ABSTRACT
This paper evaluates the performance and potential of a full-scale hybrid multi-soil-layering (MSL) system for the treatment of domestic wastewater for landscape irrigation reuse. The system integrates a solar septic tank and sequential vertical flow MSL and horizontal flow MSL components with alternating layers of gravel and soil-based material. It operates at a hydraulic loading rate of 250 L/m2/day. Results show significant removal of pollutants and pathogens, including total suspended solids (TSS) (97%), chemical oxygen demand (COD) (88.57%), total phosphorus (TP) (79.93%), and total nitrogen (TN) (88.49%), along with significant reductions in fecal bacteria indicators (4.21 log for fecal coliforms and 3.90 log for fecal streptococci) and the pathogen Staphylococcus sp. (2.43 log). The principal component analysis confirms the effectiveness of the system in reducing the concentrations of NH4, COD, TP, PO4, fecal coliforms, fecal streptococci, and fecal staphylococci, thus supporting the reliability of the study. This work highlights the promising potential of the hybrid MSL technology for the treatment of domestic wastewater, especially in arid regions such as North Africa and the Middle East, to support efforts to protect the environment and facilitate the reuse of wastewater for landscape irrigation and agriculture.
HIGHLIGHTS
High removal of TSS (97%), COD (88.57), TN (88.49), TP (79.93) achieved by MSL system.
Significant removal rates of fecal coliforms (4.21 log) and fecal streptococci (3.90 log).
Principal component analysis showed a positive correlation between COD, NH4+, TP removal, and pathogens.
Treated water met Moroccan and FAO guidelines for safe irrigation.
MSL sustainably reduces the environmental impact and health hazards of wastewater.
INTRODUCTION
Over the past few decades, Marrakech and its surrounding region have experienced significant economic, demographic, and urban growth, characterized by a notable construction boom. The expansion of the developed area has been remarkable, increasing from 2,000 ha in the early 1970s to nearly 20,000 ha by 2020 (Lachir et al. 2016). This rapid development has been particularly evident in the last 20 years, as Marrakech has established itself as the number one tourist destination in Morocco, attracting approximately 3 million visitors each year (Analy & Laftouhi 2021). The region is also a major center for mining, accounting for 28% of the country's total production, and is known for its extensive agricultural activities (Kahime et al. 2018; Analay & Laftouhi 2021). However, this rapid growth has put immense pressure on natural resources, particularly groundwater, which is used extensively for agricultural irrigation, golf courses, and various other purposes (Hssaisoune et al. 2020). The availability of groundwater in the shallow aquifer has decreased significantly, from 331 Mm3 in 1962 to 207 Mm3 in 2019 (Analy & Laftouhi 2021). More alarmingly, during the critical period between 2002 and 2019, groundwater resources declined at an average annual rate of 8.5 Mm3, resulting in a significant reduction from 353 to 207 Mm3. This represents a dramatic 41% decrease in just 17 years and the water demand for agricultural irrigation has increased dramatically, from 500 Mm3 in 1961 to 13,500 Mm3 in 2014, with 70% coming from surface water and 30% from groundwater (FAO 2015).
Like many arid countries, Morocco faces significant pressures on its water supply due to limited water resources and increasing demand from various sectors, including agriculture, industry, and domestic use. Water availability, especially in arid and semi-arid regions, is further strained by erratic rainfall patterns and recurrent droughts due to the effects of climate change (World Bank 2017; Ben-Daoud et al. 2021). Wastewater treatment and reuse is emerging as a key strategy to address this challenge. By implementing efficient wastewater treatment technologies, Morocco can recover and reuse water from a variety of sources, including domestic, industrial and agricultural wastewater, and significantly reduce the demand for freshwater resources. In addition, public health and water quality can be protected through proper wastewater management. The implementation of wastewater treatment and reuse initiatives is in line with sustainable water management practices and offers a pragmatic solution to increase water availability and resilience in the face of increasing water scarcity challenges in Morocco (Mandi et al. 2022).
To relieve pressure on traditional water sources, researchers, and policymakers are actively seeking alternative approaches to urban irrigation, and one promising solution is the use of domestic wastewater (Zidan et al. 2023). The World Bank has projected a significant increase in wastewater volumes, from 666 Mm3 in 2014 to 750 Mm3 in 2020 and 900 Mm3 in 2050 (World Bank 2017). This indicates a significant potential resource that can be harnessed for sustainable irrigation practices. However, concerns about waterborne diseases and the safety of water reuse have increased in recent years (Labhasetwar & Yadav 2023; Bibi et al. 2021). Public perception of wastewater exposure and associated health risks is a significant barrier to water reclamation and reuse. Among the various contaminants present in wastewater, pathogens are of particular concern due to their potential to cause human disease. Therefore, it is crucial to develop effective technical methods to remove pathogens from domestic wastewater to prevent contamination. The importance of such research in the context of sustainable development is paramount. As water scarcity becomes more prevalent, particularly in arid regions such as North Africa and the Middle East, the need for innovative wastewater treatment technologies that can reduce pressure on freshwater resources is urgent.
The multi-soil-layering (MSL) system, as elucidated by Guan et al. (2014), has been adopted as a decentralized approach for wastewater treatment. MSL takes advantage of the soil structure to enhance the wastewater treatment capacity, as highlighted by Sy et al. (2021). According to Masunaga et al. (2007), the standard configuration includes a permeable gravel layer surrounding a soil mixture (charcoal, iron granules, sawdust, and soil). The use of gravel is crucial to improve the treatment by increasing permeability, aeration, and preventing clogging, as emphasized by Chen et al. (2009). The soil mixture layer was found to have low porosity and lack of aeration (Latrach et al. 2018). Maintaining a balance between aerobic and anaerobic conditions is crucial for the efficiency of this system, especially for nitrogen removal through nitrification–denitrification processes (Attanandana et al. 2000). These factors are crucial for MSL technology to effectively treat wastewater and remove nitrogen (Sbahi et al. 2021). Numerous studies and adaptations using different approaches and stages have led to extensive changes in the MSL system. A vertical hybrid system, which combines the MSL system with a trickling filter, was introduced in China by Luo et al. (2013) and is effective in treating wastewater from simulated septic tanks. In this regard, the trickling filter was modified by Zhang et al. (2015) by incorporating the MSL system. Subsequently, Tang et al. (2020) used this system to remove sodium dodecyl benzene sulfonate from the effluent. Latrach et al. (2016) conducted another study in which a sand filter was integrated with the MSL system. The goal of this configuration is to improve performance while maintaining a low hydraulic retention time, particularly by removing organics and nutrients. In addition, in Morocco, Zidan et al. (2023) designed a hybrid MSL system using vertically and horizontally flowing MSL units to treat domestic wastewater in urban areas. The MSL system was shown to be capable of treating domestic wastewater by incorporating biological (biodegradation), chemical (precipitation, adsorption), and physical (sedimentation, filtration) mechanisms as highlighted by Zidan et al. (2022). The MSL system has successfully treated other types of wastewater, including polluted river water (Wei & Wu 2018), textile wastewater (Sy & Kasman 2019), turtle aquaculture wastewater (Song et al. 2015), industrial wastewater (Attanandana et al. 2000), laboratory wastewater (Sy et al. 2021), and wastewater contaminated with sodium dodecyl benzene sulfonate (Tang et al. 2020), thereby expanding its scope. This wastewater treatment plant is characterized by its ability to provide low-cost, high-performance treatment and its unique ability to accommodate higher hydraulic loads (Sbahi et al. 2020). It is highly adaptable to the environment. It minimizes the risk of clogging and requires less maintenance. Furthermore, as demonstrated in studies by Latrach et al. (2015), Guan et al. (2014), and Chen et al. (2009), the MSL system has a lifespan of more than 20 years. Zhang et al. (2015) noted that another advantage of MSL is the availability of microbial communities. In addition, the MSL system requires a relatively small area for implementation (Latrach et al. 2018) and shows resilience to negative impacts such as odor and insects as shown by Zidan et al. (2022). Very few works have been focused on hybrid MSL systems, except for the studies by Zidan et al. (2023) and Tang et al. (2020). In addition, pathogen removal by MSL and the suitability of treated wastewater for irrigation purposes have received very little attention.
The present study is designed to (1) evaluate the treatment performance and behavior of a full-scale hybrid MSL plant combining vertical flow (VF) MSL and horizontal subsurface flow (HSSF) MSL with special attention to coliform and human pathogen removal; (2) explore the correlation between physicochemical and bacteriological parameters for the MSL system; (3) investigate the effect of each compartment on the hybrid MSL performance in an arid climate; and (4) compare the treated water quality with Moroccan water reuse standards to evaluate its suitability for irrigation.
MATERIAL AND METHODS
Hybrid multi-soil-layering plant description
A schematic overview of the hybrid MSL system implemented at the Cadi Ayyad University, Marrakech (Morocco).
A schematic overview of the hybrid MSL system implemented at the Cadi Ayyad University, Marrakech (Morocco).
Each unit of the hybrid MSL system consists of stratified layers filled with gravel (approximately 1–2 cm in diameter) alternating with a soil-based layer (SBL). The SBL consists of soil (70%) associated with granular ferrous metal (10%), charcoal (10%), and sawdust (10%) and has an effective pore space of 30%. SBL characteristics are shown in Table 1.
Physicochemical characteristics and mineral composition of the SBL (mean ± standard deviation)
Parameter . | Unit . | Value . |
---|---|---|
pH | _ | 8.335 ± 0.22 |
EC | μS/cm | 448 ± 0.31 |
Total organic carbon | % | 12.48 ± 0.226 |
Total organic matter | % | 19.67 ± 0.424 |
Mineral composition | ||
LE | mg/kg | 86.85 |
Ca | % | 2.45 |
Cl | % | 3.87 |
Fe | % | 4.06 |
Si | mg/kg | 5,685 |
K | mg/kg | 2,643 |
Al | mg/kg | 3,860 |
P | mg/kg | 1,542 |
Ti | mg/kg | 737 |
S | mg/kg | 675 |
Mn | mg/kg | 460 |
Sr | mg/kg | 244 |
Zr | mg/kg | 105 |
Zn | mg/kg | 70 |
Rb | mg/kg | 45 |
Pb | mg/kg | 27 |
Cu | mg/kg | 19 |
Y | mg/kg | 15 |
As | mg/kg | 8 |
Mg | % | 1.15 |
Parameter . | Unit . | Value . |
---|---|---|
pH | _ | 8.335 ± 0.22 |
EC | μS/cm | 448 ± 0.31 |
Total organic carbon | % | 12.48 ± 0.226 |
Total organic matter | % | 19.67 ± 0.424 |
Mineral composition | ||
LE | mg/kg | 86.85 |
Ca | % | 2.45 |
Cl | % | 3.87 |
Fe | % | 4.06 |
Si | mg/kg | 5,685 |
K | mg/kg | 2,643 |
Al | mg/kg | 3,860 |
P | mg/kg | 1,542 |
Ti | mg/kg | 737 |
S | mg/kg | 675 |
Mn | mg/kg | 460 |
Sr | mg/kg | 244 |
Zr | mg/kg | 105 |
Zn | mg/kg | 70 |
Rb | mg/kg | 45 |
Pb | mg/kg | 27 |
Cu | mg/kg | 19 |
Y | mg/kg | 15 |
As | mg/kg | 8 |
Mg | % | 1.15 |
The VF-MSL unit contains six layers aerated by four PVC pipes that allow atmospheric air to penetrate and is continuously fed by vertical flow (VF). The HF-MSL unit contains only three layers without aeration, fed by a continuous subsurface flow.
To ensure a continuous inflow of wastewater, a primary solar pump first lifts the raw domestic wastewater from the sanitary manhole to the septic tank. The MSL units are then fed continuously by gravity (Figure 1). The treated water is temporarily stored in a storage tank and reused for landscape irrigation at the Cadi Ayyad University.
A hydraulic loading rate (HLR) between 160 and 250 L/m2/day and a continuous flow rate of 16 m3/day were applied to the hybrid MSL system. The system was monitored from December 2021 to June 2023 (1.5 years) with the exception of the period from August to October 2022. This period coincides with the closing of the institution for vacation, which renders the system inoperable. During this period, self-cleaning and maintenance activities will be carried out and no treatment will take place due to the absence of wastewater. When the academic year begins, the system resumes normal operation until the next vacation begins.
Sampling




Statistical analysis
In this study, the application of Fisher's exact test (F) and Fisher's test allows the comparison of the means of different parameters to determine the presence of statistically significant effects. The principal component analysis (PCA) is used to evaluate the influence of each compound on the removal of pollutants. A correlation test is used to investigate possible relationships between the different parameters.
RESULTS AND DISCUSSION
Characteristics of urban domestic wastewater
Raw wastewater has pH values ranging from 7.62 to 8.71, indicating a slightly alkaline nature (Table 2). This alkalinity can be attributed to the prevalence of anaerobic conditions in these waters, as observed by Luanmanee et al. (2002). The concentrations of parameters such as COD, TP, TN, FC, FS, and S ranged from 288.40 to 820.89 mg/L, 4.76 to 11.30 mg/L, 20.63 to 62.87 mg/L, 6.04 to 6.29 log, 6.28 to 6.08 log, and 5.29 to 5.59 log, respectively. These concentrations are consistent with the previous findings of Zidan et al. (2023) who did monitoring of the system some years ago.
Composition of raw wastewater
Variables . | Unit . | Min . | Max . | Mean . | SD . |
---|---|---|---|---|---|
pH | Unit | 7.62 | 8.71 | 8.18 | 0.047 |
DO | mg/L | 0.79 | 1.99 | 1.40 | 0.061 |
EC | μS/cm | 953.33 | 1,762.66 | 1,286.64 | 47.44 |
TSS | mg/L | 101.6 | 326 | 213.8 | 0.92 |
COD | mg/L | 288.40 | 820.89 | 520.90 | 0.036 |
![]() | mg/L | 1.79 | 7.73 | 4.43 | 0.11 |
TP | mg/L | 4.76 | 11.30 | 6.72 | 0.017 |
TN | mg/L | 20.63 | 62.87 | 41.75 | 0,13 |
TKN | mg/L | 13.06 | 42 | 24.23 | 0.092 |
![]() | mg/L | 13.32 | 39.48 | 18.08 | 0.013 |
![]() | mg/L | 0.28 | 1.88 | 0.77 | 0.002 |
FC | log (CFU/100 mL) | 6.04 | 6.29 | 6.18 | 0.083 |
FS | log (CFU/100 mL) | 5.89 | 6.28 | 6.08 | 0.12 |
S | log (CFU/100 mL) | 5.29 | 5.59 | 5.44 | 0.069 |
Variables . | Unit . | Min . | Max . | Mean . | SD . |
---|---|---|---|---|---|
pH | Unit | 7.62 | 8.71 | 8.18 | 0.047 |
DO | mg/L | 0.79 | 1.99 | 1.40 | 0.061 |
EC | μS/cm | 953.33 | 1,762.66 | 1,286.64 | 47.44 |
TSS | mg/L | 101.6 | 326 | 213.8 | 0.92 |
COD | mg/L | 288.40 | 820.89 | 520.90 | 0.036 |
![]() | mg/L | 1.79 | 7.73 | 4.43 | 0.11 |
TP | mg/L | 4.76 | 11.30 | 6.72 | 0.017 |
TN | mg/L | 20.63 | 62.87 | 41.75 | 0,13 |
TKN | mg/L | 13.06 | 42 | 24.23 | 0.092 |
![]() | mg/L | 13.32 | 39.48 | 18.08 | 0.013 |
![]() | mg/L | 0.28 | 1.88 | 0.77 | 0.002 |
FC | log (CFU/100 mL) | 6.04 | 6.29 | 6.18 | 0.083 |
FS | log (CFU/100 mL) | 5.89 | 6.28 | 6.08 | 0.12 |
S | log (CFU/100 mL) | 5.29 | 5.59 | 5.44 | 0.069 |
In situ parameters
Temporal changes of DO (a), pH (b), and EC (c) during the treatment process in the hybrid MSL system.
Temporal changes of DO (a), pH (b), and EC (c) during the treatment process in the hybrid MSL system.
At the ST-outlet, DO values range from 1 to 2.31 mg/L. At the VF-MSL outlet, DO concentrations range from 2.43 to 5.11 mg/L, while at the HF-MSL outlet, DO concentrations range from 4.02 to 5.72 mg/L (Figure 2(a)). The increase in oxygen levels at the hybrid MSL system outlet is attributed to the introduction of aeration pipes at the VF-MSL level, which results in increased oxygen levels in the effluent. Placement of the HF-MSL unit after the VF-MSL unit, which benefits from increased oxygen transfer capacity due to the permeable gravel layer, resulted in the highest DO concentration (Zidan et al. 2022). Previous studies by Latrach et al. (2018) have shown that wastewater treatment systems combining two stages of VF-MSL have increased levels of oxygenation in the effluent. Similarly, Ávila et al. (2015) obtained consistent results using a system combining VF–HF constructed wetlands (CWs). The pH values showed a significant decrease, starting at 8.14 in the hybrid MSL influent, reaching 8.01 in the S-outlet, followed by a further decrease to 7.71 in the VF-MSL outlet. However, a slight increase to 7.72 was observed in the HF-MSL effluent (Figure 2(b)). The decrease in pH values in the VF-MSL effluent can be attributed to the release of hydrogen ions resulting from oxidation processes and organic acids generated by the decomposition of organic matter (Latrach et al. 2016). These pH values can be used as an indicator to adjust the aeration rate in the MSL system to improve contaminant removal (Luanmanee et al. 2002). Throughout the hybrid MSL system study, pH values generally ranged between 6.84 and 8.73, an interval considered optimal for microbial growth and nutrient adsorption in porous systems. In addition, it is noteworthy that nitrification and denitrification processes can have a small effect on pH values (Sbahi et al. 2022).
For EC, Figure 2(c) shows significant variations in EC values among the different components of the hybrid MSL system. The average values recorded in the hybrid MSL system were 1,212.86 μS/cm at the inlet, 1,093.13 μS/cm at the S-outlet, 917.96 μS/cm in the VF-MSL outlet, and 850.18 μS/cm in the HF-MSL outlet. The decrease in EC is primarily attributed to the presence of free ions, such as , that enhance EC. Furthermore, the decrease in EC is associated with the conversion of
and
to molecular nitrogen (N2), as noted by Rasool et al. (2018).
Organic matter removal
Temporal changes in TSS (a) and COD (b) during the treatment process in the hybrid MSL system.
Temporal changes in TSS (a) and COD (b) during the treatment process in the hybrid MSL system.
The Fisher exact text (F) statistic was used to measure the ratio of the variance observed between groups and within groups. A value close to one indicates similarity between samples, while a larger deviation indicates greater differences. However, the F value alone does not indicate the significance of the difference. To determine significance, the F value is compared to the degrees of freedom and used to calculate a p-value. In this study, we obtained a very low p-value, which provides strong evidence against the null hypothesis (H0) that the COD concentrations are similar. This supports the alternative hypothesis (H1) that COD concentrations differ between groups. It is important to note that a significant p-value only confirms the presence of a difference, but does not quantify the magnitude or significance of that difference. To further evaluate the impact of the hybrid MSL system on COD concentrations, we calculated the effect size using omega (ω). The analysis revealed an effect size of 0.72, indicating a significant effect of the system on COD concentrations. This indicates that the MSL system has a significant effect on the observed COD levels.
Phosphorus removal








Temporal changes in (a) and TP (b) during the treatment process in the hybrid MSL system.
Temporal changes in (a) and TP (b) during the treatment process in the hybrid MSL system.
The treatment performance of TP reached 60.43% for the VF-MSL effluent and 78.47% for the hybrid MSL effluent. These results exceed the previous results of Zidan et al. (2023), who studied the same hybrid MSL system and observed average TP removal efficiencies of 47% in VF-MSL and 76% in hybrid MSL effluents. Furthermore, these results exceed those obtained by Luo et al. (2014), who achieved a TP removal efficiency of 47.3% in a one-unit MSL pilot. Compared to CW technologies, MSL technology has demonstrated superior efficiencies in TP removal. For example, Elfanssi et al. (2018) worked with a hybrid CW plant and reported a TP removal rate of 69%, while Zhai et al. (2011) and Lesage et al. (2007) documented TP removal rates of 68.1 and 44.5%, respectively, in two-stage CW treatment systems.
Nitrogen removal











Temporal changes in TKN (a) and (b) during the treatment process in the hybrid MSL system.
Temporal changes in TKN (a) and (b) during the treatment process in the hybrid MSL system.




Temporal changes in nitrates, (a) and TN (b) during the treatment process in the hybrid MSL system.
Temporal changes in nitrates, (a) and TN (b) during the treatment process in the hybrid MSL system.
The efficiency of TN removal in the MSL system is strongly influenced by the coexistence of aerobic and anaerobic processes. The concentration of TN was significantly reduced from 38.38 mg/L at the inlet to 4.38 mg/L at the outlet of the hybrid MSL system (Figure 6(b)). Therefore, the combination of aerobic and anaerobic processes plays an important role in the overall efficiency of TN removal in the MSL system.
Microbial parameters removal
Temporal changes of FC (a), FS (b), and Staphylococcus, S (c) during the treatment process in the hybrid MSL system.
Temporal changes of FC (a), FS (b), and Staphylococcus, S (c) during the treatment process in the hybrid MSL system.
MSL technology uses a variety of mechanisms, including physical filtration, adsorption, microbial death, and predation, to effectively reduce bacterial concentrations (Latrach et al. 2018; Sbahi et al. 2022). Filtration primarily occurs in the soil mixture layers, which have lower porosity compared to the permeable gravel layers, making them more efficient at reducing bacteria. Adsorption of bacteria is expected to occur primarily in the soil mixture layers. These initial stages of filtration and adsorption are followed by microbial degradation and natural mortality of bacteria. The mechanism of natural mortality in the soil mixture layers also plays a critical role in the removal of bacteria from domestic wastewater (Stevik et al. 2004).
Predation of bacteria by protozoa has been documented in several studies. Favorable aeration conditions in the PLs of the MSL system stimulate the formation of predator communities (Garcıa et al. 2013). Additional predators, such as Bdellovibrio and Ensifer adhaerens, contribute significantly to bacterial reduction (Wang et al. 2021). Previous research has emphasized that maintaining a minimal bacterial population in porous environments promotes the presence of predators, whose concentration increases proportionally to the bacterial concentration. For example, in a study where Escherichia coli was introduced into the soil, a sixfold increase in the protozoan population was observed, accompanied by a reduction in the number of E. coli bacteria (Garcıa et al. 2013). Natural mortality of living cells is another critical factor in the bacterial elimination process, which has been studied in both planted and unplanted sand columns (Wand et al. 2007). The conditions for microbial mortality are influenced by temperature, while pH emerges as an essential factor affecting bacterial survival. The consistent removal of bacterial indicators during the experimental period indicates a continuous elimination of bacteria at a certain rate in the hybrid MSL system (Figure 3). This observation implies the absence of bacterial accumulation, and instead suggests continuous removal by microbial degradation or natural death. The bacterial removal rate in the MSL system is likely explained by either bacterial adsorption or filtration (Stevik et al. 2004).
Correlation between studied parameters
Analyzing correlations between physicochemical and bacteriological parameters during monitoring.
Analyzing correlations between physicochemical and bacteriological parameters during monitoring.
Notably, a positive correlation (r = 0.586) was observed between the removal rate of TP and the pH of the treated wastewater by MSL. As the pH of the treated wastewater decreased, the efficiency of TP removal decreased. In addition, a strong negative correlation (r = −0.904) was found between DO and TP. This suggests that improved oxygenation in the MSL system, achieved through enhanced iron oxidation, can effectively reduce TP levels in the MSL units. Similar results have been reported by Hong et al. (2019) and Wei & Wu (2018). Gallionellaceae, as identified by Song et al. (2020), contributes to phosphorus removal through anaerobic iron oxidation and binding with ferric hydroxide. In addition, Acidimicrobiia, known to promote phosphorus removal by producing ferric hydroxide colloids in SBLs, plays a role in phosphorus removal in the hybrid system. In addition, a significant and strong correlation between TN removal efficiency and pH of the treated water was observed in the MSL system (r = 0.677). In the MSL system, aeration enhances nitrification but inhibits denitrification (r = −0.907). This leads to a significant increase in the concentration of (r = 0.628) and a subsequent decrease in the pH of the effluent (Figure 8(a)). Conversely, denitrification occurs in the anaerobic zones.
Significant linear relationships were also found between , TP, and COD in the hybrid MSL system. This indicates that the system is efficient in reducing both organic matter and nutrients. Degradation of COD and nitrogen (N) (r = 0.912) occurs primarily through biological processes within the MSL pores (Chen et al. 2009). Since most denitrifiers are heterotrophic organisms that rely on carbon as an electron donor, COD, which represents organic matter, becomes a critical carbon source for microorganisms involved in the denitrification process (Zidan et al. 2023). Consequently, the increase in NO3 concentration corresponds to a decrease in COD content (r = −0.732).
Physicochemical parameters play an important role in the removal and destruction of pathogenic bacteria in wastewater (Zidan et al. 2023). In order to understand the relationship between the concentrations of pathogenic bacteria (FC, FS, and S) during their passage through the MSL system and various physicochemical parameters, a correlation analysis was performed. The results showed positive correlations between COD, TSS, TN, TP, and the levels of FC, FS, and ST in the effluent during treatment. The correlation between COD and pathogenic bacteria was statistically significant. Similarly, TSS and TN showed significant relationships with FC, FS, and S (Figure 8(b)). These results indicate that the physicochemical parameters (COD, TSS, TN, and TP) decrease as the wastewater passes through the MSL system. These observations are in agreement with previous studies. For example, Zidan et al. (2023), working on the same plant in Morocco, reported a positive correlation between TSS and nitrogen content with pathogenic bacteria. Furthermore, Tyagi et al. (2008) reported positive correlations between TSS and FCs in the effluent of the hybrid MSL plant. The efficient removal of pathogenic bacteria by sedimentation, especially when they are attached to larger particles, may reveal the association between their removal and the reduction of TSS (Wei & Wu 2018). Thus, this mechanism may explain the positive correlation between the TSS and indicator bacteria. Previous studies have also found a positive correlation between the levels of bacterial indicators (E. coli and Enterococci) and TN concentrations in CWs, suggesting that the presence of available nitrogen could increase bacterial survival (Díaz et al. 2010).
Statistical analysis by PCA of physicochemical and bacteriological parameters


Conversely, the high concentrations of TP, COD, and TN at the inlet of the system and at the outlet of the septic tank indicate the presence of pollutants in the incoming wastewater. Based on the cos2 analysis (Figure S1(B), supplementary material), it can be concluded that the variables and individuals are well represented in the plane (PC1 and PC2), with the majority of them having cos2 values greater than 0.65.
Figure S1(B) (supplementary material) shows a variation in the representation of the parameters within the PCA plane, mainly characterized by the decrease of COD cos2. This decrease can be attributed to the absence of oxygen, which affects the degradation of organic matter by aerobic bacteria.
Quality of treated water
The combination of the VF-MSL unit with the HF-MSL unit is effective in successfully removing organic matter, nutrients, and pathogenic bacteria from urban domestic wastewater, which explains the positive performance of the hybrid MSL system. In addition, no significant problems such as clogging, odor, or insect infestation were observed throughout the monitoring period. Table 3 shows that the levels of pH, EC, TSS, , and FC in the treated effluent were within acceptable ranges according to Moroccan irrigation water quality standards. This indicates that the effluent from the hybrid MSL system maintained a quality within the permitted limits for direct wastewater discharge, making it potentially suitable for reuse in irrigation in accordance with Moroccan regulations.
Guidelines for reusing treated wastewater in irrigation
Parameters . | Unit . | Raw wastewater . | Treated wastewater . | Removal . | Admissible limits for direct discharge (Valeurs Limites Spécifiques de Rejet Domestique 2006) . | Admissible limits for wastewater reuse (Normes de qualité des eaux destinées à l'irrigation au Maroc 2007) . |
---|---|---|---|---|---|---|
DO | mg/L | 1.47 ± 0.047 | 4.63 ± 0.055 | – | – | – |
pH | Unit | 8.10 ± 0.061 | 7.57 ± 0.031 | – | – | 6.5–8.5 |
EC | μS/cm | 1,286.64 ± 47.44 | 847.90 ± 32.2 | – | – | 12,000 |
COD | mg/L | 520.20 ± 0.92 | 47.97 ± 0.74 | 87.76% | 250 | – |
TSS | mg/L | 128.63 ± 0.036 | 1.68 ± 0.071 | 97% | 150 | 100 |
![]() | mg/L | 5.05 ± 0.11 | 1.15 ± 0.23 | 70.59% | – | – |
TP | mg/L | 6.72 ± 0.017 | 1.11 ± 0.046 | 78.47% | – | – |
![]() | mg/L | 18.06 ± 0,13 | 0.61 ± 0.17 | 95.71% | – | – |
TKN | mg/L | 24.23 ± 0.092 | 3.73 ± 0.064 | 84.79% | – | – |
![]() | mg/L | 0.77 ± 0.013 | 13.02 ± 0.021 | – | – | 30 |
TN | mg/L | 14.24 ± 0.002 | 1.38 ± 0,012 | 91.65% | – | – |
FC | log (CFU/100 mL) | 6.23 ± 0.083 | 2.02 ± 0.089 | 4.21 log | – | ≤3 |
FS | log (CFU/100 mL) | 6.15 ± 0.12 | 2.29 ± 0.33 | 3.90 log | – | – |
S | log (CFU/100 mL) | 5.46 ± 0.069 | 3.06 ± 0.089 | 2.43 log | – | – |
Parameters . | Unit . | Raw wastewater . | Treated wastewater . | Removal . | Admissible limits for direct discharge (Valeurs Limites Spécifiques de Rejet Domestique 2006) . | Admissible limits for wastewater reuse (Normes de qualité des eaux destinées à l'irrigation au Maroc 2007) . |
---|---|---|---|---|---|---|
DO | mg/L | 1.47 ± 0.047 | 4.63 ± 0.055 | – | – | – |
pH | Unit | 8.10 ± 0.061 | 7.57 ± 0.031 | – | – | 6.5–8.5 |
EC | μS/cm | 1,286.64 ± 47.44 | 847.90 ± 32.2 | – | – | 12,000 |
COD | mg/L | 520.20 ± 0.92 | 47.97 ± 0.74 | 87.76% | 250 | – |
TSS | mg/L | 128.63 ± 0.036 | 1.68 ± 0.071 | 97% | 150 | 100 |
![]() | mg/L | 5.05 ± 0.11 | 1.15 ± 0.23 | 70.59% | – | – |
TP | mg/L | 6.72 ± 0.017 | 1.11 ± 0.046 | 78.47% | – | – |
![]() | mg/L | 18.06 ± 0,13 | 0.61 ± 0.17 | 95.71% | – | – |
TKN | mg/L | 24.23 ± 0.092 | 3.73 ± 0.064 | 84.79% | – | – |
![]() | mg/L | 0.77 ± 0.013 | 13.02 ± 0.021 | – | – | 30 |
TN | mg/L | 14.24 ± 0.002 | 1.38 ± 0,012 | 91.65% | – | – |
FC | log (CFU/100 mL) | 6.23 ± 0.083 | 2.02 ± 0.089 | 4.21 log | – | ≤3 |
FS | log (CFU/100 mL) | 6.15 ± 0.12 | 2.29 ± 0.33 | 3.90 log | – | – |
S | log (CFU/100 mL) | 5.46 ± 0.069 | 3.06 ± 0.089 | 2.43 log | – | – |
CONCLUSION
This study focuses on the investigation of a full-scale hybrid MSL ecotechnology for the treatment of urban domestic wastewater. Monitoring of the treatment process revealed high removal efficiencies of various pollutants. The hybrid MSL plant demonstrated significant reductions in organic matter, nitrogen, and phosphorus, with removal rates of 97% for TSS, 88.57% for COD, 88.49% for TN, and 79.93% for TP. In addition, the plant effectively removed bacterial indicators present in the domestic wastewater, achieving removal rates of 4.21 log for FC, 3.90 log for FS, and 2.43 log for S. PCA showed a positive correlation between COD, , TP removal, and bacterial pathogens. The treated water from the hybrid MSL system met the irrigation standards set by Moroccan regulations and the FAO guidelines for water reuse. Therefore, the treated water can be safely reused for irrigation purposes in green spaces or agriculture. This hybrid MSL ecotechnology represents a promising solution that effectively treats wastewater and provides a quality of water that can be reused for landscape irrigation and agriculture without health risks. This dual benefit is consistent with the principles of sustainable development by promoting environmental protection and resource conservation. By enabling the safe reuse of treated wastewater, the hybrid MSL system contributes to water conservation efforts and reduces reliance on dwindling freshwater sources, thereby supporting long-term sustainability goals. Thus, this study highlights the critical role of technological innovation in addressing water challenges and promoting sustainable development goals, particularly in water-stressed areas.
ACKNOWLEDGEMENTS
This work was supported by the project I-MAROC ‘Artificial Intelligence/Applied Mathematics, Health/Environment: Simulation for Decision Support’, the APRD Program (Morocco) and the National Center for Studies and Research on Water and Energy, Cadi Ayyad University, Morocco.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.