1.1 Guiding principles
The Guidelines contain a great deal of information about management of drinking water systems, monitoring and the vast array of contaminants that may be present in drinking water. An ever-increasing knowledge base means that the document has continued to grow in both detail and complexity. Although the increased information needs to be included, a danger is that the fundamental principles vital to ensuring safe drinking water quality become obscured in the detail. These fundamental principles, described below, should always be remembered.
The greatest risks to consumers of drinking water are pathogenic microorganisms. Protection of water sources and treatment are of paramount importance and must never be compromised.
Waterborne pathogens can cause outbreaks of illness affecting a high proportion of the community and, in extreme cases, causing death. How much treatment is needed will depend on the level of protection of water supplies. If water supplies are not completely protected from human and livestock waste, filtration is likely to be required. The principles and approaches for assessing the microbial contamination of source water and management of those risks to meet the microbial health-based targets are set out in chapter 5 and in Appendix 3.
Disinfection is the single process that has had the greatest impact on drinking water safety. There is clear evidence that the common adoption of chlorination of drinking water supplies in the 20th century was responsible for a substantial decrease in infectious diseases. Disinfection will kill all bacterial pathogens and greatly reduce numbers of viral and most protozoan pathogens. Combined with protection of water sources from human and livestock waste, disinfection can ensure safe drinking water. In the absence of complete protection of source water, filtration is likely to be required to improve the removal of viruses and protozoa. Residual disinfection should be maintained wherever possible to provide protection against contamination and limit regrowth problems.
All waterborne disease outbreaks are avoidable. Pathogens can only cause disease and death in humans if water source protection, pathogen removal by disinfection or filtration, or integrity of distribution systems fail. Chemical by-products of disinfection have been suggested as potential health risks. However, the possibility of such health risks remains highly uncertain in comparison to the well-established risks from inadequate disinfection and contamination of water supplies with pathogens. Therefore, although concentrations of by-products should be kept as low as possible, efforts to achieve this should never jeopardise effective disinfection.
The drinking water system must have, and continuously maintain, robust multiple barriers appropriate to the level of potential contamination facing the raw water supply.
The multiple barrier approach is universally recognised as the foundation for ensuring safe drinking water. No single barrier is effective against all conceivable sources of contamination, is effective 100 per cent of the time or constantly functions at maximum efficiency. Robust barriers are those that can handle a relatively wide range of challenges with close to maximum performance and without suffering major failure.
Although it is important to maintain effective operation of all barriers, the advantage of multiple barriers is that short-term reductions in performance of one barrier may be compensated for by performance of other barriers. Prevention of contamination provides greater surety than removal of contaminants by treatment, so the most effective barrier is protection of source waters to the maximum degree practicable. Knowing how many barriers are required to address the level of potential contamination in individual systems is important. This requires a thorough understanding of the nature of the challenges and the vulnerabilities of the barriers in place. In terms of reliability, there is no substitute for understanding a water supply system from catchment to consumer, how it works and its vulnerabilities to failure.
Finally, a robust system must include mechanisms or fail safes to accommodate inevitable human errors without allowing major failures to occur.
Any sudden or extreme change in water quality, flow or environmental conditions (e.g. extreme rainfall or flooding) should arouse suspicion that drinking water might become contaminated.
Disease outbreaks from drinking water are almost invariably linked to changes in measurable water quality parameters or to the failure of treatment processes to cope with extreme weather events such as high rainfall and flooding. Water treatment processes generally function best under steady state conditions, and performance can seriously deteriorate when there are major fluctuations in quality or flow. It is vitally important that water quality after treatment remain as constant as possible, no matter how much the quality of the source water varies. Operators and managers need to be aware of normal operating requirements, the measurement criteria that define normal operation and the enormous risks that can be associated with operating outside normal limits.
System operators must be able to respond quickly and effectively to adverse monitoring signals.
Sudden changes in water quality or flow are likely to be a sign of imminent problems; such variations should always trigger appropriate responses. Wherever possible, key processes should be monitored continuously. Operators and managers must have the knowledge and appropriate responsibility to implement the necessary responses, which could range from modifying treatment processes to, in extreme cases, advising health regulators to consider issuing public advice such as ‘boil water’ notices or shutting down water supplies.
Previous water quality failures or ‘close calls’ should be studied so that operators are aware of the relationship between operational indicators and subsequent water quality failures. Even seemingly small faults should be addressed because these can accumulate and lead to a serious incident. Many waterborne disease outbreaks are caused by a combination of faults.
System operators must maintain a personal sense of responsibility and dedication to providing consumers with safe water, and should never ignore a consumer complaint about water quality.
Consumers are the ultimate assessors of water quality. Consumers may not be able to detect trace concentrations of individual contaminants, but their ability to recognise change should not be discounted. In some cases, consumer complaints may provide valuable information on potential problems not detected by testing water quality or monitoring treatment processes. Water quality testing has limitations and there are many possibilities for contamination of water in reticulation systems after treatment. All consumer complaints should be investigated to ensure that otherwise undetected problems that might compromise drinking water safety have not occurred. Meeting reasonable consumer expectations and maintaining confidence in the water supply is vitally important.
Ensuring drinking water safety and quality requires the application of a considered risk management approach.
The process of keeping drinking water safe is one of risk management. This requires steering a sensible course between the extremes of failing to act when action is required and taking action when none is necessary. Lack of action can seriously compromise public health, whereas excessive caution can have significant social and economic consequences. Corrective action or system upgrades should be undertaken in a considered, measured and consultative manner. Failure to act when required (e.g. failing to shut down a system when disinfection is not working effectively) may lead to an outbreak of waterborne disease. Acting when not required (e.g. issuing a ‘boil water’ notice when that is not necessary) is usually less severe in the short term, but repeated occurrences waste resources and are likely to cause complacency in the long term, leading to failure to respond when it is truly necessary. Similarly, failing to install a treatment process when required could lead to waterborne disease; however, installing treatment processes that are not required could have a high financial cost and divert funds needed elsewhere.
Risk management is about taking a carefully considered course of action. As the obligation is to ensure safe water and protect public health, the balancing process must be tipped in favour of taking a precautionary approach.
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