Overnight stagnation of drinking water in household taps induces microbial growth and changes in community composition

Abstract

Drinking water quality is routinely monitored in the distribution network but not inside households at the point of consumption. Fluctuating temperatures, residence times (stagnation), pipe materials and decreasing pipe diameters can promote bacterial growth in buildings. To test the influence of stagnation in households on the bacterial cell concentrations and composition, water was sampled from 10 separate households after overnight stagnation and after flushing the taps. Cell concentrations, measured by flow cytometry, increased (2–3-fold) in all water samples after stagnation. This increase was also observed in adenosine tri-phosphate (ATP) concentrations (2–18-fold) and heterotrophic plate counts (4–580-fold). An observed increase in cell biovolume and ATP-per-cell concentrations furthermore suggests that the increase in cell concentrations was due to microbial growth. After 5 min flushing of the taps, cell concentrations and water temperature decreased to the level generally found in the drinking water network. Denaturing gradient gel electrophoresis also showed a change in the microbial composition after stagnation. This study showed that water stagnation in household pipes results in considerable microbial changes. While hygienic risk was not directly assessed, it emphasizes the need for the development of good material validation methods, recommendations and spot tests for in-house water installations. However, a simple mitigation strategy would be a short flushing of taps prior to use.

Introduction

The Council Directive of the European Union states that water supplied from a distribution network should fulfill quality requirements (including microbiological parameters) “at the point, within premises or an establishment, at which it emerges from the taps that are normally used for human consumption” (Council Directive 98/83/EC of November, 1998). Similarly, according to the Swiss Health Directive, drinking water is regarded as “consumable” when it meets hygienic and microbial requirements at the point of consumption, again implying specifically the household tap (Verordnung des Eidgenössischen Deparement des Innern (EDI), 2005). However, in most European countries, drinking water quality is not monitored routinely at household level but rather directly in the distribution system, as waterworks and authorities have limited access to private homes, as well as limited control over household plumbing and operation. Household pipes can, in fact, have a considerable impact on the water quality, which was already shown in several large-scale studies addressing heavy metal concentrations in tap water after overnight stagnation (Zietz et al., 2007, Zietz et al., 2003, Haider et al., 2002). These studies all reported increased concentrations of lead, cadmium, copper and nickel after stagnation in household tap water in Germany and Austria.

The impact of stagnation on the microbiological quality is not clear. In the distribution network, bacterial growth is limited by low nutrient concentrations, disinfectant residuals, low temperatures and short residence times (Servais et al., 1992, Kerneys et al., 1995, Niquette et al., 2000). However, it is possible that the microbial quality may change in household pipes, where higher temperatures, longer residence times, depletion of disinfectant residuals and nutrient contamination from pipe material or from the household environment could lead to bacterial growth. This may cause problems, since bacterial growth can lead to adverse aesthetic changes such as the development of taste, odour and colour in drinking water (Mallevialle and Suffet, 1987). Moreover, it bears the potential risk of pathogenic proliferation. The presence of coliform bacteria in distribution systems was previously associated with temperature, disinfectant type and the concentration of assimilable organic carbon (AOC), all factors which might change during stagnation (LeChevallier et al., 1996, WHO, 2006). Similar observations have been reported for Aeromonas hydrophila, Mycobacteria, Escherichia coli O157 and Vibrio cholerae (Van der Kooij and Hijnen, 1988, Torvinen et al., 2004, Vital et al., 2008, Vital et al., 2007). It is likely that those conditions that limit the growth of general heterotrophic bacteria also influence the growth of pathogens. Therefore, a clear understanding of regrowth in general is of considerable value.

Bacterial regrowth on household level was shown in a study in Tuscon (USA) where increased heterotrophic plate count (HPC) concentrations above guideline values were found in the consumers’ taps after stagnation compared to the distribution network (Pepper et al., 2004). However, the study was limited to culturable bacteria, which represent only a small fraction of the total microbial community (Van der Kooij et al., 2003; Siebel et al., 2008, Hammes et al., 2008). A previous study from our group also found significantly higher cell numbers in tap water in the morning compared to the evening in two office buildings, indicating overnight growth (Siebel et al., 2008). However, this study was restricted to two adjacent office buildings and it was not addressed whether changes in cell concentrations were accompanied by a change in microbial communities.

To assess the impact of stagnation on the microbial water quality in more detail, taps from 10 separate households fed with non-chlorinated drinking water from the same distribution system were sampled after overnight stagnation and then after brief flushing. Bacterial cell concentrations and activity were determined with flow cytometry, HPC and adenosine tri-phosphate (ATP) measurements, while microbial diversity was analysed with denaturing gradient gel electrophoresis (DGGE). The aim of the study was to assess the extent of overnight regrowth on drinking water in various households and the influence of regrowth on the stability of the microbial community.