Present-day monitoring underestimates the risk of exposure to pathogenic bacteria from cold water storage tanks.

Aji Peter,Edwin Routledge,Carla A Ng

doi:10.1371/journal.pone.0195635

Abstract

Water-borne bacteria, found in cold water storage tanks, are causative agents for various human infections and diseases including Legionnaires’ disease. Consequently, regular microbiological monitoring of tank water is undertaken as part of the regulatory framework used to control pathogenic bacteria. A key assumption is that a small volume of water taken from under the ball valve (where there is easy access to the stored water) will be representative of the entire tank. To test the reliability of this measure, domestic water samples taken from different locations of selected tanks in London properties between November 2015 and July 2016 were analysed for TVCs, Pseudomonas and Legionella at an accredited laboratory, according to regulatory requirements. Out of ~6000 tanks surveyed, only 15 were selected based on the ability to take a water sample from the normal sampling hatch (located above the ball valve) and from the far end of the tank (usually requiring disassembly of the tank lid with risk of structural damage), and permission being granted by the site manager to undertake the additional investigation and sampling. Despite seasonal differences in water temperature, we found 100% compliance at the ball valve end. In contrast, 40% of the tanks exceeded the regulatory threshold for temperature at the far end of the tank in the summer months. Consequently, 20% of the tanks surveyed failed to trigger appropriate regulatory action based on microbiological analyses of the water sample taken under the ball valve compared to the far end sample using present-day standards. These data show that typical water samples collected for routine monitoring may often underestimate the microbiological status of the water entering the building, thereby increasing the risk of exposure to water bourne pathogens with potential public health implications. We propose that water storage tanks should be redesigned to allow access to the far end of tanks for routine monitoring purposes, and that water samples used to ascertain the regulatory compliance of stored water in tanks should be taken at the point at which water is abstracted for use in the building.

Highlights

Potable water is typically produced at water treatment facilities where incoming water is treated to remove pathogens and is disinfected before it leaves the treatment works [1]
incoming mains (IM) water temperature varied between 7–19 ̊C, which was below the regulatory threshold of 20 ̊C for both mains water and stored water [25]
The microbiological analysis of water samples taken from a limited number of tanks throughout a year shows that identical regulatory actions would have occured in 80% of instances irrespective of where the sample was taken, despite consistently higher numbers of bacteria at the far end of the tank

Summary

Introduction

Potable water is typically produced at water treatment facilities where incoming water is treated to remove pathogens and is disinfected before it leaves the treatment works [1]. Treated water can become contaminated with microorganisms during transportation throughout the pipework network, and during storage [2]. Long horizontal installations of pipework, the types of materials used for the pipework and fittings, deadlegs (isolated sections of piping) and excessive water storage or stagnation can all affect water quality and encourage the proliferation of many species of bacteria [3]. In order to protect society against the harmful effects of exposure to pathogenic bacteria, many countries throughout the world have developed and adopted standards used for the evaluation of microbiological status of point-of-use and pointof-entry potable water in buildings. The United States Environmental Protection Agency (USEPA) and the European Environment Agency (under the EU Water Framework Directive) have implemented monitoring and sampling strategies to ensure that the health of building occupants is protected from unabated proliferation of pathogenic bacteria [4,5]. In the UK, all water samples taken for microbiological assessment are taken, transported and analysed under UKAS accredited conditions as stipulated by drinking water inspectrate (DWI) for compliance [6]

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Results

Discussion

Conclusion