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LED fluorimeter developed for accurate assessment of potable water quality

Device offers potential to promote a proactive means of monitoring water quality.

4 bacteria isolated from water show a clear and different response between strains.

Characterising the organic and microbial matrix of water are key issues in ensuring a safe potable water supply. Current techniques only confirm water quality retrospectively via laboratory analysis of discrete samples. Whilst such analysis is required for regulatory purposes, it would be highly beneficial to monitor water quality in-situ in real time, enabling rapid water quality assessment and facilitating proactive management of water supply systems.

A novel LED-based instrument, detecting fluorescence peaks C and T (surrogates for organic and microbial matter, respectively), was constructed and performance assessed. Results from over 200 samples taken from source waters through to customer tap from three UK water companies are presented. Excellent correlation was observed between the new device and a research grade spectrophotometer (r2 = 0.98 and 0.77 for peak C and peak T respectively), demonstrating the potential of providing a low cost, portable alternative fluorimeter. The peak C/TOC correlation was very good (r2 = 0.75) at low TOC levels found in drinking water. However, correlations between peak T and regulatory measures of microbial matter (2 day/3 day heterotrophic plate counts (HPC), E. coli, and total coliforms) were poor, due to the specific nature of these regulatory measures and the general measure of peak T. A more promising correlation was obtained between peak T and total bacteria using flow cytometry. Assessment of the fluorescence of four individual bacteria isolated from drinking water was also considered and excellent correlations found with peak T (Sphingobium sp. (r2 = 0.83); Methylobacterium sp. (r2 = 1.0); Rhodococcus sp. (r2 = 0.86); Xenophilus sp. (r2 = 0.96)). It is notable that each of the bacteria studied exhibited different levels of fluorescence as a function of their number. The scope for LED based instrumentation for in-situ, real time assessment of the organic and microbial matrix of potable water is clearly demonstrated.

Keywords

  • Fluorescence;
  • LEDs;
  • Organic matter;
  • Microbial matter;
  • Potable water quality

1. Introduction

Regulations place a duty on water companies to supply water that is wholesome at the time and point of supply, with wholesomeness defined by reference to prescribed concentrations assigned to various microbiological, chemical and physical parameters. Prescribed concentrations or values for microbiological parameters rely on proven, but dated indicator organisms, such as coliform bacteria, E. coli and colony counts. In addition to meeting standards, water must not contain any micro-organism or parasite at a concentration which would constitute a potential danger to human health.

The challenge to ensure microbial quality is addressed by water companies through disinfection, both during treatment processes and via the maintenance of a residual in most distribution systems. The most commonly used disinfectant in the developed world is chlorine. The use of chlorine, ozone, or chlorine dioxide as a disinfectant in water rich in organic matter (OM) (which also acts as a microbial food source) can lead to the occurrence of potentially carcinogenic disinfection byproducts (DBP). Consequently, water companies must manage the competing needs of biological and chemical compliance. Thus, it is imperative that companies monitor the microbial and chemical quality of drinking water at various stages of treated water supply systems, including trunk mains, service reservoirs and the distribution network.

Organic matter is ubiquitous in every water supply system and monitoring of its concentration and attributes is important for issues such as source water ecological health, treatment cost and efficacy, control of disinfection by-products and biological regrowth in distribution. Organic matter concentrations are typically assessed as total organic carbon (TOC) and/or dissolved organic carbon (DOC). Assessment requires complex and time-consuming laboratory procedures such that the data can only be used retrospectively rather than for proactive or pre-emptive management.

Another primary concern to water utilities is to ensure that the drinking water that is supplied does not pose an unacceptable health risk to consumers. As the number and type of different pathogens present in waters is extensive, varied and dependent on a range of environmental factors, it is not feasible to isolate and identify each specific pathogen on a regular basis. Hence, reliance has traditionally been placed on the measurement of total plate counts, as an overall indicator of microbial load and detection of faecal indicator bacteria and other coliform bacteria for contamination. Although these culture based tests give precise enumeration, they can take more than 30 h to perform from sample receipt to results. It is also known that, in the natural environment, often only 1% or less of microbes can be cultured in this way, leading to what has become commonly known as the “great plate count anomaly” (Staley and Konopka, 1985, Amann et al., 1995 and Allen et al., 2004).

The measurement of both organic and microbial matter thus currently relies on the collection and transportation of discrete samples that are then analysed by time consuming techniques (taking days). Such approaches are only able retrospectively to confirm (or otherwise) water quality. In-situ real time measurement is therefore highly desirable, as it would enable assurance of water quality, optimisation of process control and overall proactive and preventative operation of water supply systems. It would also be of great value to improve spatial and temporal coverage of water supply systems to fully develop and implement proactive management. Hence, there exists a pressing need for the development of novel technologies that will enable real time, in-situ, low powered and, ideally, continuous analysis of drinking water quality to improve the robustness of water supply system management. Brock (1987) affirmed the necessity of in-situ studies for describing environmental microbiology. Only then is it possible to identify the affinities, symbiosis and interactions with the surrounding environment, which are not applicable with indirect techniques ( Souza et al., 2007). Similarly, the measurement of organic matter in an unaltered state makes results directly applicable only to the specific reaction conditions and water (Goslan, 2003).

Water quality is, of course, a function of inorganic pollutants as well as organics. Such inorganic pollutants may include nitrates and phosphates arising from run-off from agricultural land, heavy metals from highway run-off, ammonia from wastewater effluent discharges, arsenic occurring naturally in groundwater, or copper from household plumbing. Whilst the presence of these inorganics in water is recognised, the focus of the work reported here is on the detection of organic and microbial matter.

In the work reported in this paper, the development and deployment of a novel, LED-based instrument, capable of the in situ detection of fluorescence peaks T and C (surrogates for organic and microbial matter respectively) in water is described. The novelty of the work (and instrument) is demonstrated via the robust correlations observed between instrument results and those provided by well-established analytical methods including, for the first time, a comparison with flow cytometry data and an assessment of the fluorescence response of four individual bacteria isolated from drinking water.

2. Background

2.1. Current methods of organic and microbial determination

A range of different techniques have been developed and applied to characterise and quantify the organic and microbial matrix within water. Total organic carbon (TOC) is the most comprehensive and widely adopted operational measure used to quantify the presence of organic carbon atoms in waters. TOC is often synonymous with natural organic matter (NOM) because organic contaminants in naturally sourced water supply systems generally represent an insignificant fraction of the TOC (Leenheer and Croué, 2003). More sophisticated analytical techniques which differentiate physico-chemical properties are also available, including: resin abstraction (used principally to determine hydrophobicity), high performance size exclusion chromatography (HPSEC), gas-chromatography–mass spectrometry (GC–MS) and UV–visible absorbance optical properties (Chin et al., 1994, Visco et al., 2005 and Bridgeman et al., 2011). Such techniques are predominantly laboratory-based and labour intensive, requiring extensive sample preparation and analysis, rendering them impractical in studies involving many sampling sites or frequent monitoring.


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