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This work investigates three laboratory-scale vertical flow-constructed wetlands (VFCWs) for treating a secondary effluent of wastewater under arid conditions to investigate the efficiency of two plants Canna indica and Typha latifolia in mono and mixed culture. The VFCWs were operated under hydraulic load (0.057 m3/m2d) and 5 days retention time. The results indicated no significant differences (P > 0.05) between the mono and mixed cultures. The C. indica gives the best efficiency of pollutant removal as COD (71.34%), NO2− (69.34%), and PO43− (69.67%). The uptake of TSS (83.98%) was best in the case of mixed culture. The mean percentages of BOD5 were convergent for mono and mixed culture, and it exceeds 89.80% in mixed culture. The mean percentages in NH4+ (98.69%) in mixed culture, elimination of NO2−, and the increase in the concentration of NO3− in the treated effluent showed the presence of nitrification in the VFCWs units. The two plant species exhibit high efficiency in the elimination of pollution compared to the unplanted control, with a slight superiority in the mixed culture. Therefore, it can be concluded that the application of these plants can be effective in arid conditions.

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  • Study of some plant species (comparison) in the field of water treatment in arid regions.

  • Comparison of monoculture and mixed.

  • Helps in the prevention of pollution.

  • Low-cost treatment solution for the rural areas.

  • Helps to optimize the design of the treatment units in arid regions.

Graphical Abstract

Graphical Abstract
Graphical Abstract
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Graphical Abstract
Graphical Abstract
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arid climate, Canna indica, mono and mixed culture, Typha latifolia, vertical flow
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Presently, in Algeria, a significant amount of wastewater from domestic use is mainly discharged directly into rivers without proper treatment and generates increasingly important bacterial and physicochemical pollution (Remini 2010). This wastewater contains concentrated pollutants, including a significant chemical oxygen demand (COD) concentration and nitrogen (N) content, and can impair the quality of aquatic environments and pose a risk to human health (Abdelhamid & Chocat 2004; Jacobs & van ‘t Klooster 2012). In addition to the lack of water in Algeria, the utilization rate of treated water and the number of wastewater treatment plants (WWTP) available are insufficient. This situation leads to thinking about finding alternative solutions to improve it (Benblidia & Thivet 2010). For this reason, it has become necessary to treat wastewater discharged into aquatic environments (Konnerup et al. 2009).

The use of conventional strategies for wastewater treatment has become a problem for developing countries due to the high cost, excessive use of energy, the need for a large area for this system, and the difficulty of adapting it to a few types of climate (Gedda et al. 2021). Constructed wetlands (CWs), which contain soil, sand, miniature life forms, and vegetation, are an innovative technique for purifying wastewater (Konnerup et al. 2009). In the 1950s, German scientists discovered that aquatic plants could reduce wastewater contamination. After that, many countries used various wetlands for this technology in wastewater treatment (Vymazal 2005). Today, the application of CWs for wastewater treatment not only reduces the costs of financing, maintenance, and energy consumption but also reduces environmental pollution and addresses the problems of scattered rural communities. In developing countries like Algeria, CWs have not taken their share as conventional treatment methods. Their use is very little compared to European countries and the United States of America (USA), despite the distinctive features of this technology, especially the economic aspect, which is a significant element in supporting development in these countries, and the aspect of the warm climate that helps with various types of wastewater and plays a fundamental role in many concepts of ecological (Hoffmann et al. 2011).

A vertical flow-constructed wetland (VFCW) is a planted filter bed. Wastewater is poured or dosed onto the surface. Treated water is collected in a drainage pipe at the bottom of the basin after vertically descending through the filter matrix. Vertical wetlands differ from other types in terms of vertical flow and air conditions within the medium (Kadlec & Wallace 2009).

The selection of plants employed in wastewater purification is one of the main factors in the efficiency of wastewater purification by CWs, as the plants utilized should have the option to endure the changeability and likely poisonous impacts of wastewater (Calheiros et al. 2007). The zone of energetic reaction of CWs is the root zone (or the rhizosphere). Plants, microorganisms, soil, and pollutants interact to induce physicochemical and biological processes (Brix et al. 2002). For CWs, research has primarily concentrated on design issues and guidelines. Understanding the essential components of wetlands is required to achieve very high efficiency in wastewater treatment. The effectiveness of various plant species, which includes the rhizosphere zone, must be known before creating wetlands on a large scale (Brix et al. 2002).

Different plant composition designs have been applied to VFCWs to test their effects on pollutant removal because nutrient absorption rates vary between plant species (Sirianuntapiboon & Jitvimolnimit 2007; Van de Moortel et al. 2010). In the same region, many studies have experimented on plants in monoculture (Amiri et al. 2019; Bebba et al. 2019; Yahiaoui et al. 2020). More research on these plant species is necessary to ascertain the effectiveness of plants in mixed cultures.

In this study, Canna indica and Typha latifolia plants were chosen because they appear to be the most tolerant to the expected arid climate conditions during the wastewater treatment period. This species is locally present, can grow in a wide range of conditions, and can withstand relatively high levels of contamination (Healy et al. 2007).

The main objective of this study is to verify the efficiency and evaluate the performance of the mentioned plant species in removing municipal wastewater contaminants under dry climatic conditions. Experiments were carried out using a pilot scale of VFCWs cultivated with the selected plant species in mono and mixed culture systems. Particular attention was paid to the evaluation of the studied CWs' capacities for the elimination of organic matter and nutrients from municipal wastewater.

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Site location

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The WWTP of Touggourt City, located in southeast Algeria (33° 16′ 00″ N and 6° 04′ 00″ E), was the site of a laboratory-scale sewage treatment system. The studied area has an arid climate. The average air temperature is between 3.4 °C in January and 41.6 °C in July. The annual average rainfall was approximately 60 mm, and evaporation was 2,458 mm/year (NMO 2019). The temperature (T) distribution inside and outside the wetland system is significant since the temperature difference between the facility and the surrounding area may affect the disposal of pollutants and all living organisms in CWs.

Laboratory-scale VFCWs units

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The vertical bed is dimensioned by considering a surface aeration coefficient of Ka = 30 g of O2 per m2 of surface per day (Cooper 1999). The area obtained is increased by 25%. The VFCWs were dimensioned by using the equation (Noorvee et al. 2005).
(1)
(2)
where Ka is the surface aeration coefficient, DO is the oxygen demand (in kg/d), CBOD5 is the concentration in biochemical oxygen demand (BOD5) in 5 days (in kg/m3), Q is the average flow of wastewater.
Four experimental VFCW systems, namely: system Unplanted (U0), kept to control the effectiveness of the filters; (U1) and (U2), one for each species (C. indica and T. latifolia); (U1+2), built for mixed culture. The pilot VFCW bed consisted of plastic tanks (64 l) with the following design features: a surface area of 0.35 m2 Equations (1) and (2) and a depth of 0.25 m. The substrate was a single layer of river gravel of 4–25 mm in diameter and 0.33% average porosity, washed and added to each container, resulting in a depth of about (h0 = 0.20 m). The design parameters of the VFCWs' are summarized in Table 1. The influent undergoes preliminary treatment within the WWTP before being directed to the primary sedimentation tank of 80 l, before being sent to the VFCWs for secondary treatment. For the conduction of the water, polyvinyl chloride (PVC) pipes of 30 mm in diameter were used, and for water, sampling valves were placed at the end of each unit. The general scheme of the constructed wetland system is shown in Figure 1.
Table 1

Design parameters of experimental VFCWs units

ParametersVFCWs
U(0)U(1)U(2)U(1+2)
Flow direction Vertical 
Units type Control Mono culture Mono culture Mixed culture 
Surface (m2) 0.35 0.35 0.35 0.35 
Depth (m) 0.25 0.25 0.25 0.25 
Plant species None C. indica T. latifolia C. indica and T. latifolia 
Depth, type and porosity of material 0.20 m, single layer river gravels, 4–25 mm, 33% 
Hydraulic loading rate (HLR) (m/day) 0.057 
Hydraulic retention time HRT (day) 5 
ParametersVFCWs
U(0)U(1)U(2)U(1+2)
Flow direction Vertical 
Units type Control Mono culture Mono culture Mixed culture 
Surface (m2) 0.35 0.35 0.35 0.35 
Depth (m) 0.25 0.25 0.25 0.25 
Plant species None C. indica T. latifolia C. indica and T. latifolia 
Depth, type and porosity of material 0.20 m, single layer river gravels, 4–25 mm, 33% 
Hydraulic loading rate (HLR) (m/day) 0.057 
Hydraulic retention time HRT (day) 5 
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Figure 1
(a) Schematic representation of VFCWs, and (b) picture of VFCWs system.
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(a) Schematic representation of VFCWs, and (b) picture of VFCWs system.

Figure 1
(a) Schematic representation of VFCWs, and (b) picture of VFCWs system.
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(a) Schematic representation of VFCWs, and (b) picture of VFCWs system.

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Plant material and acclimatization

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As for the vegetation cover, young plants of C. indica and T. latifolia were obtained from the wastewater garden (WWG) of Tamacine (33°01′ 19″ N, 6° 01′ 22″ E) province of Touggourt, Algeria, and planted at the beginning of December 2020. The plants were fixed in the bed at a density of 36 rhizomes per square meter (Kipasika et al. 2016; Bebba et al. 2019). The system was operated for one month to achieve acclimatization of the plant to wastewater. An irrigation plan began with 75% drinking water and 25% wastewater, increasing the percentage of wastewater in the mixture by 25% every week until it reached 100%. Water content was maintained for a total of 72 h in each constructed wetland. This measure was adopted to reduce the possible effects of stress on plants (Horn et al. 2014). During this period, daily monitoring was carried out to check the development of the plants. The arrangement of different plant species is illustrated in Figure 1.

Operational procedures

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The system started operation in December 2020, but the experiments described here were conducted from January to December 2021. The VFCWs operated with a hydraulic loading rate (HLR) of 0.057 m/day determined by Equation (3) (Campbell & Ogden 1999), and five days of retention time HRT determined by Equation (4) (Tousignant et al. 1999). The wastewater was collected from Touggourt WWTP. This sewage was tapped after the primary treatment and is collected in the storage tank before being sent to the VFCWs. The containers were fed from this tank to ensure gravity flow. The frequency of irrigation is weekly. Five sampling points were identified: one at the inlet to the storage tank and four at the outlet of each unit throughout the 12 months of monitoring for the following physical and chemical parameters: Water (T), potential hydrogen (pH), DO, electrical conductivity (EC), salinity, Total dissolved matter (TDS), Suspended solids (TSS), ammonium nitrogen (⁠⁠), Nitrite nitrogen (⁠⁠), Nitrate nitrogen (⁠⁠), Orthophosphate (⁠⁠), BOD5, and COD were analyzed.
(3)
where HRL is the hydraulic loading rate (m/day), V is the volume of water per day (m3/l), As is the area of the bed (m2).
(4)
where n is the effective porosity of media, L (m) is the length of the bed, W (m) is the width of the bed (m), h (m) is the average depth of liquid in the bed (m), V is the volume of the bed (m3), Q (m3/d) is the average flow through the bed.

Analytical methods

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The sampling of influent and effluent was carried out immediately for temperature, DO, pH, EC, salinity, and TDS using a portable multimeter instrument Model HI9829. TSS was measured according to the standard method for water and wastewater examination (NF T90-105) (AFNOR 1986). BOD5 was quantified by the 5-day BOD test with OxiTop head gas sensors (OxiTop ® WTW box). COD was measured using the dichromate method following ISO guideline 6060 (ISO 1989). was measured by the manual spectrometric following ISO guideline 7150 (ISO 1984a). was carried out by the method following ISO guideline 7150 (ISO 1984). was carried out by the method ISO guideline 6777 (ISO 1984b). was carried out by the method ISO guideline 6878 (ISO 2004).

Calculations and statistical analysis

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Simple ANOVA was used for all statistical analyses to determine significant statistical differences in the water treatment performance used by VFCWs. An ANOVA test was used and the level of statistical significance was set at p ≤ 0.05, with species types and culture types as factors. The analysis of variance (ANOVA) was performed using the (OriginLab software – 2018). The pollutant removal efficiency (RE) was calculated as below:
(5)
where RE is the removal efficiency (%), Cin and Cout are the influent and effluent concentrations (mg/l), respectively.
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Variations in physicochemical parameters and their details before and after treatment with VFCWs are discussed below.

Physicochemical characteristics of primary treated sewage

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The experimental pilot is fed with municipal wastewater after primary treatment to allow suspended solids to settle (Brix & Arias 2005; Munavalli et al. 2020) because VFCWs are weak in handling solids. The main characteristics of sewage influent were sample analyzed during the period of 12 months from January to December 2021 for the raw wastewater after preliminary treatment and primary sedimentation. Table 2 summarizes the characteristics of primary treated sewage.

Table 2

Initial characteristics of primary treated sewage influent, between January 2021 and December 2021. (Min, Max), and (average ± S.D mg/l), except temperature (°C) and EC (mS/cm), number of samples (n = 12)

Influent primary treated: Min, Max and (Avg ± SD)
ParametersMinMaxAverage ± SD
T 21.40 34.20 28.21 ± 4.76 
pH 7.31 7.89 7.52 ± 0.17 
EC 4.04 5.75 4.76 ± 0.45 
DO 0.09 0.79 0.37 ± 0.21 
Salinity 2.10 3.30 2.56 ± 0.32 
TDS 2,164.14 4,098.80 2,891.20 ± 547.04 
TSS 92 268 159.41 ± 53.07 
COD 114 373 232.76 ± 68.91 
DBO5 80 220 124.50 ± 38.85 
 18.60 46.40 29.70 ± 8.00 
 0.025 0.141 0.068 ± 0.033 
 0.161 0.936 0.440 ± 0.231 
 1.19 3.77 2.43 ± 0.649 
Influent primary treated: Min, Max and (Avg ± SD)
ParametersMinMaxAverage ± SD
T 21.40 34.20 28.21 ± 4.76 
pH 7.31 7.89 7.52 ± 0.17 
EC 4.04 5.75 4.76 ± 0.45 
DO 0.09 0.79 0.37 ± 0.21 
Salinity 2.10 3.30 2.56 ± 0.32 
TDS 2,164.14 4,098.80 2,891.20 ± 547.04 
TSS 92 268 159.41 ± 53.07 
COD 114 373 232.76 ± 68.91 
DBO5 80 220 124.50 ± 38.85 
 18.60 46.40 29.70 ± 8.00 
 0.025 0.141 0.068 ± 0.033 
 0.161 0.936 0.440 ± 0.231 
 1.19 3.77 2.43 ± 0.649 
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Stefanakis (2020) shows that CWs limit many physical and chemical pollutants associated with secondary treated wastewater. Table 3 shows the characteristics of the wastewater collected from each pilot unit's outflow, and Table 4 shows the pilot units' efficiency.

Table 3

Mean concentrations ± S.D (mg/l) in the effluent water of the VFCWs, between January 2021 and December 2021, except temperature (°C) and EC (mS/cm), number of samples (n = 12)

Effluent: (Avg ± SD)
ParametersInfluentU0 UnplantedU1 Mono C. indicaU2 Mono T. latifoliaU1+2 Mixed culture
T 28.21 ± 4.76 20.91 ± 6.55 20.90 ± 6.65 20.80 ± 6.48 20.88 ± 6.62 
pH 7.52 ± 0.17 7.59 ± 0.48 6.88 ± 0.24 6.91 ± 0.25 6.93 ± 0.22 
EC 4.76 ± 0.45 7.42 ± 1.51 9.45 ± 2.48 10.48 ± 2.99 10.83 ± 4.55 
DO 0.37 ± 0.21 2.54 ± 1.55 4.04 ± 1.18 4.14 ± 1.38 3.05 ± 1.13 
Salinity 2.56 ± 0.32 4.42 ± 0.84 5.80 ± 1.47 6.22 ± 1.47 6.35 ± 2.31 
TDS 2,891.20 ± 547 5,180.9 ± 864.9 6,523.1 ± 1,156.7 7,262.7 ± 1,493.3 7,451.7 ± 2,499.1 
TSS 159.41 ± 53.07 29.75 ± 13.92 23.25 ± 8.23 25.83 ± 12.43 22.25 ± 9.94 
COD 232.76 ± 68.91 73.55 ± 22.35 62.90 ± 21.92 69.60 ± 27.30 66.09 ± 24.99 
DBO5 124.50 ± 38.85 19.33 ± 13.64 14.50 ± 9.96 13.75 ± 7.39 12.33 ± 6.58 
 29.70 ± 8.00 14.97 ± 10.83 0.932 ± 1.419 0.615 ± 0.783 0.383 ± 0.472 
 0.068 ± 0.033 0.020 ± 0.014 0.017 ± 0.010 0.019 ± 0.012 0.018 ± 0.012 
 0.440 ± 0.231 0.749 ± 0.432 0.762 ± 0.344 0.850 ± 0.451 0.809 ± 0.479 
 2.43 ± 0.649 1.281 ± 0.595 0.742 ± 0.431 0.809 ± 0.536 0.762 ± 0.490 
Effluent: (Avg ± SD)
ParametersInfluentU0 UnplantedU1 Mono C. indicaU2 Mono T. latifoliaU1+2 Mixed culture
T 28.21 ± 4.76 20.91 ± 6.55 20.90 ± 6.65 20.80 ± 6.48 20.88 ± 6.62 
pH 7.52 ± 0.17 7.59 ± 0.48 6.88 ± 0.24 6.91 ± 0.25 6.93 ± 0.22 
EC 4.76 ± 0.45 7.42 ± 1.51 9.45 ± 2.48 10.48 ± 2.99 10.83 ± 4.55 
DO 0.37 ± 0.21 2.54 ± 1.55 4.04 ± 1.18 4.14 ± 1.38 3.05 ± 1.13 
Salinity 2.56 ± 0.32 4.42 ± 0.84 5.80 ± 1.47 6.22 ± 1.47 6.35 ± 2.31 
TDS 2,891.20 ± 547 5,180.9 ± 864.9 6,523.1 ± 1,156.7 7,262.7 ± 1,493.3 7,451.7 ± 2,499.1 
TSS 159.41 ± 53.07 29.75 ± 13.92 23.25 ± 8.23 25.83 ± 12.43 22.25 ± 9.94 
COD 232.76 ± 68.91 73.55 ± 22.35 62.90 ± 21.92 69.60 ± 27.30 66.09 ± 24.99 
DBO5 124.50 ± 38.85 19.33 ± 13.64 14.50 ± 9.96 13.75 ± 7.39 12.33 ± 6.58 
 29.70 ± 8.00 14.97 ± 10.83 0.932 ± 1.419 0.615 ± 0.783 0.383 ± 0.472 
 0.068 ± 0.033 0.020 ± 0.014 0.017 ± 0.010 0.019 ± 0.012 0.018 ± 0.012 
 0.440 ± 0.231 0.749 ± 0.432 0.762 ± 0.344 0.850 ± 0.451 0.809 ± 0.479 
 2.43 ± 0.649 1.281 ± 0.595 0.742 ± 0.431 0.809 ± 0.536 0.762 ± 0.490 
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Table 4

Removal efficiency (%) of pollution

Removal efficiency (%)
ParametersU0 Non-plantedU1 Mono C. indicaU2 Mono T. latifoliaU1+2 Mixed culture
TSS 79.57 83.54 82.36 83.98 
COD 66.56 71.34 68.11 69.62 
BDO5 84.03 87.96 88.31 89.80 
 49.43 96.57 97.93 98.69 
 68.16 69.34 67.97 69.22 
 −87.10 −97.46 −114.17 −170.20 
 47.50 69.67 68.03 69.23 
Removal efficiency (%)
ParametersU0 Non-plantedU1 Mono C. indicaU2 Mono T. latifoliaU1+2 Mixed culture
TSS 79.57 83.54 82.36 83.98 
COD 66.56 71.34 68.11 69.62 
BDO5 84.03 87.96 88.31 89.80 
 49.43 96.57 97.93 98.69 
 68.16 69.34 67.97 69.22 
 −87.10 −97.46 −114.17 −170.20 
 47.50 69.67 68.03 69.23 
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Turbidity and color

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Turbidity and color are very effectively removed by the wetland. The result can be seen with the naked eye also in Figure 2.
Figure 2
Turbidity and color observation.
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Turbidity and color observation.

Figure 2
Turbidity and color observation.
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Turbidity and color observation.

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Variation of T, pH, and DO

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Table 3 indicates the mean values of influent and effluent concentration profiles of environmental parameters (T, pH, and DO) across the influent wastewater and the experimental wetland units. These elements are very important for removal processes. The mean water temperature of primary treated sewage was (28.21 ± 4.76 °C), at the units U0, U1, U2, and U1+2, the mean temperature marks the values (20.91 ± 6.55 °C), (20.90 ± 6.65 °C), (20.80 ± 6.48 °C), and (20.88 ± 6.62 °C) respectively, as shown in Figure 3(a). There was no significant difference in water temperature between VFCWs with plants and the unplanted control and no observed difference between VFCWs planted with mono and mixed culture (p > 0.05). This can be very favorable for microbial activity and for the efficient removal of nutrients (Kadlec & Knight 1996; El Fanssi et al. 2019). Many wetland processes, such as plant growth, microbial organisms, and microbial-mediated reactions, are affected by temperature; optimal conditions are between 19 and 34 °C (Kadlec & Wallace 2009). On the other hand, the pH values of planted VFCWs U1 (6.88 ± 0.24), U2 (6.91 ± 0.25), and U1+2 (6.93 ± 0.22) were found to be lower than unplanted control (7.59 ± 0.48) and primary treated sewage (7.52 ± 0.17) as shown in Figure 3(b), the same results were recorded by Sharma & Sinha (2016). Also, there is no significant difference between the pH of the mono and mixed culture (p > 0.05). The pH reduction in effluents is due to nitrification that produces the H+ in VFCWs and CO2 accumulation due to plant metabolism or degradation of organic matter by heterotrophic bacteria. pH values (6.5–8.5) are optimal values for nitrogen transformations (Vymazal 2007). In addition, dissolved oxygen concentrations were low at the inlet and in all samples taken from primary treated sewage (<0.79 mg O2/l) and slightly higher at the outlet from unplanted control (2.54 ± 1.55 mg O2/l), Meanwhile, the average DO concentrations of planted cells U1, U2, and U1+2 were (4.04 ± 1.18 mg O2/l), (4.14 ± 1.38 mg O2/l), and (3.05 ± 1.13 mg O2/l) (see Figure 3(c)). DO was significantly lower in primary treated sewage than in effluent (p < 0.05). However, there is a significant difference in the effluent for the DO in the monoculture and the unplanted control (p < 0.05); we note in fact, an increase in the average values of oxygen concentrations of treated water compared to primary treated sewage. This increase can be explained by the conversion of plants to oxygen by the roots (Pérez et al. 2014) and by aeration of untreated water during its application in the vertical flow CW beds used in this study (Rehman et al. 2017).
Figure 3
(a) Water temperature, (b) pH, (c) DO, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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(a) Water temperature, (b) pH, (c) DO, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

Figure 3
(a) Water temperature, (b) pH, (c) DO, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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(a) Water temperature, (b) pH, (c) DO, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

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Variation of EC, salinity, and TDS

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Figure 4(a) shows the variation in the EC of the planted cells and the unplanted control. The EC ranges from 4.04 to 5.75 mS/cm in the primary treated sewage and (7.42 ± 1.51), (9.45 ± 2.48), (10.48 ± 2.99), and (10.83 ± 4.55) mS/cm at the units U0, U1, U2, and U1+2, respectively. The increase in the EC at the outlet was significant due to the evaporation of water as a result of the high temperature, plant transpiration (Wagner et al. 2021), and the accumulation of salts produced by the mineralization of organic matter in the planted filters (Guerrouf & Seghairi 2022), by the biological transformation of organic compounds into inorganic forms of nutrients, carbon dioxide, and CH4 gases, and simpler organic compounds (Bridgham et al. 2014). However, the EC of mixed cultures is higher than that of monocultures. On the other hand, there is no significant difference in the EC between cultured cells, which shows the influence of the plant culture on the EC in filters. The main factor in increasing EC is the density of the roots in the lower zone (Abissy & Mandi 1999). The salinity of the water at the inlet of the systems was (2.56 ± 0.32) mg/l. This salinity increased significantly at the outlet of the systems. It increased from (4.42 ± 0.84) mg/l for the U0 to (5.80 ± 1.47) mg/l, (6.22 ± 1.47) mg/l for U1, U2 and (6.35 ± 2.31) mg/l for U1+2, respectively (see Figure 4(b)). Based on these results, the saline load of treated wastewater from planted and unplanted systems increases compared to primary treated sewage. The conductivity is higher, which reflects high salinity due to extremely arid climatic conditions that cause very high evaporation, which concentrates the soil solution (Gouaidia et al. 2012). Visual observations of the plant's color, plant height, and leaf density show that C. indica and T. latifolia have adapted to increasing salinity. This proves that these plant species are suitable choices for CWs treating saline water in arid climates (Wagner et al. 2021). TDS values highlight the mineralization of wastewater by converting biomass to water, salts, minerals, and other dissolved materials. Solid carbon converts to carbon dioxide and water and produces methane through an aerobic or anaerobic decomposition process (Kumari & Chaudhary 2020). TDS are often related to conductivity, salinity, alkalinity, and hardness measures (Selvaraj & Joseph 2009). According to the results obtained (Figure 4(c)), the mean influent concentration of TDS was (2,893.53 ± 649.60) mg/l at the outputs of the VFCWs TDS values oscillate between (5,014.38 ± 680.06) mg/l and (7,324.66 ± 2,884.9) mg/l. The values recorded are generally high and show that this wastewater is rich in minerals. Higher TDS values indicate a relatively higher proportion of groundwater used for non-potable purposes than surface water (Munavalli et al. 2020). And the melting of suspended particles at elevated temperatures increases the concentration of both salinity and suspended matter due to evapotranspiration (Rhoades et al. 1999); we note that the values of dissolved solids in wastewater analyzed with irrigation water quality standards (JORA 2012) are unacceptable for crop irrigation (Fipps 2003).
Figure 4
(a) EC, (b) salinity, (c), and TDS, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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(a) EC, (b) salinity, (c), and TDS, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

Figure 4
(a) EC, (b) salinity, (c), and TDS, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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(a) EC, (b) salinity, (c), and TDS, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

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TSS removal

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The mean influent concentration of TSS was (156.41 ± 53.07) mg/l, which decreased significantly (p < 0.05) to mean concentrations of (29.75 ± 13.92) mg/l, (23.25 ± 08.23) mg/l, (25.85 ± 12.43) mg/l, and (22.25 ± 09.94) mg/l in U0, U1, U2, and U1+2, respectively (see Figure 5(a)). The U1, U2, and U1+2 systems had a mean TSS removal of 83.54, 82.36, and 83.98%, respectively. In U0, TSS removal efficiency was 79.57% (see Figure 5(a)), which confirms the importance of primary treatment in the disposal of suspended solids (Calheiros et al. 2009; Ávila et al. 2013). Moreover, according to the findings of Sirianuntapiboon & Jitvimolnimit (2007) and Marín-Muñiz et al. (2020), there are no appreciable differences between mono and mixed culture or between planted and non-planted systems (p > 0.05). In addition, the convergence of TSS removal values in unplanted and planted cells is due to interception, filtration, and decantation, which represent the processes of TSS removal (Ciria et al. 2005). Root zone treatments are considered the main step in the removal of suspended matter in VFCWs during the flow path. There is no influence of temperature changes on the removal of TSS (Kadlec & Wallace 2009; Avila et al. 2013).
Figure 5
Organic matter concentration and RE (%) of (a) TSS, (b) COD, and (c) BOD5, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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Organic matter concentration and RE (%) of (a) TSS, (b) COD, and (c) BOD5, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

Figure 5
Organic matter concentration and RE (%) of (a) TSS, (b) COD, and (c) BOD5, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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Organic matter concentration and RE (%) of (a) TSS, (b) COD, and (c) BOD5, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

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COD removal

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The COD concentration of the influent ranged from 114.00 to 373.00 mg/l with an average value of (232.76 ± 68.91). According to Figure 5(b), the average effluent concentrations for the U0, U1, U2, and U1+2 were (73.55 ± 22.35) mg/l, (62.90 ± 21.92) mg/l, (69.60 ± 27.30) mg/l, and (66.09 ± 24.98) mg/l, respectively. Results obtained from VFCWs operation showed high levels of COD removal in both planted and unplanted cells (Table 4). The average removal efficiencies of COD for the U0, U1, U2, and U1+2 were 66.65, 71.34, 68.11, and 69.62%, respectively (Figure 5(b)). The mean percentages of COD reduction were largely convergent. This result agrees with AL-Rekabi & AL-Khafaji (2021), by VFCW systems, planted by Cyperus Alternifolius and Aquatic Canna in Basrah City in Iraq. The COD removal efficiency was not significantly different (p > 0.05) between the planted cells (Ui) and the unplanted control (U0) and the type of culture (mono and mixed). The same results were recorded by Qiu et al. (2011) and Perdana et al. (2018). Deposited organic matter is rapidly removed by sedimentation and filtration in the unplanted control, which is responsible for a higher elimination of COD than biodegradability, while organic compounds are degraded to aerobic and anaerobic by heterogeneous microorganisms as a function of oxygen concentration in the planted filters (Aslam et al. 2007). It is obvious that with the increase in oxygen, the elimination efficiency of COD was gradually promoted through the supply of oxygen (Vymazal & Kröpfelová 2009; Tanveer & Sun 2012). In addition, the uptake of organic substances by plants is less, so one should not rely too much on the presence or absence of plant species in wetlands built to remove organic matter. In our experience, treatment with the single species C. indica in monoculture was evaluated with the best yield to reduce COD values. This is due to the availability of oxygen provided by C. indica in the root zone and microbial decomposition, which plays a key role in the degradation of COD (Xu & Cui 2019).

BOD removal

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Higher reductions have been observed for BOD5. Mean concentration of the BOD5 in influent was (124.50 ± 38.85) mg/l, and the average BOD5 effluent concentrations for the U0, U1, U2, and U1+2 were (19.33 ± 13.64) mg/l, (14.50 ± 09.96) mg/l, (13.75 ± 07.39) mg/l, and (12.33 ± 06.58) mg/l, respectively (Table 4 and Figure 5(c)). This may be due to the effect of plants that mimic natural treatment processes involving wetland vegetation, soils, and their associated microbial assemblages to improve water quality. In addition, high aeration provided by the aerenchyma cells of C. indica and T. latifolia roots may be the main reason for the high rate of BOD5 elimination (Vymazal et al. 1998). The results showed that there was no significant difference (p > 0.05) in the removal capacity of BOD5 between the planted cells (Ui) and the unplanted control U0 and the type of culture (mono and mixed culture). Treatment concentrations and efficacy averages are presented in Figure 5(c). The overall removal efficiency in order of performance was (Mixed culture, 89.80%) > (T. latifolia, 88.31%) > (C. indica, 87.96%) > (Unplanted, 84.03%). The mixed culture had a slightly higher removal rate compared to the monoculture. Results were recorded by Qiu et al. (2011).

Nitrogen removal

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As shown in Figure 6(a), all three treatment cells (U1, U2, and U1+2) were highly effective in removing ⁠. The mixed culture achieved a mean removal efficiency of 98.69%, which was its highest level. Within the U1 and U2 units planted by C. indica and T. latifolia, the average elimination efficiency reached 96.57 and 97.93%, respectively. Removal of planted VFCWs was significantly different from the unplanted control (p < 0.05), which amounted to 9.43%. These findings demonstrate that C. indica and T. latifolia are the plants that facilitate ammonia elimination by their absorption in tissues (Zhang et al. 2007), because they had better growth of root volume (Marín-Muñiz et al. 2020), and a high rate of nitrification (Abdelhakeem et al. 2016). It was also found that the plant species and type of culture slightly influenced the uptake efficiency of (p < 0.05). The obtained results show the occurrence of nitrification in the planted VFCWs, which is confirmed by the decrease in and by the elimination of in units: (U0, 68.16%), (U1, 69.34%), (U2, 67.97%), and (U1+2, 69.22%) (Figure 6(a) and 6(b)), and by the increase in the concentration in the treated effluent from 0.44 to 0.75 mg/l, 0.76, 0.85, and 0.81 mg/l in the U0, U1, U2, and U1+2, respectively (see Figure 6(c)). This finding agrees with the study of Zhang et al. (2007). The results showed that plant roots have a bacterial activity suitable for nitrification and denitrification of nitrogen (Calheiros et al. 2009). The average effluent temperature was between 20.80 °C and 20.90 °C for VFCWs, this range of temperature was suitable for nitrogen form removal (Huang et al. 2013). And the pH values of planted VFCWs ranged from 6.88 to 6.93. Plants in wetlands play an important role in controlling pH and temperature, which in turn leads to increased nitrification in vertical flow wetlands (Lu et al. 2009).
Figure 6
Nutrients concentration and RE (%), (a) , (b) , and (c) , in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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Nutrients concentration and RE (%), (a) ⁠, (b) ⁠, and (c) ⁠, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

Figure 6
Nutrients concentration and RE (%), (a) , (b) , and (c) , in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.
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Nutrients concentration and RE (%), (a) ⁠, (b) ⁠, and (c) ⁠, in the influent, unplanted control, and the VFCWs planted with C. indica, T. latifolia, and mixed culture.

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Phosphorous removal

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The variation of orthophosphate was depicted in Figure 7. A decrease of concentration from (2.20 ± 0.67) mg/l for the inlet to (1.27 ± 0.61) mg/l, (0.62 ± 0.43) mg/l, (0.66 ± 0.46) mg/l, and (0.68 ± 0.45) mg/l for U0, U1, U2, and U1+2, respectively. The purification yield in the cultivated basins U1, U2, and U1+2 was 69.67, 69.23, and 68.03%, respectively, and 47.50% in the unplanted basin U0 (Figure 7). Statistical analysis showed that there is a significant difference between the planted VFCWs and unplanted control, while there is no difference between the mono and mixed culture (p > 0.05) results confirmed by Zhang et al. (2007). The decrease of in the treated water for all units is caused by the uptake by gravel in the filter for the unplanted cell (Molle 2003; Sim 2003), and by the interaction of bacteria and plants (Quan et al. 2016), but it is a very small amount (Lantzke et al. 1998).
Figure 7
Phosphorous concentration , and RE (%).
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Phosphorous concentration ⁠, and RE (%).

Figure 7
Phosphorous concentration , and RE (%).
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Phosphorous concentration ⁠, and RE (%).

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Comparison between VFCW systems

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VFCW planted with mixed culture showed better removal efficiency than monoculture in terms of TSS, DBO5, and ⁠, and to a lesser extent COD, ⁠, and (Table 5), while it has not been very effective in removing ⁠. Thus, mixed culture can be considered a sustainable alternative to the secondary treatment of domestic wastewater in the climate conditions of the site.

Table 5

Comparison between VFCW systems

 
 
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The objective of this work was to study the application of different plant species, namely C. indica and T. latifolia, in VFCWs receiving municipal wastewater after primary treatment under an arid climate with mono and mixed culture. The two plant species tested all grew well and their presence significantly improved the removal of pollutants in VFCWs and showed tolerance to primary treated municipal wastewater. Plant diversity in mixed culture was important for TSS, BOD5, and elimination, while elimination of other parameters such as COD, and were very close in mono and mixed culture. However, treatment with the mixed species C. indica and T. latifolia in mixed cultures was assessed to have the best removal performance of pollution parameters. In addition, both plants showed great adaptability to arid climatic conditions and high salinity.

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The authors would like to thank all the managers of the Touggourt STP for allowing the author members to use the water of the WWTP initially treated first, as well as the use of all the devices in this research.

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

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The authors declare there is no conflict.

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