Optimization of Selected Parameters in Vertical, Horizontal, and Hybrid Surface Flow Constructed Wetland Systems for Improving the Treatment Efficiency of Textile and Sewage Effluents

Abstract

Constructed wetland systems (CWSs) can offer cost-effective wastewater treatment in developing countries like Pakistan. This study focused on optimizing design and operational parameters of CWSs in horizontal surface flow (HSF), vertical surface flow (VSF), and hybrid mesocosms for treating sewage and textile effluents using local hydrophytes: Lemna minor, Typha latifolia, and Eichhornia crassipes. Pollutants and heavy metals (Cd, Cr, Cu, Pb, Ni, and Zn) were removed under different flow configurations, bedding materials, hydrophyte species, and hydraulic retention times (HRT) to optimize the overall contaminant removal efficiency (RE). Key findings indicated that the hybrid CWS achieved a maximum RE of 63.62% for total suspended solids (TSS) and 57.9% for biochemical oxygen demand (BOD) at an HRT of 3 days, with efficiencies declining at longer retention times. Additionally, the hybrid system showed maximum metal removal, with Cd and Cr RE reaching 75.2% and 70.5%, respectively. The study also highlighted the critical role of hydrophyte species and HRT in optimizing RE. Furthermore, the choice of hydrophyte species significantly influenced pollutant removal, with treatment cells containing mixed hydrophytes achieving the highest removal efficiencies (63.62%), followed by Eichhornia crassipes with high Cd (643.33 mgkg⁻¹) and Cr (1103.72 mgkg⁻¹) uptake. A lower HRT of 3 days resulted in the highest overall removal efficiency of 57.5%, which decreased with longer HRTs (from 6 to 9 days). Optimizing design and operational parameters is crucial for maximizing CWS treatment potential.

1. Introduction

The increasing volume of wastewater generated globally poses significant environmental and public health challenges, particularly in developing countries where conventional treatment methods often prove impractical due to high costs and energy demands. Constructed wetland systems (CWSs) have emerged as a viable nature-based solution for wastewater treatment, utilizing natural processes involving soil, plants, and microorganisms to effectively remove contaminants [1]. These systems consist of treatment cells filled with various bedding materials (BMs) and planted with hydrophytes, which play a crucial role in pollutant degradation [2]. However, the application of CWSs in regions like Pakistan remains limited, primarily due to substantial land requirements and insufficient research on the influence of hydraulic parameters on their performance [1]. To address these challenges, this study introduced a novel hybrid CWS design that integrated both horizontal and vertical surface flow configurations, aiming to enhance treatment efficiency for complex wastewaters, particularly from the textile industry and municipal sewage.

The emergence of hybrid constructed wetland systems, which integrate both horizontal and vertical surface flow configurations, represents a significant advancement in wastewater treatment technology. These hybrid systems leverage the strengths of both designs, enhancing pollutant removal efficiencies, particularly for heavy metals and organic matter [1,2]. By maintaining optimal oxygen levels and promoting diverse microbial communities, hybrid CWSs can adapt to varying flow rates and pollutant loads, making them a robust solution for urban wastewater challenges [2,3]. The optimization of hydraulic retention time (HRT) within these systems further contributes to their effectiveness, underscoring the need for site-specific designs tailored to local wastewater characteristics [4].

A sustainable and operational model of a CWS encompasses appropriate types of BM and hydrophytes to proficiently remove contaminants [1,2]. Typically, wastewater is introduced into the treatment cells, permitted to percolate through the BM, and subsequently discharged through the outlet [3,4]. The fundamental components of a CWS include the treatment cells, BM, plants, and hydrological processes [5,6]. Numerous variables, such as the selection and combinations of hydrophytic species, hydrological conditions, landscape configuration, site-specific design considerations pertaining to the dimensions and form of the wetland system, characteristics of the BM, and operational methodologies, influence the treatment efficacy and optimization of a CWS concerning its treatment performance [7,8,9].

Hydraulic dynamics within CWSs, specifically hydraulic loading rates (HLRs) and hydraulic retention time (HRT), are critical determinants of their treatment efficacy [9]. Additionally, factors such as the surface area of constructed wetlands, flow rates, and porosity of BMs play a pivotal role in defining the HRT [9,10,11]. A reduction in the HLR enhances the overall treatment efficiency, while an increased HRT facilitates superior contaminant removal [7,11,12,13,14]. Consequently, the optimization of HRT duration emerges as an essential component for maximizing pollutant removal performance in constructed wetlands, effectively targeting infectious microorganisms as well [15,16,17].

Despite the recognition of CWSs as an advanced alternative for wastewater treatment, their application in Pakistan remains constrained. While the nation possesses notable expertise in treating various wastewater types using CWSs, this is predominantly on a minor scale. The widespread adoption of CWSs faces a major hurdle due to their substantial land requirements. This issue is aggravated by limited research on the influence of specific hydraulic parameters of the CWS performance across varying climates, coupled with a lack of comprehensive evaluations on the pollutant removal efficiency of different hydrophytic species.

Hydrophytes exhibit a heightened vulnerability to water loss via evapotranspiration (ET), particularly under elevated temperature and wind conditions [18,19]. This loss can significantly affect the wastewater volume processed, thereby compromising the efficiency of CWSs in wastewater treatment [20,21,22]. Unlike traditional treatment facilities that account for evaporation in their efficiency calculations, CWSs primarily focus on pollutant concentrations at the intake and outflow stages [23,24]. Contaminants such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), N, and P are examples of such pollutants [25,26]. In CWSs, where ET is often significant, determining pollutant removal efficiency based on concentration measurements can be inaccurate. This is because ET reduces the water content in treatment cells, resulting in higher concentrations of dissolved substances [18,27].

By sedimentation, filtration, and adsorption, BMs and constructed wetland hydrophytes filter pollutants out of wastewater. The BMs promote the development of biofilms and maintain the bacteria and plants in CWSs [3,17,28]. For effective treatment efficiency in CWSs, the selection of BMs is essential, for instance, gravel, limestone, coal ash, demolished concrete, clay minerals, and industrial wastes [29,30]. The overall treatment efficiency of CWSs is greatly influenced by the physical and chemical characteristics of the BM. The ability of CWSs to eliminate pollutants is enhanced by the high specific surface area of the BM. This enhances the hydrodynamic and mechanical attributes of the CWS and offers microorganisms extra surface area. As a result, the efficacy of CWSs relies on their BMs [3,17,31]. When the hydrophytes establish roots in the BM, they assist gas exchange of the microbial biofilm, accelerating the process of contaminant degradation.

With 280,000 tons of textile dyes emitted yearly into the world’s textile industrial effluent [32,33], this industry is among the most important and major sectors globally. The global economy has been affected negatively by the growing environmental degradation brought on by this industry [34]. Significant volumes of effluents are produced by the various dyeing and finishing procedures employed in the textile dyeing industry. The majority of chemicals, including dyes, only undergo partial treatment, and untreated textile effluent wastewater is released into rivers, rendering the water unfit for human use, agriculture, and aquatic life [11,35,36,37]. Moreover, numerous dyes prevalent in the textile industry exhibit carcinogenic and toxic properties [8,38].

In developing countries like Pakistan, rapid urbanization and industrialization have caused a surge in wastewater volume, threatening both the environment and public health. Although effective, conventional wastewater treatment methods demand high capital, skilled operation, and energy-intensive processes, making them impractical for many resource-limited communities.

This study addressed the pressing need for sustainable and cost-effective wastewater treatment solutions in developing countries by investigating the performance of different CWS configurations (HSF, VSF, and hybrid) for treating both domestic sewage and textile effluents. The research specifically focuses on optimizing key design and operational parameters, including hydrophyte species selection, bedding material type, hydraulic retention time, and flow configuration, to maximize the pollutant removal efficiency and minimize the environmental impact.

By identifying the most effective CWS configuration and optimizing its performance, this research aims to provide valuable insights for the development and implementation of sustainable and affordable wastewater treatment solutions in resource-limited settings, contributing to improved environmental quality and public health in developing countries.

The objectives of this study were designed to enhance the understanding and effectiveness of CWS for wastewater treatment. First, the study aimed to evaluate the physicochemical characteristics of sewage and textile effluents, focusing on parameters such as pH, COD, total suspended solids (TSS), and heavy metal concentrations to establish baseline data for treatment performance. Second, it compared the pollutant removal efficiencies of hybrid CWSs against horizontal surface flow (HSF) and vertical surface flow (VSF) systems, specifically analyzing the removal rates of organic matter and heavy metals. The research also determined the optimal HRT for maximum removal efficiency of specific pollutants in hybrid CWSs, identifying conditions that led to enhanced treatment outcomes. Additionally, the study investigated the influence of different bedding materials on the performance of hybrid CWSs, aiming to identify the most effective combinations for pollutant removal. By refining these objectives, the study aimed to provide clearer guidance for its methodology and enhance its relevance to real-world applications.

This study introduced a novel hybrid CWS design, which is an innovative approach to enhance the treatment efficiency for heavy metals and organic matter. The research demonstrated significant improvements in contaminant removal addressing the challenges posed by complex wastewaters from textile industries and sewage. Furthermore, the study investigated hydraulic retention time in optimizing pollutant removal, revealing an optimal range for specific effluents. The findings emphasized the need for site-specific designs tailored to local conditions and the potential for scaling up hybrid CWS designs in resource-limited settings, providing environmental sustainability in wastewater management.

2. Materials and Methods

2.1. Constructed Wetland Mesocosms for Operating Parameters and Structural Design Optimizations

A CWS mesocosm, capable of treating 3000 L of wastewater at a time without energy input, was constructed in the botanical garden of the University of the Punjab, Lahore, based on a gravity-driven principle. The experimental design consisted of three CWS configurations within the mesocosm: HSF, VSF, and a hybrid system combining both. The study evaluated the impacts of various factors on treatment efficiency, such as hydrophyte selection, testing different local hydrophyte species, and assessing different bedding materials for their effects on pollutant removal. The research also investigated the relationship between HRT and treatment performance. The research aimed to determine the optimal combination of these factors to determine the maximum pollutant removal, particularly for organic pollutants and heavy metals.

The developed CWS consisted of three primary assemblies. Each assembly had a central feeding tank connected to four series of treatment cells of the same shape. The first assembly consisted of three HSF treatment cells, which represented one of the CWS series. The second assembly was composed of four cells in series, two VSF and two HSF treatment cells, which formed a hybrid-type CWS. The third assembly was made up of three VSF treatment cells in series. The feeding tank was linked to 12 cells of three in a series for HSF and VSF CWS types and 16 treatment cells of four in series (two VSF and HSF each) for the hybrid flow CWS mesocosm. Figure A1 in Appendix A depicts an outline of the top orientation view and the lateral orientation view for the developed CWS. The lateral orientation view of the CWS reveals a slope gradient between the wastewater sample feeding tank and the terminal treatment cell in each of the series connected to it.

The system was designed with regulated HRT to provide optimal hydrophyte vegetation cover in the treatment cells. All treatment cells were coated with an anti-algae agent, and the entire CWS area was protected with a geomembrane. Zinc oxide, which inhibits the growth of algae, was evenly distributed throughout the cement mixture and applied on the surfaces of the treatment cells, which is generally not toxic to the environment and humans.

2.2. Structural Configuration and Functional Layout of Horizontal and Vertical Surface Flow Treatment Cells in Constructed Wetland Mesocosms

Figure 1 depicts an actual image of the HSF and VSF treatment cells, exhibiting their shapes. Both HSF and VSF were aligned separately and together to form three treatment assemblies: (i) the HSF CWS, (ii) the VF CWS, and (iii) the HVF hybrid CWS treatment assembly. Each assembly consisted of three independent HSF/VSF treatment cells that were joined together by hydraulic flow pipes and flow control valves. Figure A2 depicts a satellite view of the research location (31°29.9856′ N, 74°18.0667′ E) in the Botanical Garden, University of Punjab, Lahore, Pakistan. Each two HSF and VSF cells were alternately organized to form a hybrid series.

The established slope gradient maintained the flow through all the treatment cells in sequence while pumping the sample into the feeding tank using an electric motor pump. The feeding tank in each series was set at a height of 2.6 m above ground level, where the final treatment cell was located (Figure A1).

Table A1. Specifications of treatment cells of the constructed wetland system (CWS).

Hybrid CWS Treatment Cell Capacity
Treatment Cell Flow Configuration	Cell Dimensions (m)			Total Volume (L)
	Depth	Breadth	Length
Horizontal surface flow	0.30 ± 5	0.61 ± 1	1.22 ± 1	223 ± 2
Vertical surface flow	0.81 ± 5	0.65 ± 1	0.69 ± 1	359 ± 3

Notes: All data are shown as mean ± standard deviation. Values were obtained by taking the means of triplicate measurements.

shows the exact size of the treatment cells in each assembly. Each VSF treatment cell had an average capacity of 359 L, whereas the HSF treatment cells had an average capacity of 223 L. The difference in volume in the VSF and HSF treatment cells presented in Table A1 was due to their distinct structural and design configurations. VSF treatment cells, without bedding material and water percolation piping, naturally had more internal space compared with HSF treatment cells. This led to a higher initial volume capacity for the VSF cells.

2.3. Sampling of Wastewater

The textile effluent was collected from a factory area drain in Lahore, which received textile effluents from local textile dying units (GPS coordinates: 31°23′18.3” N, 74°21′02.8” E). The sewage effluent was collected from a point source of a domestic sewage plant at Hudiara Drain (GPS coordinates: 31°18′51.2” N, 74°13′32.8” E), which collected a major amount of the municipal wastewater of Lahore.

2.4. Determination of Hydraulic Retention Time

The influence of the HRT on the treatment efficiency was assessed by determining the HRT of the treatment cells using a modified approach as described in Equation (1) [39].

$$\mathrm{H}\mathrm{R}\mathrm{T}={\displaystyle \frac{∆V}{∆Q}}$$

(1)

where ∆V (m³) = volume of treatment cell and ∆Q = rate of flow (m³d⁻¹).

2.5. Determination of Water Quality Parameters

The pH, EC, TDS, and NaCl of the water samples were measured using multiparameter equipment (HI-9835 Hanna). The total suspended solids (TSS) were assessed using the APHA’s standard method 2540 [40]. The chemical oxygen demand (COD) was measured before and after treatments using the standard method [41]. The biochemical oxygen demand (BOD₅) was calculated using the APHA’s standard method 5220 [40]. The total Kjeldahl’s nitrogen content of the water samples was determined according to the 4500-N standard method [40] using an automated system (VELP Scientifica-UDK 152, Milan, Italy). The plant matter was digested following the acid digestion method described by [41]. The heavy metal estimation of the digested samples of water and plants was performed using an atomic absorption spectrophotometer (GBC SAVANT AA, Braeside, Australia).

The sludge volume index (SVI) of the wastewater samples was determined following the standard method 2710-D [40] and expressed by Equation (2):

$$\mathrm{S}\mathrm{V}\mathrm{I}={\displaystyle \frac{\mathrm{S}\mathrm{e}\mathrm{t}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{d} \mathrm{S}\mathrm{l}\mathrm{u}\mathrm{d}\mathrm{g}\mathrm{e} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\left(\mathrm{m}\mathrm{L}{\mathrm{L}}^{-1}\right)}{\mathrm{S}\mathrm{u}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{d} \mathrm{S}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{s}\left({mgL}^{-1}\right)}}\times 100$$

(2)

2.6. Determination of the Removal Efficiency of Pollutants

The removal of the pollutants from wastewater was determined to assess the post-treatment reduction in pollutant concentrations and was expressed as the removal efficiency, which was calculated by the method described by [42] and expressed by Equation (3):

$$\mathrm{R}\mathrm{E}\left(\mathrm{\%}\right)=\left(1-{\displaystyle \frac{\mathsf{\mu }{\mathrm{C}}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}-\mathsf{\mu }{\mathrm{C}}_{\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}}{\mathsf{\mu }{\mathrm{C}}_{\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}}}\right)100$$

(3)

where RE (%) is the percent removal efficiency, and μC_outlet and μC_inlet are the mean concentrations of specific pollutants at the outlet and inlet of the CWS, respectively.

2.7. Influence of Bedding Materials

The three CWSs each linked to the same tank had three cells in the cases of the HSF and VSF scenarios, while there were four cells in the case of the hybrid CWS. The cells in each of the four similar mesocosms in the above mentioned CWS types were filled up with the following types of bedding materials:

• Concrete demolished aggregates (CDAs);
• Road demolished aggregates (RDAs);
• Gravel based on dolomite stone;
• Glass beads (for control purposes).

Concrete demolition aggregates were collected from a structure undergoing destruction near the University of the Punjab, Lahore. Additionally, RDAs were sourced from a nearby location where an old road was being dismantled. Both types of aggregates were sieved using a large gauze with a 2.2 cm grid, yielding particles approximately 2 cm in diameter. Glass beads, each with a diameter of 2 cm, were procured from a sports shop in the walled city of Lahore. This effluent was transported to the experimental site and used to fill the central tank of the CWS. The effluent was retained for a period of three days in the feeding tanks before being transferred to the subsequent treatment cells. Figure 1 and Figure 2 depict an actual view of CWS mesocosm planted with hydrophytes and BMs incorporated in respective treatment cells.

2.8. Data Analysis

Data from replicate samples were analyzed using Microsoft Excel. One-way ANOVA was employed to assess statistically significant differences in pollutant removal efficiencies among CWS types and effluent sources. The null hypothesis (H₀) posited no significant differences in removal efficiency, while the alternative hypothesis (H₁) predicted significant differences among at least one treatment group. Statistical significance was determined at p ≤ 0.05. ANOVA enabled the simultaneous comparison of multiple groups, providing a robust framework for evaluating CWS effectiveness.

3. Results

3.1. Physicochemical Characteristics of Sewage and Textile Effluents

The physicochemical properties of sewage effluent and textile effluent fed to the CWS mesocosm are given in

Table 1. Physicochemical properties of sewage and textile effluents.

Parameters	Sewage Effluent	Textile Effluent
Color (HU)	149 ± 2.54	92 ± 3.22
pH (–)	8.6 ± 0.3	10.40 ± 3
EC (μScm−1)	328 ± 5	908 ± 5
NaCl (%)	3.9 ± 0.2	1.4 ± 0.2
TDS (mgL−1)	871 ± 4	1403 ± 4
TSS (mgL−1)	125 ± 2	85 ± 2
TKN (mgL−1)	43 ± 2	76 ± 3
BOD5 (mgL−1)	600 ± 5	710 ± 5
COD (mgL−1)	850 ± 5	900 ± 5
SVI (mgL−1)	224 ± 3	235 ± 3
Cd (mgL−1)	115.2 ± 7.34	984.3 ± 32.4
Cr (mgL−1)	318.6 ± 35.1	1448.1 ± 18.9
Cu (mgL−1)	284.8 ± 24.61	1050.5 ± 25.7
Pb (mgL−1)	294.2 ± 30.56	871.2 ± 50.1
Ni (mgL−1)	281.5 ± 28.01	607.8 ± 30.5
Zn (mgL−1)	316.4 ± 17.31	572.3 ± 25.2

Notes: HU, Hazen units; EC, electrical conductivity; TDS, total dissolved solids; TSS, total suspended solids; SVI, sludge volume index; TKN, total Kjeldahl’s nitrogen; BOD, biochemical oxygen demand; COD, chemical oxygen demand; Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Nim nickel; Zn, zinc.

.

3.2. Pollutant Removal Efficiency with a Variation of Hydrophytes Planted in Treatment Cells

The percent RE was calculated for treatment cells with individual species of hydrophytes and those with mixed species of hydrophytes (

Table 2. Comparison of pollutant removal efficiencies in treatment cells planted with Eichhornia crassipes, Lemna minor, Typha latifolia, and mixed species for sewage effluent and textile effluent during 10-week experimental periods.

RE (%)
Parameter	Mixed		E. crassipes		L. minor		T. latifolia
	SE	TE	SE	TE	SE	TE	SE	TE
NaCl	61	54	69	50	20	13	59	51
TDS	53 *	62	53	59	21 *	35 *	47	57 *
TSS	55	64	52 *	54	40	39 *	53	53
TKN	65	76	57	65	52	62	49	64
BOD5	81 *	88 *	94 *	85 *	79 *	78 *	95 *	92 *
COD	88	75	82	77	82	75	78	86
Cd	50	57	62	53 *	53	49	40	76
Cu	48	62 *	40	76	65	53	53 *	57
Cr	65 *	53	76	62	57 *	49	62	49 *
Pb	64	76 *	57	76	42	53	65	36
Ni	41	49	65 *	53 *	76	40 *	62	57
Zn	77	57	70	40	49	53	76 *	65 *

Notes: Statistical significance was determined by a one-way analysis of variance (ANOVA). * Significance at p ≤ 0.05. SE, sewage effluent; TE, textile effluent; RE, removal efficiency; TKN, total Kjeldahl’s nitrogen (%); BOD, biochemical oxygen demand; COD, chemical oxygen demand; TDS, total dissolved solids; TSS, total suspended solids; Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Ni, nickel; Zn, zinc.

), with mixed-cultured hybrid CWS achieved the highest REs of 64% for textile wastewater and 58% for sewage. A strong positive correlation was observed between the reductions in BOD₅ and COD concentrations (Figure 3), alongside significant decreases in TSS, TKN, and NaCl concentrations throughout the experimental period (Table 2).

The BOD₅ to COD ratio in wastewater samples was recorded as 0.6 for sewage and 0.7 for textile effluents, indicating a substantial reduction in contaminants despite a slow rate removal efficiency. The box–whisker plot (Figure A3) illustrates a comprehensive performance overview, showcasing effective organic pollutant breakdown through a strong correlation between BOD₅ and COD concentration reductions (Figure 3), with biweekly data recordings from each treatment cell. This was evident from the data presented, which showed that as BOD concentrations decreased, the corresponding COD concentrations did not follow a straight-line pattern, indicating a more complex interaction between these two parameters during the treatment process.

3.3. Wetland Pollutant Removal Efficiencies Subject to Different Hydraulic Retention Times

The removal efficiencies of selected parameters increased during the initial stage and gradually declined. Three HRT schemes were followed, i.e., 3 days, 6 days, and 9 days, while all treatment units were mix-cultured.

Table 3. Removal efficiencies of selected parameters at 3, 6, and 9 days hydraulic retention time (HRT) for sewage and textile effluents.

	HRT (d)
Parameter	3		6		9
	SE	TE	SE	TE	SE	TE
	RE (%)
NaCl	65.34 *	58.41	42.08 *	54.45	37.62	13.86
TDS	25.74	40.59	19.80	24.75 *	20.30	20.79
TSS	82.67	67.82	64.35	53.46	35.15 *	28.71
TKN	65.84 *	69.30 *	24.75	26.73	14.85	26.73 *
BOD5	50.49	67.32	41.09 *	59.40	20.79 *	11.88 *
COD	49.50	52.47 *	39.11	46.53	24.26	28.71

Notes: Statistical significance was determined by a one-way analysis of variance (ANOVA). * Significance at p ≤ 0.05. HRT, hydraulic retention time; SE, sewage effluent; TE, textile effluent; RE, percent removal efficiency (%); TKN, total Kjeldahl’s nitrogen; BOD, biochemical oxygen demand; COD, chemical oxygen demand; TDS, total dissolved solids; TSS, total suspended solids.

shows a comparison between REs at 3, 6, and 9 days HRT. There was a significant drop in pollutant removal efficiency with the increase in HRT. The results (Table 3) depicted that the RE was maximum at 3 days (57.9%) HRT, followed by 6 days (41.3%) and 9 days (23.6). TKN and organic pollution (COD) removal showed relatively high REs in each effluent during the first stage. The RE of TSS (75.2%) was also notable over 3 days of HRT as it declined for 9 days HRT (31.9%).

Figure 4 depicts the RE of various heavy metals, which are categorized according to effluent types and stages of treatment, as well as their effects on RE. The six heavy metals measured were cadmium, chromium, copper, lead, nickel, and zinc.

The overall Cd removal efficiency was 30% in SE, while it was slightly higher in TE (36%). Stage 1 achieved the least removal for both effluents, with SE at around 16% and TE at close to 10%. The RE increased in stages 2 and 3 for both effluents, with SE slightly higher and nearly 57% for the third stage.

Table 4. Uptake of heavy metals by selected hydrophytes with reference to different hydraulic retention times for wetland cells treating textile effluent.

Stage	Cd (mgkg−1)			Cr (mgkg−1)			Cu (mgkg−1)			Pb (mgkg−1)			Ni (mgkg−1)			Zn (mgkg−1)
	1	2	3	1	2	3	1	2	3	1	2	3	1	2	3	1	2	3
Mixed	104.44	492.22	488.89	59.75	515.22	683.20	134.44	443.33	426.67	82.22	402.22	694.44	53.33	140.00	480.00	223.75	242.50	463.75
Eichhornia crassipes	131.11	336.67	643.33	157.84	521.98	1103.72	150.00	363.33	648.89	172.22	247.78	244.44	28.89	211.11	254.44	16.25	341.25	237.50
Lemna minor	137.78	216.67	561.11	60.88	633.60	1098.08	34.44	264.44	424.44	75.56	408.89	167.78	67.78	240.00	437.78	17.50	251.25	298.75
Typha latifolia	102.22	177.78	293.33	73.28	483.65	600.90	42.22	360.00	307.78	177.78	465.56	654.44	54.44	150.00	434.44	91.25	203.75	286.25

Notes: Statistical significance was determined by a one-way analysis of variance (ANOVA). Stage 1, 3 days HRT; stage 2, 6 days HRT; and stage 3, 9 days HRT. Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Ni, nickel; Zn, zinc.

and

Table 5. Uptake of heavy metals by selected hydrophytes with reference to different hydraulic retention times in wetland cells treating sewage effluent.

	Cd (mgkg−1)			Cr (mgkg−1)			Cu (mgkg−1)			Pb (mgkg−1)			Ni (mgkg−1)			Zn (mgkg−1)
Stage	1	2	3	1	2	3	1	2	3	1	2	3	1	2	3	1	2	3
Mixed	18.89	21.11	66.67	5.56	175.56	105.56	78.89	93.33	153.33	2.22	126.67	62.22	29.89	117.24	136.78	43.33	112.22	216.67
Eichhornia crassipes	10.00	26.67	55.56	6.67	112.22	156.67	45.56	104.44	144.44	24.44	91.11	106.67	31.03	93.10	174.71	54.44	101.11	126.67
Lemna minor	23.33	44.44	56.67	51.11	213.33	148.89	38.89	110.00	174.44	33.33	133.33	74.44	63.22	106.90	140.23	42.22	110.00	230.00
Typha latifolia	14.44	42.22	84.44	25.56	197.78	244.44	4.44	97.78	190.00	31.11	113.33	85.56	8.05	109.20	147.13	38.89	93.33	262.22

Notes: Statistical significance was determined by a one-way analysis of variance (ANOVA). Stage 1, 3 days HRT; stage 2, 6 days HRT; and stage 3, 9 days HRT. Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Ni, nickel; Zn, zinc.

represent the uptake of heavy metals by various selected hydrophytes under different HRTs in treatment cells designed to treat textile effluent and sewage effluent, respectively. The data highlight the efficiency of each hydrophyte species in absorbing specific heavy metals over varying HRTs. This provides insights into optimal retention times and plant species for effective heavy metal removal.

The Cr removal efficiency was like that for Cd, consistently lower in sewage compared with textile effluents. In stage 1, the removal of both Cd and Cr was lowest, with sewage effluent removal at 18.6% and textile effluent removal at approximately 7%. The RE increased for stages 2 and 3, although sewage effluent reached a higher maximum efficiency of approximately 52% by stage 3, while textile effluent reached a maximum at around 48% by stage 2.

Cu removal efficiency was slightly higher in sewage effluent (38%) than textile effluent (32%) throughout all three stages. Stage 1 had the lowest efficiency for both effluents with sewage effluent at around 28% and textile effluent close to 13%. The removal efficiency increased for stages 2 and 3 for both effluents, particularly concerning sewage effluent (53% for stage 3). The textile effluent RE showed a significant rise at stage 2 (42%) but then decreased slightly at stage 3.

3.4. Comparison of Pollutant Removal Efficiency in Horizontal Surface Flow and Hybrid Flow Constructed Wetland Systems

This study investigated the efficacy of the HSF and hybrid flow CWS for Cd, Cr, Cu, Ni, and Zn removal from textile and sewage effluents. The results demonstrated a clear trend in metal REs across the three stages of the CWS (

Table 6. Removal efficiencies (REs) of heavy metals at different stages of hydraulic retention times (HRTs), where stage 1 = 3 days HRT, stage 2 = 6 days HRT, and stage 3 = 9 days HRT for sewage and textile effluents (Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Ni, nickel; Zn, zinc).

Metal		Stage	RE (%)
			HSF	Hybrid
Textile Effluent	Cd	Stage 1	23.27	29.08
		Stage 2	22.30	26.17
		Stage 3	20.36	25.20
	Cr	Stage 1	6.79	26.17
		Stage 2	22.63	38.78
		Stage 3	20.66	35.51
	Cu	Stage 1	23.27	32.96
		Stage 2	14.54	15.51
		Stage 3	28.11	46.53
	Ni	Stage 1	29.08	25.20
		Stage 2	39.69	19.40
		Stage 3	29.9	41.69
	Zn	Stage 1	27.14	17.45
		Stage 2	29.08	34.90
		Stage 3	9.69	17.45
Sewage Effluent	Cd	Stage 1	24.24	42.65
		Stage 2	19.39	48.47
		Stage 3	4.85	20.36
	Cr	Stage 1	29.08	10.66
		Stage 2	11.63	41.69
		Stage 3	12.60	20.36
	Cu	Stage 1	10.66	45.56
		Stage 2	20.36	47.50
		Stage 3	19.39	49.44
	Ni	Stage 1	17.45	15.51
		Stage 2	13.57	42.65
		Stage 3	19.39	11.63
	Zn	Stage 1	7.76	19.39
		Stage 2	4.85	9.69
		Stage 3	14.54	30.05

). Both Cu and Cd exhibited the highest removal rates in the hybrid CWS compared with the HSF system. This pattern was consistent across all three stages, with the hybrid CWS achieving the following maximum RE. Similarly, Cd removal peaked in the hybrid CWS.

Table 6 represents the REs of heavy metals at different stages of HRT for both sewage effluent and textile effluents. The REs for specific heavy metals across three stages were as follows: stage 1 (3 days HRT), stage 2 (6 days HRT), and stage 3 (9 days HRT). The results indicated that the CWS demonstrated varying REs for heavy metals (such as Cd and Cr) across different stages of HRT. Specifically, the RE tended to increase with longer HRT, suggesting that an extended contact time allowed for more effective treatment of heavy metals. However, the data also showed that the RE declined at certain stages, indicating potential limitations in the system performance over time. Overall, the findings highlight the importance of optimizing HRT to enhance the effectiveness of CWSs in treating sewage and textile effluents.

3.5. Effect of Bedding Material

The experiment carried out with four different bedding materials was carried out in all three types of wetlands, namely, HSF, VSF, and hybrid flow CWSs (

Table A2. Changes in pollution parameters of textile effluents in the horizontal surface flow (HSF), vertical surface flow (VSF), and hybrid constructed wetland system (CWS) with four different bedding materials (CDA, concrete demolished aggregates; RDA, road demolished aggregates).

Parameters	Textile Effluent	HSF CWS				VSF CWS				Hybrid CWS
		CDA	RDA	Dolomite Gravel	Glass Beads	CDA	RDA	Dolomite Gravel	Glass Beads	CDA	RDA	Dolomite Gravel	Glass Beads
pH	10.40 ± 3	9.8 ± 2.4	9.3 ± 2.1	11.5 ± 1.2	8.5 ± 2.4	9.6 ± 2.6	8.7 ± 1.8	11.3 ± 2.6	8.2 ± 2.4	9 ± 1.2	8.6 ± 1.6	9.2 ± 2.4	8.1 ± 1.2
EC (mS cm−1)	908 ± 5	1050 ± 6	905 ± 3.6	1250 ± 6	708 ± 4.8	980 ± 1.2	710 ± 1.2	1020 ± 4.8	699 ± 4.8	820 ± 6	716 ± 2.4	950 ± 4.8	680 ± 2.4
TDS (mg L−1)	1403 ± 4	985 ± 1.2	840 ± 6	1150 ± 1.2	650 ± 1.2	950 ± 4.8	907±2.4	1040 ± 1.2	560 ± 4.8	830 ± 1.2	790 ± 4.8	910 ± 2.4	480 ± 4.8
NaCl (%)	1.4 ± 0.2	0.25 ± 3.6	0.2 ± 2.4	0.3 ± 3.6	0 ± 2.4	0 ± 1.2	0.1±3.6	0.2 ± 1.2	0 ± 6	0 ± 1.2	0 ± 2.4	0 ± 3.6	0 ± 1.2
BOD5 (mg L−1)	710 ± 5	227.2 ± 4.8	248.5 ± 4.8	284 ± 2.4	177.5 ± 2.4	213 ± 3.6	170.4±3.6	269.8 ± 4.8	142 ± 1.2	191.7 ± 4.8	163.3 ± 4.8	220.1 ± 4.8	113.6 ± 4.8
COD (mg L−1)	900 ± 5	342 ± 6	324 ± 1.2	369 ± 1.2	279 ± 4.8	324 ± 1.2	306±2.4	360 ± 2.4	288 ± 1.2	288 ± 3.6	270 ± 2.4	324 ± 3.6	252 ± 4.8
SVI (mL g−1)	35 ± 5.4	10 ± 2.9	13 ± 1.2	11 ± 2.4	15 ± 1.2	9 ± 4.8	8.5±2.5	9.7 ± 4.8	11 ± 1.2	10 ± 1.6	11 ± 2.4	12 ± 2.4	8.5 ± 1.6
Cd (mg L−1)	984.3 ± 32.4	521 ± 4.8	482 ± 2.4	635 ± 4.8	476 ± 4.8	535 ± 2.4	484±2.4	657 ± 1.2	456 ± 6	552 ± 2.4	475 ± 4.8	653 ± 2.4	431 ± 1.2
Cr (mg L−1)	1448.1 ± 18.9	797 ± 2.4	712 ± 2.4	878 ± 1.2	657 ± 2.4	782 ± 4.8	693±6	854 ± 2.4	637 ± 6	754 ± 3.6	663 ± 3.6	822 ± 1.2	607 ± 6
Cu (mg L−1)	1050.5 ± 25.7	632 ± 1.2	605 ± 1.2	688 ± 6	592 ± 6	574 ± 4.8	544±3.6	624 ± 6	531 ± 6	514 ± 6	495 ± 6	591 ± 3.6	482 ± 3.6
Pb (mg L−1)	871.2 ± 50.1	454 ± 3.6	416 ± 1.2	522 ± 3.6	408 ± 1.2	425 ± 4.8	406±4.8	455 ± 1.2	395 ± 1.2	397 ± 1.2	372 ± 4.8	416 ± 1.2	354 ± 1.2
Ni (mg L−1)	607.8 ± 30.5	393 ± 2.4	372 ± 4.8	436 ± 4.8	359 ± 4.8	344 ± 4.8	308±1.2	442 ± 1.2	306 ± 6	324 ± 4.8	302 ± 3.6	362 ± 6	286 ± 1.2
Zn (mg L−1)	572.3 ± 25.2	276 ± 3.6	256 ± 4.8	352 ± 2.4	228 ± 4.8	252 ± 3.6	222±2.4	313 ± 2.4	215 ± 3.6	213 ± 2.4	251 ± 4.8	269 ± 3.6	198 ± 6
Thickness of biofilm (mm)	-	0.5	0.5	1	1	0.86	0.8	1	1.2	0.6	0.7	0.8	1.3

Notes: All data are shown as mean ± standard deviation. Values are based on triplicate measurements. BOD, biochemical oxygen demand; COD, chemical oxygen demand; TDS, total dissolved solids; EC, electrical conductivity; SVI, sludge volume index; Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Ni, nickel; Zn, zinc.

and

Table A3. Changes in pollution parameters of sewage effluents in horizontal surface flow (HSF), vertical surface flow (VSF), and hybrid constructed wetland system (CWS) with four different bedding materials (CDA, concrete demolished aggregates; RDA, road demolished aggregates).

Parameter	Sewage Effluent	HSF CWS				VSF CWS				Hybrid CWS
		CDA	RDA	Dolomite Gravel	Glass Beads	CDA	RDA	Dolomite Gravel	Glass Beads	CDA	RDA	Dolomite Gravel	Glass Beads
pH	8.6 ± 0.3	7.7 ± 2.4	8 ± 2.1	7.4 ± 1.2	7.4 ± 2.4	7.3 ± 2.6	7.6 ± 1.8	7.4 ± 2.6	7.6 ± 2.4	7.7 ± 1.2	7.5 ± 1.6	7.6 ± 2.4	7.5 ± 1.2
EC (mS cm−1)	328 ± 5	244 ± 6	231 ± 3.6	195 ± 6	214 ± 4.8	210 ± 1.2	232 ± 1.2	186 ± 4.8	179 ± 4.8	194 ± 6	188 ± 2.4	208 ± 4.8	291 ± 2.4
TDS (mg L−1)	871 ± 4	774 ± 1.2	684 ± 6	599 ± 1.2	742 ± 1.2	701 ± 4.8	642 ± 2.4	633 ± 1.2	604 ± 4.8	611 ± 1.2	607 ± 4.8	621 ± 2.4	681 ± 4.8
NaCl (%)	3.9 ± 0.2	0.8 ± 3.6	0.5 ± 2.4	0.6 ± 3.6	0.4 ± 2.4	0.9 ± 1.2	1.2 ± 3.6	0.4 ± 1.2	0.4 ± 6	1.1 ± 1.2	0.9 ± 2.4	1.4 ± 3.6	1.2 ± 1.2
BOD5 (mg L−1)	600 ± 5	197 ± 4.8	188 ± 4.8	204 ± 2.4	147 ± 2.4	166 ± 3.6	170 ± 3.6	159 ± 4.8	166 ± 1.2	200 ± 4.8	331 ± 4.8	314 ± 4.8	288 ± 4.8
COD (mg L−1)	850 ± 5	260 ± 6	244 ± 1.2	288 ± 1.2	209 ± 4.8	239 ± 1.2	270 ± 2.4	259 ± 2.4	266 ± 1.2	277 ± 3.6	202 ± 2.4	222 ± 3.6	177 ± 4.8
SVI (mL g−1)	224 ± 3	16 ± 2.9	22 ± 1.2	12 ± 2.4	16 ± 1.2	11 ± 4.8	20 ± 2.5	18 ± 4.8	14 ± 1.2	12 ± 1.6	10 ± 2.4	10 ± 2.4	11 ± 1.6
Cd (mg L−1)	115.2 ± 7.34	66 ± 4.8	61 ± 2.4	45 ± 4.8	52 ± 4.8	71 ± 2.4	66 ± 2.4	55 ± 1.2	74 ± 6	48 ± 2.4	52 ± 4.8	66 ± 2.4	51 ± 1.2
Cr (mg L−1)	318.6 ± 35.1	145 ± 2.4	111 ± 2.4	133 ± 1.2	149 ± 2.4	125 ± 4.8	131 ± 6	140 ± 2.4	119 ± 6	118 ± 3.6	200 ± 3.6	158 ± 1.2	188 ± 6
Cu (mg L−1)	284.8 ± 24.61	126 ± 1.2	127 ± 1.2	133 ± 6	114 ± 6	140 ± 4.8	144 ± 3.6	106 ± 6	109 ± 6	148 ± 6	166 ± 6	98 ± 3.6	101 ± 3.6
Pb (mg L−1)	294.2 ± 30.56	56 ± 3.6	58 ± 1.2	66 ± 3.6	83 ± 1.2	97 ± 4.8	100 ± 4.8	66 ± 1.2	81 ± 1.2	71 ± 1.2	66 ± 4.8	94 ± 1.2	93 ± 1.2
Ni (mg L−1)	281.5 ± 28.01	66 ± 2.4	91 ± 4.8	88 ± 4.8	67 ± 4.8	89 ± 4.8	104 ± 1.2	119 ± 1.2	160 ± 6	122 ± 4.8	96 ± 3.6	100 ± 6	105 ± 1.2
Zn (mg L−1)	316.4 ± 17.31	241 ± 3.6	238 ± 4.8	222 ± 2.4	264 ± 4.8	204 ± 3.6	208 ± 2.4	255 ± 2.4	249 ± 3.6	237 ± 2.4	241 ± 4.8	237 ± 3.6	251 ± 6
Thickness of biofilm (mm)	-	1.1	1.1	0.9	0.88	0.87	1.2	1.1	0.88	1.1	1.2	0.9	1.2

Notes: All data are shown as mean ± SD. Values obtained by triplicate measurements. BOD, biochemical oxygen demand; COD, chemical oxygen demand; TDS, total dissolved solids; EC, electrical conductivity; SVI, sludge volume index; Cd, cadmium; Cr, chromium; Cu, copper; Pb, lead; Ni, nickel; Zn, zinc.

). The data were collected after a period of three days, with samples taken from each treatment cell of the respective CWS type for each combination of bedding material. The four series of each type of CWS had cells filled with four types of bedding material, and textile effluent was allowed to pass through the system with an HRT of three days. The results indicated that glass beads consistently exhibited the highest removal efficiency, while RDA showed the lowest efficiency (Figure 5). Overall, the best results for pollution reduction were observed for the hybrid type of CWS as compared with only HSF and VSF.

Comparing the bedding materials, CDA and RDA were the cheapest in terms of being waste materials with only transportation expenses as the main cost line. Gravel of dolomite stone was easily available as a construction material, but it was relatively uneven in size, providing a tight packing of BM in the individual cells, leaving fewer exposed surfaces for the biofilm to form and providing limited space for the movement of wastewater within the cells. However, when these materials were used as BMs in the treatment cells and initially wet, dissolvable salts and ions were released into the effluent. This showed the reason for the high pH, EC, and TDS in the effluents linked to these materials. In particular, dolomite showed the highest dissolved mineral content followed by CDA and RDA (Table A2 and Table A3). The waste of demolished roads had a coating of hydrocarbon tars that prevented minerals from dissolving. Finally, glass beads, on the other hand, had a uniform size due to which they provided more space between the beads to form a thick biofilm allowing contact of wastewater with the entire biofilm.

4. Discussion

The study provided valuable insights into the optimization of constructed wetlands, emphasizing the interplay between design factors, operational parameters, and the selection of materials and plant species in achieving effective wastewater treatment. The hybrid CWS demonstrated a significant reduction in various pollutants, including TSS, TKN, and NaCl concentrations, and heavy metals. This highlighted the CWS ability to address multiple wastewater contaminants.

The presence of a diverse microbial community within the hybrid CWS, when compared with HSF or VSF systems likely contributed to the corresponding enhanced treatment efficiency. Previous research has indicated that the strategic combination of CWS configurations, such as integrating a vertical flow unit followed by a horizontal flow unit, can yield improved nutrient removal, particularly in terms of nitrogen [43]. Through rigorous experimentation, valuable insights into the factors influencing the performance of these systems showed a decline in removal efficiency during the later stages of treatment.

The observed reduction in BOD and COD concentrations in hybrid CWSs was significant, particularly at lower pollutant levels. This finding aligned with previous research that indicated constructed wetlands were effective in treating wastewater due to their ability to utilize microbial processes for organic matter degradation [44,45,46]. However, as concentrations increased, the correlation shifted, suggesting a saturation effect where microbial activity became less efficient. This phenomenon has been documented in other studies, which noted that high pollutant loads can inhibit microbial growth and activity, leading to diminished treatment performance [47,48].

The majority of pollutants are typically removed during the initial stages of treatment within the CWS. This is due to the rapid adsorption, precipitation, and biodegradation of readily available contaminants by the diverse microbial communities and the high surface area provided by the CWS. Second, the performance of the hydrophytes in pollutant removal can diminish over time due to various factors [49]. The wetland plants may experience stress, nutrient deficiencies, or the accumulation of toxins in their tissues over the course of the treatment process. These physiological changes can negatively impact on the plants’ ability to uptake and sequester pollutants, leading to a gradual decline in the overall pollutant removal efficiency of the CWS. Additionally, the gradual clogging and fouling of the CWS media can also contribute to the reduced performance observed in the later stages of treatment. This pattern is consistent with findings from other studies that have reported a gradual reduction in pollutant removal over time due to factors such as clogging, fouling, and accumulation of organic matter [9,50].

Previous studies have shown that pollutant removal from wastewater generally increases with increasing hydraulic retention time up to an optimal point, after which it gradually declines. This trend is often observed due to factors such as the saturation of treatment mechanisms, potential clogging or fouling of the system, and the limited capacity of the wetland plants and microorganisms to continue removing pollutants at a high rate over extended periods [17,51,52,53].

The superior performance of hybrid CWS in removing metals like Cu and Cd compared with HSF and VSF systems can be attributed to several factors. This system utilizes a combination of treatment mechanisms, including adsorption onto the wetland media, precipitation through chemical reactions, and uptake by the wetland plants [51,54].

The hybrid CWS features a surface water zone that promotes the establishment of diverse microbial communities. These microorganisms actively engage in bioaccumulation and biodegradation processes, significantly contributing to the removal of metals. The increased surface area of the hybrid CWS creates a more favourable habitat for these microbial populations, enhancing their activity and, consequently, improving the metal removal efficiency compared with the HSF and VSF systems, which have limited surface water interactions [55,56]. Direct uptake of metals by plant roots further contributes to their extraction from the wastewater. The synergistic relationship between plants and microbes is more pronounced in the hybrid CWS, leading to improved metal removal [23,57].

However, chromium (Cr) removal exhibited a different pattern. While the hybrid flow CWS demonstrated significantly better Cr removal overall compared with the HSF system, the mechanisms effective for Cu and Cd removal in the hybrid CWS might not be as efficient for Cr. This suggests that specific mechanisms or adaptations may be required for optimal Cr removal within the hybrid CWS design [58].

Mixed-cultured treatment cells within the hybrid CWS design achieved superior RE for almost all pollutants in both effluents. This aligns with the understanding that diverse microbial communities in the rhizosphere of hydrophytes can break down a wider range of pollutants compared with monocultures. This finding emphasizes the critical importance of developing a rich microbial environment within the hybrid CWS for effective treatment of both sewage and textile effluents [16,57,59,60].

This trend was observed in the current study. Organic pollution and TKN exhibited relatively high removal across all hydraulic retention times, suggesting a strong degrading capability within the constructed wetland system. Notably, the TSS RE was highest at short HRTs and declined at longer HRTs [61]. This decrease in TSS removal at longer HRTs may be attributed to optimized ET loss during shorter retention periods. While longer HRTs allow for more complete treatment, they also provide more time for the settlement of suspended solids, potentially leading to their resuspension in the water column at longer retention times [55,62]. Optimizing the HRT is crucial for balancing pollutant degradation with solids management within the CWS [21,30].

Glass beads, owing to their uniform size, create more space for the development of a robust biofilm. This enables intimate contact between the wastewater and the entire biofilm surface, leading to significant degradation of organic waste and maximizing the reduction in BOD and COD. Glass beads offer several advantages: they represent a one-time investment, provide an inert surface that is easy to clean and maintain, and foster the formation of better and thicker biofilms, facilitating rapid degradation of organic load. These characteristics contribute to more efficient metal removal, likely due to adsorption on the biofilm. Consequently, this study advocates for the use of glass beads to improve metal removal within CWS systems. The working of a CWS depends on the type of bedding material and the health of biofilms formed [17]. It is the bedding material that keeps the plants and microbes alive in constructed wetlands [3].

The biofilm causes a pronounced degradation of organic waste, causing more than 70% of reduction in BOD and COD in the cells. Even the cost of glass beads is not much in terms of the fact that it is a single-time investment. Glass beads provide inert surfaces, are easy to clean and wash, and form better and thicker biofilms, which facilitate a quick degradation of organic load. Even the metals are removed at a faster pace due to adsorption on the biofilm. So, this study proposes the use of glass beads for better removal efficiency in all types of wastewaters. Previous studies have shown biofilm formation on the surface of the wetland media to be instrumental in the degradation of organic matter and other pollutants making it a crucial factor in the performance of constructed wetlands [63,64].

This study presented a comprehensive examination of the factors influencing the successful performance of the hybrid CWS. It delved into the pivotal role of the impact of HRT and the advantages of utilizing glass beads as a BM. Additionally, the research highlighted the significance of considering the unique characteristics of target pollutants and optimizing design parameters to achieve optimal treatment efficiency. This work contributes to a deeper understanding of the hybrid CWS and its potential for effective wastewater treatment.

The correlations between organic, inorganic and metal contaminants in textile and sewage effluents can be understood through various mechanisms that govern their removal in CWSs. Organic contaminants, measured as BOD, are primarily broken down by microorganisms (bacteria and fungi) in CWSs [62]. Hydrophytes enhance this process by providing surfaces for microbial colonization and facilitating oxygen transfer through their roots, which is essential for aerobic degradation. Previous studies showed hydrophytes and microbial degradation work symbiotically in constructed wetlands to improve wastewater treatment efficiency [65,66].

Inorganic contaminants, such as N and P are removed through biological uptake and chemical precipitation. Nitrifying bacteria assist in the removal of N, while P precipitates with metal ions in the bedding materials, removing it from the wastewater. Heavy metals like Cd, Cu, and Pb are removed via adsorption, precipitation and plant uptake. Bedding materials with high surface area and specific chemical properties enhance adsorption, while hydrophytes uptake metals through their roots, accumulating them in plant tissues and reducing their concentration in the water. Organic matter breakdown releases nutrients (N and P), which promote the growth of hydrophytes and microbial communities, improving the removal of contaminants [52,53,55,62]. However, heavy metals can inhibit microbial activity, slowing organic matter degradation. Phosphorus precipitation with metal ions can be influenced by the presence of organic matter, which alters the pH and ionic strength of the effluent, affecting metal solubility and adsorption. The removal of organic, inorganic, and metal contaminants in CWSs involves interconnected biological, chemical, and physical mechanisms. A holistic approach is essential for optimizing wastewater treatment processes [50,57,58].

In summary, the current study provided insights into optimizing wastewater treatment systems by highlighting the effectiveness of a hybrid CWS design, the importance of HRT, the role of BM and the selection of hydrophytes. The research offered valuable guidance for enhancing CWS performance. The implications for developing countries underscore the potential of CWS to address urgent wastewater management challenges, promoting environmental sustainability and public health. Future research directions identified in the study can pave the way for continued advancements in this vital field.

5. Conclusions

Constructed wetland systems are highly effective for treating various effluents and ideal for urban areas with diverse contamination. Among different designs, the hybrid flow CWS showed superior performance in removing pollutants from both textile and sewage effluents. The hybrid flow CWS enhances wastewater treatment by maintaining higher oxygen levels and optimizing microbial activity. Its adaptability to varying flow rates and pollutant loads makes it a robust solution for urban wastewater challenges, highlighting its potential for widespread use in cities. It promotes aerobic and anaerobic microbial activities essential for the removal of organic and inorganic pollutants from different wastewater types.

The findings highlighted that CWS efficiency was influenced by multiple temporal and spatial factors, including the flow configuration, the choice of hydrophytic species, HRT, and bedding material types. By optimizing the design and operation of CWSs subject to local climatic conditions and the availability of hydrophytes, it is possible to significantly enhance the treatment of industrial and domestic wastewater. Such optimization not only improves pollutant removal but also ensures the sustainable and cost-effective operation of these systems in different contamination levels.