Clostridium perfringens as a Drinking Water Quality Parameter

Transcription

1 Clostridium perfringens as a Drinking Water Quality Parameter A summary review prepared by David Sartory on behalf of the European Microbiology Advisory Group (EMAG) for the Article 12 Technical Committee as a contribution to the rolling revision of the Drinking Water Directive 1998 Introduction The Council Directive on the quality of water intended for human consumption¹, published in 1998, introduced a new microbiological indicator parameter of Clostridium perfringens (including spores) for check monitoring, with a parametric value of 0/100 ml. The purpose of check monitoring is regularly to provide information on the organoleptic and microbiological quality of the water supplied for human consumption as well as information on the effectiveness of drinking water treatment. Monitoring for the Cl. perfringens parameter is only necessary if the water originates from, or is influenced by, surface water. The Directive also states In the event of non-compliance with this parametric value, the Member State concerned must investigate the supply to ensure that there is no potential danger to human health arising from the presence of pathogenic micro-organisms, e.g. Cryptosporidium. This implies that the value of Cl. perfringens is believed to be that it is an indicator for the presence of robust pathogens that may survive water treatment processes rather than being a pathogenic organism in its own right. This summary overview assesses the role of Cl. perfringens as an indicator of drinking water quality, particularly with regard to the potential occurrence of Cryptosporidium oocysts in treated drinking water, with reference to recent research and experience by Member States. Monitoring Requirements of the Directive The Directive requirements for standard annual check monitoring frequencies for Cl. perfringens range from four for treatment works producing less than 1000 m 3 /d to four plus three for each 1000 m 3 /d for treatment works supplying greater than 100,000 m 3 /d. As an example, a surface water treatment works supplying 725,000 m 3 /d requires 2194 samples per year for Cl. perfringens, which is the equivalent of six samples per day. Daily sampling is required at all water treatment works supplying more than 119,999 m 3 /d. A reduced frequency of up to 50 % may be applied to this parameter. These monitoring frequencies mean, therefore, that it is the intention of the Directive to apparently value Cl. perfringens on a par with other parameters such as turbidity, iron and aluminium used as flocculants, hydrogen ion concentration, colour and taste and odour in providing information on the effectiveness of water treatment. This paper suggests that its value in this respect is not validated by research findings. 1

2 Historical Use of Clostridium perfringens The testing of water for the organism now known as Clostridium perfringens was first proposed in England in Although other researchers in Europe and USA also remarked on its potential usefulness the test was not adopted widely outside the UK. In Europe, particularly France, a much broader group of clostridia, which includes Cl. perfringens, known as the sulphite-reducing clostridia (SRC) was more often tested for, and it was this parameter that was included in the first European Drinking Water Directive published in In the UK Cl. perfringens was regarded as a secondary indicator useful in situations of infrequent sampling to detect remote contamination (either in time or location) due to the environmental resistance of the spores, especially for groundwater supplies which received little or no treatment. Additionally, several researchers worldwide have advocated the use of Cl. perfringens (and closely related species) for the assessment of faecal contamination of environmental waters. There has been wider interest in the use of Cl. perfringens in drinking water quality assessment in the last ten years where researchers in Canada and the Netherlands have indicated that testing for the spores of the bacterium (or SRC) may be useful in assessing efficiency of water treatment, as a potential surrogate indicator for the inactivation of viruses and protozoan pathogens. However, it should be borne in mind that these studies used large sample volumes ( litres) for analyses, whereas the typical volume used for regulatory microbiological analyses is 100 ml. Assessing Clostridium perfringens as a Drinking Water Quality Parameter Occurrence in Source Waters A number of studies worldwide, particularly North America, Europe, South Africa and Australia, provide data on the numbers of Cl. perfringens or SRC in freshwaters used for the production of drinking water. Despite geographical and analytical differences the typical levels reported are remarkably similar with counts ranging from < 1 cfu/100 ml for unpolluted waters to up to 3000 cfu/100 ml for waters receiving wastewater effluent. These levels are typically 100- to 1000-fold lower than those recorded for E. coli or enterococci. Any use of a surrogate indicator parameter with respect to a specific group of pathogens, in this case the robust pathogens surviving water treatment, as exemplified by oocysts of Cryptosporidium, will depend upon any relationship between the chosen indicator and the pathogen in environmental waters and through treatment. Studies on the potential value of Cl. perfringens or SRC for predicting the occurrence and level of Cryptosporidium oocysts have presented conflicting data, with some reports from Canada reporting significant correlations, whilst others from Canada, UK, Italy and South Africa failing to find any correlation. This lack of any consistent relationship in source waters means that the numbers of Cl. perfringens or SRC cannot be used to assess the risk of Cryptosporidium challenge 2

3 at surface water treatment works, with any potential relationship being source dependant. One of the main reasons for this may be that equivalent volumes of water are typically not tested for both organisms (100 ml for clostridia and litres for Cryptosporidium). Impact of Water Treatment Processes and Occurrence in Treated Final Waters There is limited direct data on the removal of clostridial spores by conventional water treatment (coagulation/flocculation/sedimentation/filtration), although it is generally accepted that they are removed efficiently by these processes. Log 10 removal rates of 3 to 7 are typically reported, depending upon the number of treatment processes involved. Clostridium spores have been shown to be more resistant to conventional disinfectants than the other indicator bacteria (e.g. between 30- and 90-times more resistant than E. coli to chlorine), but are still inactivated by the concentrations and contact times typically used in water treatment. In relation to the other classic bacterial indicators (E. coli and enterococci) numbers of Cl. perfringens or SRC in source waters are relatively low, and given their fairly effective removal by conventional treatment processes, this results in infrequent detection in final treated water using the standard sample volume of 100 ml. For these bacteria to be useful in the assessment of treatment works efficacy large volumes need to be analysed, such as those used in the Canadian and Dutch studies where litres were sampled and the detection methods used targeted the enumeration of the robust spores. A study undertaken for the UK Drinking Water Inspectorate at four water treatment works using 24 hour continuous Cryptosporidium sampling and hourly sampling for microbiological parameters did not find any correlation between occurrence of presumptive Cl. perfringens (in 400 ml hourly samples) and Cryptosporidium oocysts (in litres over approximately 24 hours), both being very infrequently detected. The method of analysis for Cl. perfringens stipulated in the Directive requires their enumeration including spores. This means that vegetative cells are included and are considered to have a role in treatment works performance, whereas it is only the spores of either sulphite-reducing clostridia or Cl. perfringens that are recognised by some researchers as being potentially appropriate for such monitoring with regard to removal of robust pathogens. Recent research in the USA, Canada and the UK has resulted in more interest in the use of the aerobic spore-forming bacteria (typically species of Bacillus), which occur naturally in source waters in much higher numbers than Cl. perfringens or the sulphite-reducing clostridia, as a more appropriate and more easily analysed indicator of treatment works performance. However, as with all indicators, there are potential problems with their use. Studies in Spain have shown growth of aerobic spore-forming bacteria in carbon filters, and in the UK growth of sulphite-reducing clostridia in slow sand filtration beds. Where such growth occurs, this would limit the use of these indicators as surrogates for robust pathogen removal. However, 3

4 research into the use of these bacteria as treatment works performance indicators is ongoing. Occurrence in Distribution System Waters There are very few publications giving information on the occurrence of Cl. perfringens or SRC in distribution systems. Data from the UK indicates a low frequency of occurrence and numbers detected for SRC (1 3 % of samples), with, for one water supplier, up to 40 % of detections being Cl. perfringens. Some Member States, ranging from the northernmost to the Mediterranean, have reported not having isolated Cl. perfringens from potable water supplies. Some European studies have reported regular occurrence of SRC, and these appear to be more related to growth of SRC within distribution systems and associated with corrosion processes. Some species, including Cl. perfringens, are capable of growing in the presence of low levels of oxygen, with spore production being enhanced in its absence. Clostridia have been isolated from corrosion tubercules (up to 460 cfu/g) in distribution system pipes. This indicates that SRC may be considered to be a natural part of drinking water microbiota and their presence a potential indicator of corrosion processes. However, SRC are widely distributed in soils and their presence may also indicative of soil or rainwater intrusion into a distribution system. Given the robust nature of clostridial spores and the ability for several species to grow in corrosion environments, their presence, including that of Cl. perfringens, may be more indicative of operational issues (accumulation in distribution systems sediment or biofilms or growth in corrosion nodules) rather than risks to public health. This is an area, however, that needs further research but the Directive requirement for monitoring Cl. perfringens is not targeted at distribution systems. Analytical Methods for the Enumeration of Clostridium perfringens The currently available analytical methods for the enumeration of Cl. perfringens from water are not particularly good. The method stipulated in the Directive suffers from poor recoveries and lack of specificity. Whilst the method being proposed in the revised ISO/CD (draft 20 th December 2002) standard has been found by most Member States to be an improvement in both recovery efficiency and specificity, it can still significantly underestimate numbers of Cl. perfringens when compared with numbers recovered using non-selective media. Neither method can be regarded as ideal. Conclusions Clostridium perfringens may be useful as an indicator of faecal contamination in environmental waters, but as numbers in source waters are relatively low compared to other indicator bacteria, and conventional water treatment processes are effective in their removal, they are of much less use in drinking water quality 4

5 assessment, particularly with regard to the indication of the potential removal efficiency of robust pathogens. Data on the putative relationship between occurrence of Clostridium perfringens and robust pathogens such as Cryptosporidium, in source and treated waters is inconclusive (and probably variable between source waters and treatment processes) and limited to the use of specialist large volume sampling. Enumeration of spores of sulphite-reducing clostridia or Clostridium perfringens may have a role to play in assessing the efficacy of treatment works processes, but as a specialist operational tool employed by water suppliers and using large volume sampling rather than for routine check monitoring. However, recent studies on the use of the more abundant aerobic spore-forming bacteria (e.g. Bacillus spp) indicate that these may be a more suitable operational monitoring/assessment tool. Further research on the potential value of either spores of sulphite-reducing clostridia or aerobic spore-forming bacteria as an operational performance monitoring tool should be encouraged. Recommendation It is recommended that the inclusion of Clostridium perfringens as a check parameter in the Drinking Water Directive is reconsidered with regard to its limited value as an indicator of public heath protection and effectiveness of water treatment. The options for consideration are:- a) removal from the list of microbiological parameters, or b) reduction in significance to Audit Monitoring for all water types. Option a) is the preferred option of the majority of Member State representatives on EMAG. ¹ Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. Official Journal of the European Communities, , L330/32-L330/53. Selected Bibliography Bonadonna L., Briancesco R., Donia D., Ottaviani M., Veschetti E. and Divizia M. (in press) Un indagine sulla diffusione di protozoi patogeni nel fiume Tevere: qualità igienico-sanitaria delle acque e rapporti di correlazione con parametri microbiologici e chimico-fisici [An investigation on the occurrence of pathogenic 5

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