Identifying the root cause of bacteriological failures from routine drinking water analyses in the UK is difficult because investigative re-samples are often compliant with the standards. The aim of this paper is to develop understanding of the reasons for sporadic changes in microbiological quality.

Investigative reports for bacteriological failures from one UK water company demonstrated that two thirds of non-compliances had no cause identified; of the remaining third, taps and sample lines were named as the cause. Results from the first five months of an on-going 12-month customer tap study demonstrate a decline in chlorine concentration with an increase in water temperature; no trend in heterotrophic plate counts was observed with either of these parameters. No coliforms, E. coli, enterococci or C. perfringens were enumerated. On two occasions, heterotrophic bacteria were detected on the post-tap-disinfection sample only; on two separate occasions (following tap cleaning with an abrasive sponge) non-coliforms were detected. A separate study using a proprietary hygiene monitoring tool indicated that the cleanliness of internal tap surfaces differed if they were left to run permanently compared with if they were turned on intermittently.

These experimental observations suggest that the tap may either promote the growth of biofilms from indigenous treated-water microorganisms or be an entry point for bacteria sourced from the tap's environment. Further work is proposed to investigate the hypothesis that intermittent tap use leads to the development of weak biofilms and thus to increased bacteriological non-compliance during sampling.