Increasing knowledge about taps and samples

Work carried out by a research engineer at the Stream Industrial Doctorate Centre will improve understanding of sporadic bacteriological non-compliance – and help Severn Trent Water to improve its compliance further.

In 2009, Kate Ellis (below right) enrolled as one of nine research engineers in the newly-formed Stream Industrial Doctorate Centre. Previously, she had studied environmental sciences before working as a wastewater analyst.

Looking for a new challenge, Kate studied for a Master’s degree in water technology and completed a project with the Research and Development team at Severn Trent Water. Kate was subsequently appointed to help them achieve their business target of full compliance with water quality standards, in collaboration with Professor Catherine Biggs, Sheffield University’s Department of Chemical and Biological Engineering, and Dr Michael Templeton, at the Department of Civil and Environmental Engineering, Imperial College London.

Severn Trent Water operates 141 treatment works, 495 service reservoirs and supplies water to eight million customers via a network of underground pipes. Routine water quality monitoring for a range of parameters is carried out at treatment works, service reservoirs and customers’ taps in order to assess compliance with European and national standards.

The company has a business target of zero water quality failures and achieves near excellent compliance with the regulations (consistently above 99.9% since 1997;

A small number of samples do not meet water quality standards; these are investigated immediately and action is taken to ensure quality is restored as soon as possible.

The water treatment process is designed to remove bacteria from raw water and quality is maintained in the distribution system with a residual level of chlorine. Once bacteria has been detected, identification of the root cause is fundamental for preventing recurrence.

This is made difficult to achieve due to the fact that re-samples tend to be compliant, providing no indication as to the reason for the change in quality. A variety of research tools have already been applied to improve understanding of the root causes of sporadic bacteriological non-compliance.

Two key areas remain under-examined: sources and pathways for bacteria in water samples – that is, where they come from and how they get there; and the development and application of site-specific risk management strategies. The tap from which a sample is taken has long been held responsible for bacteriological non-compliance.

To address this, at the start of 2008 a new tap disinfection protocol was initiated by Severn Trent Water; when comparing data for two years either side of this amendment it was observed that the numbers of failures had been halved through this change. By reducing the impact of taps and sampling technique on compliance it becomes easier to identify remaining root causes in terms of sources and pathways.

Kate has started an investigation to characterise the microbial diversity of water samples using routine analyses and molecular microbiological tools. The study area comprises a treatment works and service reservoir separated by 12km of pipe – the simplest section of distribution system that could be identified.

Autumn and winter samples have been collected from the regulatory sample points and analysis is in progress at the University of Sheffield. Single customer tap sampling has also commenced using on-site analysis of residual chlorine and routine bacteriological tests at Severn Trent Water’s laboratory.

Regulatory customer tap sampling involves collecting from properties at a variety of points in the distribution system and thus rarely involves regularly collecting from the same property. This study will therefore enable Kate to monitor the water quality at a single point for a full year to look for seasonal changes.

Other studies have shown that changes to the flow regime of water in pipes can lead to the release of bacteria from stable communities on the pipe wall, and it is a natural extension of this principle when considering the taps and local pipework. In most of Severn Trent Water’s treatment works, sample taps are left running between sampling, whilst at some treatment works and all service reservoirs taps are turned off and flushed prior to collecting samples.

Future work shall determine the impact of operating some sample taps in a flush-first manner. The bacteriological quality of the water and the tap surfaces will be investigated via analyses at Severn Trent Water’s laboratory, specialised methods at Sheffield University and tools developed by collaborators.

The outcomes of this project will have follow-on implications for water collected from customers’ taps too. The next phase of work will involve developing strategies to reduce the impact of tap flushing on the quality of water collected, in conjunction with cost-benefit and environmental impact analyses.

Kate’s research into the conditions under which taps are operated and samples are collected will improve understanding of the occurrences of sporadic bacteriological non-compliance. In adopting optimised methods and tools, Severn Trent Water will be able to improve its near excellent compliance further and make progress toward achieving the company’s challenging business target.

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