The Reuse of Washings from Pool Filtration Plants After the use of Simple Purification Processes

The necessity of regular filter backwashing in the pool water technological systems generates enormous, unproductive water losses. In order to properly wash the filter bed, a water consumption of 4÷6 m³ per 1 m² of bed is required. According to the guidelines, such backwashing should take place every 2÷3 days [1–6]. It can be estimated that the monthly water consumption for a typical installation, consisting of two filters with a diameter of 1800 mm, can be over 450 m³. Every year, it is over 5 000 m³ of unproductive wastewater discharged usually into the sanitary sewage system. In Poland, there are over 560 pools equipped with at least 2 filters [7]. Therefore it can be estimated that annually more than 2 800 000 m³ of backwash water is lost. Assuming the average price for wastewater discharges in large cities, 1.48 €/m³, it is easy to estimate that over 4 million € per year is spent on discharging “pool” washings [8]. In addition, the water used to backwash filtration beds is usually taken from the technological system where it was previously heated. The temperature of washings ranges from 25°C to 36°C (average about 30°C). For this reason, its discharging into the sewage system is also a waste of energy used to heat it.

In the Institute of Water and Wastewater Engineering of the Silesian University of Technology, the research on the quality of backwash water from swimming pool installations is being conducted. Their main purpose is to check the possibility of managing the backwash water after applying simple and relatively inexpensive individual processes and devices (for example: sedimentation tanks, settling tanks, clarifiers or settling tanks coupled with a coagulant chamber). It is being considered whether the washings may be drained to watercourses, to the ground, used for watering plants, sprinkling tennis courts and playing fields, flushing toilets or even being recycled back to swimming pool systems [9, 10]. Modern devices also allow for the recovery of heat from backwash water. The system combining these devices with the currently developed new water treatment technology, using membrane techniques (ultrafiltration and microfiltration), could effectively facilitate rational water, sewage and energy management in swimming pools [11–14].

The research included a physico-chemical analysis of washings samples taken during the filter backwashing (mean mixed sample). The research concept consisted of comparing the results with the Polish Minister Regulation, which determines the limit values for pollutants in wastewaters discharged to ground [15].

As regards the possibility of managing the washings from swimming pool facilities, it is important that this regulation implements the European Parliament and Council Directives of 2006 on the protection of groundwater against pollution and deterioration [16], of 2008 on environmental quality standards in the field of water policy [17] and of 2010 on industrial emissions [18].

Based on own research experience [9, 10] and literature review [19–22], the following parameters determining the possibility of reuse of washings from filtration swimming pool systems were considered: pH, temperature, total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD₅), free chlorine, chlorides, aluminum, sulfates, total suspended solids (TSS) and turbidity.

The samples were collected and marked in accordance with applicable standards and methods [23–25]. pH was determined by potentiometric method, temperature by direct measurement method, total suspended solids by direct weigh method, COD by dichromat method, total phosphorus by molybdat/antimon method, total nitrogen by Koroleff-reaktion method, free chlorine by DPD method, chlorides by thiocyanate method, sulfates by bariumchlorid method and BOD₅ by dilution method. COD, TP, TN free chlorine, chlorides and sulfates were measured using the spectrophotometer DR5000 UV/VIS and BOD₅ was measured using the Oxi Top®OC 100.

The preliminary studies have shown that presence of TSS and too high concentrations of free chlorine are the main impurities that make the direct management of washings impossible [9, 10]. For this reason, an attempt was made to reduce the content of suspensions in washings using simple processes, i.e. 120-minute sedimentation carried out in Imhoff sedimentation funnel and coagulation under laboratory conditions (1 minute of fast mixing at 200 RPM + 20 minutes of slow mixing at 20 RPM + 30 minutes of sedimentation) [26]. The preliminary studies have also shown the possibility of using a coagulant used in the pool water purification process [10, 14]. For this reason it was decided to use two types of coagulants: dialuminium pentahydroxychloride solution for washings from pools P2, P3, P4, P5 and aluminum sulphate solution for other pools.

The research was carried out for 20 pool water treatment installations with a similar technological system. The basic elements of each of them are filters with a multilayer bed of sand and hydroanthracite. In Table 1 the types of analyzed pools and the characteristics of their filtration system (number and diameter of filters, method and medium used for rinsing) are summarized.

The quality analysis showed that any raw washings from the 20 studied objects could not be directly managed because the measured pollution indicators exceeded the limit values (Table 2). Fig. 1–5 present the values of selected pollutants in comparison to the permissible values according to the Polish Regulation on conditions to be met when introducing wastewater into watercourses or into the ground [15].

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

The pH values of washings discharged from the tested pool facilities, with the exception of P4 pool, ranged from 6.50 to 9.00. The washings from P4 had the highest pH value, i.e. 9.13. The temperature of the washings samples ranged from 20.3°C (P13) to 35.0°C (P18). The temperature level depended mainly on the function of the pool. Higher temperatures of around 30.0°C were usually found in the washings from pools for children (e.g. P5, P6) and from circulations supplying hot tubs (e.g. P3, P18). Lower temperatures were measured from the circuits of sports and recreational pools (e.g. P4, P20). The concentration of total nitrogen in 8 out of 17 tested samples exceeded the limit value (10.00 mgTN/L) for wastewater discharged to the ground. The concentrations of total phosphorus in 6 out of 19 tested samples exceeded the limit value (1.00 mgTP/L). A similar relationship was noted for COD. In 8 out of 20 washings’ samples, COD values exceeded the permissible value (125.0 mgO₂/L). As a result of the sedimentation or coagulation process the washings were subjected to, the content of suspended solids and the related parameters have been significantly reduced, including TN, TP, COD and turbidity. In turn, the content of chlorides, aluminum and sulphates in the tested washings did not raise any objections.

The total suspension solids and free chlorine concentration in raw washings exceeded the permissible values. Attempts to remove the suspended matter in the sedimentation process have been proved to be very effective. After the sedimentation process the concentration of suspensions was significantly reduced in all tested samples (Fig. 6). An average efficiency of the sedimentation process was about 66%. However, despite a significant reduction in the content of suspensions in the washings for some samples the results were not low enough to meet the limit values allowed in the Polish Regulation [15]. For 4 washings samples the effects of the coagulation process were also evaluated. Two samples were selected for which the sedimentation process proved to be insufficient and comparatively 2 samples from the systems for which sedimentation was sufficient to reduce the content of suspensions. The used coagulation process was highly effective. For the tested samples the content of suspensions after coagulation was below the limit value, i.e. 35 mg/L. The coagulation, removed, on average, 56% more of the suspension than the sedimentation (Fig. 7).

Figure 6.

Figure 7.

The effectiveness of sedimentation and coagulation processes was also evaluated based on the comparison of washings and supernatants turbidity values resulting from the pre-treatment of washings. The sedimentation reduced the turbidity by an average of 68% and the coagulation by approx. 90%. Turbidity as an indicator of pollution (content of insoluble suspended particulates) has not been included in the Polish Regulation [15] but its value is an important indicator of the water or wastewater pollution degree. Therefore, it can be concluded with a high probability that the decrease in turbidity will lead to the reduction of the impurities indicators related to the content of suspended solids (COD, BOD₅, total nitrogen and total phosphorus).

The reuse of washings for watering plants, sprinkling the courts or fields is synonymous with putting them into the ground. Because filter beds are usually washed with circulating water with a significant content of free chlorine (with highly oxidizing properties) its effect on animal life and plant growth should be taken into account [9, 27]. In almost every sample of raw washings the concentration of free chlorine exceeded the value allowed by the Polish Regulation [15] i.e. 0.20 mgCl₂/L (Fig. 8). Fig. 9 compares the content of free chlorine in 30 samples of raw washings (0'), from various facilities, and in the supernatant water after a two-hour sedimentation (120') in the Imhoff funnel with the limit values specified in the Polish Regulation [15].

Figure 8.

Figure 9.

The two-hour sedimentation provided a reduction of free chlorine concentration in the tested samples by an average of 47%. This was caused both by the consumption of chlorine for the oxidation of impurities contained in the backwash water and by the release of free chlorine to the atmosphere. The average concentration of chlorine after a two-hour sedimentation was 0.27 mgCl₂/L. Only in nine samples of supernatant water the content of free chlorine was below the limit value (Fig. 9). For the remaining 21 samples chlorine decay test was carried out. A decrease in free chlorine content over time was demonstrated. This experiment was carried out by simulating the conditions that are present in settling tanks, i.e. in open vessels with a large surface area of contact with the atmosphere. The time needed for the required reduction in free chlorine concentration in the analyzed 19 washings samples ranged from 2 to 8 hours. For two samples (No. 27 and No. 29) it took 12 hours.

The quality of washings discharged from swimming pool installations was analyzed in laboratory conditions in order to determine the possibilities of their rational management. It has been shown that the concentration of suspension and free chlorine are the main parameters that make their drainage into the natural environment impossible. Limit values of these parameters are specified in the Regulation of the Minister of the Environment regarding the conditions to be met when introducing sewage into waters or into the ground [15]. Processes of sedimentation or sedimentation supported by coagulation allow to lower the concentration of total suspended solids below the admissible value specified in the Polish Regulation. The concentration of free chlorine is reduced to the acceptable concentration after allowing the washings to settle for up to several hours. The additional intensive few minutes of aeration causes acceleration of free chlorine decay.

The obtained research results allow us to assume that the management of backwash water in all tested swimming pools would be possible after applying a simple system of their treatment, for example, a settler or a settling tank with a coagulant chamber and then a chamber for intensive aeration.

The quality of washings depends on many factors: the type and the number of filters, the type of filter bed, the duration of the filtration cycle, the hydraulics of the pool basin, the volume of water used for backwash process, the quality of water in the pool system, the quality of source water, the swimming pool operating conditions and the applied pool water technology. The analysis of the backwash water quality in swimming pools with similar water treatment systems (prefiltration – surface coagulation – pH correction with sulfuric acid solution – disinfection with sodium hypochlorite solution) showed significantly different results for each pool. For this reason it is necessary to determine the optimal parameters of the treatment processes and to determine the optimal dose and type of coagulant individually for each swimming pool.