Multi-soil-layering (MSL) is a sustainable wastewater treatment method that uses natural materials. Taiwan has successfully employed MSL to treat rural domestic sewage for reuse and to improve the water quality. However, coastal and riverside areas in Taiwan face challenges with sodium chloride contamination due to seawater intrusion, making conventional purification methods less effective. This study examined the effectiveness of MSL in treating sodium chloride-contaminated wastewater using three experimental tanks. Results showed that when sodium chloride concentration exceeded 2%, the reduction rates for ammonia nitrogen (NH4+-N) and chemical oxygen demand (COD) dropped below 50% from 90%. However, phosphate (PO43−) and nitrate nitrogen (NO3) reduction rates were less affected. Replacing gravel with zeolite in the MSL system's permeable layers increased sodium chloride tolerance from 2 to 3%, improving NH4+-N reduction to 50% but only slightly affecting COD reduction. Furthermore, low concentration (0.5%) sodium chloride solution to condition the MSL system, fostering the growth of salt-tolerant bacteria (halophiles). Subsequently, at a 50% pollution reduction rate, experiments showed that the tolerance of NH4+-N increased from 2 to 3%, and the sodium chloride tolerance for COD reduction improved from 2.5 to 3.8%. This significantly enhances the applicability of MSL in treating sodium chloride-containing wastewater.

  • To evaluate the MSL systems capacity to treat sodium chloride-containing domestic wastewater.

  • Solutions were proposed to improve the reuse of water resources in such wastewater.

  • These solutions are cost-effective and do not require advanced technology.

  • This approach is particularly effective for purifying domestic wastewater in areas impacted by soil salinization or seawater intrusion.

Rapid urbanization and economic growth have led to increased wastewater production, which poses significant environmental risks if untreated. In most urban areas, sewage is collected through sewer networks and treated in centralized wastewater treatment plants to ensure suitable water quality before being directed to water resource recovery centers for appropriate recycling. However, in less densely populated rural areas, challenges such as high costs, long construction durations, and the need for professional maintenance often result in untreated domestic wastewater being discharged directly into the environment. This phenomenon particularly affects villages located within reservoir catchment areas, where long-term discharge of untreated wastewater threatens water quality.

To improve water quality in reservoirs and rivers and to produce recycled water that meets water quality standards, the Taiwanese government has adopted decentralized and small-scale natural treatment systems (NTSs). NTSs primarily entail the use of natural elements such as oxygen, soil, microorganisms, and plants to purify wastewater to acceptable standards for discharge into rivers and streams. Common NTSs include constructed wetlands, gravel contact oxidation facilities, and soil infiltration trenches (Chen et al. 2009a, 2009b; Mendieta-Pino et al. 2021). Conventional NTSs require regular operation and maintenance to prevent treatment efficiency degradation and internal clogging. Compared with conventional NTSs, multi-soil-layering (MSL) systems offer several advantages, such as lower maintenance requirements, smaller footprint, more readily available construction materials, lower installation costs, reduced energy consumption, ability to withstand higher hydraulic loading ratios, and higher resistance to clogging; MSL systems constitute a type of NTS because they use natural materials for water purification (Masunaga et al. 2007). Moreover, the natural fill materials within an MSL system enhance its environmental sustainability because they can be recycled for other purposes such as soil improvement after a system's lifespan of 15–20 years (Ho & Wang 2015). Therefore, MSL systems align with the principles of the Sustainable Development Goals of the United Nations and serve as a nature-based solution for water purification, which highlights their importance in environmental conservation. Kammoun et al. (2024) implemented an MSL system for treating rural sewage and reusing the treated water for irrigation and washing purposes. The results demonstrated that the MSL system effectively reduced nutrients in the wastewater and significantly decreased fecal coliforms, fecal streptococci, and pathogenic Staphylococcus species. Additionally, the system exhibited the capability to minimize microcystins, effectively mitigating cyanobacterial blooms in water bodies (Aba et al. 2024; Mugani et al. 2024). These findings confirm the efficacy of MSL systems in treating domestic wastewater. However, further research is needed to investigate whether the system can maintain such exceptional performance when the domestic wastewater contains salinity.

Since 2013, Taiwan has adopted MSL systems, successfully implementing such systems in 11 locations existing primarily in mountainous areas and reservoir catchment areas far away from the coast. However, in the past decade, coastal areas and reservoir catchment areas on the outlying islands of Taiwan have faced severe water shortages and considerable eutrophication, necessitating urgent water quality improvements. As displayed in Figure 1, Taiwan has 24 domestic water supply reservoirs, with the sodium chloride (NaCl) concentrations in the 21 reservoirs on the main island ranging from 0.029 to 0.037%. Reservoirs with such low NaCl concentrations would not impact the pollution reduction efficiency of the MSL systems. However, the NaCl concentrations in the three reservoirs on Taiwan's outlying islands are approximately 10 times higher than those of the aforementioned 21 reservoirs, with the NaCl concentrations in the Lienchiang, Kinmen, and Penghu reservoirs being 0.213, 0.293, and 0.185%, respectively. Such high NaCl concentrations increase the difficulty of using MSL systems to improve water quality on these outlying islands. An examination of domestic wastewater from coastal areas in Taiwan revealed that the NaCl concentrations in this wastewater range from 0.3 to 2.2% (Yang 2023). Accordingly, the present study conducted a series of laboratory experiments by using wastewater with varying NaCl concentrations to investigate the effect of NaCl concentration on the purification efficiency of an MSL system. Moreover, this system's materials and operational methods were modified to enhance its capability to reduce the pollution caused by wastewater with a high NaCl concentration, thus expanding the potential applications of the system.
Figure 1

Sodium chloride concentrations in Taiwan's domestic water supply reservoirs.

Figure 1

Sodium chloride concentrations in Taiwan's domestic water supply reservoirs.

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When untreated nutrients from wastewater flow into water bodies such as rivers, lakes, or reservoirs, eutrophication can occur, thus increasing water purification costs and possibly rendering the water resources unusable. MSL has been widely adopted since its development in Japan in 1999 (Luanmanee et al. 2001; Boonsook et al. 2003; Sato et al. 2005, 2011; Chen et al. 2007, 2009a, 2009b; Masunaga et al. 2007; Latrach et al. 2016, 2018). Since its introduction in Taiwan in 2013, MSL has been successfully used in various applications, including the cyclic purification of reservoir, lake and rivers water, and treatment of domestic wastewater.

An MSL system consists of soil-mixing layers (SMLs) and permeability layers (PLs). The SMLs comprise varying proportions of soil, organic matter, activated carbon particles, and iron particles, whereas the PLs contain gravel, pumice, or zeolite (Attanandana et al. 2000; Sato et al. 2019; Sbahi et al. 2021; Zhou et al. 2021). When wastewater passes through an MSL system through trickling filtration, organic substances are initially adsorbed onto the surface of the PLs through physical adsorption. The MSL system has an environment conducive to microbial survival and reproduction; thus, it accelerates microbial growth and metabolic activities, which facilitates the removal of organic pollutants from wastewater and effectively reduces the chemical oxygen demand (COD) to enable water purification (Ho & Wang 2015; Koottatep et al. 2018).

In an MSL system, ammonium nitrogen () in wastewater is primarily removed through the activities of nitrifying and denitrifying bacteria. undergoes nitrification in the aerobic environment of the PLs; it is first converted into nitrite nitrogen () and then further oxidized into nitrate nitrogen () through nitrification. Subsequently, in the anaerobic environment of the SMLs, undergoes denitrification to form nitrogen gas (N2) and water (H2O) (Sato et al. 2002; Unno et al. 2003; Hong et al. 2019).

Phosphate is removed from the wastewater in an MSL system through its binding with iron ions, resulting in solid-phase phosphate precipitation within the system. Iron particles in the SMLs release ferrous ions (Fe2+), which then combine with phosphate ions () in the wastewater to form solid-phase iron phosphate (Fe3(PO4)2). In MSL system areas that have sufficient oxygen, ferrous ions might further oxidize into ferric ions (Fe3+), which combine with phosphate ions () to form solid-phase iron phosphate (FePO4). The precipitated Fe3(PO4)2 and FePO4 settle in the gravel layer at the bottom of the system, effectively removing phosphate from the water (Masunaga & Wakatsuki 1999; Guaya et al. 2015; Amarh et al. 2021).

Luanmanee et al. (2002) installed an MSL facility at the student cafeteria of Kasetsart University in Thailand to treat domestic wastewater. In an experiment, they measured the pollutant reduction efficiencies of this facility at hydraulic loading rates ranging from 100 to 600 L/m2/day. Their results indicated that this facility had , total phosphorous, and COD reduction rates of 89.5% ± 7.2%, 73.3% ± 8.1%, and 69.6% ± 15.1%, respectively.

Masunaga et al. (2003) established six MSL systems along Uya River in Japan. These systems comprised SMLs and PLs containing different types of soil: two systems comprised SMLs containing Andisol (volcanic ash soil) and PLs containing zeolite with a diameter of 1- to 3-mm zeolite, two systems comprised SMLs containing Entisol (granitic sandy soil) and PLs containing zeolite in 1- to 3-mm diameter, and two systems comprised SMLs containing Andisol and PLs containing 3- to 5-mm diameter of zeolite. They assessed the performance of these systems at hydraulic loading rates of 1,000–4,000 L/m2/day over five years of operation, and their results revealed that these systems exhibited total nitrogen, total phosphorous, and biochemical oxygen demand reduction rates of 22.4–50.5, 51.9–66.8, and 72.2–83.5%, respectively.

MSL has been successfully applied in Japan for the removal of nutrients from water; it has also been successfully implemented in China, Morocco, Thailand, Tunisia, and other regions (Khalifa et al. 2020; Song et al. 2021; Aba et al. 2021; Liu et al. 2022; Sbahi et al. (2022); Zidan et al. 2022). In addition to exploring the nutrient removal ability of MSL systems, studies have investigated the effectiveness of such systems in removing antibiotics, surfactants, Escherichia coli, and Enterococcus from wastewater. However, the effects of the NaCl concentration in water on the pollutant removal efficiency of MSL systems have not been investigated. Understanding the effects of NaCl on the filling materials used in MSL systems is crucial for evaluating these systems' capability to treat saline wastewater. Hanson et al. (1999) noted that cations on soil particles can create repulsive forces between these particles, thus causing soil separation and expansion, which can lead to the blockage of pores and to the obstruction of wastewater flow in an MSL system. Mazloomi & Jalali (2016) conducted experiments on the effects of different cations (K, Na, Ca, and Mg) on the ammonia adsorption capacity of zeolite; they revealed that increasing the cation concentrations negatively affected this capacity, with the strongest effect being observed for K, followed by that for Na.

When NaCl is dissolved in water, it dissociates into sodium ions (Na+) and chloride ions (Cl), which bind with water molecules, thus reducing water activity. This phenomenon leads to cellular dehydration and can induce cell death in microorganisms or inhibit microorganism growth through osmotic shock (Davidson et al. 2013). Ingram (1940) studied bacteria and observed that their respiratory rates decrease and mortality rates increase when the NaCl concentration exceeds 10 g/L. Uygur & Kargi (2004) observed that the removal efficiencies for various pollutants decreased with an increase in the NaCl concentration in a sequencing batch reactor system, indicating that NaCl has an adverse effect on biological treatment systems. However, microorganisms have evolved halophilic or halotolerant adaptations to cope with saline environments, enhancing their capability to remove pollutants from water (DasSarma & Arora 2002; Satyanarayana et al. 2005).

Woolard & Irvine (1995) used halophilic bacteria isolated from the Great Salt Lake in the United States to treat phenolic wastewater with salinity levels of 1–15%; they achieved a 99% removal rate for phenols. This finding demonstrates that halophilic bacteria can effectively treat wastewater containing high concentrations of NaCl. MSL systems primarily remove pollutants through microbial processes such as nitrification, denitrification, and nitritation. Gerardi (2003) noted the presence of halophilic species within nitrifying and denitrifying bacteria that can thrive in marine environments. Denitrification in MSL systems is primarily conducted by heterotrophic denitrifying bacteria, often Pseudomonas stutzeri (Ho & Lin 2022). Seip et al. (2011) conducted growth experiments on P. stutzeri DSM5190 in saline environments and observed that its growth was not significantly affected by NaCl concentrations of 1–5%; this result suggests that effective cultivation of halophilic bacteria in MSL systems can improve the feasibility of these systems for treating NaCl-contaminated wastewater.

In summary, wastewater in Taiwan's coastal areas and offshore reservoirs often contains notable NaCl concentrations. To effectively use MSL systems for pollutant reduction and water quality improvement in these areas and reservoirs, the influence of NaCl on these systems must be considered. Studies have suggested that NaCl influences MSL systems; however, this influence can be mitigated through strategies such as replacing system materials and cultivating halophilic and halotolerant bacteria. Accordingly, the present study investigated the effects of different concentrations of NaCl on MSL systems through a series of laboratory experiments. In addition, two solutions were designed to mitigate the influence of NaCl on the pollutant removal mechanisms of an MSL system: (1) one of the solutions entails using zeolite instead of gravel as the material of the PL layers to leverage zeolite's adsorption properties for NaCl and ammonia nitrogen () and (2) the other entails cultivating halophilic or halotolerant bacteria within the system.

Experimental equipment and materials

This study designed an indoor experimental setup for MSL systems. The setup comprised three main components: acrylic MSL tanks, a water supply system, and a wastewater collection tank (Figure 2). The dimensions of the acrylic MSL tanks were 30 cm (width) × 60 cm (length) × 70 cm (height). The water supply system contained a square acrylic tank with dimensions of 18.5 cm (width) × 18.5 cm (length) × 110.5 cm (height). This tank was equipped with inlet pipes, overflow pipes, and wastewater supply pipes. Floating balls were used to maintain a constant water head. The wastewater collection tank was a polyvinyl chloride tank capable of storing 300 L of wastewater; this tank was equipped with a submersible pump for transferring wastewater to the water supply system.
Figure 2

Schematic of the experimental setup for testing MSL systems.

Figure 2

Schematic of the experimental setup for testing MSL systems.

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Three MSL tank configurations were created for the experiment. The first configuration, denoted as Test 1 (Figure 3(a)), had the same internal filling arrangement as that described by Ho & Wang (2015). Initially, an 8-cm-thick gravel layer was placed at the bottom of the tank. Subsequently, SMLs and PLs were added to the tank. Each SML comprised 75% sand (sandy loam soil with a median particle size of 0.121 mm), 10% iron grains (1–3 mm in diameter), 10% carbon powder (particle size < 0.075 mm), and 5% rice husks; these components were mixed uniformly and packed into nonwoven geotextile bags with dimensions of 10 cm (width) × 60 cm (length) × 5 cm (height). The normal bulk density of SML is around 1.2 g/cm3. The gaps between the SMLs were filled with aggregate particles with a diameter of 3–5 mm (PLs). Finally, a 2-cm-thick gravel layer was placed over the SMLs and PLs. The second configuration, denoted as Test 2 (Figure 3(b)), had an identical arrangement and SML composition to the Test 1 configuration, but its PLs comprised zeolite with a diameter of 3–5 mm. The third configuration, denoted as Test 3 (Figure 3(a)), had the same internal filling materials as those of the Test 1 configuration; however, the initial microbial conditioning methods differed between these configurations.
Figure 3

Schematics of the internal layouts of different MSL systems used in the conducted experiments: (a) Test 1 and Test 3 configurations and (b) Test 2 configuration.

Figure 3

Schematics of the internal layouts of different MSL systems used in the conducted experiments: (a) Test 1 and Test 3 configurations and (b) Test 2 configuration.

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Experimental methods and procedures

The three experimental systems were filled with tap water and left undisturbed for 48 h to ensure complete saturation of the internal filling. Subsequently, the water was drained from the tanks. The same microbial conditioning method was used in Tests 1 and 2; specifically, diluted domestic wastewater (diluted 10 times) was evenly applied within the tank at a rate of 300 L/m2/day. By contrast, in Test 3, diluted domestic wastewater (diluted 10 times) with 0.5% NaCl was uniformly distributed within the tank at a rate of 300 L/m2/day. Water samples were collected weekly from both the inlet and outlet of the tanks for analysis. When the , , , and COD reduction rates in the effluent stabilized, indicating the completion of microbial system conditioning, a series of tests were conducted at different NaCl concentrations.

To ensure the stability of the wastewater concentration, this study produced wastewater with approximately the same composition as the average wastewater composition determined by Taiwanese domestic wastewater treatment plants. The wastewater was produced using the method of Chen (2017), with the approximate concentrations of , , and COD being 25, 5, and 150 mg/L, respectively. Furthermore, to simulate NaCl-containing wastewater, sea salt was used as an additive. The typical concentration of NaCl in seawater ranges from 3 to 5%. Therefore, domestic wastewater with NaCl concentrations of 0, 0.5, 1, 2, 3, 4, and 5% was prepared for the experiment (Table 1).

Table 1

Amount of chemicals to be added to synthetic sewage

Amount of synthetic sewage solution = 120 L
PollutantsAdd chemicalsSimulated concentrationAmount of chemicals added
 NHCL 25 mg/L 0.095 g 
PO4 KH2PO4 5 mg/L 2.636 g 
COD Milk powder 150 mg/L 11.043 g 
NaCl Sea salt 0–5% 0–6,000 g 
Amount of synthetic sewage solution = 120 L
PollutantsAdd chemicalsSimulated concentrationAmount of chemicals added
 NHCL 25 mg/L 0.095 g 
PO4 KH2PO4 5 mg/L 2.636 g 
COD Milk powder 150 mg/L 11.043 g 
NaCl Sea salt 0–5% 0–6,000 g 

After its production, the wastewater was transferred into the wastewater storage tank and pumped into the water supply system. The wastewater was distributed uniformly in the MSL systems by using a constant-head method; the hydraulic loading rate was controlled at 2,000 L/day/m2. Samples of water flowing into and out from the MSL systems were collected for water quality analysis to evaluate the effectiveness of pollutant reduction. To ensure the reliability of the experimental data, water samples were collected and tested for water quality at least three times for each experiment. If the variation among the test results was within 10%, the three data points were averaged to represent the results of that particular experiment. As unified testing procedures, the measurement methods for , , , and COD adhered to the Standard Method for the Examination of Water and Wastewater promulgated by the Taiwan's Ministry of Environment to ensure the consistency and rationality of the test data quality.

Target reduction rates for different pollutants

The pollutant removal capabilities of the investigated MSL systems were evaluated on the basis of target reduction rates set for different pollutants; these target rates were set in accordance with several studies that have applied MSL systems to treat domestic wastewater (Sato et al. 2011; An et al. 2016; Latrach et al. 2016; Shen et al. 2018; Song et al. 2018; Li et al. 2021; Sbahi et al. 2021). The target reduction rate set for , NO3-N, and COD was 50%. Moreover, because of the excellent performance of MSL systems in phosphate reduction, the target reduction rate for was set to 65%.

This study uses the aforementioned target reduction rates as criteria to assess the operation of the MSL systems. An MSL system was considered to have satisfactory or unsatisfactory performance if it did or did not achieve the target reduction rates, respectively.

The experimental results obtained for the three investigated MSL systems are described as follows.

Ammonia nitrogen

Figure 4 displays the removal efficiency of the MSL system across varying NaCl concentrations. In Test 1, when the wastewater in the MSL system did not contain NaCl, the concentration decreased from 27.2 to 1.16 mg/L, achieving a 95.74% reduction rate (Figure 5). However, when the wastewater in the MSL system had a NaCl concentration of 0.5%, the concentration of the wastewater decreased to only 7.6 mg/L after purification by the MSL system, with the reduction rate being 72.06% Thus, the presence of NaCl in the wastewater affected the MSL system's ability to remove from the wastewater. As the NaCl concentration increased, the reduction rate exhibited a nearly linear decline. At NaCl concentrations of 4 and 5%, the reduction rates stabilized; this suggests that the biochemical degradation capability of the MSL system might have degraded considerably or disappeared, and physical adsorption by the fill materials primarily contributed to removal. Thus, the presence of NaCl in the wastewater strongly affected the MSL system's biochemical degradation capacity for but weakly affected its physical adsorption capacity. Consequently, the system retained some capability for removal. However, the reduction rate dropped below 50% (48.28%) at 2% NaCl. Therefore, an MSL system with PLs composed of gravel should not be used for water purification when the NaCl concentration in wastewater is ≥2%. For these NaCl concentrations, alternative purification methods or modified fill materials are recommended.
Figure 4

concentrations in wastewater before and after purification in Tests 1–3.

Figure 4

concentrations in wastewater before and after purification in Tests 1–3.

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Figure 5

reduction rates achieved in Tests 1–3.

Figure 5

reduction rates achieved in Tests 1–3.

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In Test 2, replacing gravel with 3–5 mm zeolite particles in the PLs effectively enhanced the reduction rate. Without NaCl, the reduction rate of the MSL system reached 100%, attributed to zeolite's superior microbial support and adsorption capabilities. However, NaCl still affected system performance, with the reduction rate dropping below 50 at 3% NaCl. At 4% NaCl, the effectiveness of zeolite decreased rapidly, as the alkaline environment caused desorption of . Despite these challenges, the zeolite replacement extended the NaCl tolerance of the system from 2% to approximately 3%, achieving the target reduction rate of 50%.

In Test 3, wastewater with a NaCl concentration of 0.5% was used to condition an MSL system with the same configuration as that used in Test 1; wastewater purification experiments were conducted after the completion of the conditioning process. The results of Test 3 indicated that as the NaCl concentration in the wastewater increased, the reduction rate of the MSL system decreased, although to a smaller extent than that in Test 1. This finding demonstrates that when an MSL system is conditioned with wastewater having a low NaCl concentration during the initial operational phase, the cultivated halophilic nitrifying bacteria can effectively adapt to and treat wastewater containing relatively high concentrations of NaCl. Therefore, at NaCl concentrations of <2%, the reduction rates in Test 3 were consistently higher than those in Test 1. This approach enhanced the system's NaCl tolerance from 2% to approximately 3%, achieving the target reduction rate of 50%.

Figure 5 illustrates the reduction rates achieved by the three MSL systems at different concentrations of NaCl in the wastewater within the systems. This figure indicates that the optimal reduction rates were achieved for wastewater without NaCl, with the highest rate being observed in Test 2 (100%). The reduction rates in Tests 1 and Test 3 also exceeded 90% under the aforementioned condition. When the NaCl concentration in the wastewater increased, the reduction rate decreased, and the greatest decrease was observed in Test 1; the rates of decrease in Tests 2 and 3 were similar. When a reduction rate of 50% was considered the standard for pollutant removal, the MSL system used in Test 1 had a NaCl tolerance of approximately 2%, whereas those used in Tests 2 and 3 had a NaCl tolerance of approximately 3%. Therefore, an MSL system's tolerance to NaCl in wastewater can be enhanced by replacing the gravel in its PLs with zeolite or by conditioning it with wastewater containing low concentrations of NaCl during initial operation to cultivate halophilic nitrifying bacteria.

Nitrate

Figure 6 presents the concentrations observed in the three tests. In all tests, the PLs of the MSL system effectively converted in the wastewater within the system into . When wastewater containing entered the SMLs, which had anaerobic environments, anaerobic bacteria within the SMLs performed denitrification. This process resulted in the decomposition of into N2 and H2O, which were then released into the atmosphere. Thus, the MSL systems had excellent removal capability. Moreover, the reduction rate did not decrease markedly with an increase in NaCl concentration in the wastewater; the concentration was <5 mg/L after purification at all NaCl concentrations.
Figure 6

concentrations in wastewater before and after purification in Tests 1–3.

Figure 6

concentrations in wastewater before and after purification in Tests 1–3.

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Regardless of the NaCl concentration of the wastewater, the MSL systems used in all three tests achieved a reduction rate of >50%; the reduction rate exceeded 70% in most cases (Figure 7). Test 2 had the highest reduction rates, followed by Test 3. However, at NaCl concentrations of 4 and 5% in Test 2, the adsorption capacity of zeolite was affected by the increased pH, leading to the release of that was previously adsorbed onto the zeolite. Consequently, the reduction rate decreased under the aforementioned conditions. This phenomenon was not observed in Test 1 or Test 3.
Figure 7

reduction rates achieved in Tests 1–3.

Figure 7

reduction rates achieved in Tests 1–3.

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The stable removal efficiency across varying NaCl concentrations demonstrates that the anaerobic denitrification process in the SMLs is less affected by salinity compared to the aerobic nitrification process in the PLs. This can be attributed to the adaptability of anaerobic bacteria to osmotic stress, which maintains denitrification activity even under high salinity. The superior performance in Test 2 under low salinity conditions highlights the potential of zeolite to enhance removal. However, the observed decline at high salinity emphasizes the need for pH stabilization strategies or alternative fill materials for consistent performance. Test 3 demonstrated a balanced approach, where pre-conditioning with low NaCl wastewater improved the system's resilience without significant sensitivity to pH changes.

In conclusion, while the MSL system effectively removes across a range of salinity levels, modifications such as zeolite use or system conditioning can further enhance its performance. However, the choice of strategy should consider the potential trade-offs, such as sensitivity to pH or operational complexity under high salinity conditions.

Phosphate

Figure 8 illustrates the phosphate () concentrations observed in the three tests. This figure indicates that all MSL systems had excellent removal ability. In Test 2, zeolite adsorbed sodium ions, thereby reducing the NaCl concentration in the SMLs. This reduction in NaCl concentration caused a decrease in the formation of soluble sodium phosphate (Na3PO4) through the combination of phosphate ions with sodium ions. This thus led to most phosphate ions binding to iron ions in the SMLs, which resulted in a high reduction rate.
Figure 8

concentrations in wastewater before and after purification in Tests 1–3.

Figure 8

concentrations in wastewater before and after purification in Tests 1–3.

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In Test 1, because gravel cannot adsorb sodium ions, the NaCl concentration in the SMLs was high, which led to the formation of Na3PO4 from sodium and phosphate ions. This process affected the opportunities for iron ions to bind to to form solid FePO4. Therefore, an increase in the NaCl concentration in the wastewater had a stronger effect on the removal efficiency in Test 1 than in Test 2; thus, under the presence of NaCl in the wastewater, the concentration of purified wastewater was higher in Test 1 than in Test 2.

In Test 3, salt-tolerant bacteria cultured within the MSL system absorbed sodium ions to sustain their survival, thereby reducing the formation of Na3PO4. Consequently, the concentration in purified wastewater in Test 3 was lower than that in Test 1 but higher than that in Test 2.

When the NaCl concentration of the wastewater was <1%, the three MSL systems exhibited comparable reduction rates (Figure 9). However, when the NaCl concentration was >1%, only the system used in Test 2 exhibited no notable decrease in its reduction rate. The formation of Na3PO4 in Test 1 resulted in the corresponding MSL system's reduction rate decreasing from 98.43% at a NaCl concentration of 0 to 82.44% at a NaCl concentration of 5%, a decrease of approximately 16.25%. The reduction rate decreased marginally at high NaCl concentrations in Test 3. However, once the growth of salt-tolerant bacteria within the MSL system stabilized in this test, the reduction rate no longer declined with increasing NaCl concentration. Overall, the reduction rate of MSL systems is only marginally influenced by NaCl concentration. Hence, even at a NaCl concentration of 5% in wastewater, the MSL systems examined in this study maintained a reduction rate of >80%, surpassing the target reduction rate of 65%.
Figure 9

reduction rates achieved in Tests 1–3.

Figure 9

reduction rates achieved in Tests 1–3.

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Chemical oxygen demand

Figure 10 illustrates the COD values before and after purification across the three tests. At low NaCl concentration, the MSL systems exhibited high COD removal ability (>95%). However, as the NaCl concentration increased, the COD removal ability of these systems decreased considerably, and the greatest decrease was observed when the NaCl concentration was increased to 2%. This phenomenon is similar to that observed for . As indicated in Figure 10, similar COD removal efficiency levels were observed in Tests 1 and 2. Thus, using zeolite to treat wastewater with a high NaCl concentration does not directly facilitate COD removal. However, the COD removal rate achieved in Test 3 was greater than those achieved in Tests 1 and 2 when the NaCl concentration was 2 and 3%. This result can be attributed to the salt-tolerant bacteria cultivated in Test 3, which effectively reduced COD even at high NaCl concentrations. Nevertheless, when the NaCl concentration was >4%, even the MSL system used in Test 3 failed to maintain a high COD removal efficiency level; the COD removal efficiency level in Test 3 was similar to those in Tests 1 and Test 2.
Figure 10

COD values observed in Tests 1–3.

Figure 10

COD values observed in Tests 1–3.

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Figure 11 displays the COD reduction rates achieved in the three tests. This figure indicates that similar COD reduction rates were achieved in Tests 1 and 2 at different NaCl concentrations in the wastewater. When the wastewater did not contain NaCl, COD reduction rates of >95% were achieved in both tests. However, as the NaCl concentration increased above 1%, the COD reduction rates in these tests decreased considerably. When the NaCl concentration exceeded 2%, the COD reduction rates in both tests decreased below 50%. This finding indicates that replacing gravel with zeolite was insufficient for improving the MSL system's COD reduction rate for wastewater with high NaCl concentrations. However, the salt-tolerant bacteria cultivated in Test 3 enabled the MSL system to maintain a COD reduction rate of >50% even at a NaCl concentration of 3%. Thus, wastewater with a low NaCl concentration should be used during the initial operation of an MSL system to cultivate salt-tolerant bacteria. This approach enables the COD to be effectively reduced for wastewater with NaCl concentrations of up to 3%. However, when the NaCl concentration exceeded 4%, the MSL system used in Test 3 also failed to achieve a sufficient COD reduction rate. Therefore, alternative strategies should be employed for reducing the COD when the NaCl concentration in wastewater is ≥4%.
Figure 11

COD reduction rates achieved in Tests 1–3.

Figure 11

COD reduction rates achieved in Tests 1–3.

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The results emphasize the limitations of conventional MSL systems in treating saline wastewater. For wastewater with NaCl concentrations >3%, alternative strategies, such as introducing halophilic microbial consortia or optimizing system configurations to reduce osmotic stress, should be explored. When NaCl concentrations reach ≥4%, the use of advanced treatment technologies, such as membrane filtration or chemical oxidation, may be necessary to achieve effective COD reduction.

This study investigated whether MSL systems can effectively treat domestic wastewater containing NaCl to facilitate its reuse. Three MSL systems were designed to purify wastewater with varying concentrations of NaCl. The experimental results of Test 1, Test 2, and Test 3 indicate that the MSL systems effectively purified wastewater without NaCl, with the , , , and COD reduction rates being 96 ± 4, 94 ± 4, 99 ± 0.5, and 96 ± 1%, respectively.

First, as the NaCl concentration in wastewater was increased, the MSL system used in Test 1, which comprised gravel-containing PLs, exhibited a continuous linear decrease in its reduction rate; the reduction rate decreased below 50% when the NaCl concentration exceeded 2%. Hence, NaCl indeed influenced this MSL system's reduction rate. The use of zeolite instead of gravel in the PLs of the MSL system (system used in Test 2) or the implementation of system conditioning with wastewater having low NaCl concentrations (system used in Test 3) considerably enhanced the MSL system's tolerance to NaCl. The highest removal rates were observed in Test 2. This result indicates that when an MSL system is used to purify wastewater with a high NaCl concentration, zeolite should be used instead of gravel in the PLs of the MSL system. An MSL system with PLs containing zeolite can achieve an NH4+-N reduction rate of >50% even when the NaCl concentration in the wastewater is 3%.

Second, the MSL systems exhibited relatively stable removal rates, which did not decrease considerably as the NaCl concentration in the wastewater increased. For all three MSL systems, the levels in the purified water remained below 5 mg/L at NaCl concentrations of 0–5%. Compared with the MSL system used in Test 1 and Test 3, the system used in Test 2 exhibited higher removal rates; this is because zeolite has higher adsorption capacity of the cation than does gravel. However, a high NaCl concentration can increase the water pH, thereby reducing zeolite's adsorption capacity and resulting in the potential release of and again.

Third, the MSL system used in Test 1 exhibited a marginal reduction of as the NaCl concentration in the wastewater increased. This is because in Test 1, sodium ions interfered with the binding of phosphate ions to iron ions. By contrast, in Test 2, zeolite's ability to adsorb sodium ions reduced the NaCl concentration of the SMLs, thus facilitating the effective binding of phosphate and iron ions for reducing the concentration. Therefore, the reduction rate of the MSL system used in Test 2 was almost unaffected by the NaCl concentration. The reduction rates of the MSL systems used in Tests 1 and 3 were influenced by the NaCl concentration; however, this influence was minor. The three MSL systems maintained reduction rates of >80% even at a NaCl concentration of 5%.

Finally, the COD reduction rates of all MSL systems decreased as the NaCl concentration in the wastewater increased. The use of zeolite instead of gravel in the PLs did not enhance the COD reduction rate. When the NaCl concentration exceeded 1%, the COD reduction rates of the MSL systems used in Tests 1 and 2 declined sharply. However, the halophilic nitrifying bacteria cultivated in the MSL system used in Test 3 enabled a satisfactory COD reduction rate even at an NaCl concentration of 3%. At higher NaCl concentrations, all three systems exhibited unsatisfactory COD reduction rates. Therefore, the MSL system used in Test 3 (system conditioned with wastewater having a low NaCl concentration and containing halophilic nitrifying bacteria) can be employed for reducing the COD of wastewater with a NaCl concentration of up to 3%.

This study demonstrated the potential of MSL systems for treating domestic wastewater with varying NaCl concentrations, and future recommendations could assess the long-term performance and sustainability of MSL systems, particularly under continuous exposure to high salinity wastewater. Potential issues such as clogging, material degradation, and shifts in microbial communities over time remain unexplored.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

Aba
R. P.
,
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