Page 3: Guidance on Waterborne Bacterial Pathogens
This document provides information as background for those interested in drinking water quality and safety. There is a particular focus on waterborne bacterial pathogens that may not have the classical faecal-oral transmission route, as these may be less well known to the water industry and public health professionals. Although there is information available from many countries, this document emphasizes studies most relevant to the North American situation.
Throughout history, consumption of drinking water supplies of poor sanitary quality has been linked to illnesses in human populations. These illnesses most commonly present as gastrointestinal-related symptoms, such as diarrhoea and nausea. The organisms identified within this document as waterborne faecal bacterial pathogens are those that have been well established as having a history of being responsible for waterborne outbreaks of gastrointestinal illness.
There are standardized methods available for detecting and measuring certain pathogenic bacteria in drinking water. However, routine monitoring for these organisms still remains difficult and impractical. This is because there are a number of types of bacterial pathogens that can be present in human and/or animal wastes, which can vary significantly in their distribution, depending on the sources of contamination affecting the water supply; and because conducting detection and identification procedures for each possible type can be difficult, requiring significant resources. As a result, monitoring for a broad indicator of faecal contamination such as Escherichia coli is useful in verifying the microbiological quality and safety of the drinking water supply. The presence of E. coli in drinking water system indicates that the source or the system has likely been affected by recent faecal contamination; as a result, the water should be deemed as unsafe to drink.
In recent decades, there has been an increasing amount of interest in waterborne bacterial pathogens that occur naturally in the water environment and thus have the potential to be transmitted through water. The waterborne non-faecal bacterial pathogens discussed within this document can cause human infection, resulting in gastrointestinal and non-gastrointestinal illnesses (particularly respiratory illnesses). Routine monitoring for these pathogens remains difficult and expensive as well. As these organisms occupy different environmental niches and have primary sources other than human or animal faeces, there are at present no satisfactory microbiological indicators for their presence. To date, none of these organisms has been associated with outbreaks of illness as a result of ingestion of drinking water in Canada. However, as they do have the potential to be spread through drinking water, it is important to ensure that the treatment and disinfection strategies in place are capable of providing adequate control of these organisms. Health Canada maintains that the best means of safeguarding against the presence of waterborne pathogens (including non-faecal bacterial pathogens) in drinking water is the application of the multibarrier approach, which includes adequate treatment, a well-maintained distribution system and, in the case of enteric bacteria, source protection. Treatment and disinfection requirements in the provision of microbiologically safe drinking water are based on health-based treatment goals for the removal and inactivation of the enteric protozoa Giardia and Cryptosporidium and enteric viruses. These organisms present a significant challenge to water treatment and disinfection technologies because of their difficulty of removal, high infectivity and high disinfectant resistance. As a result, current drinking water treatment and disinfection practices applied to meet the treatment goals for viruses and protozoa are expected to be similarly capable of controlling waterborne bacterial pathogens in drinking water. This approach can reduce both faecal and non-faecal pathogens to non-detectable levels or to levels that have not been associated with human illness.
Consequently, it remains unnecessary and impractical to establish maximum acceptable concentrations for the waterborne bacterial pathogens described in this document. The monitoring of E. coli continues to be used in the verification of the microbiological quality of drinking water. Information on the adequacy of drinking water treatment and on the microbial condition of the distribution system is provided by the monitoring of E. coli and other indicators, such as disinfectant residual and turbidity.
Under the multibarrier approach to safe drinking water, numerous process controls are required to function alongside bacteriological analysis in order to reliably produce drinking water of an acceptable quality.Important individual elements under this approach include:
- source water protection (where possible);
- optimized treatment performance (e.g., for turbidity reduction and particle removal);
- proper application of disinfection technologies;
- a well-designed and well-maintained distribution system; and
- maintenance of a disinfectant residual.
The potential for the introduction of waterborne bacterial pathogens into the distribution system and their ability to survive and regrow in biofilms are of concern in drinking water treatment. Recurring patterns of elevated heterotrophic plate counts downstream of water treatment may indicate the presence of biofilms, which could be a source of waterborne pathogens. As microorganisms in biofilms may survive, multiply and be released into the distribution system, water that meets the bacteriological guidelines may be recontaminated over time.
Additional considerations specific for aiding in the control of biofilms in the distribution system can include:
- use of proper construction materials;
- control measures to reduce levels of natural organic matter, scaling and corrosion;
- measures to prevent low flow rates or water stagnation and to control temperatures (where possible);
- maintenance activities, such as flushing and cleaning; and
- ensuring adequate disinfection after installation of new pipes and after maintenance or repair of existing pipes.
Contamination problems involving waterborne bacterial pathogens can occur in water systems beyond the water treatment plants' distribution network, such as plumbing systems or heating, ventilation and air conditioning systems. Specific information pertaining to guidance and requirements for these systems can be found by consulting the proper regulatory authority.
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