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Drinking Water Standards

Drinking water quality standards describe the quality parameters set for drinking water. 

Many developed countries specify standards to be applied in their own country. Europe has the European Drinking Water Directive, in the USA the US Environmental Protection Agency (EPA) establishes standards as required by the Safe Drinking Water Act. For countries without a legislative or administrative framework for such standards, the World Health Organisation publishes guidelines on the standards that should be achieved.

The WHO Guidelines widely used throughout the world. They have evolved over a period of 50 years from standards to guidelines, reflecting the fact that they have no legal force. The WHO emphasises that it is important that member states should adapt the Guidelines to local circumstances in setting standards, taking into account geographical, social and economic considerations. The Guidelines are based on the best scientific evidence and are adapted through a process of rolling revision to take account of new thinking and new evidence.

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Content Table

What are standards for?

Drinking water standards fulfil several purposes:

1.   To protect public health.

2.   To ensure drinking water is acceptable to consumers.

3.   To provide a benchmark for water supplier operations.

4.   To provide reassurance to consumers that drinking water is safe.

However, to achieve this in a reasonable manner it is important that standards are proportionate and appropriate to cover the primary threats to drinking water, which may vary significantly between countries and regions.

In order for standards to be effective they need to be monitored and enforced. Monitoring itself can impose significant costs and so it is important to ensure that the number of parameters for which monitoring is required is not excessive and that the levels set do not require methods of analysis that are excessively complex without demonstrable need.

WHO Guidelines

This 4th edition of the World Health Organization’s Guidelines for Drinking-water Quality builds on over 50 years of guidance by WHO on drinking-water quality, which has formed an authoritative basis for the setting of national regulations and standards for water safety in support of public health. It is the product of significant revisions to clarify and elaborate on ways of implementing its recommendations of contextual hazard identification and risk management, through the establishment of health-based targets, catchment-to-consumer water safety plans and independent surveillance. 

WHO’s Guidelines have no legal force. WHO emphasise that it is important that member states should adapt the Guidelines to local circumstances in setting standards, taking into account geographical, social and economic considerations. The Guidelines are based on the best scientific evidence and are adapted through a process of rolling revision to take account of new thinking and new evidence. The third edition of the Guidelines introduced a step change in the approach to assuring drinking based on hazard and risk assessment from source to tap with the establishment of barriers to mitigate the risks and operational management procedures to ensure that the barriers are optimised and operating at all times.  This approach is called Water Safety Planning and has been widely introduced and adopted in partnership with IWA.

The 4th edition of the Guidelines further develops concepts, approaches and information introduced in previous editions, including the comprehensive preventive risk management approach for ensuring drinking-water quality that was introduced in the third edition. 

A number of countries develop their drinking water standards from first principles but follow a very similar approach to WHO, although local policy may result in some differences. In addition, standards may differ because of the process of adaptation to local circumstances.

Microbial Contaminants

The greatest risk to health comes from waterborne pathogens that primarily cause gastrointestinal infections. Such infections remain a major cause of morbidity and mortality in many parts of the world, particularly in children. The problem with setting standards for waterborne pathogens is that measuring actual pathogens is very difficult and expensive at this time. However, the vast majority of waterborne pathogens are present as a consequence of faecal contamination from humans or animals. The approach to setting standards has been to measure organisms that are indicators of faecal pollution. The best is Escherichia coli (E.coli), which is not usually a pathogen in its own right, although some strains are, but is always present in faecal matter and is relatively easy to measure. Sometimes a less specific test for faecal coliforms is used and more frequently faecal enterococci are also being used. The standard of no E. coli per 100 ml sample is a measure has proved extremely valuable over many decades. It is not specifically health-based but is a surrogate to demonstrate that no faecal contamination is present and, by inference, no pathogens. Total coliforms are also used as an indicator measure but are not specific for faecal contamination and include organisms that grow naturally in the environment. However, they are still a very useful check for possible ingress of contamination into treated water reservoirs or the distribution system.

Because infection by pathogens is acute, a single exposure of often a small number of organisms can give rise to disease in a susceptible individual, and pathogens are not evenly dispersed through a water supply. It is only possible to obtain a small snapshot of the status of a supply from end of pipe sampling so it is necessary to use a number of supporting operational parameters, such as turbidity and chlorine residual, to ensure the barriers to pathogens are working all of the time. In addition, some pathogens, such as Cryptosporidium, are resistant to chlorine, while the faecal indicators are susceptible. It is, therefore, possible for there to be no faecal indicator present while certain pathogens are. The above are two of the key reasons for the introduction of Water Safety Plans.

Chemical constituents (naturally present) and contaminants are also an issue for drinking water. In contrast to pathogens, only a small number of substances have been shown to cause human health effects through exposure in drinking water. These are arsenic, fluoride nitrate, lead and to a lesser extent selenium and uranium in exceptional circumstances. Lead is present due to lead plumbing materials and connections and through the use of lead solder in plumbing. Arsenic, fluoride, uranium and selenium are naturally occurring and nitrate is present from agriculture, sewage and leaking septic tanks. Other substances are those that might be present in concentrations that are potentially of concern, usually following long-term exposure and so guidance as to safe concentrations are required to help in controlling source contamination and in water supply operations.

Chemicals

The WHO Guidelines only present formal guidelines for chemicals that are health-based. In a small number of cases, there are difficulties in achieving the health-based value and so the guideline may be set at a slightly higher value to reflect this, however, the guideline value is always set so that it still affords a suitable level of protection. This approach also takes advantage of the high level of precaution that is generally built into the health-based values.

In developing guidelines/standards for chemicals there are several factors that influence the outcome.

1.  Scientific knowledge

2.  Quasi-scientific considerations, e.g. uncertainty factors used in deriving tolerable daily intakes(TDI), the models used in deriving risk-based values and the proportion of a TDI allocated to drinking water as opposed to other sources such as food.

3.  Socio-political climate. Public chemophobia drives regulators to be excessively risk-averse.

4.  Practicalities and costs. Can the substance be controlled or is the precaution built into a standard affordable?

It would be easy to set highly precautionary values for a long list of chemicals but with a very high cost and little, if any, health benefit.

The way in which guideline values or standards are derived is broadly as follows.

1.  Human data,  mostly epidemiology but often data on exposure (from all sources) in epidemiological studies is very limited and may not lend itself to the rigorous quantitative analysis required for setting standards.

2.  Animal data allocation from which a TDI is calculated by applying an uncertainty factor to a no effect level or similar level and allocation of a proportion of the TDI to drinking water.  Application of a mathematical model to extrapolate cancer risk from high doses in animal experiments.

3.  Consumer acceptability, a level which will not cause consumers to object to taste, odour or appearance of the water.

There are differences in the policy requirements of differing regulatory authorities that result in differing values based on similar datasets, it is, therefore, important that the basis of standards and guidelines is transparent. This is particularly important in determining the action to be taken in the event of an exceedence of the standard.

Monitoring

In order to determine whether a standard or guideline is being met it is necessary to carry out monitoring. The point at which samples are taken depends on the nature and source of the constituent/contaminant. Pathogens can enter the system at a range of points, although the largest threat is usually from raw water. As indicated above the introduction of WSPs and operational monitoring provides a more comprehensive assessment of the microbiological safety of a supply in the absence of methods of continuous monitoring for pathogens.

Chemicals may be in source water and, if they do not change in distribution, can be monitored immediately post-treatment. If they are stable then monitoring may be relatively infrequent, but some will vary in time, particularly in surface waters, and monitoring may need to be more frequent and targeted to particular periods. Some are formed or introduced during treatment and change in distribution, such as trihalomethanes, and need to be monitored at the end of distribution. Some come from plumbing materials, such as lead and copper and these need to be monitored at the tap. However, the concentration will increase after a period in contact with the pipes or fittings and will vary from building to building so that a great deal of care needs to be applied to the monitoring strategy. Some substances, such as acrylamide are difficult to measure in water at the required levels. A few large water suppliers are able to measure acrylamide in water and can use this for a control mechanism, but for others the standard must be met by specifying the maximum level of acrylamide in polyacrylamide and controlling the dose of the polymer used to maintain levels that meet the standard.

There are also different ways in which results of monitoring may be presented in relation to the standard. In some jurisdictions the standard for chemicals are regarded as a simple maximum, in others results may be presented as an average over a specified time, while yet others may require that results are presented as a ninety-five percentile, reflecting the long-term nature of the standard.

Exceedence of a standard

When a standard or guideline is exceeded, this needs to trigger action to correct the situation. For microbial faecal indicators this requires an immediate investigation and, potentially, involvement of health authorities. At the very least it requires resampling and assessment of operational parameters to determine if there could have been a failure in treatment or ingress into distribution. If the finding is in a tap sample it is important to demonstrate that there is no problem in the main water supply to the property. For other microbial indicators such as total coliforms, a failure should also trigger investigation to determine why the problem has arisen and whether this problem could lead to the ingress of pathogens.

For most chemical parameters the impact will only be apparent over time and most standards incorporate a significant margin of safety. The response can be more measured to determine if the exceedence is a rare event or if there is likely to be frequent exceedence of the standard. It then becomes important to consult appropriate health authorities regarding the significance of the exceedence and the actions that may be required. In some cases a derogation may be given that will allow the water supplier time to correct the problem or for environmental regulators to correct a problem of contamination of source water.

Small supplies

Small supplies are often a significant problem as expertise and resources are often very limited. This does not mean that small supplies should be ignored but that they require a somewhat different approach. Often this approach is to focus on the most important parameters for health and acceptability. The introduction of WSPs for small supplies is one that has proven to be highly cost effective in identifying hazards and taking simple actions to prevent contamination.

Final thoughts

Standards and guidelines are important for helping to assure drinking water safety and quality but they should not be just a list of parameters with associated numbers. The introduction of WSPs has shown that, although numbers are important, they must be supported by an appropriate management framework. The values associated with standards for chemical constituents and contaminants provide the water quality targets in WSPs. However, standards for microbiological contaminants require much more than meeting the standard for faecal indicator bacteria if the safety of drinking water is to be assured.

Standards need to be established with the local geographical, social and economic considerations in mind so that drinking water quality and safety is assured but without excessive cost. Highly precautionary standards have the potential to deliver cost without related benefit.

Finally, if standards are not monitored and enforced, they are just words and numbers on a piece of paper and are of no value in protecting public health.

Related Articles

References

WHO Guidelines for drinking-water quality

Water Safety Plan Manual: Step-by-step risk management for drinking-water suppliers

Chemical safety of drinking-water: Assessing priorities for risk management

Fawell J (2010) Drinking Water Safety and Standards for Drinking Water. In Textbook of Environmental Medicine. Eds J Ayres, R Harrison, R Maynard and G Nichols. Hodder Arnold. In Press.

See also WHO Guidelines for Drinking-water Quality - 4th Edition (2011). IWA Publishing.

WHO Water, Sanitation and Hygiene

WHO Drinking Water Quality

US-EPA Drinking Water Contaminants

Australian Drinking Water Guidelines

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