Current online water quality monitoring methods and their suitability ...

Monochloramine is a suitable disinfectant for Barriers 3-4 to ensure RO membrane performance ismaintained. However, it should be noted that online free chlorine analysers are only suitable forBarriers 5, 6 and 7, as they are based on direct adaptations of laboratory-based methods. Thereforethey require strictly controlled measurement conditions and frequent calibration. For Barriers 2 (postclarification) and 3 there are no suitable commercially available methods with sufficient sensitivity tomonitor free chlorine levels below 0.05 mg/L. This is due to the limitations of the method andnonlinear responses at the lower concentrations. As free chlorine is a CCP in Barrier 2 that may affectthe performance of upstream treatment barriers, it is of paramount importance that there is onlinemonitoring technology available that can detect in real-time if the system is approaching critical limits.In addition, reagent stability is of concern, as is signal drift.Therefore, there is still work to be done before commercially available online sensors can be used forlarge-scale autonomous water quality monitoring. This is very important in Barriers 5-6 and for thedistribution system in Barrier 7, as appropriate chlorine residuals must be maintained to ensure aconsistent, safe supply of potable water.As microbiological quality is the most important parameter required to ensure a safe supply ofdrinking water, the development of new online water quality monitoring tools that could rapidly assessthe treatment process would be highly beneficial and further complement the WCRW plan. Althoughthere have been some rapid methods developed for pathogens, e.g. flow cytometry and fluorescent insitu hybridisation (FISH), these still have several inherent drawbacks that inhibit their widespreadapplications, such as detection problems, the presence of large numbers of interfering species, and therequirement for highly skilled personnel 87 .Although there has been significant research in this area, viruses and protozoa are difficult to detect infresh water due to their low concentrations. Consequently, large volumes of water are required tomake a measurement with any degree of confidence. This can make analytical methods unsuitable forreal-time online process control, especially within Barriers 3-7 87, 88 . Methods that requirecentrifugation as a pre-concentration step are also not suitable for online applications 88 .Therefore, there are still no suitable online microbial analysis tools available that have the robustness,sensitivity and limited maintenance requirements necessary to be an effective monitoring tool for theWCRW system.Research and DevelopmentBiosensors offer an alternative for measuring microorganisms such as pathogens, viruses and bacteriain real-time to complement existing standard monitoring practices. For example, a cell-basedbiosensor has been demonstrated for measuring an E. coli strain that was responsive to mercury. Itwas immobilised on a micro-well fibre array and its response to mercury was measured 86 .Alternatively, DNA hybridization microarrays have been suggested as a platform for the paralleldetection of pathogenic organisms, which has potential applications in the PRW system 86 . Furtherdevelopment is required to improve their selectivity, durability and resistance to biofouling before theycould be used in applications such as the PRW system. Stewart-Pullaro et al. (2006) investigated asimple, rapid and inexpensive method for monitoring F + RNA, however no continuous, real-timemethods with suitable accuracy were available 89 . Another promising development by Delahaye et al.(2003) has been the titration of adenosine triphosphate (ATP) using bioluminescence, which couldhave applications for monitoring coliforms and entercocci 90 . However despite these researchadvances, there are no biosensors available that have the operational performance and simplicity ofcomparable inorganic sensors (e.g. pH sensors) 86 .Once again electronic tongues have been suggested as an alternative tool for pathogen detection. Theyshow some promise, as measurements can be obtained rapidly with limited sample pre-treatment beingrequired, provided the correct sensors can be obtained to indicate microbial populations. However,sensor fouling would still be a problem, especially within Barriers 2, 6 and 7. Additionally, issuessuch as signal drift, reproducibility and calibration still need to be overcome to make this techniquesuitable for real-time continuous monitoring.Current Online Water Quality Monitoring Methods and Their Suitability for the Western CorridorPurified Recycled Water Scheme Page 28