According to the German standard DIN 19643, 4 to 6m3 of water per 1 m2 of filter bed is required for proper performance of the swimming pool filter bed rinsing process. In the case of large swimming pools with several systems, the demand just for water for rinsing filter beds is very large. A typical swimming pool treatment system, consisting of 4 filters with the diameter of 1.800 mm that are washed on average once every two days, requires 600 to 900 mm3 per month. Due to the huge demand for water and the high cost of water consumption and wastewater drainage, there is a growing interest in recovery of water from swimming pool washings [1]. Washings contain a large quantity of suspended solids and dissolved substances. Particularly problematic, from the standpoint of use of washings, is their high content of byproducts of disinfection, admixtures, and pollutants present in chemicals used in the surface coagulation process [2, 3].
Pressure-driven membrane processes with different distribution of substances make it possible to separate pollutants on the molecular level and, consequently, broadly use both in processes of preparation of ultrapure water and for removal of micro pollution from water streams. Ultrafiltration is based on a sieve transport mechanism and stops particles with dimensions larger than the diameter of the membrane pores. Ultrafiltration membranes make it possible to stop small suspended solids, colloids, bacteria, and viruses. Nanofiltration is based on a sieve transport mechanism and the Donnan exclusion principle, and nanofiltration membranes make it possible to completely stop particles with the diameter of 1 nm or larger and to divide ions with different valence [4, 5]. Given the above, both techniques were evaluated with regard to treatment of washings from swimming pool systems. The main trends are based on using membrane processes in the treatment of drinking water circuits [6, 7], there is not much research on membrane processes in pool water treatment [8]. Therefore, it is necessary to broaden the relevant knowledge in this area.
The present paper presents the results of studies on treatment of samples of washings taken from swimming pool systems in the process of ultra- and nanofiltration based on selected physico-chemical parameters (turbidity, ultraviolet absorbance). Additionally, a toxicological assessment was performed using the Microtox® bacteria test in order to document the changes in the toxicological quality of the washings before and after the membrane filtration process.
Flat ultrafiltration and nanofiltration membranes made by Osmonics Inc. (USA), with different physico-chemical parameters, were used in the tests. The characteristics of the membranes and the operating parameters of the processes are given in Table 1.
Characteristics of membranes and operating parameters of the process
Process | Membrane symbol | Membrane material | Limit molar mass, Da | Process pressure, MPa | Volume flow rate of deionized water Jw * , 105 m3/m2·s | Recovered permeate % |
---|---|---|---|---|---|---|
UF | MW | Polyacrylonitrile (PAN) | 200,000 -500,000 | 0.5 | 5.46 | 50 |
V5 | Polyvinylidene difluoride (PVDF) | 200,000 | 0.2 | 2.06 | ||
0.5 | 1.24–1.46 | |||||
NF | DK | Composite (epidermal layer - polyamide) | 150-300 | 3.0 | 3.03 | |
HL | 3.03 |
Tested independently for each filtration cycle.
The membranes were placed in a steel filtration cell with the volume of 380 cm3 where the active surface of the membrane was equal to 38.5 cm2. Before the filtration started, the new membranes were conditioned by filtering deionized water in order to stabilize the volume of the permeate stream. The process was performed in a one-direction filtration layout for collection of 50% of the feed. The treatment process was performed in three consecutive cycles without changing the membrane. After each cycle, the membrane was rinsed with deionized water. This was done in order to document the occurrence of disadvantageous phenomena accompanying membrane filtration, i.e. fouling and scaling, caused by organic and inorganic pollution.
In order to evaluate the transport properties of the membranes, the volumetric flow rate of deionized water,
Where:
v – volume of water or permeate [m3],
F – active surface area of the membrane [m2],
t – filtration time [s].
In order to determine the separation properties of membranes, the retention (R) was determined based on the reduction of the values of pollution indicators:
Where:
R – retention of the pollutants [%],
cp – concentration (indicator value) of pollutants in the permeate stream [NTU or m-1],
cn – concentration (indicator value) of pollutants in the feed [NTU or m-1].
The intensity of the reduction of the hydraulic performance of the membrane was determined by way of determination of an intermediate parameter – the relative volumetric permeate stream (
Where:
In order to evaluate the quality of the washings and of the filtrates, selected physicochemical parameters were measured. The conductivity (C) and the reaction (pH) of the samples were measured with the inoLab® 740 (WTW, (Measurement and Analytical Equipment) multi-parameter meter. Ultraviolet absorbance, at the wavelength of 254 nm, was measured using the UV VIS Cecil 1000 made by Analytik Jena AG, with the optical path length of the cuvette d equal to 1 cm. The UV254 value was determined using the measurement method presented by US EPA [9], and the final result of the analysis is presented in m-1. Ultraviolet absorbance
The tested washings samples taken before and after the membrane filtration process underwent an additional toxicological assessment performed by way of the Microtox® bacteria test. This was done in order to evaluate the effectiveness of the membrane processes performed in elimination of toxic substances present in the washings. The test was performed in accordance with the
The tests were performed on samples of washings taken from systems of pools used for different purposes (sports pool (SP), recreational pool (RP), jacuzzi bathtub (JB)) located in indoor swimming pool complexes. The washings taken from SP and RP systems were a mix of streams from two separate systems drained to a common settling tank. The washings were taken during the night in the course of washing of the sand and gravel pressure filter beds that are the main elements of the swimming pool water treatment systems in those facilities. The physicochemical analyses described in the Analytical procedures chapter were performed in the morning on the following day. The quality characteristics of the washings are shown in Table 2.
Characteristics of post-process washings from swimming pool systems
Parameter | Unit | System | |
---|---|---|---|
SP + RP (sports pool and recreational pool) | JB (jacuzzi bathtub) | ||
Color | m-1 | 9.00 | 2.00 |
Turbidity | NTU | 27.60 | 9.22 |
pH | - | 7.37 | 7.65 |
Conductivity (PVC) | µS/cm | 1,350 | 3,300 |
|
m-1 | 20.90 | 6.80 |
Total chlorine | mgCl2/dm3 | 1.28 | 0.73 |
Free chlorine | mgCl2/dm3 | 0.37 | 0.33 |
Chlorine, combined | mgCl2/dm3 | 0.91 | 0.40 |
Ammoniacal nitrogen | mgN-NH4/dm3 | 1.90 | 0.10 |
Nitrate nitrogen | mgN-NO3/dm3 | 19.00 | 4.00 |
The results presented herein are the average values determined in the tests and the standard deviation did not exceed 5% in any of the cases presented.
In the course of the process of ultrafiltration of washings from the systems of the sports pool (SP) and the recreational pool (RP) performed using a polyacrylonitrile membrane (MW) at transmembrane pressure equal to 0.5 MPa, a reduction of the permeate stream compared to the initial value determined for deionized water (5.46·10-5 m3/m2·s) was observed; on average, the reduction was equal to approx. 26%, regardless of the filtration cycle (Fig. 1). The membrane rinsing performed between successive cycles did not restore the initial value of the volumetric flow rate of permeate (JV). In the same process conditions, filtration of washings was performed using a Polyvinylidene difluoride membrane (V5). This membrane was characterized by a lower initial volumetric flow rate of deionized water (1.46·10-5 m3/m2·s), compared to the MW membrane. The polyacrylonitrile UF membrane was characterized by a larger reduction of the permeate stream in the course of the process (Fig. 1). Compared to the initial value determined for deionized water, the permeate stream was reduced by approx. 28-31%, depending on the filtration cycle.
Later, washings taken from the jacuzzi bathtub (JB) system were filtered in a UF process using the V5 membrane at two different transmembrane pressures: 0.2 MPa and 0.5 MPa (Fig. 2). The observed values of the average relative permeability coefficients were lower at the pressure of 0.2 MPa than at the pressure of 0.5 MPa. Moreover, a significant impact of turbidity and absorbance
Fig. 3 shows the average values of average volumetric flow rate of permeate (
Fig. 4 shows the separation properties of the tested UF membranes with reference to the value of ultraviolet absorbance (RUV254) and turbidity (Rturbidity). In the course of filtration of washings from the SP and RP systems, the MW membrane demonstrated high values of the average pollution retention coefficients measured using those indicators. In the case of ultraviolet absorbance, the value of the retention coefficient was on the level of 85% and the reduction of turbidity in all the tested cycles was larger than 96%. The high values of removal of suspended solids and fine particles in the course of filtration of the same washings were also observed in the case of the V5 membrane. Initially, the retention coefficient RUV254 was smaller in the case of the V5 membrane than the MW membrane. However, in successive filtration cycles, the separation ability of the membrane increased. This was due to the formation on the surface of the membrane of the so-called secondary membrane characterized by lower porosity compared to the original membrane [14-15].
Fig. 5 shows a comparison of the separation abilities of the V5 membrane at different transmembrane pressure values, i.e. 0.2 MPa and 0.5 MPa. In most of the filtration cycles performed, the values of average pollution retention coefficients were higher at the pressure of 0.5 MPa compared to filtration cycles performed at the pressure of 0.2 MPa.
Fig. 6 shows a comparison of the separation properties of the DK and HL nanofiltration composite membranes during treatment of washings. The DK membrane was a little more effective in eliminating the pollution present washings characterized by the value of ultraviolet absorbance. The values of the retention coefficients were equal to 100%. With regard to reduction of turbidity, the HL membrane was more effective than the DK membrane, and its retention coefficient was more than 99%.
The toxicological assessment was performed using the Microtox® test on the samples from the SP and RP systems before and after the filtration process. Fig. 7 shows the changes in the values of inhibition of bioluminescence of samples of washings: raw and after the UF and NF process. In accordance with the toxicity classification, the washings taken before treatment in the UF process were considered as toxic for the
The membrane processes (UF and NF) performed in the tests significantly improved the quality of the washings from swimming pool systems. Nanofiltration separated pollutants more effectively but its hydraulic performance was much lower than that of ultrafiltration. Thus, it can be concluded that preliminary preparation of washings is necessary before membrane filtration (e.g. by way of a coagulation and/or sedimentation process). The quality of the washings had a significant impact on the effectiveness of the UF process. Both lower turbidity and lower ultraviolet absorbance