The pursuit of good microbiological conditions in domestic softeners: a new improvement

Effective resin disinfection is mandatory to ensure the microbiological quality of water treated by domestic softeners. The wet and sometimes warm environment inside the softener is ideal for bacteria growth. Our research was focused on the evaluation of the microbial quality of water from softeners sanitized by chlorine solutions or by electrolytic systems. We employed the heterotrophic plate count and specific tests to monitor the presence of opportunistic and pathogenic bacteria (Pseudomonas aeruginosa, Escherichia coli, enterococci, and coliforms). Completely new devices were equipped with a commercially available electrolytic system or with a newly patented one or sanitized by automatic or manual addition of chlorine solutions. In all cases, the contamination was reduced, not completely avoided. In particular, the patented electrolytic system significantly reduced bacterial proliferation in strongly contaminated devices. Our data confirm the difficulties encountered to solve the problem of microbiological quality of softened water and offer encouraging information on new possible solutions. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wh.2020.136 om http://iwaponline.com/jwh/article-pdf/18/2/200/709049/jwh0180200.pdf er 2021 L. Bolelli (corresponding author) Elida Nora Ferri Stefano Sangiorgi Stefano Girotti University of Bologna, Via S. Donato 15, 40127 Bologna, Italy E-mail: luca.bolelli@unibo.it Giuseppe Novelli Culligan Italiana SpA, Via Gandolfi 6, 40057 Cadriano di Granarolo Emilia, Bologna, Italy This article has been made Open Access thanks to the generous support of a global network of libraries as part of the Knowledge Unlatched Select initiative.


INTRODUCTION
Most people consider the quality of tap water unsatisfactory.
Millions of families prefer to drink mineral water or to employ water micro filtering devices to obtain chilled, sparkling, or simply drinking water with a better taste.
Water softeners reduce water hardness avoiding damage to machinery and plants. Softener and micro filtering devices have the same widespread use. Unfortunately, all systems for water distribution or treatment suffer from microflora proliferation and biofilm formation, with consequent inadequate microbiological quality of the delivered water Peter & Routledge ). According to national and international directives establishing the limit values for the microbiological quality of water intended for human consumption, softened water included, the presence of nonpathogenic organisms is admitted in limited amounts while pathogens such as Pseudomonas aeruginosa, enterococci, and Escherichia coli must be completely absent (European Council ; WHO ).
To this aim, domestic softeners undergo manual or automatic sanitization, based mostly on chlorine solutions.
However, various aspects such as the scheduling, the chlorine concentration, and the addition procedure influence the effectiveness of the process. Moreover, maintenance or drainage activities can contaminate the softener's components, such as the outlet and the tubes downstream from the resin container, which are never reached by the flow of sanitizing solution. An electrolytic cell producing chlorine, activated by the brine flow during the resin regeneration phase represents a built-in disinfection system. The electrolytic cells now commercially available produce some chlorine, but its amount and/or the treatment interval are not suitable for effective resin sanitization. The present research was focused on the study of the performance of a newly patented electrolytic system (Novelli & Scaranari ). The microbiological quality of softened water sanitized by the current methods and by the new device was monitored, obtaining promising results.

Softeners and experimental setups
We employed eight new domestic water softeners (WSs) 'Medallist M910' model (Culligan Italiana SpA, Cadriano, BO, Italy) (Figure 1, left). A softener includes the ionic exchange resin container (A) and the brine container (B) connected by a pipe where the commercial electrolytic system is placed when present. Conversely, the newly patented electrolytic system was placed directly inside the brine container.
We planned two experimental periods. First, we compared the effectiveness of three sanitization strategies: 1. Automatic, by a commercially available electrolytic cell (E.C.) (WS no. 1).
The WS no. 8 was the control device, without sanitization.
WSs were connected in parallel to a mechanical filter blocking the particulate impurities, in turn, connected to the main supply (see Figure 1, middle).
During the second period, we evaluated the performance of the new electrolytic system. The WSs no. 1, 2, and 3 were manually sanitized by 14% NaClO solution and each one equipped with the new electrolytic cell. It was activated every 4 h for 2, 8, and 15 min, respectively (see Figure 1, right). The WS no. 8 was still the control, without sanitization.
After 1 month of activity, we switched off the softeners, simulating 7 weeks of inactivity. We restarted the WSs without any sanitization.

Water sampling
Once a week, two water samples (type A and B, 100 mL) were collected in sterile containers (IDEXX Laboratories, Inc., Westbrook, ME, USA) following the Watercoolers Europe Association () procedure. Briefly: once a day for 3 days, we drained off 60 L of water, five times the whole content of the resin container. The day after, we drained off 6 L of water before to collected the first sample (sample A). Immediately after, we drained off 120 L of water and we collected the second sample (sample B).
Each week, we collected from the mains supply a sample of influent water (sample A) and a second one (sample B) after 24 h.

Microbiological assays
The heterotrophic plate count (HPC) of colony-forming

RESULTS
The analyses on the samples collected during the first research period revealed that about half of the A-type samples did not respect the admitted limits concerning or the CFUs or the presence of pathogens. The B samples resulted in definitely below the legal limits. We reported in Table 1 the series of cases of the A-type samples, which can be considered a risk for human health. Presence of P. aeruginosa (▴), presence of coliforms (▪). In both cases, the admitted limit was 0 CFU/100 mL. HPCs above the threshold at 22 C (❍) and 37 C (•). The threshold value was in both cases 100 CFU/mL. Table 2 concern both the A-and B-type samples collected during the 18th week.

The quantitative data reported in
They indicate the performance of the different sanitization strategies. As expected, the B samples were always less contaminated than the respective A ones. The 120 L of water drained off before collecting the B samples were able to remove the main part of bacteria proliferating on the inner surfaces of the device.
During the second experimental period, we employed only four devices, no. 1, 2, and 3, sanitized by the new electrolytic system activated for 2, 8, and 15 min, respectively.
The results from the microbiological tests on all the A samples collected during this period are reported in Table 3. The control, never sanitized, was the WS no. 8.
To evaluate the possibility of correlating a high number of CFUs with the presence of pathogens, we calculated the Pearson index of correlation between the CFU values both at 22 and 37 C and the number of pathogens identified in the A samples. As shown in Table 4, A, in all cases the Index values were far from 1, clearly demonstrating that the presence or absence of pathogens was independent of the total plate counts.    The sanitizing solutions (4, 7, and 15% of NaClO) were introduced automatically at each regenerating phase (kit × %) or manually once a month (man × %). The A and B samples collected from the main supply were always negative for colony growth. Bolded values were those above the respective admitted limits (see Table 1). a E.C. ¼ electrolytic cell.
Bolded values were above the respective admitted limits (see Table 1). a During weeks 5-11, no water samples were collected and the softeners were switched off. Presence of P. aeruginosa (▴), presence of coliforms (▪).
samples collected during the 10th week showed the presence of coliforms in two softeners and of P. aeruginosa in three of them. The contamination occurred during the maintenance manipulations since the technicians did not use sterile gloves. These data, together with those concerning the outlets' sanitization, confirmed that great attention must be paid to hygiene rules during any interventions on the softeners' internal components or on the outlet. Especially at 37 C, the agreement between the data was very good, confirming the negligible effect produced by the presence of the electrolytic cell. The reasons for the low effectiveness of the commercial electrolytic system can be ascribed to its placement and activation time. Placed inside the pipe connecting the two containers, the electrolytic cell interacts with only a little volume of brine.
Moreover, the contact time is very short, corresponding just to the resin regeneration phase.
The automatic and manual sanitization resulted, at all concentrations, in a better effect than the electrolytic system (Table 2). Nevertheless, P. aeruginosa contamination was found in WS no. 2 samples. P. aeruginosa and coliforms were present in samples from WS no. 3 and 4. The addition of NaClO solution was not active enough to remove the contamination by pathogens. The formation of biofilms on the wet surfaces and the particular resistance of P. aeruginosa The newly patented electrolytic system offered good perspectives to solve the problem. It proved to be effective in maintaining the HPCs under the admitted limit when the starting contamination level is quite low, as it must be for new devices, and it is activated at regular intervals longer than a few minutes. Further studies will be necessary to assess the best working parameters for the new electrolytic system. With the collaboration of the manufacturer, the study will continue changing the material of the electrode to increase the efficiency of the sanitization process. With this new configuration of the device, we will attend the increase of chlorine production to reduce the bacteria proliferation.