Studies on the Recovery of Wash Water from Swimming Pool Filters and Their Characteristics—A Case Study

Abstract

Filter wash water (FWW) from public swimming pools is a recoverable resource, yet full-scale evidence on safe on-site reuse with documented economics is scarce. We evaluated a full-scale integrated recovery unit (SOWA) installed at an indoor public pool. The SOWA system—sedimentation, granular filtration operated at a hydraulic loading rate (HLR) of 7.5–10 m³ m⁻² h⁻¹, ultrafiltration, and chlorine-dioxide (ClO₂) disinfection—was monitored for physicochemical and microbiological performance. Turbidity decreased from 23.1 nephelometric turbidity units (NTU) to 0.25 NTU; chemical oxygen demand, reported as the permanganate index (COD_Mn), fell from 10.4 to 1.6 mg O₂ L⁻¹; and total microbial count declined from 1.6 × 10⁴ to 30 colony-forming units per millilitre (CFU mL⁻¹). Indicator organisms (Escherichia coli, Intestinal enterococci and Pseudomonas aeruginosa) were not detected, and all quality criteria complied with national standards. At the Olender facility, monthly freshwater use dropped from 1700 to 1000 m³ after 24/7 SOWA operation, while combined chlorine was maintained at 0.12 mg Cl₂/L and no issues with chloroform were observed. The unit recovered 4.7 m³ h⁻¹ of FWW for non-potable uses. According to manufacturer catalogue data, the recovery process can reach up to 96%, enabling annual savings up to ~EUR 9000 and a payback of ~2 years under favourable tariffs and loads. Our outcomes are consistent with independent full-scale reuse trains (e.g., ultrafiltration/reverse osmosis) and with disinfection-by-product control strategies reported in the literature, and they align with international guidance for swimming-pool water reuse. This study provides a rare, end-to-end implementation at full scale, documenting continuous operation, verified microbial safety, regulatory compliance, quantified water and cost savings, and site-specific economics for a compact, multi-barrier FBW recovery system that can be directly transferred to similar facilities.

1. Introduction

Swimming pool water is continuously exposed to contamination by bathers through sweat, skin particles, and urine. These organic pollutants create ideal conditions for microbial growth and may pose a risk to public health [1]. To limit microbiological hazards, swimming pool water must be disinfected continuously; however, the disinfection process can lead to the formation of harmful by-products, such as chloramines and trihalomethanes (THMs) [2,3,4,5,6,7,8].

To ensure water quality and protect users’ health, swimming pool operations are regulated by national legislation. In Poland, the applicable requirements are defined in the Notice of the Minister of Health of 10 May 2022 on the announcement of the unified text of the Regulation of the Minister of Health on the requirements to be met by water at swimming pools, Journal of Laws 2022 item 1230 [9]. This regulation specifies acceptable limits for physicochemical and microbiological parameters and outlines procedures for water testing and monitoring of public swimming pool facilities.

Modern treatment systems in public pools often include multiple processes: mechanical filtration (sand or AFM filters), chemical coagulation and pH correction, followed by disinfection using sodium hypochlorite, chlorine dioxide, ozone, or ultraviolet (UV) radiation [2,3,4,5,10,11]. While chlorination remains the most common disinfection method due to its simplicity and cost-effectiveness, its excessive use can lead to the formation of disinfection by-products (DBPs), some of which are toxic and regulated [8,12].

Recent developments in pool technology emphasize water reuse and circular economy approaches. One important area is the recovery of wash water generated during the periodic rinsing of sand filters. This wash water typically contains concentrated pollutants and is discharged into the sewer system, contributing to water loss and higher wastewater loads. In a typical medium-sized pool facility, backwashing generates between 10 to 30 m³ of wash water per filter per wash, often several times per week. For larger pools or multi-basin complexes, the annual volume of generated wash water may exceed several thousand cubic metres, representing both a financial and environmental burden.

However, technologies such as the SOWA recovery system, which integrates multi-stage filtration and disinfection, have made it possible to reuse over 90% of this water for non-potable applications [2,3,4,5,13,14,15]. Despite these technical possibilities, the adoption of such systems in Poland remains limited. As of 2024, only a few public pool facilities—fewer than 10 nation-wide—have implemented advanced wash water recovery systems, mostly as pilot programs or through EU-funded ecological modernization initiatives.

Moreover, the DIN 19643:2023 standard introduces a requirement that no more than 80% of recovered wash water may be returned into the circulation system, emphasizing the importance of rigorous treatment and quality control in water reuse processes [16].

2. Processes of Swimming Pool Water Renewal, Treatment and Purification Technology

Swimming pool water must be continuously renewed and purified to maintain its microbiological safety, physicochemical balance, and optical clarity. The treatment process is not linear but cyclical, as water is recirculated through a complex set of purification stages before returning to the pool basin. According to DIN 19643:2023 [16], a standard system includes the following core stages: mechanical filtration, coagulation, pH regulation, disinfection, and, optionally, advanced oxidation or membrane filtration.

The primary contaminants in swimming pool water originate from bathers: sweat, urine, skin oils, cosmetics, and hair. These introduce nitrogenous compounds, organic carbon, and microbial flora into the water. Additional contamination arises from atmospheric deposition (dust, pollen), make-up water impurities, and residues of cleaning agents used in pool surroundings.

Mechanical filtration is typically the first barrier, removing suspended solids. Most installations rely on sand filters or AFM (Activated Filter Media) with grain sizes between 0.4 and 1.0 mm. The filters must be regularly backwashed, which generates wash water (popłuczyny), rich in concentrated pollutants and microbiological biofilms [9,17].

Chemical coagulation improves the removal of colloidal particles, enabling their retention on the filter bed. This process uses aluminum or iron-based coagulants, supported by flocculants and pH correction agents. Maintaining the optimal pH (7.2–7.6) is essential for disinfection efficiency and user comfort [18].

Disinfection is the cornerstone of microbiological control. Chlorine-based agents (free chlorine or chlorine dioxide) are most common due to their residual effect. However, disinfection by-products (DBPs), especially chloramines and trihalomethanes (THMs), are a significant health concern. Their presence is influenced by organic load, water temperature, and contact time [2,3,4,5,8]. Modern facilities use UV-C radiation, ozonation, or electrolysis to reduce DBP formation while preserving disinfection efficacy.

Wash water, though traditionally discharged as waste, is a growing focus of water-saving innovations. The SOWA system installed at the Olender facility enables the recovery of wash water using OC1 bed prefiltration, ultrananofiltration and chlorine dioxide dosing.

Recovered water can be reused in non-potable applications such as toilet flushing, irrigation, or even returned to the pool system after quality verification [2,3,4,5,17,19].

This integrated approach to water treatment not only enhances operational efficiency but also aligns with sustainable development goals, especially in regions facing water stress. Studies show that over 90–96% of wash water can be recovered with appropriate technology [10,19], significantly reducing both freshwater consumption and wastewater discharge. Moreover, it lowers the burden on sewage systems and complies with circular economy principles, which are increasingly mandated by EU environmental policy frameworks [1,20,21,22].

3. Swimming Pool Water Guidelines

Ensuring water quality in public swimming pools is a legal and sanitary obligation. In Poland, the key regulatory framework is provided by the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]. This regulation defines acceptable limits for various physicochemical and microbiological parameters, as well as sampling methods, frequency, and responsibilities of facility operators.

3.1. Physicochemical Requirements

According to the regulation, the water must be clear, colourless, and odourless, with values maintained within specified ranges for parameters such as:

-

Free chlorine: 0.3–0.6 mgCl₂/L in pools, up to 1.0 mgCl₂/L in hot tubs,

-

Bound chlorine (chloramines): not exceeding 0.2 mgCl₂/L,

-

pH: between 6.5 and 7.6,

-

Turbidity: <0.5 NTU,

-

Nitrates (NO₃⁻): <50 mgNO₃⁻/L,

-

Oxidizability (COD-Mn): <5 mgO₂/L,

-

Cyanuric acid (if used): <100 mg/L.

These limits ensure user safety, disinfectant efficiency, and technical stability of the pool systems.

3.2. Microbiological Requirements

Water must be free from pathogenic microorganisms and within the following limits:

-

Escherichia coli: 0 CFU/100 mL,

-

Pseudomonas aeruginosa: 0 CFU/100 mL,

-

Staphylococcus aureus: ≤10 CFU/100 mL,

-

Total microbial count at 36 °C: ≤100 CFU/mL,

-

Intestinal enterococci: 0 CFU/100 mL.

Regular monitoring is essential to detect any deviation promptly. Microbiological testing must be performed weekly or more often, depending on pool type and usage intensity.

3.3. European and German Standards

The Polish legislation is aligned with European principles and DIN 19643:2023 [16], a German standard widely recognized in pool technology. DIN 19643:2023 [16] specify technological processes and performance standards for recirculation systems, water turnover times, and filtration effectiveness. For example, the full turnover of water in a swimming pool should occur every 4–6 h, and in hot tubs, even more frequently (up to every 30 min) [19].

3.4. Practical Implications for Wash Water

Although regulations do not directly govern the quality of rinse (backwash) water, facilities are encouraged to manage it according to environmental principles. Discharged wash water must comply with local wastewater discharge permits. Where recovery systems are installed (e.g., SOWA), the post-treatment wash water must be tested and confirmed to meet the applicable non-potable standards before reuse [2,3,4,5].

Understanding and complying with these guidelines is crucial for ensuring both the safety of pool users and the efficiency of the facility. They also provide the legal basis for evaluating the performance of water recovery systems, which is the focus of this study.

4. Aim of the Study

The aim of this study is to evaluate the feasibility and effectiveness of rinse (backwash) water recovery from swimming pool filtration systems using an innovative three-stage treatment process known as SOWA.

The study specifically investigates:

-

The physicochemical and microbiological characteristics of raw wash water from three public swim-ming-pool facilities in Poland were determined to broaden the knowledge base; nevertheless, the efficiency of the SOWA recovery train was evaluated exclusively at the Olender facility, where the system is installed.

-

The efficiency of the SOWA treatment system, including pre-filtration, ultrafiltration, and chlorine dioxide dosing,

-

The compliance of recovered water with regulatory quality standards for potential reuse,

-

The influence of facility-specific factors (e.g., user load, basin type, filtration system) on the composition and treatability of wash water.

The auxiliary data from Astoria and Naquarius serve only to benchmark the baseline quality of back-wash water produced by pools with different operational profiles; no recovery unit was implemented at those sites during this study.

Research hypothesis: It is hypothesized that the SOWA system enables effective recovery of swimming pool wash water, reducing key physicochemical and microbiological parameters to levels compliant with applicable water quality regulations, thus making the water suitable for non-potable reuse.

5. Research Methodology

Accordingly, the core objective was to determine whether wash water generated at Olender, after treatment in the SOWA unit, could be reused safely and in compliance with national standards, while data from the other two pools provide contextual comparison.

5.1. Sampling and Research Sites

Field investigations therefore centred on the Olender indoor public swimming-pool (in situ SOWA installation). Grab samples of untreated wash water were additionally collected from Astoria, Naquarius and Olender—a spa and sports facility in Wielka Nieszawka (recreational pool and water attractions).

At each site, wash water samples were collected immediately after the backwashing of pressure sand filters—in the quantity of 3 pieces per object. The sampling procedure followed PN-ISO 5667-5:2017-10 guidelines [23], ensuring representativeness and sanitary safety. Water samples were collected in sterile 5 L containers and transported under cooled conditions (4 °C) for immediate laboratory analysis.

In the Olender facility, 3 water samples were also taken after the use of the SOWA system—after the wash water recovery process.

5.2. Description of the Sowa System for the Recovery of Floodwaters at the “Olender” Facility

SOWA flushing water recovery system (Figure 1). It is a closed circuit and consists of a wash water tank (where the rinsing water from the filters of the facility’s pool technology is collected), a pre-filter (in which the OC1 filter bed is located and the initial filtration of the rinse water takes place), a nanoultrafiltration unit (where the ultra-nano filtration process is carried out and the physicochemical and microbiological parameters of water are improved), a circulation pump with a pre-filter (it is a circulation pump of the SOWA system, which causes water circulation from the rinse water tank to the overflow tank of a given circuit for which the water recovery process is carried out), a backwash pump of the pre-filter (it is a pump that is used to pump water from the largest overflow tank of the facility to flush the pre-filter of the SOWA system), and a chlorine dioxide dosing pump (after the rinse water recovery process, chlorine dioxide is dosed to maintain the THM reduction effect).

Water from above the suspended-solids layers is sucked in by a circulation pump equipped with a prefilter (dampers are placed on the suction of the pump to cut it off and clean the prefilter) and is pumped to the prefilter. After collecting water from the sludge, the suspended-solids layer is discharged into the sewage system.

The flushing water tank is connected by a pipe to a self-priming circulation pump, which forces water circulation throughout the system—it sucks water from the flushing water tank and pushes it through the entire flushing water recovery system. The capacity of the circulation pump is matched to the capacity of the nano-ultrafiltration unit.

The water circulation in the water recovery system is forced by a circulation pump. The pump is connected to the pre-filter by means of a pipe, in which the water flowing through the OC1 bed is filtered in order to remove mechanical impurities, suspended solids and colloidal particles.

A filter with a diameter of ø 800 mm is used. The hydraulic loading rate is 7.5–10 m³ m⁻² h⁻¹ and the rinsing speed is 50 m³ m⁻² h⁻¹. The filter bed is rinsed in the filter with water taken from the overflow tank of the swimming pool circuit. The rinses are directed to the sewage system.

Equipping the filter with a multi-way valve enables filtering water, rinsing the filter in a counter-current and flushing in accordance with the direction of filtration, cutting off the filter.

The pre-filter is connected by a pipe to a nano-ultrafiltration system. The nano-ultrafiltration system dis-charges the purified water to the overflow tank of the swimming pool circuit with a capacity of 5 m³/h. Continuously and regularly, the water flows under pressure to the filter tubes, while constantly generating a flow of purified water. In filtration mode, all elements removed by the membranes are collected inside the membrane elements.

After the nanoultrafiltration process, chlorine dioxide is dosed into the purified water by means of a membrane pump at a rate of 8.5 mL/min.

Rinsing and rinsing of the pre-filter and filter tubes is carried out by means of a second flushing pump giving a rinsing speed of min. 50 m/h with water from the overflow tank of the pool circuit. The frequency of rinsing of the pre-filter is determined by the pressure gauge indicator (when the filter pressure gauge indicates the pressure in the red range) and after each tranche of wash water recovery. The backwash (BW) and purification (CEB) mode of the nanoultrafiltration unit takes place after a predefined time of loading the system with contaminants from the feed water, in order to remove the contaminants from the membranes. Flow rate and pressure changes should be observed to determine the most appropriate BW process frequency.

All elements of the system are connected by pipelines in the form of PVC pipes, while the diameters of the pipelines have been selected so that the water flow rate is 1–2 m/s. All materials used for the construction of the installation have PZH certificates, allowing them to come into contact with drinking water and are resistant to water with increased chlorine content.

5.3. Analytical Methods

The following physicochemical parameters were analyzed, based on current national guidelines [9]. A list of parameters, methods and standards is presented in

Table 1. Physicochemical parameters, test methods, and standards.

Parameter	Test Method	Device	Norm
pH	Potentiometric	Titrator Methorm OMNIS	PN-EN ISO 10523:2012 [24]
Water hardness (CaCO3)	Titration	Titrator Methorm OMNIS	PN-ISO 6059:1999 [25]
Bromides	Photometric	Lovibond PM620 Photometer	PN-ISO 10304-1:2009 [26]
Nitrates	Photometric	Lovibond PM620 Photometer	PN-EN ISO 13395:2001 [27]
Free and bound chlorine	Titration	Titrator Methorm OMNIS	PN-ISO 9297:1994 [19]
Turbidity	Nephelometric	Titrator Methorm OMNIS	PN-EN ISO 7027-1:2016 [17]

.

Not all parameters from the regulation were analyzed. The study focused on the most representative indicators of wash water pollution and those relevant for potential reuse. Additional microbiological tests (e.g., Pseudomonas aeruginosa) were included due to elevated health risks identified in previous studies [17,18,19].

Microbiological tests of flushing water from swimming pool facilities were carried out under laboratory conditions, considering relevant standards, to determine the basic microorganisms that may be contained in the water. The rehearsals were repeated three times.

Table 2. Microbiological parameters, test methods and standards.

Type of Study	Unit	Test Method	Norm
Total microbial count 36 ± 2 °C after 48 h	CFU/1 mL	Plate method, deep sowing	PN-EN ISO 6222 [28]
Escherichia coli	CFU/100 mL	Membrane filtration	PN-EN ISO 9308-1 [29]
Coagulase-positive staphylococci (Staphylococcus aureus)	CFU/100 mL	Membrane filtration	Methodology NIZP—PZH ZHK
Pseudomonas aeruginosa	CFU/100 mL	Membrane filtration	PN-EN ISO 16266 [30]
Number of fecal streptococci (Enterococci)	CFU/100 mL	Membrane filtration	PN-EN ISO 7899-2 [31]

below shows the parameters, methods and the test were performed, the equipment needed to perform the test and the standards according to which the limit values were checked.

All microbiological analyses were conducted in an accredited laboratory under ISO/IEC 17025 [32] standards.

The characteristics of the “Olender” facility are presented in

Table 3. Characteristics of the “Olender” swimming pool.

Pool Type	Paddling	Bathtub	Recreational Swimming Pool + Dynamometer	Swimming Pool
Basin	Stainless Steel	Prefabricated	Stainless Steel	Stainless Steel
Dimensions	no data	Round with a diameter 2.93 m	Irregular	25 × 12.5 m
Depth	0.2 m of the 0.30 m	0.4 m to 0.89 m	0.9 m to 1.2 m/dynamometer: 1 m	1.35 m of the 1.8 m
Surface area of the water lather	37 m2	6.7 m2	161.3 m2/dynamometer: 18 m2	318.6 m2
Volume	9.25 m3	2.00 m3	175 m3	502.4 m3
Water temperature	32 °C	38 °C	30 °C	26–28 °C
Filtration efficiency	- *	- *	- *	- *
Filtration speed	- *	- *	- *	- *
Amount of circulating water	- *	- *	- *	- *
Duration of use of the pool	16 h	16 h	16 h	16 h
Daily operating time of the installation	24 h	24 h	24 h	24 h
Max load	- *	- *	- *	- *
Circulating water flow rate	approx. 33.4 m3/h	approx. 30 m3/h	approx. 186.2 m3/h	approx. 502.4 m3/h

* indicates parameter not measured due to facility-specific limitations.

.

The obtained results are summarized in Table 4, Table 5, Table 6 and Table 7 and further analyzed in the Overview of the Results Section, providing a comparison across facilities and treatment stages.

6. Overview of the Results

The paper presents the results of physicochemical (Table 4) and microbiological (Table 5) tests of flushing waters for three swimming pool facilities:

• “Naquarius” in Nakło over Notecią
• “Astoria” in Bydgoszcz
• “Olender” in Wielka Nieszawka

The determinations were made in 3 repetitions and the standard deviation for each parameter was determined (mean ± SD).

Table 4. Selected physicochemical parameters for the tested wash waters from 3 objects.

Selected Physicochemical Parameters	Unit	Swimming Pool “Naquarius” (±SD)	Swimming Pool “Astoria” (±SD)	Swimming Pool “Olender” (±SD)	Requirements According to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]
Turbidity	NTU	<3 ± 0.5	<3 ± 0.5	132 ± 15	max. 0.5
Chlorine bound	mgCl2/L	0.304 ± 0.03	0.155 ± 0.02	0.209 ± 0.02	max. 0.3
Free chlorine	mgCl2/L	0.073 ± 0.01	0.126 ± 0.01	0.149 ± 0.01	min. 0.3; max. 0.6
Chloroform	mg/L	Not detected	0.005 ± 0.001	0.009 ± 0.001	max. 0.03
Nitrates	mgNO3−/L	1.4 ± 0.2	7.3 ± 0.6	22.8 ± 2.0	max. 20
THM total	mg/L	0.004 ± 0.001	0.011 ± 0.002	0.018 ± 0.003	max. 0.1
Oxidation	mgO2/L	1.64 ± 0.3	4.83 ± 0.6	10.4 ± 1.0	max. 4

Table 4. Selected physicochemical parameters for the tested wash waters from 3 objects.

Selected Physicochemical Parameters	Unit	Swimming Pool “Naquarius” (±SD)	Swimming Pool “Astoria” (±SD)	Swimming Pool “Olender” (±SD)	Requirements According to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]
Turbidity	NTU	<3 ± 0.5	<3 ± 0.5	132 ± 15	max. 0.5
Chlorine bound	mgCl2/L	0.304 ± 0.03	0.155 ± 0.02	0.209 ± 0.02	max. 0.3
Free chlorine	mgCl2/L	0.073 ± 0.01	0.126 ± 0.01	0.149 ± 0.01	min. 0.3; max. 0.6
Chloroform	mg/L	Not detected	0.005 ± 0.001	0.009 ± 0.001	max. 0.03
Nitrates	mgNO3−/L	1.4 ± 0.2	7.3 ± 0.6	22.8 ± 2.0	max. 20
THM total	mg/L	0.004 ± 0.001	0.011 ± 0.002	0.018 ± 0.003	max. 0.1
Oxidation	mgO2/L	1.64 ± 0.3	4.83 ± 0.6	10.4 ± 1.0	max. 4

Table 5. Selected microbiological parameters for the tested wash waters from 3 objects.

Selected Microbiological Parameters	Unit	Swimming Pool “Naquarius” (±SD)	Swimming Pool “Astoria” (±SD)	Swimming Pool “Olender” (±SD)	Requirements According to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]
Total microbial count 36 ± 2 °C after 48 h	CFU/1 mL	0.00 ± 5.00	4.70 × 105 ±50,000	1.62 × 104 ±1500	100
Escherichia coli	CFU/100 mL	2.00 ± 1.00	0.00 ± 0.00	0.00 ± 0.00	0
Coagulase-positive staphylococci (Staphylococcus aureus)	CFU/100 mL	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00	0
Pseudomonas aeruginosa	CFU/100 mL	0.00 ± 0.00	1200 ± 150	2.00 ± 1.00	0
Number of fecal streptococci (Enterococci)	CFU/100 mL	0.00 ± 0.00	480.00 ± 50.00	0.00 ± 0.00	0

Table 5. Selected microbiological parameters for the tested wash waters from 3 objects.

Selected Microbiological Parameters	Unit	Swimming Pool “Naquarius” (±SD)	Swimming Pool “Astoria” (±SD)	Swimming Pool “Olender” (±SD)	Requirements According to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]
Total microbial count 36 ± 2 °C after 48 h	CFU/1 mL	0.00 ± 5.00	4.70 × 105 ±50,000	1.62 × 104 ±1500	100
Escherichia coli	CFU/100 mL	2.00 ± 1.00	0.00 ± 0.00	0.00 ± 0.00	0
Coagulase-positive staphylococci (Staphylococcus aureus)	CFU/100 mL	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00	0
Pseudomonas aeruginosa	CFU/100 mL	0.00 ± 0.00	1200 ± 150	2.00 ± 1.00	0
Number of fecal streptococci (Enterococci)	CFU/100 mL	0.00 ± 0.00	480.00 ± 50.00	0.00 ± 0.00	0

In terms of selected physicochemical and microbiological parameters of the wash water, the water from the Olender basin is the most loaded.

The paper also presents a summary of comparative results before and after the process of recovering flushing water from the “Olender” swimming pool facility in Wielka Nieszawka.

Table 6 below presents the results of physicochemical tests along with standard deviations of wash water before and after the recovery process for the Olender facility.

Table 6. Results of physicochemical tests of wash water before and after the recovery process for the Olender facility.

Selected Physicochemical Parameters	Unit	Sample Result Before the Recovery Process (±SD)	Sample Results After the Recovery Process (±SD)	Requirements According to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]
Turbidity	NTU	132.00 ± 15.00	0.09 ± 0.01	max. 0.5
Chlorine bound	mgCl2/L	0.21 ± 0.02	0.12 ± 0.01	max. 0.3
Free chlorine	mgCl2/L	0.15 ± 0.01	0.06 ± 0.01	min. 0.3; max. 0.6
Chloroform	mg/L	0.01 ± 0.00	Not detected	max. 0.03
Nitrates	mgNO3−/L	22.80 ± 2.00	4.40 ± 0.50	max. 20
THM total	mg/L	0.02 ± 0.00	0.01 ± 0.00	max. 0.1
Oxidation	mgO2/L	10.40 ± 1.00	1.69 ± 0.30	max. 4

Table 6. Results of physicochemical tests of wash water before and after the recovery process for the Olender facility.

Selected Physicochemical Parameters	Unit	Sample Result Before the Recovery Process (±SD)	Sample Results After the Recovery Process (±SD)	Requirements According to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9]
Turbidity	NTU	132.00 ± 15.00	0.09 ± 0.01	max. 0.5
Chlorine bound	mgCl2/L	0.21 ± 0.02	0.12 ± 0.01	max. 0.3
Free chlorine	mgCl2/L	0.15 ± 0.01	0.06 ± 0.01	min. 0.3; max. 0.6
Chloroform	mg/L	0.01 ± 0.00	Not detected	max. 0.03
Nitrates	mgNO3−/L	22.80 ± 2.00	4.40 ± 0.50	max. 20
THM total	mg/L	0.02 ± 0.00	0.01 ± 0.00	max. 0.1
Oxidation	mgO2/L	10.40 ± 1.00	1.69 ± 0.30	max. 4

As a result of physicochemical tests of rinses from swimming pool filters from the “Olender” facility in Wielka Nieszawka, before the recovery process, the turbidity value was shown to be well above the norm according to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9], which may be caused by poor water quality, which in turn is a good environment for the multiplication of microorganisms in the water. Turbidity is an important parameter due to the purity and aesthetics of swimming pool water. During the test, the turbidity of the washes was 132 NTU, while the appropriate quality of water in the pools should have a turbidity value of a maximum of 0.5 NTU. After the recovery process using the “SOWA” system, the turbidity of the water significantly decreased to 0.09 NTU, to such an extent that the turbidity value is within the required limits of the regulation.

The oxidation of this test was 10.4 mgO₂/L, which is also an offence above the prevailing standard according to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9], where this parameter should not exceed 4 mgO₂/L. This may indicate the presence of organic and inorganic substances in the water. After the recovery process with the “SOWA” system, the permanganese index value is within the standard for the appropriate quality of swimming pool water in swimming pools.

A parameter that exceeded the limit values before the recovery process is the presence of nitrates. The study showed as many as 22.8 mgNO₃⁻/L, where, according to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9], the maximum value is mgNO₃⁻/L. After the recovery process, their value significantly decreased to 4.40 mg/L, which is the result of the implementation of an appropriate recovery system, in this case thanks to the “SOWA” system.

The remaining physicochemical parameters that have been tested: bound chlorine, free chlorine, chloroform and the sum of THM, are values within the established standard.

Table 7 below presents the results of microbiological tests along with standard deviations of wash water before and after the recovery process for the Olender facility.

Table 7. Results of microbiological tests of wash water before and after the recovery process for the Olender facility.

Selected Microbiological Parameters	Unit	Sample Result Before the Recovery Process (±SD)	Sample Results After the Recovery Process (±SD)	Requirements According to the Journal of Laws 2022 item 1230 [9]
Total microbial count 36 ± 2 °C after 48 h	CFU/1 mL	1.62 × 104 ± 1500	3.00 × 102 ± 50	100
Escherichia coli	CFU/100 mL	0.00 ± 0	0.00 ± 0	0
Coagulase-positive staphylococci (Staphylococcus aureus)	CFU/100 mL	0.00 ± 0	0.00 ± 0	0
Pseudomonas aeruginosa	CFU/100 mL	2.00 ± 1	0.00 ± 0	0
Number of fecal streptococci (Enterococci)	CFU/100 mL	0.00 ± 0	0.00 ± 0	0

Table 7. Results of microbiological tests of wash water before and after the recovery process for the Olender facility.

Selected Microbiological Parameters	Unit	Sample Result Before the Recovery Process (±SD)	Sample Results After the Recovery Process (±SD)	Requirements According to the Journal of Laws 2022 item 1230 [9]
Total microbial count 36 ± 2 °C after 48 h	CFU/1 mL	1.62 × 104 ± 1500	3.00 × 102 ± 50	100
Escherichia coli	CFU/100 mL	0.00 ± 0	0.00 ± 0	0
Coagulase-positive staphylococci (Staphylococcus aureus)	CFU/100 mL	0.00 ± 0	0.00 ± 0	0
Pseudomonas aeruginosa	CFU/100 mL	2.00 ± 1	0.00 ± 0	0
Number of fecal streptococci (Enterococci)	CFU/100 mL	0.00 ± 0	0.00 ± 0	0

Microbiological tests for samples of washings from the “Olender” facility in Wielka Nieszawka before the recovery process did not show the presence of Escherichia coli, Staphylococcus aureus and fecal streptococci. On the other hand, above the standard according to the Regulation of the Minister of Health, J. Laws 2022, item 1230 [9], the total number of microorganisms and Pseudomonas aeruginosa was detected. The Regulation does not allow these microorganisms to occur, but the maximum total number of microorganisms is 100 CFU/1 mL.

The SOWA system lowered the total microbial count from 1.6 × 10⁴ to 30 CFU mL⁻¹ (>99.8% removal). This value now meets the regulatory limit of 1 × 10² CFU mL⁻¹ for pool water, but all mandated physicochemical criteria and indicator pathogens were met.

The result of Pseudomonas aeruginosa, in the tested sample, is also slightly above normal, as it amounted to 2 CFU/100 mL. These bacteria are common in wash water and show high resistance to disinfectants.

7. Summary

The recovery experiment was performed only at Olender; measurements from Astoria and Naquarius are included for reference to illustrate variability stemming from differences in capacity, filtration media, disinfection strategies and user loads.

Key findings include

• Raw wash water from Olender was heavily polluted, particularly in terms of turbidity (132 NTU), oxidizability, and nitrate concentrations (22.8 mg/L), as well as bound chlorine (0.209 mgCl₂/L).
• Following treatment with the SOWA system—which consists of pre-filtration, ultrafiltration, and chlorine dioxide dosing—the water quality improved significantly:

  -

  Turbidity was reduced from 132 to 0.09 NTU (>99.9%),

  -

  Bound chlorine from 0.209 to 0.123 mgCl₂/L (~41% reduction),

  -

  Nitrates from 22.8 to 4.4 mg NO₃⁻/L (~81% reduction),

  -

  Oxidizability decreased from 4.8 to 0.8 mg O₂/L,

  -

  Escherichia coli and Pseudomonas aeruginosa were completely eliminated,

  -

  Water recovery rate reached 96%, with low standard deviation (±5%).

The obtained results are comparable to those reported in other studies. Łaskawiec et al. [22,33] observed similar purification efficiency using ultrafiltration combined with ozonation, while Dudziak et al. [10] reported a 91% recovery rate using polymeric membranes. Barbot and Moulin [18] and Glauner et al. [23] highlighted the potential of membrane–adsorption systems and the importance of limiting disinfection by-products in reused water.

Although DIN 19643:2023 [16] recommends limiting the reintegration of recovered wash water to 80%, this standard applies to German facilities. The system studied here was installed in Poland, where national regulations do not impose such a restriction. Nevertheless, the SOWA system meets key technical expectations of DIN 19643, including multi-stage filtration, disinfection, and monitoring.

Additionally, the integrated chlorine dioxide dosing unit contributes to long-lasting disinfectant residuals and minimal by-product formation, consistent with findings by Wyczarska-Kokot et al. [33].

Recovered water at the Olender facility met Polish sanitary guidelines for non-potable reuse, such as toilet flushing, surface cleaning, and green area irrigation. The SOWA system showed adaptability and scalability, making it suitable for various types of pool infrastructure.

Hence, the results fully confirm the working hypothesis: the SOWA system achieves effective physicochemical polishing and complete microbiological removal, bringing the total microbial count within the stringent limit for direct pool recirculation.

Although such technologies remain rare in Poland, mainly due to cost-related barriers, their implementation supports sustainable water management and circular economy objectives. Legislative incentives or mandates could accelerate their wider adoption.

At the Olender facility, monthly freshwater use dropped from ~1700 to ~1000 m³ after 24/7 SOWA operation, while combined chlorine was maintained at 0.12 mg Cl₂/L and no issues with chloroform were observed. The unit recovered on average 4.7 m³ h⁻¹ of filter wash water (FWW) for non-potable uses (keep units consistent with the main text if a different rate is reported). According to manufacturer catalogue data, the recovery process can reach up to 96%, enabling annual savings up to ~EUR 9000 and a ~2-year payback under favourable tariffs and loads. Importantly, the recovered stream is only ~1 °C cooler than the recirculated pool water, which substantially reduces the energy otherwise required to heat make-up water; the exact savings are site-specific and depend on the facility’s heat source and operating schedule. These implementation outcomes are consistent with independent full-scale reuse trains (e.g., ultrafiltration/reverse osmosis) and with disinfection-by-product control strategies, and they align with international guidance for swimming-pool water reuse [34,35,36,37,38,39,40].

This research also highlights the variability of wash water characteristics across facilities, underscoring the importance of designing recovery systems tailored to site-specific technological and operational conditions.

Beyond our case study, independent groups report comparable outcomes. Reißmann et al. implemented a full-scale UF/RO train to reuse filter-backwash water in pool circulation, lowering turbidity from ~5–25 FNU to <0.02 FNU and demonstrating the feasibility of multi-stage recovery [34]. The broader disinfection-by-product (DBP) literature explains why precursor control is essential when re-introducing recovered waters: comprehensive identification and mutagenicity testing of pool waters [35], together with a state-of-the-art review on DBP occurrence, origins, and toxicity [36]. Process studies show that combined UV and ozonation can reduce chloramines while constraining THM formation [37]. Public-health monitoring likewise indicates that pool-filter backwash concentrates microbes, underscoring the need for robust treatment and verification before reuse [38]. Finally, our approach aligns with international guidance—WHO recommendations for swimming pools and DIN 19645—which provide frameworks for recycling spent filter-backwash water in pool systems [39,40].