Microbial sensors and risk-based monitoring
Escherichia coli is often used as a microbial indicator of faecal contamination of (drinking) water and its resources. Additionally, other microbial parameters are of interest for the production of safe drinking water, such as Enterococci, Pseudomonas aeruginosa, Clostridium perfringens and – to be introduced in the revised European Drinking Water Directive – somatic coliphages. Until recently, the development of sensor technologies for water quality monitoring has largely been focused on (physico-)chemical parameters, but sensors for the detection of microbial parameters are catching up fast. New methods for E. coli have been developed based on the rapid detection of specific enzymes (e.g. BACTControl or Colifast) or by using other detection techniques than plate culturing, such as raman spectroscopy, flow cytometry, light scattering or a combination of these. Detailed information on these technologies can be found in Sensileau’s Sensor Database. Depending on the detection method and the target enzyme chosen, these methods produce very general results (presence/absence), a semi-quantitative result (most probable number – MPN) or a fully quantitative result. More generic methods may not be fully specific for the bacteria under investigation, thereby introducing a certain level of uncertainty in the result. However, the time to result for quantitative methods varies from several hours to about one day, which is in many cases considerably faster than the traditional laboratory method.
Thus, the question arises: Can bacterial sensors be used in monitoring programmes under the EU Drinking Water Directive? Well, yes and no.
Methods for the detection and quantification of microbial contaminants in drinking water at the tap are legally required to meet certain standards. ISO standards are specified for some parameters, and the methods used to detect and quantify certain bacterial parameters need to be in accordance with this standard or proven to produce equivalent results. Many of the currently available sensor technologies for microbial parameters do not meet this requirement, as they have not been through the process of ‘equivalence testing’, which is necessary to prove that the method produces equivalent results to the specified ISO standards. However, this does not mean that these sensor technologies cannot be applied or do not have any added value.
In the current EU drinking Water Directive (98/83/EC), risk-based monitoring according to WHO’s Water Safety Plan approach or similar risk assessment strategies is voluntary. The proposed revision of the EU Drinking Water Directive goes a step further and makes the risk-based approach mandatory for all EU Member States. And this is where the microbial sensors come in handy. Although their detection methods may not be suitable for legal compliance monitoring of drinking water at the tap, these sensors are very useful to provide a warning signal if microbial contamination is detected in the source water or during the treatment and transport of drinking water. In order to provide an indication of microbial contamination, it is not necessary to achieve similar sensitivity or accuracy levels as the ISO-standards do, but the information throughout the treatment process can support a risk management strategy based on early warning signals. With the help of bacterial sensors, a dedicated monitoring programme can be developed, and additional samples taken when the presence of microbial contamination is detected. This helps to focus monitoring efforts on locations and times where it is most needed, and reduces the number of negative samples which do not add any valuable information. In this way, cost savings can be achieved, and the monitoring results collected contribute to a better insight into the public health risks of microbial drinking water contamination.
A few examples
The current ISO method for C. perfringens uses membrane filtration (MF) and TSCA medium followed by subculture and confirmation of isolates by testing for acid phosphatase. This method is rather time-consuming and may take up to 48 hours. In the UK, a new membrane filtration method has been developed which features a simplified isolation medium (TCA) and a membrane transfer procedure for the acid phosphatase test. This reduces the time to result to 18–24 hours.
The US EPA has developed and described various tests for somatic coliphages:
- EPA 1601: presence/absence assay;
- EPA 1602: enumerative assay.
The development of commercial test kits for somatic coliphages based on these EPA test methods is underway (e.g. Bluephage), and this development is expected to continue in the coming years. Uptake of this parameter in the new European Drinking Water Directive may boost developments even more.
Many new and simpler methods for microbiological parameters are currently being developed, and it is expected that this development will continue in the future. Very often, the objective is to simplify existing methods so that they require less expertise or experience to be performed, thereby allowing these methods to be used on-site at treatment plants or pumping stations, rather than in a laboratory. Another major objective is to shorten the time to result by developing new methods which are significantly faster than the conventional laboratory methods. It is expected that large-scale deployment of such methods in the future will lead to a more rapid detection of potential health risks throughout the entire process of drinking water production and distribution, thus contributing significantly to increasing overall public health.
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Are you using on-site sensors for the detection of microbial parameters in source water, during treatment or after transport of drinking water? Tell us what you think. Submit a comment to this article, and we shall get in touch.