3.1. Plant growth response and wastewater treatment in CWs
3.1.1. Plant growth response
The mean of above-ground biomass and stump diameter of plant species produced onto the CWs are denoted in table 2. Overall, above-ground biomass of the plants ranged from 1.8 ± 0.7 to 15.2 ± 0.7 kg.m-2, while their stump diameters varied from 8.6 ± 3.6 to 21.3 ± 9.5 cm. Indeed, above-ground biomasses were classified in this order following the plants: P. purpureum (15.2 kg.m-2) > T. laxum (13.1 kg.m-2) > E. pyramidalis (8.3 kg.m-2) > A. gayanus (2.6 kg.m-2) > C. zizanioides (1.8 kg.m-2). However, Mann Whitney test (p < 0.05) showed that the biomasses developed by E. pyramidalis, P. purpureum and T. laxum were significantly higher than those of A. gayanus and C. zizanioides. Unlike above-ground biomasses, the sequence mean values of diameter stumps was in this order: P. purpureum (21.3 cm) > T. laxum (16.4 cm) > E. pyramidalis (12.1 cm) > C. zizanioides (10.7 cm) > A. gayanus (8.7 cm). However, no significant difference observed between the diameters of the different plant stumps (ANOVA test: p > 0.05). Although the stump diameter of C zizanioides was higher than that of A gayanus, it developed the lowest above-ground biomass.
3.1.2. Wastewater treatment in CWs
22.214.171.124. Assessment of physical parameters
The mean values of pH, dissolver oxygen (DO) and water volume at the inlet and outlet of all the beds (planted and unplanted) are denoted in table 3. As for pH, values obtained in the bed outlets were higher than those of the raw water. Moreover, the average pH values of the planted bed exits (between 6.92 and 7.17) were slightly lower than those of the unplanted bed (7.33). However, the sequence of pH mean values were between raw water and unplanted bed was ranked as decreasing in this order: wastewater pH (6.81) < pH (A. gayanus) (6.92) < pH (E. pyramidalis) (6.93) < pH (C. zizanioides) (7.05) < pH (P. purpureum) (7.06) < pH (T. laxum) (7.17) < pH (unplanted) (7.32). Besides, significant differences were observed between wastewater pH and those of planted beds, as well as those of the different beds between them (Mann Whitney test: p < 0.05).
DO values measured outlet the beds (i.e., 5.41 ± 0.9 and 7.53 ± 1.6 mg.L-1) were high compared with that of raw water (inlet) (i.e., 2.13 ± 0.6 mg.L-1), whereas those of the planted beds were greater than those of the unplanted bed. However, some signiﬁcant differences were noted among those of the planted beds (Mann Whitney test: P < 0.05).
As for the water volume collected at the outlet of the beds, this remain less than that of wastewater applied (80 liters). Indeed, the average water volume collected in the unplanted bed (72.4 ± 1.9 L) was the highest and was followed by those of the different beds planted with C. zizanioides (62.2 ± 3.6 L), A. gayanus (60.3 ± 3 L), E. pyramidalis (58.6 ± 5.8 L), T. laxum (55.6 ± 3.8 L) and P. purpureum (54.2 ± 4.3 L). Using ANOVA (p < 0.05), we observed significant difference between the unplanted bed and the planted beds, while those of the beds planted with P purpureum and T. laxum were of the same order of magnitude and significantly lower than water volume collected at the outlet of the other planted beds.
126.96.36.199. Removal efficiencies for chemical parameters
Table 4 showed inlet and outlet mean concentrations as well as removal efficiencies of CWs for different parameters such as COD, BOD5, TN, NH4+, NO3- and PO43-. In fact during the experimental tests, the concentrations of these pollutants in synthetic wastewater varied of 535-623.3 mgO2.L-1 (COD), 356.1-373.8 mgO2.L-1 (BOD5), 37.6-44.5 mg.L-1 (TN), 31-35 mg.L-1 (NH4+), 1.1-1.97 mg.L-1 (NO3-) and 6.8-7.9 mg.L-1 (PO43-), with average values matching to 611.8 mgO2.L-1, 369.8 mgO2.L-1, 41.4 mg.L-1, 33.4 mg.L-1, 1.8 mg.L-1 and 7.4 mg.L-1, respectively. After treatment, the average removal efficiencies within CWs ranked from 90.9 to 95.9% for COD, 95.2 to 98.5% for BOD5, 74.3 to 84% for TN, 76 to 84% for NH4+, 77.4 to 96.9% for PO43, regardless of the planted beds. Otherwise, the average removal efficiencies varied from 53.3 to 89.3% within unplanted beds, regardless of the parameter. We noted that the presence of the plants onto the CWs improved its yields from 5.9 to 24.1% whatever the pollutant. It is worthy that some negative values were obtained for NO3- in the case of A. gayanus, C. zizanioides and E. pyramidalis beds. Thus, the CWs removal efficiencies were ranked following order of performance: (P. purpureum) > (T. laxum) > (E. pyramidalis) > (A. gayanus) > (C. zizanioides) > (unplanted). However, the planted beds were significantly efficient than the unplanted beds (Mann Whitney test: p < 0.05). Moreover, the beds with P. purpureum and T. laxum were more significantly efficient than those of E. pyramidalis, A. gayanus and C. zizanioides (Kruskal-Wallis test, p ˂ 0.05).
188.8.131.52. Estimation of bacterial density in CWs
Aerobic and anaerobic bacteria densities were assessed in the beds’ sediments (Figure 2). Overall, aerobic bacteria density was higher than that of anaerobic bacteria in all the CWs beds, excluding unplanted bed. Regarding to aerobic bacteria (Figure. 2A), the median density was ranked in this order according to plants: P. purpureum (7.7 106 CFU.g-1) > T. laxum (6.4 106 CFU.g-1) > E. pyramidalis (4.8 106 CFU.g-1) > A. gayanus (4.5 106 CFU.g-1) > C. zizanioides (2.4 106 CFU.g-1) > Unplanted bed (1.4 106 CFU.g-1). Indeed, a significant difference was observed between the bed planted with C. zizanioides and the other beds showing no difference between them (Mann Whitney test: p < 0.05).
For anaerobic bacteria in the bed sediment (Figure 2B), no difference was observed between all beds (Mann Whitney test: p > 0.05), regardless the planted bed or not. Thus, anaerobic bacteria number ranged generally from 0.3 106 to 2 106 CFU.g-1. Meanwhile, the sequence of their median densities in the beds corresponded to this order with a slightly high number in the control: 0.7 106 CFU.g-1 (P. purpureum) < 0.8 106 CFU.g-1 (T. laxum) < 0.8 106 CFU.g-1 (E. pyramidalis) < 0.9 106 CFU.g-1 (A. gayanus) < 1.1 106 CFU.g-1 (C. zizanioides) < 1.3 106 CFU.g-1 (Unplanted).
To better insights the biological activity in the beds, the total bacteria density was assessed during the treatment trial (Figure 3). In fact, the total bacteria density oscillated between 2.9 106 and 12.3 106 CFU.g-1 whatever the bed, while the median density of the bed planted with P. purpureum was obviously the highest and the control the lowest as already demonstrated above (i.e., 8.4 106 CFU.g-1 (P. purpureum) > 7.1 106 CFU.g-1 (T. laxum) > 5.6 106 CFU.g-1 (E. pyramidalis) > 5.4 106 CFU.g-1 (A. gayanus) > 3.5 106 CFU.g-1 (C. zizanioides) > 2.4 106 CFU.g-1 (Control)). Finally, the total bacteria density of the planted beds was significantly higher than that obtained in the control, whereas among the planted beds, the total bacteria density of C. zizanioides bed remained significantly lower than the other's (Mann Whitney test: p < 0.05).
To better understand the bacteria evolution within all the bed sediments, the vertical distribution of aerobic, anaerobic and total bacteria densities in the different sediment layers was investigated. Indeed, the figures 4, 5 and 6 showed the aerobic, anaerobic and total bacteria densities distributions, respectively. Overall, from the upper layer [0; 10 cm] to the bottom layer [50; 60 cm] of the beds, the number of aerobic bacteria decreased (17.4 106 to 0.1 106 CFU.g-1) (Figure 4), while that of anaerobic bacteria increased (0.1 106 to 2.1 106 CFU.g-1) (Figure 5). However, the total bacteria density decreased from upper surface towards the bottom of the beds (1.5 106 to 17.4 106 CFU.g-1) (Figure 6). Aerobic bacteria density of beds surface layers ([0; 10 cm] and [10; 20 cm]) were statistically greater than those of deep layers ([40; 50 cm] and [50; 60 cm]) in planted beds, while for those of the first three (3) layers ([0; 10 cm], [10; 20 cm] and [20; 30 cm]), the bacteria densities of the planted beds were significantly higher than those of the unplanted bed. Among the planted beds, the bacteria density of C zizanioides was lower than those of the other beds, excluding that of A gayanus's bed in the [20; 30 cm] layer (Mann Whitney test: p < 0.05).
Unlike the aerobic bacteria, statistical analysis showed that in all beds, the densities of the anaerobic bacteria of the two first upper layers ([0; 10 cm] and [10; 20 cm]) were lower than those obtained in the two last layers ([40; 50 cm] and [50; 60 cm]) (Mann Whitney test: p < 0.05). Likewise, the anaerobic bacteria densities in unplanted beds were not clearly distinguished in the different consecutive layers. However, no difference was obtained between the different planted and unplanted beds (Kruskal Wallis test: p > 0.05).
Like the aerobic bacteria case, the total bacteria density in the beds was significantly greater in the upper horizons ([0; 10 cm] and [10; 20 cm]) than those of the deep layers ([40; 50 cm] and [50; 60 cm]) (Mann Whitney test: p < 0.05). In planted beds, total bacteria density in bed planted with C. zizanioides was also lower, whereas those of other beds do not differ significantly. On the other hand, the total bacteria densities in the different planted beds were significantly higher than those obtained within the control in the first two upper layers ([0; 10 cm] and [10; 20 cm]). However, the sequence of the total bacteria densities was more important in the bed planted with P. purpureum, and followed by those of T. laxum, E. pyramidalis, A. gayanus, C. zizanioides, and unplanted bed.