Biological methods for online estimation of ultraviolet disinfection dose in drinking water treatment

Abstract

Ultraviolet (UV) disinfection of drinking water is now a widely-accepted practice and in particular, is one of the most effective water treatment processes for inactivating the waterborne pathogen Cryptosporidium. Because UV disinfection is a physical, rather than chemical, disinfection method (i.e. there is no disinfectant residual), the measurement and monitoring of operational UV disinfection performance relies on indirect measures, such as UV intensity, taken by sensors. These indirect measurements are linked via previously validated algorithms, determined by the UV disinfection system supplier, to the level of disinfection of a target pathogen being provided at any given time. The objective of this research was to test and optimise a molecular technique of detecting UV dose-dependent changes in the DNA of the pool of microorganisms indigenous to pre-disinfection water. Such a method could be used by water utilities for verifying the ultraviolet (UV) disinfection dose that is applied to drinking water by operational UV systems. The method would be a direct, sample-based measure, without the need to spike any test microorganisms or chemicals into the water or take the UV disinfection system offline. Initially, this work focused on an approach based on culturing bacteria to attempt to identify heterotrophic organisms indigenous to water treatment processes. Water samples from a UK water treatment works were collected and exposed to 40 mJ/cm2 of monochromatic (254 nm) UV light using a collimated beam device. Survivors were grown on low nutrient agar, and representative example of survivors were identified by 16s rRNA partial gene sequencing. The UV dose-response profiles of two of the thus isolated organisms, Flavobacterium succinicans and Sphingopyxis chilensis, were determined for the first time. Sphingopyxis chilensis has a UV dose-response profile which suggests it has the potential for use as an indigenous surrogate for the inactivation of Cryptosporidium, a commonly chosen target pathogen for UV disinfection, up to 2.3-log inactivation. Furthermore, the ubiquity and intrinsic antibiotic resistance profile of the Sphingomonadaceae, of which Sphingopyxis chilensis is a member, suggest that this group may warrant further development as an indigenous surrogate measurement of UV disinfection in low UV dose (up to 7 mJ/cm2 ) applications. However, this work also suggested that numbers of single species are unlikely to be sufficient at the relevant locations in the water treatment processes for routine direct measurement using such an organism to be viable.

In order to address problems related to selecting and enumerating a particular species from the complex mixture of organisms present in water samples, a more generic method was sought. This research considered the total extractable genomic DNA of the indigenous microorganisms in the water passing through a UV reactor could be a generic indigenous UV dose indicator. An enzyme-linked immunosorbent assay (ELISA) was applied to the total extractable genomic DNA from water samples collected from drinking water treatment plants and exposed to collimated beam UV doses. This approach, once optimised, resulted in linear relationships between the assay response and UV dose. As such, ELISA-based enumeration of thymine dimers in total extractable genomic DNA from a mixed species population has the potential to provide a direct, relatively quick, sampling-based means of monitoring the UV dose being delivered by operating UV disinfection systems in drinking water treatment plants, without the need to spike a biodosimeter or actinometer into the treatment process, or take systems out of service.