yes	Yes	Yes	Yes	Minimal
Bactericidal	Good	Good	Good	Good	Good
Virucidal	Moderate	Poor	Good	Good	Good
Protozocidal	Poor	Poor	Good	Moderate	Good
By-products of possible health concern	Yes	Yes	Yes	Yes	No
Persistent residual	Moderate	Long	None	Moderate	None
Contact Time	Moderate	Long	Short	Moderate	Short
pH dependent	Yes	Yes	Slightly	Slightly	No
Process control	Well developed	Well developed	Developed	Developed	Well developed

Where contact tanks or clear water storages are used to achieve the desired contact time, the T₁₀ contact time needs to be taken into consideration (Church and Colton 2013; USEPA 2003). The T₁₀ contact time is the minimum detention time experienced by 90 percent of the water passing through the tank, and is based on a baffling factor or tracer studies (Church and Colton 2013).

Determining the effectiveness of ultraviolet disinfection

The C.t concept does not apply to UV disinfection. UV disinfection relies on exposure of pathogens to UV irradiation, measured as UV fluence or UV dose, in $\text{mJ/cm}^2$. Different pathogens respond differently to UV irradiation so the target UV dose is typically selected based on the main pathogen/s of concern for a given source water.

The UV dose achieved by a given unit is a function of the following:

• UV transmittance (UVT %), i.e. water quality;

• lamp power (Watts); and

• exposure time (which is typically related to flow rate).

Wherever possible, a validated UV system should be used, and preferably those systems that have been validated in accordance with the requirements of the USEPA Ultraviolet disinfection guidance manual for the final long term 2 enhanced surface water treatment rule (UVDGM) (2006). Other validation processes for UV systems also exist (DVGW, 2006a, 2006b, 2006c; ONORM, 2001, 2003; and NWRI, 2012).

The monitoring, operation and maintenance requirements that must be applied to ensure the ongoing performance of the UV unit at the required dose, as detailed in USEPA UV Disinfection Guidance Manual (2006) include:

• flow, UVT and lamp power set points;

• maintenance and calibration of essential UVT or UV intensity instrumentation; and

• UV lamp monitoring, cleaning and replacement.

Finally, it should be recognised that energy consumption is a significant contributor to operating cost for UV disinfection systems, particularly large scale systems. Consequently, the accurate and efficient setting and control of the UV dose is important to avoid any undue overdose and therefore energy wastage.

Ensuring the effectiveness of disinfection

As one of the most important processes in assuring safe drinking water, disinfection will always be a CCP (see section 3.3.2). This means the disinfection process should be continuously monitored to provide assurance that it is functioning correctly (see Appendices A1.7 and A1.8).

For chlorination, chloramination and chlorine dioxide this is best achieved by permanent online chlorine residual analysers installed downstream of disinfection.

With disinfection processes that do not provide a residual, other parameters can be monitored to confirm disinfection effectiveness, e.g. the intensity of UV dose can be monitored for UV performance. A key consideration is ensuring that effective disinfection is achieved under the most extreme operating conditions of maximum flows and minimum detention times.

Given that disinfection will be a CCP, other important issues that will need to be considered to ensure the effectiveness of the disinfection process are:

• establishing target criteria and critical limits for the disinfection process (section 3.4.2);

• preparing and implementing operational procedures (section 3.4.1) and operational monitoring (section 3.4.2) for the process;

• preparing corrective action procedures (section 3.4.3) in the event that there are excursions in the operational parameters; and

• undertaking employee training (section 3.7.2) to ensure that the treatment process operates to the established target criteria and critical limits.

It is important to note that when chlorine, chloramine, ozone and chlorine dioxide are used as disinfectants, if the disinfectant comes into contact with organic matter a range of disinfection by-products (DBP) can be formed. The by-products produced by individual disinfectants, and their potential health significance, are described in the Fact Sheets for each disinfectant.

It is also important to note that the possible presence of microbial contaminants in drinking water poses a greater risk to public health than the possible presence of DBP. Therefore, disinfection should not be compromised in order to control DBP.

Factors affecting disinfection

Disinfection processes, and their associated C.t values, are affected by a range of external factors, such as the pH and temperature of the water. Particle shielding can also reduce disinfection.

Water quality operators and managers need to understand the factors which can adversely impact on effectiveness of the disinfection process and monitor these parameters, in order to achieve effective disinfection.

The table below summarises the parameters which might impact on the effectiveness of disinfection which should be monitored for common disinfectants.

Disinfectant Parameters which may impact the effectiveness of disinfectants

Chlorine	Turbidity, pH, temperature
Chloramine	Turbidity, pH, temperature
Chlorine dioxide	Turbidity, pH, temperature
Ozone	Turbidity, pH, temperature
Ultraviolet irradiation	Turbidity, colour, iron and UV absorbance or UV transmissivity (UVT)

Ideally these parameters should be monitored continuously, especially for source waters where water quality can change rapidly. If source waters are very stable (such as confined groundwater) it may be acceptable to monitor with regular grab samples.

The table above does not include all the operational parameters which should be monitored to demonstrate the effectiveness of disinfection process. Further details can be found in the Information Sheet for each disinfectant.

If the target criterion for an operational parameter is breached, it does not necessarily mean that disinfection has been compromised. In such instances the water utility needs to:

• Validate that disinfection is still effective, specifically in the circumstance where supply has to be maintained during the period that the operational parameter is outside the target range;

• Where possible, use an alternative source water;

• Where possible, only take raw water that is within, or has returned to within, the target range for the water treatment processes that are being used; and

• Verify that the water within the distribution system is still of a satisfactory quality.

Disinfection and residual management

When operating a distribution system it is important to understand the difference between effective disinfection and maintaining a disinfectant residual. The water is effectively disinfected when the required C.t value has been achieved. After effective disinfection, enteric pathogens should not reappear within the distribution system, unless there is a failure in the integrity of the system. Therefore, unless there is a barrier breach within the distribution system the water should remain safe to drink even in the absence of adequate disinfectant residual. Barrier breaches could include such things as ingress, backflow, loss of pressure within the distribution system, or contamination within post-treatment storage tanks.

Operationally, a sudden loss of chlorine residual within the distribution system can also warn of an unusual event or potential contamination, such as backflow, within the distribution system.

Most Australian water utilities use chlorine as their disinfectant of choice with C.t target of at least 15 mg/L.min which is consistent with the World Health Organization’s recommendation that effective disinfection can generally be achieved by maintaining a free chlorine concentration of 0.5 mg/L for 30 minutes (WHO, 2011). Based on data contained in Information Sheet 1.3 a C.t of 15 mg/L.min should achieve effective inactivation of enteric bacteria and viruses. With chlorinated/chloraminated supplies, it is common practice in Australia to endeavour to maintain adequate chlorine residual throughout the entire distribution system. Maintaining chlorine residual provides protection from backflow, ingress and Naegleria, and helps inhibit biofilm growth.

Where the aim is to maintain adequate disinfectant residual across the entire distribution system, the target chlorine residual set at the chlorinator will be based on achieving a desired residual concentration through most of the distribution system, including, where possible, the extremities of the system, after allowing for chlorine decay. As most chlorinators are designed to achieve a set residual to ensure a minimum C.t near the point of dosing the resultant C.t values will increase with longer contact time in the distribution system achieving a margin of safety.

The chlorine decay from the point of disinfection to the extremity of the distribution system is influenced by many factors including:

• Water travel time/age

• Chlorine demand of the water being disinfected

• Water temperature

• Biofilm growth

• Asset condition of pipes and storages

All of these factors are dynamic. It is necessary to monitor the chlorine residual frequently within the distribution system, to enable adjustments of the chlorinator dosing rate to achieve the desired residual within the system.

If maintaining a consistent residual is difficult to achieve, or chlorine demand changes frequently, the water supplier should seek to understand the factors that are driving these changes. Some factors, such as water temperature, may be outside the water supplier’s control. However, other factors which may contribute to a sudden increase in chlorine demand (e.g. algal growth) may indicate an adverse event in the source water.

Whilst the absence of sufficient disinfectant residual does not necessarily mean the water is unsafe to drink, if the desired chlorine residual is not being achieved then corrective action needs to be taken to ensure the target value is achieved.

Managing water supplies with no residual

When used as disinfectants, ozone and UV light do not produce residuals within the distribution system. As discussed in the previous section, after effective disinfection, enteric pathogens should not reappear within the distribution system, unless there is a failure in the integrity of the system. In systems which lack residual disinfection, there is an increased need to ensure the integrity of the system to compensate for the absence of a residual.

In the Australian context, distribution systems that are suited to operating with no residual are those which have high quality source water (e.g. groundwater from a confined aquifer), or are small distribution systems, with a low water age within the distribution system, and which do not have embedded post-treatment storage tanks.

The attractions of operating distribution systems with no residual disinfection are that costs associated with maintaining residual disinfection are avoided, there are no issues with disinfection by-products, and it addresses the concerns of some consumers in relation to the use of chlorine. The main issues with no residual are that if the integrity of system is breached, there is no effective barrier to microbial contamination, and there can be additional costs involved with the cleaning and disinfection of the distribution system.

References

Austrian Standards Institute (ÖNORM) (2001). ÖNORM M 5873-1: Plants for the disinfection of water using ultraviolet radiation: Requirements and testing. Part 1. Low pressure mercury lamp plants. Österreichisches Normungsinstitut, A-1021 Vienna, Austria.

Austrian Standards Institute (ÖNORM) (2003). ÖNORM M 5873-2 Plants for the disinfection of water using ultraviolet radiation: Requirements and testing. Part 2. Medium pressure mercury lamp plants. Österreichisches Normungsinstitut, A-1021 Vienna, Austria.

Church J. and Colton J. (2013). Optimise chlorine contact Tank performance, WaterWorks, May 2013, 10-13, Water Industry Operators Association of Australia (WIOA), Shepparton, Australia.

Cutler D. and Miller G. (2005). The role of public health improvements in health advances: the twentieth-century United States. Demography 42:1-22.

German Association for Gas and Water (DVGW) (2006a). UV Devices for Disinfection in the Water Supply Part 1: Requirements Related to Composition, Function and Operation, in Technical Rule, Code of Practice W294-1. Deutsche Vereinigung des Gas- und Wasserfaches, Bonn, Germany.

German Association for Gas and Water (DVGW) (2006b). UV Devices for Disinfection in the Water Supply Part 2: Testing of Composition, Function and Disinfection Efficiency in Technical Rule, Code of Practice W294-2 Deutsche Vereinigung des Gas- und Wasserfaches, Bonn, Germany.

German Association for Gas and Water (DVGW) (2006c). UV Devices for Disinfection in the Water Supply; Part 3: Measuring Windows and Sensors for the Radiometric Monitoring of UV Disinfection Devices: Requirements, Testing and Calibration in Technical Rule, Code of Practice W 294-3 Deutsche Vereinigung des Gas- und Wasserfaches, Bonn, Germany.

National Water Research Institute (NWRI) (2012). Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse, Third Edition, California, United States of America.

US Centres for Disease Control and Prevention (1999). Morbidity and Mortality Weekly Report. Achievements in Public Health 1900-1999, Control of Infectious Diseases 48 (29) 621-629.

United States Environmental Protection Agency (US EPA) (2003). Guidance Manual for Disinfection Profiling and Benchmarking US EPA Office of Water: Washington D.C.

United States Environmental Protection Agency (US EPA) (2006). Ultraviolet disinfection guidance manual for the final long term 2 enhanced surface water treatment rule. US EPA Office of Water: Washington D.C.

World Health Organization (WHO) (2011). Guidelines for Drinking-water Quality, fourth edition, Geneva, Switzerland.