Analysis of the Effects of Using Classical and Modified Pool Water Treatment Technologies

There are approx. 760 indoor public swimming pool facilities in Poland, most of which (66%) are regular swimming pools (25 m × 12.5 m), 23% are training and recreation pools (at least 16 m long), 9% are sports-type swimming pools (25 m × 16 m) and 2% are Olympic pools (50 m long) [1]. Many of them, due to very strict requirements, struggle with the problem of maintaining adequate water quality.

The current requirements as to the quality of swimming pool water [2–5] necessitate the improvement of the classic technology of its treatment (Fig. 1). In the majority of cases, it is associated with the need to modernize existing installations or implement a new technology.

Figure 1.

The classic technology of treatment and disinfecting swimming pool water, used in many swimming pool facilities, does not allow for the sufficient removal of chemical compounds which are products of chlorine reacting with pollutants introduced into the water by bathers and which often have an adverse effect on their health [6–9]. In order to maintain the proper quality of water in swimming pools, especially in public ones with high bathers loads, the classic technological systems of swimming pool water treatment are now being abandoned in favor of systems using filtration and final disinfection with a chlorine compound supported by ozone [10–12], dosing products with nanosilver [13], irradiation with UV rays [14–17] or ultrafiltration [18, 19]. Including these processes in the technological system of swimming pool water treatment allows to lower the doses of chlorine in the final stage of its disinfection, which in turn reduces the amount of disinfection by-products (DBPs) [9, 12]. Many epidemiological studies have shown that both short-term and long-term exposure of swimmers and bathers to DBPs causes allergic reactions, the development of respiratory and digestive system diseases, changes in metabolic profiles, as well as genotoxic and mutagenic changes [20–24].

Modernization of water treatment systems in many swimming pool facilities consists in the replacement of filter units (the old, often horizontal filters, filled only with quartz sand, are replaced by highly effective multilayer filters with a layer of anthracite, activated carbon, zeolite), and the use of high-quality chemicals, e.g. coagulants containing a solution of silicic acid, sodium tetrahydroxy-aluminate, pentahydroxy-aluminum chloride, installation of automatic chemical dosing systems, measurement and control of circulating water quality, the use of equipment for cleaning the bottom and walls of swimming pools and the use of modern methods and techniques for disinfecting swimming pool water [25].

In the majority of swimming pool facilities built in the last five years, the installed water purification and disinfection systems allow for comfortable bathing and protection against bacteriological contamination, compounds that may cause allergies, irritation of mucous membranes or that have carcinogenic properties. In older facilities, where the water quality does not meet the requirements specified in the guidelines, this situation can only be changed by the modernization of the swimming pool water treatment system [9, 14, 26].

The main goal of this analysis was to compare the effects of using classic and modified technologies for the treatment and disinfection of swimming pool water in sports and recreational pools with similar technical and functional characteristics.

The research was conducted for ten sports and recreational swimming pools (P1, P2, P3, P4, P5, P6, P7, P8, P9 and P10), which were divided into two groups. The first group includes pools with a classic water treatment system (WTS) or a system resembling the classic one. This group has been designated as P classic WTS and includes pools P1, P2, P3, P4, P5 and P6. The second group includes swimming pools with a modified WTS. This group has been designated as P modified WTS and includes pools P7, P8, P9 and P10. The division of pools into groups with the characteristic features of the filtration and/or disinfection process in the water treatment system is presented in Table 1.

In addition, it should be added that in the group of pools with classic WTS, in pools P2 and P6 an additional active carbon filtration layer was used and in pools P4 and P5 chemical disinfection (dosing of NaOCl solution) supplemented with physical disinfection (UV irradiation) was used. Nevertheless, these processes do not significantly modify the WTS, and in the DIN19634 standard of 2012 and the Pool Water Treatment Advisory Group (PWTAG 2017) guidelines, they are recommended for standard use in swimming pool water circuits [3, 4].

All the analyzed pools are supplied with water from the municipal water supply, which meets the quality requirements for drinking water [27, 28]. The pools are equipped with a vertical water flow system with an active overflow, which discharges the displaced water to the retention tanks. Water is pumped from the tanks to the filtration system by means of circulation pumps integrated with pre-filters. In P8 pool, an open vacuum filter with filter elements washed with diatomaceous earth was used. The remaining pools have a filtration system consisting of pressure filters with multilayer beds. A 0.5% solution of hydrolyzed aluminum chloride was dosed into the pipeline supplying water to the filters. The average dose of this coagulant was 0.5–1.0 mL/m³. The same length of the filtration cycle was observed during the research. Each filter worked for 3 days before its filter bed was rinsed. If the pH of the water needed to be adjusted, a 30% sulfuric acid solution was used in all pools. The pool water treatment plants have been equipped with automatic systems for dosing reagents and controlling basic water quality parameters (temperature, pH, redox potential, free chlorine, combined chlorine, ozone). Each of the pools was available to bathers 16 hours a day.

The basic technical parameters of the swimming pools and the average attendance are presented in Table 2.

Water quality tests in the selected pools were carried out for 6 weeks. Samples were taken at fixed times (between 9 a.m. and 11 a.m.) twice a week (Mondays and Thursdays) for physicochemical analyses. Once every two weeks (Thursdays), samples were taken for bacteriological analyses. On the scheduled days of sampling from the pools, students from nearby schools took part in swimming lessons. In total, 12 water samples for physicochemical analyses and 3 water samples for bacteriological analyses were collected from each pool.

The main objective of the physicochemical tests was to determine the parameters of water quality, taking into account substances harmful or hazardous to health, such as combined chlorine (chloramines) and trihalomethanes (THMs). The main goal of bacteriological tests was to assess the risk associated with microbial contamination.

Water sampling was performed in accordance with the guidelines of the standard PN-EN ISO 5667-3: 2013-05 (Water quality – Sampling – Part 3: Fixation and handling of water samples). The swimming pool water samples were taken from a depth of approx. 30 cm below the surface and approx. 50 cm from the edge of the pools. The water sample for the final analyses was a mixed sample, i.e. collected in at least three characteristic points of the pool basin.

The analysed physicochemical parameters were: nitrates and ammonium ion (photometric method, DR 3900 spectrophotometer with RFID technology, Hach^®, Loveland, CO, USA), free chlorine and combined chlorine (photometric method, Pocket Colorimeter II DeviceTM, Hach^®, Loveland, CO, USA), chloroform (trichloromethane) and the sum of THMs (trichloromethane, bromodichloromethane, dibromochloromethane, tribromomethane), (gas chromatography method, Agilent Technologies GC7890B chromatograph with MSD5977A mass detector, USA), temperature, redox potential and pH (potentiometric method, SensION meter + MM150 DL, Hach^®, Loveland, CO, USA), total organic carbon – TOC (catalytic oxidation combustion method at 680°C, TOC-L series analyser, Shimadzu, Japan), permanganate index (titration method, kit for titration oxidation analysis in acid medium) and turbidity (nephelometric method, TN-100 turbidimeter, Eutech®, Singapore). All measurements/analyses were conducted in triplicate.

The analysed bacteriological parameters were colony forming units (CFU) of Escherichia coli and Pseudomonas aeruginosa (membrane filtration method), and the total number of microorganisms at 36±2°C after 48 h (horizontal method using agar medium). Bacteriological tests were performed by an accredited laboratory.

The description of the water quality parameters in the tested pools and the determination of the relationships between them were carried out using the Microsoft Excel spreadsheet and Statistica software by StatSoft. The main goal of the statistical analysis was to assess the significance of differences between water quality in the studied swimming pools and to research the reasons for their occurrence. The significance level α = 0.05 was adopted for the calculations. The occurrence of statistically significant differences between the studied parameters was found when the test probability p was lower than the adopted level of significance (p < α). Finally, the significant differences (p < 0.05) of the parameters were evaluated through a non-parametric Mann-Whitney test.

The comparison of the obtained results with the permissible levels of water pollution in the group of pools P classic WTS (P1 – P6) and the group of pools P modified WTS (P7 – P10) made it possible to assess the differences between the quality parameters of the studied basin waters treated with the use of classic and modified technologies and the legitimacy of their application.

Initial data analysis (comparison of distributions and assessment of their similarity) indicated the necessity to use non-parametric tests for the evaluation of the test results. As the distribution of data was not normal (Kolmogorov-Smirnov test and Shapiro-Wilk test), the median values were presented in order to compare the parameters of the pool water quality. The average values are also presented in Table 3 as a measure supplementing the analysis of the test results.

Based on the analysis of the treatment effects, it was found that the least effective treatment systems were those used in P5 and P6 pools, i.e. systems based on the classic solution of the swimming pool water treatment system. It should be emphasized, however, that the quality of swimming pool water is very much influenced by the intensity and method of swimming pool use. In both P5 and P6, the number of swimming lessons for primary school children, conducted as part of physical education program, was greater than in the other pools. Pro-health education, constant reminding of the rules of hygiene, can significantly improve the quality of swimming pool water.

The control and continuous regulation of water pH, free chlorine concentration and water temperature used in swimming pool facilities allow to obtain the required level of these parameters regardless of the load of the pool and the technology used.

Tests carried out for a group of pools with a classic (P classic WTS) and a modified (P modified WTS) water treatment system have shown that the use of the latter allows for obtaining water of better quality in terms of both physiochemical and bacteriological properties.

The use of additional reagents (PolaClear^®, PolaOxyd^®, APF^®, ACO^®), new types of filter beds (PolaCarb^®, AFM^®), water ozonation (SPIDWOFIL^®) and at least two-stage disinfection resulted in better water quality in the group of pools with a modified water treatment system.

From the bacteriological point of view, the water quality in all analyzed pools did not raise any objections, and the two-stage disinfection in P4, P5, P9 and P10 and the three-stage disinfection in P8 were effective protection against secondary water contamination.