2 Research and Development, Severn Trent Water Ltd., Coventry, UK. 3 Department of Civil and Environmental Engineering, Imperial College London, London, ...
Improving Drinking Water Quality Compliance K. Ellis1,2, B. Ryan2, M. Templeton3 and C. A. Biggs1
Department of Chemical and Biological Engineering, The University of Sheffield, Sheffield, UK 2 Research and Development, Severn Trent Water Ltd., Coventry, UK 3 Department of Civil and Environmental Engineering, Imperial College London, London, UK 1
Introduction Severn Trent Water (STW) operates 141 water treatment works (WTWs), 495 service reservoirs and supplies water to 8 million customers via a network of underground pipes. Routine water quality monitoring for a range of parameters is carried out at WTWs, reservoirs and customers’ taps in order to assess compliance with European and national standards. STW has a business target of zero water quality failures and achieves near excellent compliance with the regulations (consistently above 99.9 % since 1997; Drinking Water Inspectorate, www.dwi.gov.uk). A small number of samples do not meet water quality standards; these are investigated immediately to identify the root cause of the failure and action is taken to ensure quality is restored as soon as possible. Failures where no cause(s) can be identified present a problem for preventing recurrences. This research therefore seeks to improve understanding of the causes of sporadic changes in bacteriological quality and to provide options for improving compliance in the future.
Methods
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Data Analysis
Results
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Customer Tap Study
Table 1: Summary of root cause analysis findings from the bacteriological failures in 2009 at WTW finals, service reservoirs and customer taps; 43 in total, 0.25 % of routine bacteriological tests. Causes are ‘Known’ if re-samples or swabs are also non-compliant. Unknowns are divided by whether re-samples were collected from the same or a different point.
Data Analysis Despite extensive root cause investigations, the cause of many non-compliances could Root Cause not be identified even of Failure following a re-visit to the Known (single) sample point (Table 1).
Sample Point WTW Final
Reservoir
Tap
2
1
9
0
1
0
0
11
16
0
0
3
Known (multiple)
Customer Tap Study Unknown (same) Observable change in chlorine with seasonal water Unknown (different) temperature (Figure 1a). Low numbers of bacteria detected 1.00 a) (Figure 1b). It is notable that 0.90 tap disinfection did not 0.80 always lead to negative 0.70 0.60 results and that two positive 0.50 tests occurred only on post0.40 disinfection samples. Non0.30 coliform detections were from 0.20 samples collected 1 - 2 days 0.10 after cleaning the tap.
14.0 12.0
Chlorine, mg/L
10.0 8.0
6.0 4.0
Free Cl Total Cl
Temp.
2.0 0.0
25
20 NonCL Post
15
HPC37 Post 10
HPC22 Post
21.04.11
07.04.11
24.03.11
10.03.11
24.02.11
10.02.11
27.01.11
HPC37 Pre 13.01.11
0
30.12.10
NonCL Pre
16.12.10
5
02.12.10
Discussion and Conclusions The ATP test indicated that the manner in which a tap is operated (normal or constantly running) affects the level of biological contamination. The customer tap study suggests that disinfecting or cleaning taps de-stabilises the local microbial community and impacts sample quality. Most WTW taps are constantly running, reservoir taps are flushed prior to weekly sampling and customer taps are in intermittent daily use. Assuming that the initial findings are typical they go some way to explaining the increased frequency of failures at reservoirs and customer taps and the low incidence of cause identification. Further research is required to fully understand the impact of tap operation and sampling protocols.
Project Development
b)
18.11.10
Colony Count
0.00 30
ATP Swab Test
16.0
Temperature, degC
ATP Swab Test 3M’s CleanTrace™ ATP (adenosine tri-phosphate) Swab kit was used to compare levels of biological contamination inside two taps at a WTW: one constantly running and the other in normal use. Fluorescences in relative light units were 15 and 17,000 respectively. The swab blank had a reading of 15; readings in excess of 1,000 are deemed to be ‘contaminated’.
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HPC22 Pre
Date
Figure 1: Results from six months’ twice weekly customer tap sampling – a) onsite data (gaps = no data); b) total plate counts from bacteriological assessment. Heterotrophic plate counts in yeast extract agar (HPC) at 22 and 37 °C and noncoliforms (NonCL), pre- and post-tap disinfection – separated by a bar where applicable. No coliforms, E. coli, C. perfringens or enterococci were detected.
To assess the impact of tap operation, a rig has been designed which will be fed by a reservoir. Taps shall be operated as constantly running or flush first. Water samples will be collected weekly and assessed via R2A agar HPCs, live/dead staining and ATP fluorescence. Tap surface swabs will be used to assess microbial quality of internal tap surfaces and the sample tap environment.
Constantly running
Flush first
Reservoir Figure 2: Schematic of reservoir tap rig – constantly running or flush first operation in triplicate.
Acknowledgements This research is funded under the Stream Programme and is collaborative between EPSRC, Severn Trent Water and Sheffield University. Thanks are expressed to Severn Trent Water staff at Church Wilne Laboratory, Long Eaton and within the Quality and Environment data team, Coventry and Gareth Lang at 3M Food Safety, Loughborough.
