Assessment of the Impacts of Phyto-Remediation on Water Quality of the Litani River by Means of Two Wetland Plants (Sparganium erectum and Phragmites australis)

Abstract

Water pollution from human activities is largely a result of the discharge of wastewater and industrial waste into rivers. Phytoremediation, the technique that uses plants to remove pollutants from the polluted waters, is a growing field of research because of its various environmental advantages. This study aims to evaluate the efficiency of a constructed wetland in removing pollutants and treating the polluted waters of the Litani River in Lebanon, by means of two aquatic plants, Phragmites australis and Sparganium erectum. Results showed that the levels of the physicochemical and biological parameters measured on water samples at downstream of the wetland were lower than those obtained at upstream. Results revealed that average removal efficiency was 41% for chemical oxygen demand (COD), 54% for biological oxygen demand (BOD5), 97% for nitrate (NO₃⁻), 40% for nitrite (NO₂⁻), 67% for phosphate (PO₄³⁻), while it was negative (−62%) for sulfate (SO₄²⁻), indicating an increase in sulfate content in the treated effluent returning to the river. On the other hand, most of the effluent chemical and biological characteristics were within the provisional discharge limits of effluent to water body set by the Ministry of Environment (MoE) and Lebanese Wastewater Reuse Guidelines of the Food and Agricultural Organization of the United Nations (FAO). Statistical analyses also showed significant variations (p < 0.5) among the two sampling sites along the wetland. Our findings clearly demonstrate that phytoremediation is a viable solution to remove pollutants in a competitive environment and improve the quality of contaminated waters by acting as a sink for various contaminants. The gained experience may be scalable to other sites and environments across the country.

1. Introduction

Constructed wetlands are an alternative, promising technology for water/wastewater treatment and pollution mitigation [1]. They belong to the wider category of natural treatment systems, which are designed and constructed to utilize the natural processes involving wetland vegetation, soils, and the associated microbial assemblages to assist in treating wastewaters [2,3,4]. In addition, this environmentally friendly and sustainable technology provides multiple economic, ecological, technical and societal benefits, not only for domestic, municipal and industrial wastewater treatment, but also for treating agricultural runoff and agro-industrial wastewater [5]. Endowed with the advantages of cost-effectiveness and low energy consumption, the wetland technology places the overall context of the need for reliable and sustainable solutions to managing agricultural runoff and agro-industrial wastewater [6].

Phytoremediation using constructed wetlands has become a logical solution to improve the quality of contaminated waters by acting as a sink for various contaminants [7]. Phytoremediation is a technique for which aquatic plants are highly useful in removing pollutants in wastewater, by absorbing organic and inorganic pollutants in a competitive environment [8,9]. Multiple water contaminants can be eliminated by using renewable and biological processes offered by constructed wetlands, requiring limited maintenance and external energy inputs [10].

The most important advantage of this system is that it is a green technology that uses plant and microbe natural resources lowers degradation of the environment and safeguards ecosystems. Other benefits include the fact that both organic and inorganic pollutants are effectively removed by aquatic plants, making them suited for the treatment of mixed types of pollutants [8]. However, a critical assessment of the performance and effectiveness of wetland systems for removing various contaminants, for which the design parameters and operational conditions affecting the efficiency of contaminant removal [6].

Plants are the primary components of a constructed wetland, as they can influence the wetland treatment performance by several processes [11], either for enhancing the abundance and diversity of microorganisms in the rhizosphere by increasing available surface area for bacterial attachment and growth [12], or exuding a range of degradable organic compounds (including sugars, organic acids, and amino acids), which can especially provide a continuing supply of carbon for denitrification bacteria in wetland systems [13]. In addition, wetland plants absorb nutrients and contaminants into their tissues directly [14], such as heavy metals and micro-pollutants [15,16]. In [11], Wang et al. demonstrated that plant roots improve oxygen conditions, thereby supporting the aerobic processes in constructed wetlands in flooded conditions. On the other hand, the existence of plants is thought to increase and stabilize hydraulic conductivity in constructed wetlands [17]. In [18], Lama et al. demonstrated that the interaction between water flow and Phragmites australis plants significantly affects flow dynamics, hydraulic conveyance, and water quality of vegetated water bodies.

The Litani River is Lebanon’s largest river and most important water resource, suffering from widespread sewage disposal, direct drainage of unregulated industrial wastewater from urban areas, lack of riverbed protection, and illegal diversion. Today, the river is becoming a threat to public health as water contamination extends to soils, crops, and wildlife, as well as hinders the socio-economic growth and well-being of riparian ecosystems. In an attempt to address the deteriorating water quality of the Litani River, the Litani River Basin Management Support (LRBMS) has constructed a wetland system between 2012 and 2013 in a publicly owned site by the Litani River Authority (LRA), to contribute to reducing the high pollution rates of the River’s waters. The objectives of the present study were to (i) assess the performance of a constructed wetland using two aquatic plants, Sparganium erectum and Phragmites australis, in treating the contaminated waters of the Litani River, and (ii) determine the efficiency of these two plants in removing pollutants and improving the quality of the polluted waters of the River.

2. Materials and Methods

2.1. Climatic Characteristics of the Wetland Site

The climate of South Bekaa Valley is sub-Mediterranean, with hot and dry season between April and September and cold and wet season for the rest of the year. Average yearly rain and potential evapotranspiration are 696 mm and 1314 mm, respectively, based on data of the EU-SUPROMED Project (Sustainable Production in Water Limited Environments of Mediterranean Agro-Ecosystems, 2019–2022) for the calculation of Typical Meteorological Year (TMY) for South Bekaa Valley, during the period from 1994 through 2018 [19]. About 95% of the rain occurs from November to March. Ambient weather data (solar radiation, air temperature, wind speed at 2 m height, air temperature at dew point and relative humidity) were recorded on an hourly basis from an automated weather station (METOS Compact, PESSL Instruments, Austria) 80 m apart from the wetland site. The weather station is established within a standard meteorological park (40 m N–S × 40 m W–E) cultivated with rye grass (Lolium perenne), and is automatically linked to a built-in data logger, which discharges at 10 min interval the registered meteorological data via GPRS (General Packet Radio Service) standard wireless communication into a computer situated in the weather monitoring unit of the research station. Data was used to compute potential evapotranspiration according to Penman–Monteith equation [20] (Figure 1).

2.2. Characteristics of the Constructed Wetland

The designed wetland is a Free Water Surface (FWS) wetland established in 2013 by the Litani River Basin Management System (LRBMS), on a public-owned property, in the southern plains of the Bekaa Valley. The site is within Khirbet Kanafar Agricultural and Extension Center of the Litani River Authority, and 10 km away from Lebanon’s only remaining natural wetland “Ammiq Wetland” (UNESCO biosphere reserve), offering significant potential for environmental education, wetland habitat restoration, and other additional benefits. The constructed wetland area boundary is generally flat, with elevations ranging from 861.5 m above the sea level (a.s.l) at the top of the surrounding berms, to 860.0 m a.s.l in the shallow basins cultivated with Phragmites australis and Sparganium erectum, to 857.5–858.5 m a.s.l in the deep ponds (Figure 2). The wetland is approximately 3.5 ha in size, with an inner wet area (shallow basins and deep ponds) of 2.5 ha in size. It consists of three main parts:

• Inlet structure, including piping and pumping station, constructed near the riverbank, conveys inflow water from the river to the wetland. The pumping station consists of three electrical pumps, two of which are 60 L/s capacity each, and one 30 L/s capacity, impelling water directly from the bottom of the river, and conveying it into the wetland by means of a 16-inch galvanized iron pipe buried in the soil.

• An oval-shaped basin, 240 m average length (north–south) and 125 m average width (east–west), with an average outer area, including berms, of 35,000 m², and inner wet area of 25,000 m². The inner area consists of an alternation of three deep ponds (2–3 m deep) and two shallow areas (30–50 cm deep), with a ratio of 2:1 (2/3 deep ponds versus 1/3 shallow areas). The deep ponds were designed to promote mixing and uniform flow, and the shallow areas to promote growth of emergent wetland vegetation, which provides a biologically and chemically diverse environment, where much of the pollutant removal occurs.

• Adjustable outlet structure, made of a concrete weir, piping, and outlet earth channel to convey the treated water back to the Litani River. The discharge channel features initial and terminal narrow stream channels whose banks are seeded with the same mix of plant species as the outside of the wetland berms. The bed of the discharge channel ends with a large rock weir structure. The discharge channel has been sized to accommodate a normal flow of 20–60 L/s, based on expected outflows from the constructed wetland system. This corresponds to a channel width of approximately three to five meters with the exception of the widened, flattened central area.

The wetland has been designed to provide 5-day residency time for effluents and treat as much as 100% of the River waters during the dry season. From the inlet pipe to the outlet pipe, water in the wetland spends 5–6 days for treatment purposes. This interval has been designed as the time period needed for water residency in the wetland, which corresponds to BOD five days (BOD₅). With a pumping capacity of 60 L/s, total daily pumped water 5184 m³. With a storage capacity of 30,000 m³, the residency time is then 30,000 m³/5184 m³ per day = 5.72 days. This water residency-time inside the wetland corresponds to BOD₅, or the amount of oxygen needed for the biological degradation of organic substances in water. From hydraulic point of view, a wetland is considered a water catchment surface, conceptualized as a ‘reservoir’ with inflows (upstream contributions) and outflows (evaporation, infiltration, surface runoff and final drainage discharge. The storage within the wetland is conceptualized as the difference between inflows and outflows:

(1)$Q_{in}-Q_{out}=\frac{dV}{dt}$

where Q_in is inflow (m³/s), Q_out outflow (m³/s), V storage (m³) and t time.

The constructed wetland has a dense coverage of emergent vegetation in its shallow zones with species adapted to constant flooding. Phragmites australis (common reed) and Sparganium erectum are native to Lebanon and a robust emergent marsh plant species that provide habitats for a variety of bird species. Moreover, they are commonly found near the site at the Ammiq wetland and are readily propagated by planting its rhizomes (root structures). In the deep, open water areas of the constructed wetland, both floating and submerged plants will serve to enhance biodiversity and the treatment effectiveness for certain pollutants. Nymphaea alba, or water lilies, are planted in the wetlands for this purpose.

3. Methodology Used

For quality assessment, water samples were collected weekly during a 10-week period from 21 June through 29 August 2020, from both the wetland inflow and outflow ponds. The water sampling method was the extendable sampling pole method, which is fully described in the ‘Climate Change Indicators in the United States’ [10,21] and the ‘Monitoring and Sampling Manual’ of the Department of Environment and Science Government of the State of Queensland, Australia [22]. Samples were collected directly into the laboratory supplied containers at each water sampling date to reduce the risk of contamination. As described by US-EPA and DES, also [23], direct sample collection is the preferred procedure if the environment is safe, e.g., during low flow conditions, and sample bottles do not contain preservative. Collected water samples using the extendable sampling pole is recommended in isolated pools, so as not to disturb the substrate. A full description of the sampling method is available in the Monitoring and Sampling Manual of the Department of Environment and Science, State of Queensland [22]. Physicochemical and biological parameters were analyzed at the Soil and Water Laboratory of Kherbet Kanafar Agricultural and Extension Center of the Litani River Authority, 100 m apart from the constructed wetland. Physicochemical parameters included total dissolved solids (TDS) and electrical conductivity (EC), which were determined by a tracer pocket tester (JENWAY 470 conductivity meter), pH by a portable pH meter (HI-83141), and nitrate (NO₃⁻), nitrite (NO₂⁻), phosphate (PO₄³⁻), and sulfate (SO₄²⁻) by spectrometer (Thermo Helios Aquamate 2000E). Biological parameters included Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD₅), and Dissolved Oxygen (DO). The ratio of BOD₅/COD was then calculated. DO was measured directly on site by a dissolved oxygen meter (MILWAUKEE). COD was determined by using COD reagent tubes containing dichromate solution, 2 mL of water samples were added to the tubes, and then were placed in a heating reactor (VELP-Scientifica, Spain) at 150 °C for 2 h. After that, COD concentration was determined by a spectrometer (Thermo Helios Aquamate 2000E). For BOD₅ measurement, 250 mL of water sample were poured in glace bottles, a stirring bar, sodium hydroxide and nitrification inhibitor were added to the bottles that were closed by a VELP BOD sensor and placed in a BOD System 6–FTC 90–r refrigerated incubator (VELP-Scientifica, Spain) for 5 days at 20 °C. Sampling was on a regular, weekly basis starting from the week of 21–27 June 2020, through the week of 23–29 August 2020. The influent samples were collected on Monday of each week of the sampling period, while the effluent samples were collected on Friday, to abide the 5-day interval of water time-residency between the two samplings days, so that the time needed for BOD₅ is respected.

3.1. Pollutants Removal Efficiency

The reduction efficiency (RE, in %) of the concentration of pollutants was assessed according to the International Water Association [24] which proposed an equation for this intent [25]. The efficiency of the wetland in terms of the removal percentage of pollutants (COD, BOD₅, NO₃⁻, NO₂⁻, PO₄³⁻, and SO₄²⁻) was computed using the following formula:

(2)$\mathrm{RE}(\%)=100\frac{C_i-C_e}{C_i}$

where, C_i and C_e are the average influent and effluent concentrations, respectively (in mg/L).

3.2. Statistical Analyses

Statistical analyses of the physicochemical and biological parameters data obtained from water sampling at the wetland inflow and outflow during the study period were conducted by paired t-test using STATISTICA, Software version 10, which provides all the tools needed for statistical analysis [26,27]. The Student’s t-test was used to detect how significant the differences between the two water sampling groups, inflow and outflow, are in terms of pollutant’s concentration, and how the differences were repeatable for the whole sampling period.

4. Results and Discussion

4.1. Comparative Influent and Effluent Water Quality

Minimum, maximum, and mean values of chemical oxygen demand (COD), dissolved oxygen (DO), biological oxygen demand (BOD₅), phosphate (PO₄³⁻), nitrate (NO₃⁻), nitrite (NO₂⁻), sulfate (SO₄²⁻), water temperature (T), total dissolved solids (TDS), electrical conductivity (EC), and pH, measured on water samples from the wetland influent and effluent are found in Table 1 and Table 2, respectively, alongside the Environmental limit values for surface water based on MoE Decision 8/1 of the Ministry of Environment [28] and the Lebanese Wastewater Reuse Guidelines [29,30]. In addition, Table 3 presents the results of standard deviation and p value of the removal efficiency of contaminants calculated according to Equation (2) on water samples from the two sites along the wetland.

4.2. Time Course Evolution of Physicochemical Parameters

4.2.1. EC, TDS, pH and T

Figure 3a,b illustrates time course evolution of electrical conductivity (EC) and total dissolved solids (TDS), respectively, during the sampling period from June through August 2020, in inflow and outflow samples. Electrical conductivity has been shown to decrease in the wetland outflow compared to the inflow. The value of EC of the influent ranged from 530 to 993 µSm⁻¹, with an average value of 782.5 µSm⁻¹, while the range in the effluent ranged from 561.3 to 1000 µSm⁻¹, with an average value of 753.3 µSm⁻¹ (Table 1 and Table 2). This slight decrease of the EC level at the downstream of the wetland may be due to the absorption of ions such as Ca²⁺ and Mg²⁺, combined with sulfate and phosphate salts, by the wetland plants. Concerning TDS, the concentration of this indicator of water turbidity along the wetland did not mark a remarkable variation, as its concentration ranged from 469.5 mg∙L⁻¹ at the upstream to 467.5 mg∙L⁻¹ at the downstream. Natural water sources typically have a certain level of TDS, but human activity, such as irrigation or urbanization, can greatly raise the TDS level in surface water [31]. The same is implied for EC, where large variations in conductivity may be due to either natural flooding, evaporation, or man-made contamination, which may be very harmful to the quality of the water [32]. The World Health Organization (WHO) considers a TDS concentration less than 1000 mg L⁻¹ to be acceptable, and a range of 10 to 1000 μSm⁻¹ for EC to be acceptable in freshwater [33]. Therefore, the results obtained in both the wetland inflow and outflow samples presented in Table 1 and Table 2 satisfy the standards set by WHO.

On the other hand, the average pH along the wetland ranged from 7.8 at the inlet to 8.2 at the outlet (Figure 3c), and this range is within the environmental limits for surface water set by Decision 8/1 of the Ministry of Environment and FAO guidelines for wastewater reuse in Lebanon, while water temperature was found to steadily vary between the two sampling sites across the wetland from 25.0 °C to 26.3 °C (Figure 3d).

4.2.2. Nitrate and Nitrite

The evolution of nitrate and nitrite during the sampling period from the wetland inflow and outflow are presented in Figure 4a,b, respectively. Data shows a peak in the inflow concentration of nitrate at the beginning of the sampling period (Figure 4a). The mean level of nitrate (NO₃⁻) in the downstream site of the wetland (0.37 mg L⁻¹) was much lower than the level obtained from the upstream site of the river, which is 14.3 mg L⁻¹, thus showing a high removal efficiency by the wetland. The high level of NO₃⁻ found at the upstream site of the wetland is mainly due to agricultural activities in the plains near the Litani River, for which overestimation of irrigation needs of sprinkler-irrigated potatoes may have led significant loads of nitrate by surface runoff to the river.

For nitrite, the concentrations along the wetland trail ranged from an average value of 0.35 mg L⁻¹ at the inflow site to 0.30 mg∙L⁻¹ at the outflow site, with a removal efficiency of 40% (p < 0.456). However, the values of NO₃⁻ and NO₂⁻ differed significantly among the different sampling dates, as marked in Figure 4a,b. This variation might be attributed to several components, as the Litani River effluents contain excessive amounts of nitrogen, as a result of the agricultural runoff and agro-industrial wastewater typical of the Litani River Basin.

Despite substantial variability in the degree and rate of nitrogen cycling under the influence of several variables, such as air temperature, level of dissolved oxygen, pH, and other environmental conditions, the probable elimination mechanisms of nitrate and nitrite include plant and microbial assimilation, nitrification/denitrification phase, and potential release of volatile ammonia gas to the atmosphere [34].

The inflow NO₃⁻ concentrations (Table 1) fall within the range of the environmental limit values for surface water based on MoE Decision 8/1 [28]. Furthermore, the levels of NO₂⁻ found in the inflow samples were found to be lower than the guideline levels for protecting sensitive aquatic animals during short-term exposures [35]. On the other hand, outflow concentrations for both nitrate and nitrite remained relatively constant throughout the study, demonstrating high removal efficiency for nitrate (97.39%, p < 0.078) and nitrite (40%, p < 0.456). This was expected given the strong dependency of microbial denitrification on temperature, which converts NO₂⁻/NO₃⁻ to NO_x and N₂ gases [36].

4.2.3. Phosphate

Figure 4c presents time course evolution of phosphate in water samples from both the wetland inflow and outflow during the sampling period. A significant decrease in phosphate concentration measured in the outflow was observed, compared to the inflow, with a removal efficiency of 67% (Table 3). Average concentration of phosphate along the wetland upstream and downstream ranged from 5.8 mg L⁻¹ to 1.9 mg∙L⁻¹, respectively (Table 1 and Table 2). The level of phosphate in the influent at all sampling dates were higher than the discharge limit of 5 mg L⁻¹ set by the Ministry of Environment [28]. Indeed, the concentrations of phosphates at the downstream site of the wetland ranged from 0.6 to 4.5 mg∙L⁻¹ and were higher compared to the values obtained from the upstream site. This indicates that the river has increasing phosphate levels as the result of its direct discharge into its waters.

Phosphate in water is primarily due to the natural decomposition of rocks and stones, agricultural runoff, flooding, sewage, and industrial waste. Phosphorous can increase the growth of algae and aquatic vegetation contributing to eutrophication of the aqueous environment [24]. On that note, In [37] Box et al. found that changes in the riverine environment, such as vegetation growth associated with altered flow regimes, increased sediment loads and eutrophication. In order to prevent eutrophication, the environmental limit value for phosphate concentration in surface water based on MoE Decision 8/1 [28] is <5 mg/L. The constructed wetland treatment system has achieved effluent quality that satisfies these requirements in terms of PO₄³⁻ in all samples. The removal mechanism of phosphate occurs as a result of several physical, chemical, and biological processes, such as (i) sedimentation of particulate phosphorous (organic and inorganic absorbed PO₄³⁻), (ii) precipitation associated with mineral particles within the water column, (iii) sorption (adsorption/absorption) in wetland soils (fixation of phosphate by iron and aluminum in the soil), and (iv) biological uptake by plants and micro-organisms [38].

4.2.4. Sulfate

Figure 4d shows the variation of sulfate concentration in water samples from the wetland inflow and outflow during the sampling period. Unlike the nitrates and phosphate, an increase in the sulfate concentration has been reported at the two sampling sites from 35.8 mg L⁻¹ at the inlet to 57.9 mg L⁻¹ at the outlet, thus showing a removal efficiency of −61.67%, with no significant difference (p > 0.05) (Table 3). Minimum and maximum concentrations of sulfate at the upstream of the wetland were 20.8 and 46.2 mg L⁻¹, while at the downstream they were 15.1 and 181.1 mg L⁻¹. Similar results have been detected by [39], where instead of decreasing, sulfate concentration has increased in the constructed wetland outflow. These phenomena might be due to the denitrifying bacteria activity, chemolithoautotrophic, that use reduced sulfur compounds in the form of sulfide as an electron donor [40]. This nitrate reduction and S-oxidizer bacteria will oxidize sulfide back to SO₄²⁻ during denitrification [41]. The activity of these bacteria may explain the high removal of NO₃⁻ and the release of SO₄²⁻ in the wetlands [42]. However, values obtained in the wetland inflow and outflow, are within the acceptable range of 1000 mg L⁻¹ for surface water set by MoE Decision 8/1 [28] (Table 1 and Table 2), while the proposed maximum allowable limit for sulfate of the National Standard for treated domestic wastewater reuse for irrigation is 500 mg L⁻¹ [43].

4.3. Time Course Evolution of the Biological Parameters

Figure 5 displays time course evolution of BOD₅, COD, and the ratio of BOD₅/COD and DO in water samples from the wetland inflow and outflow. Results show that the wetland increased DO (average 34%) and reduced BOD₅ (average 54.3%) and COD (average 41%). Student’s t-test analysis revealed that these changes were significant at p < 0.05 (Table 3). The increasing of oxygen concentration in the wetland outflow was presumably due to the cascade input tubing and wind mixing in deep open-water areas as a result of passive aeration. Additionally, growth of oxygen provided by algae and submerged plants may also have contributed to these results [44]. COD and BOD₅ average values in the outflow samples were 154.7 mg L⁻¹ and 31.7 mg L⁻¹, respectively, and were slightly above the range of the environmental limit values for surface water set by MoE Decision 8/1 [28]. As such, the Lebanese surface water discharge limits refer COD < 125 mg L⁻¹ and BOD₅ < 25 mg L⁻¹. On the other hand, previous studies have proved that complete elimination of COD and BOD cannot be accomplished in constructed wetlands. In fact, the decomposition of plant residues and other naturally occurring organic materials in the wetland will produce BOD and COD [45,46,47]. Moreover, the comparison of the BOD and COD removal efficiency obtained with previous studies undertaken by Abi Saab et al. [48] and Amacha et al. [49] on the same wetland reveals that the removal rate of these parameters has decreased over the years. Therefore, it is worth noting that the implementation of an artificial aerated system is highly recommended in this case, as it contributes to increase DO concentration and therefore improve treatment performance, especially for BOD₅ and COD removal rate [50].

Moreover, the calculated BOD₅/COD ratio (Figure 5c), from both the wetland inlet and outlet, ranged between 0.1 and 1.0 during the sampling period, indicating a presence of biodegradable material, meaning that the limit of organic matter can be decayed by microbes in natural and artificial treatment conditions [51]. The high levels of BOD₅ and COD observed in the effluent might be due to high amount of organic matter from domestic wastewater and agricultural inputs, and the processing of hides and skins of the poultry industry, as well as various chemicals sources, mainly paper and plastic industries, largely spread in the Litani River basin, which discharge their loads directly into the environment, thereby increasing the levels of BOD₅ and COD in the river waters. In the downstream site of the wetland, the levels of BOD₅ and COD were significantly lower than the upstream site, thus indicating the capability of the aquatic plants in de-polluting the river waters through the wetland biological process. In [13], Dong et al. showed the role of vegetation should not be ignored in the process of wastewater purification in constructed wetlands, as root oxygen released contributes to pollutant removal, alongside with other environmental and hydraulic factors within a constructed wetland.

Figure 5d illustrates the time course evolution of dissolved oxygen (DO) concentration of water samples from the wetland inflow and outflow. In the influent samples, average DO concentration was 3.96 mg L⁻¹, while in the effluent samples the average concentration raised to 5.30 mg L⁻¹. This increase in the concentration of the dissolved oxygen may be due to the oxygen released by the root systems of the wetland plants, as described in [11] by Wang et al. who demonstrated that plant roots improve oxygen conditions, thereby supporting the aerobic processes in constructed wetlands in flooded conditions. On the other hand, to investigate the effect of vegetation on microbial processes by increasing oxygen concentrations in the rhizosphere, BOD₅ and COD levels in the effluent returning to the rivers have decreased, compared to the level obtained at the inflow gate of the wetland.

5. Conclusions

Results of this study showed the constructed wetland has successfully achieved high removal rate of nitrate (NO₃⁻), nitrite (NO₂⁻), and phosphate (PO₄³⁻), but not of sulfate (SO₄²⁻), as this was the one concentration was found to increase at the wetland downstream. Moreover, biological oxygen demand (BOD₅) and chemical oxygen demand (COD) reduction, along with the enrichment of the wetland waters at the downstream in dissolved oxygen (DO), resulted in improved water quality of effluents returning to the river. The rest of the parameters mean values, namely, electrical conductivity (EC), total dissolved solid (TDS), were within the recommended levels for natural surface water. Therefore, the constructed wetland has clearly contributed to reducing the level of pollution in the river and improving its deteriorated water quality and ecologic viability.

The presence of Phragmites australis and Sparganium erectum has been shown a great impact on the removal of pollutants, due to which both organic and inorganic pollutants have been effectively treated by these two aquatic plants, making them suited for the treatment of mixed types of pollutants by multiple removal mechanisms, such phytoaccumulation, phytodegradation, phyto-transformation, phytovolatilization, and Phytoextraction, to clean up or detoxify pollutants [8,52].

A deeper comprehensive performance assessment of the constructed wetland system for de-polluting the waters of the Litani River, over a longer time period, is needed. The current research shows the potential of wastewater treatment by means of a constructed wetland, as sustainable and cost effective technology, and the gained experience may be scalable to other sites and environments across the country.

Author Contributions

Conceptualization and methodology: F.K., R.H., N.A. and W.C.; writing—review and editing: F.K., R.H., N.A., W.C. and J.H.; Figures: F.K. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors wish to thank PHC CEDRE Project No 46459UE (2021) for the financial support. They also wish to extend their thanks to LRBMS (Litani River Basin Management Support), a 5-year Program (2009–2013) funded by the US Agency for International Development (USAID), for constructing the wetland, and providing technical support to the monitoring staff of the Litani River Authority. Deep thanks go to Eric Viala, Chief of Partly, for his encouragement and continuing support, and to Eng. Paul Frank, Founder and Principal Engineer at FlowWest, Oakland, California, for his leading role in designing the constructed wetland. A deep appreciation to the Euro-Mediterranean TREASURE Research Network (Treatment and Sustainable Reuse of Effluents in semiarid climates), led by INRAE-LBE, Narbonne, France (www6.inrae.fr/treasure, accessed on 23 October 2022) and to JPI project Control4reuse (http://control4reuse.net, accessed on 23 October 2022) financed by the French Research National Agency under the contract ANR-18-IC4W-0002.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Generate image description

Figure 1. Daily precipitation (P, mm), maximum (Tmax, °C) and minimum (Tmin, °C) air temperature, and potential evapotranspiration (ETo, mm day-1) recorded at the wetland site during the sampling period.

Generate image description

Figure 2. Overview of Litani River constructed wetland.

Generate image description

Figure 3. Time course evolution of electrical conductivity (a), total dissolved solids (b), pH (c) water temperature (d).

Generate image description

Figure 4. Time course evolution of nitrate (a), nitrite (b), phosphates (c), and sulfates (d).

Generate image description

Figure 4. Time course evolution of nitrate (a), nitrite (b), phosphates (c), and sulfates (d).

Generate image description

Figure 5. Time course evolution of biological oxygen demand (a), chemical oxygen demand (b), ratio of BOD5/COD (c), and dissolved oxygen (d).

Table 1

Minimum, maximum, and mean value of water quality parameters collected from the wetland influent compared to recommended limits.

Parameters	Wetland Influents			Environmental Limit Values for Surface Water Based on MoE Decision 8/1 [28]	Lebanese Wastewater Reuse Guidelines [29]
	Min	Max	Mean		WaterCategoryI	WaterCategory II	WaterCategory III
Temperature (°C)	21.0	27.5	25.1	30	-	-	-
EC (µs/m)	530.0	993.0	782.5	-	-	-	-
TDS (mg/L)	318.5	595.5	469.5	-	-	-	-
DO (mg/L)	2.0	5.6	3.9	-	-	-	-
pH	7.5	8.4	7.8	6–9	6–9	6–9	6–9
Phosphate (mg/L)	3.5	8.2	5.8	5	-	-	-
Nitrite (mg/L)	NQ *	0.35	0.1	-	-	-	-
Nitrate (mg/L)	NQ *	44.6	14.3	90	30	30	30
Sulfate (mg/L)	20.8	46.2	35.8	1000	-	-	-
BOD5 (mg/L)	28.0	159.5	69.4	25	25	100	100
COD (mg/L)	59.0	377.5	262.1	125	125	250	250

* Not quantifiable.

Table 2

Minimum, maximum, and mean value of water quality parameters collected from the wetland effluent compared to recommended limits.

Parameters	Wetland Effluents			Environmental Limit Values for Surface Water Based on MoE Decision 8/1 [28]	Lebanese Wastewater Reuse Guidelines [29]
	Min	Max	Mean		WaterCategory I	WaterCategory II	WaterCategory III
Temperature (°C)	24.0	28.0	26.3	30	-	-	-
EC (µs/m)	561.3	1000.0	753.3	-	-	-	-
TDS (mg/L)	335.0	671.5	467.5	-	-	-	-
DO (mg/L)	4.0	6.9	5.3	-	-	-	-
pH	7.8	8.9	8.2	6–9	6–9	6–9	6–9
Phosphate (mg/L)	0.6	4.5	1.9	5	-	-	-
Nitrite (mg/L)	0.0	0.3	0.04	-	-	-	-
Nitrate (mg/L)	3.3	0.0001	0.37	90	30	30	30
Sulfate (mg/L)	15.6	181.1	57.9	1000	-	-	-
BOD5 (mg/L)	5.4	99.8	31.7	25	25	100	100
COD (mg/L)	29.0	280.0	154.7	125	125	250	250

Table 3

Mean values of water quality parameters and variation percentages of the inflow and outflow of the constructed wetland of the Litani River.

Parameter	Number of Samples	Inflow	Outflow	Removal Efficiency (%) *	p Value
Temperature (°C)	10	25.02 ± 2.68	26.29 ± 1.23	−5.06	0.072
EC (µs/m)	10	782.48 ± 127.1	753.31 ± 179.1	3.73	0.407
TDS (mg/L)	10	469.51 ± 75.9	467.51 ± 142.6	0.43	0.952
DO (mg/L)	10	3.96 ± 1.16	5.3 ± 1.05	−33.8	0.032
pH	10	7.82 ± 0.28	8.22 ± 0.35	−5.12	0.006
Phosphate (mg/L)	10	5.84 ± 1.49	1.90 ± 1.19	66.9	0.000
Nitrite (mg/L)	10	0.08 ± 0.1	0.04 ± 0.09	40.27	0.456
Nitrate (mg/L)	10	14.30± 20.44	0.37 ± 1.09	97.39	0.078
Sulfate (mg/L)	10	35.86 ± 8.26	57.99 ± 49.32	−61.67	0.202
BOD (mg/L)	10	69.45 ± 39.5	31.71 ± 26.8	54.3	0.027
COD (mg/L)	10	262.09 ± 130.3	154.72 ± 119.5	41	0.012

* Values were obtained by applying Equation (2).

References

1. Stefanakis, A.I. Constructed Wetlands: Description and Benefits of an Eco-Tech Water Treatment System. Waste Management: Concepts, Methodologies, Tools, and Applications; International Management Association. IGI Global: Hershey, PA, USA, 2020; pp. 503-525. [DOI: https://dx.doi.org/10.4018/978-1-7998-1210-4.ch025]

2. Mustafa, A.; Scholz, M.; Harrington, R.; Carroll, P. Long-term performance of a representative integrated constructed wetland treating farmyard runoff. Ecol. Eng.; 2009; 35, pp. 779-790. [DOI: https://dx.doi.org/10.1016/j.ecoleng.2008.12.008]

3. Mustafa, A. Constructed Wetland for Wastewater Treatment and Reuse: A Case Study of Developing Country. Int. J. Environ. Sci. Dev.; 2013; 4, pp. 20-24. [DOI: https://dx.doi.org/10.7763/IJESD.2013.V4.296]

4. Vymazal, J. Emergent plants used in free water surface constructed wetlands: A review. Ecol. Eng.; 2013; 61, pp. 582-592. [DOI: https://dx.doi.org/10.1016/j.ecoleng.2013.06.023]

5. Mustafa, H.M.; Hayder, G. Recent studies on applications of aquatic weed plants in phytoremediation of wastewater: A review article. Ain Shams Eng. J.; 2020; 12, pp. 355-365. [DOI: https://dx.doi.org/10.1016/j.asej.2020.05.009]

6. Wang, M.; Zhang, D.Q.; Dong, J.; Tan, S.K. Application of constructed wetlands for treating agricultural runoff and agro-industrial wastewater: A review. Hydrobiologia; 2018; 805, pp. 1-31. [DOI: https://dx.doi.org/10.1007/s10750-017-3315-z]

7. Herath, I.; Vithanage, M. Phytoremediation in Constructed Wetlands. Phytoremediation: Management of Environmental Contaminants; Ansari, A.; Gill, S.S.; Gill, R.; Lanza, G.R.; Newman, L. Springer International Publishing: Cham, Switzerland, 2015; Volume 2, pp. 243-263. [DOI: https://dx.doi.org/10.1007/978-3-319-10969-5_21]

8. Ali, H.H.; Fayed, M.I.A.; Lazim, I. Use of aquatic plants in removing pollutants and treating the wastewater: A review. J. Glob. Innov. Agric. Sci.; 2022; 10, pp. 61-70. [DOI: https://dx.doi.org/10.22194/JGIAS/10.985]

9. Anning, A.K.; Korsah, P.E.; Addo-Fordjour, P. Phytoremediation of wastewater with Limnocharis flava, Thalia geniculata and Typha latifolia in constructed wetlands. Int. J. Phytoremediat.; 2013; 15, pp. 452-464. [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23488171][DOI: https://dx.doi.org/10.1080/15226514.2012.716098]

10. U.S. Environmental Prtection Agency. Climate Change Indicators in the United States; 4th ed. EPA 430-R-16-004 U.S. Environmental Protection Agency: Washington, DC, USA, 2016; Available online: www.epa.gov/climate-indicators (accessed on 23 October 2022).

11. Wang, Q.; Xie, H.J.; Ngo, H.H.; Guo, W.S.; Zhang, J.; Liu, C.; Liang, S.; Hu, Z.; Yang, Z.C.; Zhao, C.C. Microbial abundance and community in subsurface flow constructed wetland microcosms: Role of plant presence. Environ. Sci. Pollut. Res.; 2015; 23, pp. 4036-4045. [DOI: https://dx.doi.org/10.1007/s11356-015-4286-0]

12. Menon, R.; Jackson, C.R.; Holland, M.M. The influence of vegetation on microbial enzyme activity and bacterial community structure in freshwater constructed wetland sediments. Wetlands; 2013; 33, 365. [DOI: https://dx.doi.org/10.1007/s13157-013-0394-0]

13. Dong, C.; Huang, Y.H.; Wang, S.C.; Wang, X.H. Oxygen Supply and Wastewater Treatment in Subsurface-Flow Constructed Wetland Mesocosm: Role of Plant Presence. Pol. J. Environ. Stud.; 2016; 25, pp. 573-579. [DOI: https://dx.doi.org/10.15244/pjoes/61008]

14. Liu, J.G.; Zhang, W.; Qu, P.; Wang, M.X. Cadmium tolerance and accumulation in fi fteen wetland plant species from cadmium-polluted water in constructed wetlands. Front. Environ. Sci. Eng.; 2014; 10, pp. 262-269. [DOI: https://dx.doi.org/10.1007/s11783-014-0746-x]

15. Teuchies, J.; De, J.M.; Meire, P.; Blust, R.; Bervoets, L. Can acid volatile sulfides (AVS) influence metal concentrations in the macrophyte Myriophyllum aquaticum?. Environ. Sci. Technol; 2012; 46, 9129. [DOI: https://dx.doi.org/10.1021/es300816y] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/22853009]

16. Huang, J.C.; Passeport, E.; Terry, N. Development of a constructed wetland water treatment system for selenium removal: Use of mesocosms to evaluate design parameters. Environ. Sci. Technol.; 2012; 46, 12021. [DOI: https://dx.doi.org/10.1021/es301829r] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23057702]

17. Zhang, J.; Wu, H.M.; Hu, Z.; Liang, S.; Fan, J.L. Examination of oxygen release from plants in constructed wetlands in different stages of wetland plant life cycle. Environ. Sci. Pollut. Res.; 2014; 21, 9709. [DOI: https://dx.doi.org/10.1007/s11356-014-2905-9] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/24777322]

18. Lama, G.F.C.; Errico, A.; Pasquino, V.; Mirzaei, S.; Preti, F.; Chirico, G.B. Velocity uncertainty quantification based on Riparian vegetation indices in open channels colonized by Phragmites australis. J. Ecohydraulics; 2022; 7, pp. 71-76. [DOI: https://dx.doi.org/10.1080/24705357.2021.1938255]

19. Karam, F.; Nassif, N.; Mouneimne, A.H.; El Hachem, C.; Moussawi, L. Calibration and validation of MOPECO as sustainable water management tool within South Bekaa Irrigation Scheme in Lebanon. Proceedings of the AgroEcoInfo Symposium, Organized within the COST (European Cooperation in Science and Technology) Initiative; Volos, Greece, 30 June–2 July 2022; in press

20. Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements; FAO Irrig. and Drain. Pap., 56 Food and Agriculture Organization of the United Nations: Rome, Italy, 1998; 300p.

21. U.S. Environmental Protection Agency. Constructed Wetlands Treatment of Municipal Wastewaters; EPA/625/R-99/010 U.S. Environmental Protection Agency: Washington, DC, USA, 2000; Available online: www.epa.gov (accessed on 23 October 2022).

22. DES. Monitoring and Sampling Manual: Environmental Protection (Water) Policy; Department of Environment and Science Government: Brisbane, QL, Australia, 2018; 263p.

23. UN-HABITAT. Constructed Wetlands Manual. UN-HABITAT Water for Asian Cities Programme. Kathmandu, NP. 2008; Available online: www.unhabitat.org (accessed on 23 October 2022).

24. Sperling, M.V. Basic Principles of Wastewater Treatment; IWA Publishing: London, UK, 2007.

25. Singh, A.L. Nitrate and phosphate contamination in water and possible remedial measures. Environmental Problems and Plant; Essay Environmental Science, Department of Botany, Banaras Hindu University: Varanasi, India, 2013; pp. 44-51.

26. Hill, T.; Lewicki, P. STATISTICS: Methods and Applications; StatSoft: Tulsa, OK, USA, 2007.

27. StatSoft, Inc. Electronic Statistics Textbook; StatSoft: Tulsa, OK, USA, 2011; Available online: http://www.statsoft.com/textbook/ (accessed on 23 October 2022).

28. MoE. Ministry of Environment. Lebanese Standards for Wastewater Treatment and Release. Standards and Limits for Air Pollutants and Liquid Wastes Discharged by Classified Facilities and Waste Water Treatment Plants; Decision Issued by the Ministry of Environment: Decision 8/1 on 30 January 2001 The Ministry of Environment: Beirut, Lebanon, 2001.

29. FAO. Wastewater Reuse and Sludge Valorisation and Reuse. Proposition for Lebanese Wastewater Reuse Guidelines; Food and Agricultural Organization of the United Nations (FAO): Rome, Italy, 2010.

30. FAO. Coping with Water Scarcity—The Role of Agriculture Phase III: Strengthening National Capacities. Lebanon; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 2016.

31. Rosli, N.; Seca, G. Comparative Study of Water Quality at Different Peat Swamp Forest of Batang Igan, Sibu Sarawak. Am. J. Environ. Sci.; 2010; 6, pp. 416-421. [DOI: https://dx.doi.org/10.3844/ajessp.2010.416.421]

32. Mihir, P.; Nihar, S.R.; Pankaj, R.K.; Malabika, R.B. Electrical Conductivity of Lake Water as Environmental Monitoring—A Case Study of Rudrasagar Lake. IOSR J. Environ. Sci. Toxicol. Food Technol.; 2015; 9, pp. 66-71.

33. World Health Organisation (WHO). Guidelines for drinking-water quality, volume 2: Health criteria and other supporting information. Sci. Total Environ.; 2006; 61, 274. [DOI: https://dx.doi.org/10.1016/0048-9697(87)9038]

34. Lee, C.; Fletcher, T.D.; Sun, G. Nitrogen removal in constructed wetland systems. Eng. Life Sci.; 2009; 9, pp. 11-22. [DOI: https://dx.doi.org/10.1002/elsc.200800049]

35. Camargo, J.A.; Alonso, A. Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment. Environ. Int.; 2006; 32, pp. 831-849. [DOI: https://dx.doi.org/10.1016/j.envint.2006.05.002]

36. Kadlec, R.H.; Wallace, S. Treatment Wetlands; 2nd ed. CRC Press: Boca Raton, FL, USA, 2009; 1046.

37. Box, W.; Järvelä, J.; Västilä, K. Flow resistance of floodplain vegetation mixtures for modelling river flows. J. Hydrol.; 2021; 601, 126593. [DOI: https://dx.doi.org/10.1016/j.jhydrol.2021.126593]

38. Diaz, F.J.; O’geen, A.T.; Dahlgren, R.A. Agricultural pollutant removal by constructed wetlands: Implications for water management and design. Agric. Water Manag.; 2012; 104, pp. 171-183. [DOI: https://dx.doi.org/10.1016/j.agwat.2011.12.012]

39. Bezbaruah, A.N.; Zhang, T.C. Performance of a Constructed Wetland with a Sulfur/Limestone Denitrification Section for Wastewater Nitrogen Removal. Environ. Sci. Technol.; 2003; 37, pp. 1690-1697. [DOI: https://dx.doi.org/10.1021/es020912w] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/12731855]

40. Sierra-Alvarez, R.; Beristain-Cardoso, R.; Salazar, M.; Gómez, J.; Razo-Flores, E.; Field, J.A. Chemolithotrophic denitrification with elemental sulfur for groundwater treatment. Water Res.; 2007; 41, pp. 1253-1262. [DOI: https://dx.doi.org/10.1016/j.watres.2006.12.039] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/17296214]

41. Nelson, D.C.; Jørgensen, B.B.; Revsbech, N.P. Growth Pattern and Yield of a Chemoautotrophic Beggiatoa sp. in Oxygen-Sulfide Microgradients. Appl. Environ. Microbiol.; 1986; 52, pp. 225-233. [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/16347121][DOI: https://dx.doi.org/10.1128/aem.52.2.225-233.1986]

42. Sturman, P.J.; Stein, O.R.; Vymazal, J.; Kröpfelová, L. Sulfur Cycling in Constructed Wetlands. Wastewater Treatment, Plant Dynamics and Management in Constructed and Natural Wetlands; Springer: Dordrecht, The Netherlands, 2008; pp. 329-344. [DOI: https://dx.doi.org/10.1007/978-1-4020-8235-1_29]

43. Margane, A.; Steinel, A. Proposed National Standard for Treated Domestic Wastewater Reuse for Irrigation. German-Lebanese Technical Cooperation Project Protection of Jeita Spring (Project No. 2008.2162.9); Federal Republic of Germany, Federal Institute for Geosciences (BGR) and Council for Development and Reconstruction (CDR): Beirut, Lebanon, 2011; 42p.

44. Todd, M.J.; Vellidis, G.; Lowrance, R.R.; Pringle, C.M. High Sediment Oxygen Demand Within an Instream Swamp in Southern Georgia: Implications for Low Dissolved Oxygen Levels in Coastal Blackwater Streams1. JAWRA J. Am. Water Resour. Assoc.; 2009; 45, pp. 1493-1507. [DOI: https://dx.doi.org/10.1111/j.1752-1688.2009.00380.x]

45. Lu, S.; Zhang, X.; Wang, J.; Pei, L. Impacts of different media on constructed wetlands for rural household sewage treatment. J. Clean Prod.; 2016; 127, pp. 325-330. [DOI: https://dx.doi.org/10.1016/j.jclepro.2016.03.166]

46. Brix, H. Do macrophytes play a role in constructed treatment wetlands?. Water Sci. Technol.; 1997; 35, pp. 11-17. [DOI: https://dx.doi.org/10.2166/wst.1997.0154]

47. Karathanasis, A.D.; Potter, C.L.; Coyne, M.S. Vegetation effects on fecal bacteria, BOD, and suspended solid removal in constructed wetlands treating domestic wastewater. Ecol. Eng.; 2003; 20, pp. 157-169. [DOI: https://dx.doi.org/10.1016/S0925-8574(03)00011-9]

48. Abi Saab, M.T.; Jammoul, D.; Makhlouf, H.; Fahed, S.; Lebbous, N.; Hajjar, C.; Todorovic, M. Assessing the performance of constructed wetland for water quality management of a Southern Mediterranean River. Water Environ. J.; 2018; 32, pp. 508-518. [DOI: https://dx.doi.org/10.1111/wej.12348]

49. Amacha, N.; Karam, F.; Jerdi, M.; Frank, P.; Viala, E.; Hussein, D.; Baydoun, S. Assessment of the efficiency of a pilot constructed wetland on the remediation of water quality—Case Study of Litani River, Lebanon. Environ. Pollut. Clim. Chang.; 2017; 1, 119. [DOI: https://dx.doi.org/10.4172/2573-458X.1000119]

50. Nivala, J.; Murphy, C.; Freeman, A. Recent Advances in the Application, Design, and Operations & Maintenance of Aerated Treatment Wetlands. Water; 2020; 12, 1188. [DOI: https://dx.doi.org/10.3390/w12041188]

51. Samudro, G.; Mangkoedihardjo, S. Review on bod, cod and bod/cod ratio: A triangle zone for toxic, biodegradable and stable levels. Int. J. Acad. Res.; 2010; 2, pp. 235-239.

52. Karam, F.; Amacha, N.; Jalloul, H.; Haddad, R. Assessment of toxic ion removal by two wetland macrophytes (Phragmites australis and Sparganium erectum) in treating the polluted Litani River waters in Lebanon. Proceedings of the Mid-Year Research Webinar on Control4Reuse, 14 April, 2021; Control4Reuse, INRAe Mälardalen University: Västerås, Sweden, 2021; Available online: https://sites.google.com/view/control4reuse/news/webinar-14-april-2021 (accessed on 23 October 2022).