Effectiveness of multi-soil-layering systems in the treatment of sodium chloride-contaminated wastewater

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.

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.

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 .
Pollutants .	Add chemicals .	Simulated concentration .	Amount 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 .
Pollutants .	Add chemicals .	Simulated concentration .	Amount 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

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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.

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.

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.

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.

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.

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.