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Article

Differences in Microbial Communities in Drinking Water from Conventional Electronic and Manual Taps in Dependence on Stagnation and Flushing Cycles

1
Department for Integrated Sensor Systems, University for Continuing Education Krems, Dr. Karl Dorrek Straße 30, 3500 Krems, Austria
2
Department of Biotechnology, University of Natural Resources and Life Sciences, 1190 Vienna, Austria
3
WimTec, Freidegg 50, 3325 Ferschnitz, Austria
4
Transhelsa GmbH, Hirtenberger Straße 31, 2544 Leobersdorf, Austria
*
Author to whom correspondence should be addressed.
Water 2023, 15(4), 784; https://doi.org/10.3390/w15040784
Submission received: 10 November 2022 / Revised: 11 January 2023 / Accepted: 9 February 2023 / Published: 16 February 2023
(This article belongs to the Section Water Quality and Contamination)

Abstract

:
Water taps can be a reservoir for microorganisms and pose a risk for contamination and infection. In this work, water samples from different common taps were examined to determine the influence of certain parameters on the microbial load of drinking water. Methods: Four different types of taps were installed along the same water pipe. Over a period of six months, water samples were taken at specific intervals and analyzed for their colony-forming units (CFU/mL) and for the presence of the water pathogens Pseudomonas aeruginosa and Legionella pneumophilia. Two different flushing configurations were investigated: Setup A: the same flush intervals for all taps once a day to determine differences based on type, size and mode of operation. Experimental setup B: different flush cycles for manual and electronic taps to investigate the effects of water stagnation in the tap and whether electronic taps with automatic flushes improve water quality. Results: No Legionella pneumophilia and Pseudomonas aeruginosa were found during the study period. The size of the tap has a great influence on the number of CFU/mL—a maximum of 330 CFU/mL was found in the smallest tap and 1080 CFU/mL in the largest tap, with a significant difference. Stagnation in the tap leads to a significantly higher number of CFU/mL. The results of this work can be used as a basis for the development of innovative taps. There are many possibilities in terms of materials, tap size and intelligent action algorithms—such as automatic flushing—to maintain the quality of our drinking water in a resource-saving way.

1. Introduction

Drinking water is one of the most important nutrients necessary for life [1]. Drinking water is not sterile; 1 mL can contain 10³ to 106 bacterial cells. As long as there are no pathogens in it, the relatively high numbers are not problematic for human health [2]. In Austria, drinking water is classified as a foodstuff and its quality is ensured by various standards, directives and laws, especially the European Drinking Water Directive [3,4,5,6]. The country of Austria is in the fortunate position of being able to cover its demand for high-quality drinking water almost exclusively from protected groundwater resources. The water supply companies have the task of treating water from natural sources, obtaining drinking water of the highest quality and maintaining this quality along the water network up to the water meter of a domestic drinking water system [7]. The domestic water supply, as part of the distribution system, which includes the pipelines between the water meter and the consumers’ taps, is the last stage of a drinking water supply system [8]. Here, the owner is responsible for the quality of the drinking water at all taps of his house or building [9]. There are many conditions and parameters that can affect the chemical or microbial quality of drinking water within a building’s plumbing system and many studies have been conducted to identify and better understand these factors, such as temperature, biofilm formation, stagnation in the pipes, design and materials of pipes and taps [8,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24].
It is known from numerous studies that stagnation periods favor the growth of biofilms in water pipes, which can lead to a higher microbial load in terms of CFU/mL. Biofilms in drinking water systems can serve as an environmental reservoir for pathogenic microorganisms and are, thus, a possible cause of water contamination, which poses a potential health risk to humans [10]. Bédard et al. [13] used a calculated surface-to-volume ratio to estimate the number of microorganisms in water. In the case of water pipes, the surface/volume ratio is the internal surface area of the pipe divided by the volume of water in the pipe, giving the unit cm−1. In their study, they observed a strong correlation between the surface/volume ratio and the number of bacteria found in the water after a certain stagnation time. The higher the surface-to-volume ratio, the higher the number of germs in the water, suggesting that biofilm is an important factor in water contamination. The authors recommend, especially for hospitals, simple taps with minimal internal pipe surfaces and no mixing nozzles to reduce biofilm growth. Electronic taps with automatic flushing are often seen as a solution to the problem of stagnation and resulting biofilm growth. However, it is not yet clear whether electronic faucets promote or inhibit bacterial growth. Sydor et al. [22] found that electronic taps were more frequently contaminated with Legionella and other bacteria. Some components can be the foci of concentrated bacterial growth. Merrer et al. [21] demonstrated that electronic taps are much more likely to be contaminated than manual taps and can be a reservoir for Pseudomonas aeruginosa. Hargreaves et al. [20] found that only one particular type of electronic tap used in a hospital was associated with unacceptable levels of microorganisms in the water. Mäkinen et al. [23] concluded that certain types of electronic taps improve hospital hygiene as they were associated with less microbial growth in biofilms in the tap aerator than some other types of electronic taps or manual taps. A direct effect of the mixing nozzle on the presence of Legionella ssp. could not be found; correct installation was mentioned as a crucial parameter.
In this study, we analyzed how the water quality (in terms of colony-forming units per ml CFU/mL) of drinking water varies for different types of faucets and under different flushing conditions. The results and decisive parameters of this work can be used as the basis for the development of innovative tap systems in terms of materials, tap size and intelligent action algorithms, such as automatic flushing. The aim is to minimize or prevent the settlement and growth of bacteria and water pathogens in order to preserve the quality of our drinking water in a resource-saving way.

2. Materials and Methods

2.1. Construction of the Experimental Setup

Four different conventional, commercially available taps were tested for their microbial load in terms of CFU/mL and for the presence of Pseudomonas aeruginosa and Legionella pneumophilia. The taps differed in function, design and size. The taps used are shown in Figure 1. We had two electronic taps E1 (Figure 1a) and E2 (Figure 1b) and two manual taps M1 (Figure 1c) and M2 (Figure 1d). The selection was made considering the volume of stagnant water. Unfortunately, the exact stagnation water volume is not known. No drawings are available of tap M1 and the exact volume (including screw connections and hoses) is also not known for the other taps. It can only be determined optically that M2 is much larger than M1 and thus has a much larger stagnation water volume. The electronic fittings were originally intended to be used in a different version, which was not available. Therefore, two similar models were chosen. There is a difference in the volume of the stagnation water. E1 has a slightly larger stagnation water volume than E2.
Each of these taps was installed in triplicate along the same water pipeline that led directly from the water supply network into the building. In addition, another single tap was connected to this line. This tap was an electronic tap whose task was to flush the common water line every hour, it is labeled with E-FL. This additional tap ensures that differences in the water samples are only due to differences in the design and size of the taps, as the common pipe is always flushed in the same way, providing the 12 taps with the same water quality. It is important to note that in this setup, only the influence of the different taps on drinking water quality was investigated and not the other parts of the water installation pipe system. There were additional sampling points to check the water quality before and after construction. Sampling point no. 1 was used to check the water supplied from the water network, sampling points no. 2 and no. 3 were used to check the common water pipe behind the connection points of the 13 taps. The test setup is shown in Figure 2a,b. The water enters the building from the outside. One line with the cold water goes directly to the water taps E1, E2, M1, M2 and E-FL, and the other line goes via the boiler (65 °C) to the water taps. The taps are controlled by a microcontroller board (Raspberry Pi, Linux, and Phyton) to ensure regular flushing and a fixed water temperature (32.5+/−1 °C) at the taps. The draining water is led directly into the drain of the building. At certain intervals, about once a month, water samples were taken and analyzed for colony-forming units (CFUs) and contamination with water pathogens, such as Legionella pneumophilia and Pseudomonas aeruginosa. Section 2.3 explains the importance of these three microbial parameters in domestic water installations. All taps were controlled and activated via a touch panel. The manual taps M1 and M2 were operated or controlled via servo control, see Figure 2c.
There were two different experimental runs with different flush setups:
  • Setup A: Equal flushes for all types of taps: all taps were flushed once a day.
  • Setup B: Different flushing intervals for the different types of taps: the electronically controlled taps (E1 and E2) were flushed once a day and the manually controlled taps (M1 and M2) once a week.
Experimental setup A was chosen to determine any differences between the taps based on parameters such as design, size and operating principle (electronic or manual).
Experimental setup B simulated stagnation to answer the following questions: Does a longer stagnation time promote the growth of biofilms inside the tap, leading to a higher total bacterial count in the water samples? If so, can electronic taps with automatic flushing help to solve this problem? This issue is of interest for water quality in households that are only used at weekends or during holidays and are represented here by the manual taps M1 and M2.
Table 1 summarizes at a glance the flushing intervals for the two different flushing setups and the question of interest behind the chosen parameters.

2.2. Sampling and Microbial Analysis

Sampling was performed 20 h after the last flush in each setting. The sampling and subsequent laboratory work were the same for both settings. An overview of the analytical procedure is shown in Figure 3. One liter of water from each tap was filled into bottles, water temperature was measured and the samples were taken to the laboratory for further processing. To determine CFU/mL, aliquots (100–200 mL) were applied to yeast extract and incubated for 48 h at 37 °C and for 72 h at room temperature. The work was carried out according to the general requirements and guidelines for microbiological investigations by culture (ÖNORM, ISO) [6,25,26,27]. For each tap type and growth temperature, CFU/mL were counted, mean values were calculated and presented in boxplots, and statistical tests (ANOVA—analysis of variance) were performed. The plates for incubation at 37 °C were prepared in duplicates. For the detection of Legionella pneumophilia and Pseudomonas aeruginosa, 100 mL of the samples were filtered and prepared according to established guidelines [28,29,30,31].

2.3. Microbial Risk Assessment

Drinking water is a natural product and therefore not germ-free. Microorganisms occur everywhere in the environment—in the air, in water and also in humans. If fungi, viruses and bacteria are in equilibrium with their environment, they usually pose little risk, but if this equilibrium is upset by certain factors (e.g., stagnation, temperature, nutrients or pollution) growth can explode and become a health problem. The Drinking Water Ordinance defines limits that may not be exceeded and provides for strict control [32,33].
The presence of pathogenic bacteria in domestic water systems is a very common problem, with Legionella ssp. and Pseudomonas ssp. being the most significant representatives and also the most common causes of infection. In Austria and Germany, regular inspections of water installations for Legionella are already mandatory. In order to minimize the risk of bacterial contamination, appropriate precautions are necessary. In Austria, ÖNORM B 5019:2017 is intended to prevent infections caused by microbial contamination and deals with hygiene-relevant aspects of central hot water supply systems. The standards formulated therein apply specifically to bathing facilities, hospitals and spas, care and communal facilities, accommodation establishments, residential complexes and public buildings. If pathogens are detected in the water, different steps—from medium-term to immediate remediation—must be taken, depending on the detected concentration, and they also are defined in ÖNORM B 5019. The number of colony-forming units in water provides information about their general microbiological condition and their limit is specified in the Drinking Water Ordinance.
In our study, we selected CFU/mL (incubated at 22 °C and 37 °C), Pseudomonas aeruginosa and Legionella pneumophila as indicator germs for evaluating water quality. Experience has shown that these microbiological parameters are important for investigations in domestic installations and should be taken into account as investigation parameters [6].

2.3.1. Colony-Forming Units

Colony-forming units include bacteria, yeasts and molds that require an external source of organic carbon for growth. The term colony count, also called colony-forming unit, was bindingly defined in the Drinking Water Ordinance 1975. The total colony count in water provides information on the general microbiological condition of the water. In the laboratory, water samples are exposed to an incubation temperature of 22 °C or 36 °C. The subsequent different colony formation takes place at different temperatures and provides information about the origin of the organisms. An increased colony count at 22 °C indicates naturally occurring germs in the water that are potentially less hazardous to health. Much more problematic are germs that proliferate, such as colonies after incubation at 37 °C, as the body temperature of mammals is simulated here [33].
CFU levels are a good indicator for an increase in ideal bacterial growth conditions, which can lead to pipe corrosion, slime growth, altered water taste, increased need for disinfection, and harboring secondary respiratory pathogens. The increase in colony numbers at the various incubation temperatures always indicates the contamination of a house’s drinking water installation. The causes can be manifold: calcified taps, construction and maintenance work, burst pipes, drying out, stagnant water, insufficient water flow, unsuitable materials, and missing or wrong cleaning agents. Formed biofilms favor the growth of other microbes, such as legionella, mycobacteria, pseudomonads and amoebae. Germs that grow at 36 °C are particularly critical. Contaminated installations can pose a serious risk to people with weakened health, the elderly and small children. The route of transmission is either direct drinking water consumption or the inhalation of water vapor [33].

2.3.2. Pseudomonas aeruginosa

In the cold-water supply sector, contamination by pseudomonads is important in addition to fecal indicators. Pseudomonas aeruginosa is a common cold-water germ that can also colonize slightly moist niches. This germ is regularly found in sewage, surface water and cleaning utensils such as cloths and sponges, but also in sink siphons and drains. Pseudomonads are characterized by their extremely low nutrient requirements and their high ability to multiply even at temperatures below 15 °C. For this reason, all waters, including drinking water, can be contaminated. Pseudomonas aeruginosa can cause skin infections. A healthy person usually has only a low risk of contracting an infection with Pseudomonas aeruginosa. The situation is different for immunocompromised people, such as those living in hospitals, nursing homes or other medical facilities. In these facilities, Pseudomonas aeruginosa is one of the most common pathogens of various infections, often with fatal outcomes. The water for hospitals, other medical facilities and care facilities must not contain pseudomonads at the transfer point from the drinking water supply to the building [33,34].

2.3.3. Legionella pneumophilia

Legionella is an environmental germ. Accordingly, they are present in practically all freshwater bodies. Legionella can also occur in cold water. However, they do not multiply significantly at temperatures below 15 °C. Legionella finds ideal growth conditions at a water temperature of 25 to 45 °C. As soon as the water temperature exceeds 55 °C, the growth of the legionella is effectively inhibited until the bacteria finally die off above 70 °C. In principle, all Legionella should be classified as potential human pathogens. In Europe, however, most infections are due to Legionella pneumophila [26,29].
Infection with Legionella pneumophila usually occurs through inhalation of finely atomized water particles contaminated with Legionella. People with weakened immune systems, chronic lung diseases and smokers have an increased risk. Transmission from person to person is not possible. Besides wound infections, there are two different types of infections. On the one hand, a distinction is made between the usually harmless Pontiac fever—also called “summer flu”—and, on the other hand, the actual legionellosis, which is also generally referred to as “Legionnaires’ disease” or “Legionella pneumonia”. Not only the respiratory tract, but also wounds and aspiration are possible ports of entry for Legionella into the human body. However, these two infection routes occur less frequently. Swallowing water contaminated with Legionella does not pose a health risk, as Legionella are killed in the stomach by gastric acid [33,35].
The microbial risk for colony-forming units, Pseudomonas aeruginosa and Legionella, was estimated according to ÖNORM B 5021:2020-08 and EN ISO 13843:2017, and is shown in Figure 4 [36,37]. The water quality of the four tap groups was assessed using this table and is discussed in Section 3 and Section 4.

3. Results

As stated above, the water from the water supply was consistently supplied in hygienically perfect condition during all sampling. This was checked with the help of the additional sampling point no. 1 directly after the supply line into the building and even before the setup with the taps to be tested (see Figure 2a). Colony-forming units are neither found in incubation at 37 °C nor in incubation at 22 °C. No pseudomonads or legionella could be detected here either. With the help of the additional sampling points no. 2 (cold water line out) and no. 3 (hot water line out), it was possible to check for possible contamination outside the taps and the effectiveness of the free flushing of the common pipe. The values were always within the hygienically perfect range with a maximum of 10 CFU/mL (incubated at 37 °C). No pseudomonads or legionella could be detected. This leads to the conclusion that the free flushing by the additional water tap E-FL was very effective and that all increased CFU values of the water samples from the individual water taps (E1, E2, M1 and M2) are exclusively due to the type and construction and the associated stagnation volume of the water tap. The CFUs/mL for the additional sampling points are provided in the supplementary data, Table S4.

3.1. Experimental Setup A: Same flush Cycle for all Taps

All taps were flushed once a day. This experimental setup was chosen to determine whether the water samples from the different taps differ in their CFU numbers. Any differences are due to size, material, construction and mechanics.

3.1.1. Legionella pneumophila

No colony-forming units of Legionella pneumophila were detected in any group of taps.

3.1.2. Pseudomonas aeruginosa

No colony-forming units of Pseudomonas aeruginosa could be detected in any group of taps.

3.1.3. Colony-Forming Units (CFUs)

The values for CFU/mL are shown in boxplots, see Figure 5a,b. Each box represents a type of tap (in triplicates) and contains the results of all samplings. The limit ranges for the assessment of water quality based on the associated microbial risk have been drawn from the table in Figure 4. The green area stands for hygienically perfect drinking water quality, the yellow area for hygienically acceptable drinking water quality and the orange area for hygienically poor drinking water quality. CFU numbers for incubation at 37 °C ranged from 0 to 880 CFU/mL (see Figure 5a). No statistically significant differences were found between the taps (p = 0.359). The boxplots for CFU/mL for the 72 h incubation at room temperature (RT) are shown in Figure 5b. The numbers of CFU/mL for this ranged from 0 to 800 CFU/mL. No statistically significant differences were found between the taps (p = 0.159).
For both incubation temperatures (37 °C and RT), the water samples from the different taps do not differ significantly in their CFU values.
If we look at the risk assessment table (Figure 4) and the corresponding marked areas, one can see that the water quality for taps E1, E2 and M1 is in the hygienically acceptable range, but that tap M2 is of poor quality for some measurements. For CFU values incubated at 22 °C, all taps are in the acceptable range.

3.2. Setup B: Different Flushings: Flushing Once a Day versus Once a Week

In this setting, the electronic taps E1 and E2 were flushed once a day, and the manual taps M1 and M2 once a week (see Table 1). This experimental setup investigates the question of whether stagnation in the tap favors the formation of biofilms, which leads to higher CFU numbers in the drinking water. This simulates the situation in holiday or weekend homes, where stagnation often lasts for a week or more.

3.2.1. Legionella pneumophila

No colony-forming units of Legionella pneumophila were detected in any group of taps.

3.2.2. Pseudomonas aeruginosa

No colony-forming units of Pseudomonas aeruginosa could be detected in any group of taps.

3.2.3. Colony-Forming Units (CFUs)

The results are shown as boxplots in Figure 5c,d in the same way as for Setup A (Section 3.1.3) but only for incubation at 37 °C for 48 h. We found statistically significant differences between the taps. Fewer flushes lead to a significant higher number of CFU/mL.
If the faucet is large, such as faucet M2, this results in a large stagnant volume of water inside, and the number of CFU/mL can become very high. We found statistically significant differences between E1 and M2, E2 and M1, E2 and M2.
Figure 5d is a more detailed diagram for the manual tap M1 compared to the electronic taps E1 and E2. This Figure is very interesting when compared with Figure 5a. In Figure 5a we see that the M1 armature has lower CFU/mL values than the E1 and E2 armatures. Figure 5c shows a completely different picture caused by less frequent flushing of the M1 armature. Tap M1 has higher CFU/mL values than taps E1 and E2. The difference between M1 and E2 is statistically significant.
If we look at the risk assessment table (Figure 4) and the corresponding marked areas, one can see that the water quality for taps E1, E2 and M1 is in the hygienically acceptable range, but that tap M2 is of poor quality for all measurements.
Figure 6 shows a comparison between daily and weekly flushing for the taps M1 and M2. Flushing once a week leads to higher CFU/mL counts for both taps. The difference between once-daily flushing and once-weekly flushing is clearer for tap M2 than for tap M1. For tap M1 we found no statistically significant difference (p = 0.055). For tap M2, the difference is even statistically significant. The water quality here changes from acceptable/poor to poor. This suggests that the larger the tap, the more important regular flushing is to reduce microbial contamination.

4. Discussion

Water taps are perceived as a recognized reservoir for microorganisms and a risk for infections. Taps are the last link in the long chain from the water supplier to the consumer. They differ fundamentally in terms of area of application, type of attachment and functionality, and there are almost no limits to the choice of design. For several decades now, electronic sensor taps have been used in both public and private areas. They have increasingly replaced conventional manual water taps, as they can be operated without touching them and thus represent the most hygienic solution. However, criticism of these touchless taps has been mounting, and several studies have shown that the risk of contamination is significantly higher with electronic taps than with conventional manual models. In [19,20,21,38], it was found that electronic taps are much more likely to be contaminated than manual taps and could be a reservoir for water pathogens such as Pseudomonas aeruginosa. However, after reviewing those studies and some other related studies [22,38,39,40,41,42,43], we see: The main cause of microbial contamination is water stagnation as a result of inadequate use. Insufficient withdrawal quantities or flow rates often lead to the build-up of a persistent biofilm that is difficult to remove. If the normative specifications for temperatures are then also disregarded, microbiological growth can increase almost exponentially. Retrograde contamination of taps due to improper handling (cleaning, touching or spraying with contaminated objects or substances) should also not be ignored. This applies to both manual and electronic taps. If adequate use is not given, a further reduction in water consumption by electronic taps is counterproductive. Modern electronic taps, however, offer an effective tool to counteract insufficient use with the demand-oriented, intelligent automatic free-flush function. It is obvious that the use of taps with automatic free-flush systems is particularly recommended in places where manual taps are not sufficiently used [44], and this is exactly what we see in our results. In setup A with the same flushing for all taps, more CFUs are found in the water samples from taps E1 and E2 compared to the manual tap M1 (the differences are not statistically significant). At first glance, this seems to confirm the assumption that electronic taps would be more contaminated. However, when looking at the manual tap M2, it is much more likely that the stagnation caused by the size of the M2 tap is responsible. It is known from the literature that stagnation in water systems can affect water quality [13,14,15,16,45]. Stagnant water can have chemical and microbiological contaminants that pose a potential health risk to residents [45]. Most of these studies related to whole-building plumbing systems—that is, not just the taps, but also the pipes in the building that lead to the taps. The larger the building, the longer the water pipe network, and the more important intelligent flush cycles become. Our study looked at the effects of stagnation only within the tap. All external parts of the pipe system were flushed regularly every hour via an additional tap. So even though the volume of stagnant water is very small compared to the volume of water in the piping system of an entire building, and even though the tap was flushed 20 h before sampling, we found a statistically significant change for tap M2 when changing the flushing interval from once a day to once a week. The number of CFU/mL in the water samples is statistically significantly higher in setup B (flushing once a week) than in setup A (flushing once a day). Tap M1, the smallest tap, and therefore, also the tap with the smallest stagnant water volume inside, has the lowest CFU/mL value of all taps when flushed once a day. If the flush cycle is reduced to once a week, the number of CFU/mL increases distinctly, which we can see for the manual taps M1 and M2 in Figure 6. We conclude that longer stagnation times favor the formation of biofilms, which leads to a higher number of CFU/mL in the water samples. The ability of biofilm growth in pipes and taps is mainly determined by the surface area available for colonization and the nature of the material. The larger the tap, the greater the internal surface area, which provides more attachment sites for biofilm development [13]. Bédard et al. calculated the ratio between the surface area of the inside of the pipe and the volume of water in the pipe. They called this value “surface-to-volume-ratio” and calculated it for different sections of the water pipe system. They found an excellent correlation between the number of CFU/mL in the water samples and the surface/volume ratio. Tap M2, by far the largest tap of all the taps in this study, fits this pattern. For setup A, flushing once per day, M2 has the most CFU/mL in the water samples of all taps for both incubation temperatures. In setup B, flushing once a week, there are also significantly more CFU/mL in the water samples from M2 compared to water samples from M1. Elevated total germ counts can be reduced with free flushes, as already discussed in Section 3.2.3. With regard to water quality classified according to ÖNORM B 5021:2020-08, an improvement in quality from hygienically poor to hygienically acceptable was achieved.
Limitations of this study: The cold water was not kept at a constant temperature. However, this corresponds to the real application in practice. In contrast, the hot water temperature was the same over the entire test period (apart from the hysteresis between switching the DHW heater on and off). This also corresponds to the real application in practice. The two flush cycles in our study took place at different times of the year. However, by recording water temperature and total bacterial count in the supply line, we can exclude an influence on the total bacterial counts of the individual taps. We compensated for different solar irradiation and the resulting heating of the experimental room and setup as best as possible with heat protection foil. The total bacterial counts were not analyzed according to their exact composition to see whether certain microorganisms were dominant and whether there were differences in the composition of the total bacterial count depending on the tap and flush cycle. In further work, this question could be clarified with the help of DNA analytics. Moreover, at the end of a study, the taps could be disassembled and biofilm swabs could be taken from different parts of the tap.

5. Conclusions

The aim of this study was to examine both manual and sensor-controlled taps in a long-term test over a longer period of time and to analyze the bacterial load and also to substantiate the importance of automatic free flushing. The results of this study should serve as a basis for the development of a new type of tap design as well as the integration of an innovative methodology for the control of electronic taps. Design modifications, new action algorithms and advanced control elements can be derived from the data obtained, which will be used to design, develop and build a new generation of sensor installations.
  • We found no statistically significant differences in the numbers of CFU/mL between electronic and manual taps. Stagnation volume and time have a greater impact on the CFU counts.
  • Longer stagnation times in the tap lead to a higher number of CFU/mL.
  • Tap size—which correlates with a larger volume of standing water within the system—appears to have a large effect on the number of CFU/mL; the larger the tap, the greater the microbial load of CFUs/mL.
  • Regular flushing seems to be a good strategy to reduce the number of CFUs/mL in drinking water.
  • Electronic taps can be programmed to flush water pipes free at regular intervals. Depending on the tap and the place of use, this can be customized, allowing a high level of hygiene to be achieved with electronic taps.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w15040784/s1, Table S1: CFU/mL for the single taps #1–#12 for flushing Setup A, Incubation at 37 °C. Table S2: CFU/mL for the single taps #1–#12 for flushing Setup A, Incubation at room temperature (RT). Table S3: CFU/mL for the single taps #1–#12 for flushing Setup B, Incubation at 37 °C, Table S4: CFU/mL for the additional sampling points.

Author Contributions

Conceptualization, A.E.K., J.E., T.P., M.T., M.H., H.H., S.G. and M.B.; methodology, A.E.K., J.E., T.P., M.T., M.H., H.H., S.G. and M.B.; software, T.P.; data curation, A.E.K. and J.E.; writing—original draft preparation, A.E.K.; writing—review and editing, M.B.; project administration, M.B.; funding acquisition, M.B. and J.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the European Regional Development Fund (ERDF) and the state of Lower Austria, grant number K3-W-47/007-2017 and WST3-F-5030664/006-2017. Open Access Funding by the University for Continuing Education Krems.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The water taps used in this study: (a) electronic tap E1, (b) electronic tap E2, (c) manual tap M1 and (d) manual tap M2. E1 and E2 are very similar models, they differ in their volumes of stagnant water. M2 is much larger than M1 and the other taps.
Figure 1. The water taps used in this study: (a) electronic tap E1, (b) electronic tap E2, (c) manual tap M1 and (d) manual tap M2. E1 and E2 are very similar models, they differ in their volumes of stagnant water. M2 is much larger than M1 and the other taps.
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Figure 2. (a) Schematic illustration of the test setup: The water comes from the house water installation of the building. The cold water goes directly to the taps, warm water is prepared in a boiler and then directed to the taps. The taps are controlled by an electronic control unit. The runoff water goes directly to the drain of the building. The taps of the different tap groups are drawn in different colors and named in the legend as follows: yellow = E1, red = E2, blue = M1, green = M2 and orange = E-FL (additional electronic tap for flushing). The additional sampling points no. 1, 2 and 3 are marked with yellow triangles. Sampling point no. 1 was used to check the water supplied from the water network, sampling points no. 2 and no. 3 were used to check the common water pipe behind the connection points. (b) Photograph of the installation setup with the 4 different taps in triplicates plus the additional electronic tap for flushing (E-FL). (c) Side view of the installed setup. Some of the servo controls for the manual taps are marked in white circles.
Figure 2. (a) Schematic illustration of the test setup: The water comes from the house water installation of the building. The cold water goes directly to the taps, warm water is prepared in a boiler and then directed to the taps. The taps are controlled by an electronic control unit. The runoff water goes directly to the drain of the building. The taps of the different tap groups are drawn in different colors and named in the legend as follows: yellow = E1, red = E2, blue = M1, green = M2 and orange = E-FL (additional electronic tap for flushing). The additional sampling points no. 1, 2 and 3 are marked with yellow triangles. Sampling point no. 1 was used to check the water supplied from the water network, sampling points no. 2 and no. 3 were used to check the common water pipe behind the connection points. (b) Photograph of the installation setup with the 4 different taps in triplicates plus the additional electronic tap for flushing (E-FL). (c) Side view of the installed setup. Some of the servo controls for the manual taps are marked in white circles.
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Figure 3. Overview of the analytical steps for determining numbers of colony-forming units, Pseudomonas aeruginosa and Legionella pneumophila.
Figure 3. Overview of the analytical steps for determining numbers of colony-forming units, Pseudomonas aeruginosa and Legionella pneumophila.
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Figure 4. The estimated microbial risk for colony-forming units, Pseudomonas aeruginosa and Legionella in colony-forming units per milliliter (CFU/mL) according to ÖNORM B 5021:2020-08. The different areas were marked in different colors: green = hygienically perfect, yellow = hygienically acceptable, orange = hygienically poor, red = hygienically not acceptable. This colors will be used in the results to mark the ranges the diagrams.
Figure 4. The estimated microbial risk for colony-forming units, Pseudomonas aeruginosa and Legionella in colony-forming units per milliliter (CFU/mL) according to ÖNORM B 5021:2020-08. The different areas were marked in different colors: green = hygienically perfect, yellow = hygienically acceptable, orange = hygienically poor, red = hygienically not acceptable. This colors will be used in the results to mark the ranges the diagrams.
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Figure 5. (a,b) Setup A: Colony forming units per milliliter (CFU/mL) in Boxplots for flushing all taps once a day. (a) CFU/mL for the different taps for incubation at 37 °C for 48 h. (b) CFU/mL for the different taps for incubation at RT for 72 h. (c,d) Setup B: Colony forming units per milliliter (CFU/mL) in Boxplots for flushing the electronic taps E1 and E2 once a day and the manual taps M1 and M2 once a week. (c) CFU/mL for the different taps for incubation at 37 °C for 48 h. (d) A more detailed graph for tap M1 compared to taps E1 and E2. One box shows all measurements for one tap (in triplicates). Statistically significant differences are marked with *. The limit ranges for the assessment of water quality based on the associated microbial risk have been drawn from the table in Figure 4. The green area stands for hygienically perfect drinking water quality, the yellow area for hygienically acceptable drinking water quality, and the orange area for hygienically poor drinking water quality.
Figure 5. (a,b) Setup A: Colony forming units per milliliter (CFU/mL) in Boxplots for flushing all taps once a day. (a) CFU/mL for the different taps for incubation at 37 °C for 48 h. (b) CFU/mL for the different taps for incubation at RT for 72 h. (c,d) Setup B: Colony forming units per milliliter (CFU/mL) in Boxplots for flushing the electronic taps E1 and E2 once a day and the manual taps M1 and M2 once a week. (c) CFU/mL for the different taps for incubation at 37 °C for 48 h. (d) A more detailed graph for tap M1 compared to taps E1 and E2. One box shows all measurements for one tap (in triplicates). Statistically significant differences are marked with *. The limit ranges for the assessment of water quality based on the associated microbial risk have been drawn from the table in Figure 4. The green area stands for hygienically perfect drinking water quality, the yellow area for hygienically acceptable drinking water quality, and the orange area for hygienically poor drinking water quality.
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Figure 6. Comparison between daily and weekly flushing for the manual taps (a) M1 and (b) M2. One box contains all measurements of colony forming units per milliliter (CFU/mL) for one flushing setup. Statistically significant differences are marked with *. The limit ranges for the assessment of water quality based on the associated microbial risk have been drawn from the table in Figure 4. The green area stands for hygienically perfect drinking water quality, the yellow area for hygienically acceptable drinking water quality, and the orange area for hygienically poor drinking water quality.
Figure 6. Comparison between daily and weekly flushing for the manual taps (a) M1 and (b) M2. One box contains all measurements of colony forming units per milliliter (CFU/mL) for one flushing setup. Statistically significant differences are marked with *. The limit ranges for the assessment of water quality based on the associated microbial risk have been drawn from the table in Figure 4. The green area stands for hygienically perfect drinking water quality, the yellow area for hygienically acceptable drinking water quality, and the orange area for hygienically poor drinking water quality.
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Table 1. Overview of the taps, their labels and flushing intervals for the two setups and the question behind the setup.
Table 1. Overview of the taps, their labels and flushing intervals for the two setups and the question behind the setup.
Electronic (E1, E2)Manual
(M1, M2)
Electronic Tap for Flushing
(E-FL)
Question
Setup A:Once a dayOnce a dayhourlyAre there differences in microbial load because of tap type (size, design, operating principle)?
Setup B:Once a dayOnce a weekhourlyDoes stagnation within the tap favor biofilm growth? Can electronic faucets with automatic flushing improve water quality?
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MDPI and ACS Style

Knecht, A.E.; Ettenauer, J.; Posnicek, T.; Taschl, M.; Helmecke, M.; Haller, H.; Gölß, S.; Brandl, M. Differences in Microbial Communities in Drinking Water from Conventional Electronic and Manual Taps in Dependence on Stagnation and Flushing Cycles. Water 2023, 15, 784. https://doi.org/10.3390/w15040784

AMA Style

Knecht AE, Ettenauer J, Posnicek T, Taschl M, Helmecke M, Haller H, Gölß S, Brandl M. Differences in Microbial Communities in Drinking Water from Conventional Electronic and Manual Taps in Dependence on Stagnation and Flushing Cycles. Water. 2023; 15(4):784. https://doi.org/10.3390/w15040784

Chicago/Turabian Style

Knecht, Anja E., Jörg Ettenauer, Thomas Posnicek, Martin Taschl, Marcus Helmecke, Hannah Haller, Stefanie Gölß, and Martin Brandl. 2023. "Differences in Microbial Communities in Drinking Water from Conventional Electronic and Manual Taps in Dependence on Stagnation and Flushing Cycles" Water 15, no. 4: 784. https://doi.org/10.3390/w15040784

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