**4. Discussion**

In this study, pH, EC, TDS, TSS, COD, BOD and color of the dye-enriched water and heavy metals contents were significantly decreased in the vegetated and vegetated-inoculated floating treatment wetlands. The reductions in pollutants load in treated dye-contaminated water emphasize the prominent role of vegetation and bacteria in floating wetlands.

The pH might be decreased due to the release of organic acids by the roots of the plants as reported in earlier studies [31,41]. The decrease in EC might be associated with the uptake of nutrients by plants and the biological and physicochemical binding of pollutants to roots and soil particles [13,36]. The pH and EC reduction was highest in treatment vegetated with plants and augmented with bacteria. This suggests the key role of plants and bacteria in pH and EC reduction through the release of organic acids and the uptake of nutrients by plants and bacteria [25,34,35]. The TDS and TSS loads were reduced due to the combination of physical and biological processes supported by floating wetlands [41]. The suspended particles in the water are trapped in the biofilm of the roots of macrophytes, and there they either precipitate at the bottom or adsorb on biofilm where they might be degraded [42]. Physical entrapment in roots, sorption and settlement at the bottom might contribute to the removal of TDS and TSS from treated water [16,19,43]. Further, the roots of plants act as physical filters and provide appropriate organic matter that acts as a bio-sorbent and contributed to the removal of particulate matter [11,21].

Roots allow microbial communities to assimilate carbon compounds and reduce BOD and COD [44]. In this study, the high removal of BOD in wetland systems might be attributed to the deposition and filtration of organic compounds that can be settled. The speedy and high removal rate in bacterial augmented FTWs could be attributed to the biofilm on roots, which contributes to the removal of organic matter by decomposing it into simple nutrients, thus aiding in the direct uptake by the plant [20,45]. Uptake by plants' roots is an important method of nutrient removal [42]. The nutrients in the wastewater might be taken up by the roots of the plants. There, they can either accumulate in the plant biomass or be degraded by endophytic bacteria present inside the plants [25,46]. The similar findings have been reported by earlier studies, where plants and bacterial combinations enhanced the removal of organic pollutants from highly polluted wastewater [31,47,48].

Color was also removed to a grea<sup>t</sup> extent in this study by the vegetated treatment and the vegetated-inoculated treatment. It has been well reported that COD, a measure of oxidizable contaminants, has a positive correlation with color in textile wastewater [11]. Correspondingly, in this study color was reduced with the reduction in COD. However, the rate of decolorization was high in vegetated-inoculated floating wetlands. This could be associated with the combined action of plant and bacteria in the degradation of dyes and removal of color [11]. This emphasized the key role of bacteria in the decolorization of dye from textile effluent. The previous studies also showed that many bacteria are helpful in the removal of dyes, and that bacteria have the ability to degrade dyes by aerobic as well as anaerobic mechanisms [11,49].

In this study, the concentration of six heavy metals (Cr, Ni, Zn, Fe, Pb and Mn) was decreased significantly in the treated dye-containing wastewater. The unique potential of *P. australis* to remove heavy metals has been reported by many researchers [25,34]. In the previous studies, *P. australis* showed similar pattern of removal of heavy metals from industrial e ffluent [11,28,36]. These previous studies also demonstrated that the heavy metals from wastewater were taken up by the *P. australis* in its roots and shoots [41,50,51]. The maximum concentrations of heavy metals were found in the roots of the plant, meaning that the root has most potential to uptake heavy metals [50].

In the case of inoculation of *P. australis* with bacteria, the heavy metal removal capacity was further enhanced. The improved performance of bacterially augmented FTWs emphasized the role of bacteria in the removal of heavy metals from polluted water. The inoculated bacteria reduced the metals load in polluted water by their bioaccumulation potential [31]. These bacteria might contribute to reducing metal-induced toxicity and increase the bioavailability and metals uptake of plants [27]. It is well reported that in FTWs the inoculated bacteria may boost the metals removal process by entrapment of metals in root biofilms, sorbing of metallic ions on the bacterial cell wall and oxidation of metal ions [52,53]. Further, the plaque formation by the combined action of plant and bacteria on plant roots may increase the Fe, Mn, Cu and Zn binding in roots biofilms [13,54]. This emphasizes that *P.australis* and inoculated bacterial combined role, which contributes to metals removal from treated dye-contaminated wastewater. The significantly substantial removal of metals from bacterial inoculated treatments relative to non-inoculated vegetated treatments could be attributed to a high population of bacteria in the inoculated treatment.

The inoculated bacteria showed persistence in polluted water being treated by floating treatment wetlands. The periodic analysis of water from all three treatments showed the high population of bacteria in inoculated treatments as compared to the non-inoculated treatments. The higher population of bacteria in the water of inoculated treatments confirmed that inoculated bacteria showed persistence and were responsible for dye removal and pollutant removal. This could be due to the fact that the inoculated bacteria successfully made mutualistic relationships with plants, which supported the survival of inoculated bacterial [55]. This finding is consistent with previous studies in which inoculated bacteria improved the pollutant removal process [24,34]. The survival of inoculated bacteria depends upon the nutrient supply, pH, temperature and the interaction with the host [56,57]. In this study, the bacterial population in the roots and shoots of inoculated plants were found to be higher as compared to non-inoculated vegetated treatment. This could be due to the preferential survival of bacteria in roots and shoots of *P. australis* in inoculated treatments, as reported in previous studies [28,58]. Further, these bacteria were initially isolated from the roots and shoots of the plants; hence these bacteria possibly have an adaptive mechanism to survive and grow in these parts of the plant in this hostile environment [18,27]. In order to make FTWs a potential wastewater treatment method, periodic inoculation of bacteria should be performed in order to overcome the problem of decreasing bacteria with time in inoculated water [57,59].

Toxic pollutants in the environment inhibit plant growth [27]. Dyes containing toxic chemicals and potentially toxic heavy metals also inhibit plant growth [28]. In this study, the *P. australis*, synergistic with bacteria, achieved high root and shoot growth as compared to plants without inoculation. The control tank having only water and plants with no added dye showed maximum growth of roots and shots of plants due to the absence of any toxic pollutant. The bacteria present in the system can promote plant growth by decreasing biotic and abiotic stress [60]. Bacteria also positively affect plant growth by releasing phyto-hormones and by the solubilisation of essential nutrients [61]. Pollutant-degrading rhizospheric and endophytic bacteria have been proven as e ffective to enhance plant growth development and phytoremediation e fficacy [53]. Similar results have been reported by previous studies where inoculated bacteria promoted plant growth by alleviating pollutant-induced toxicity and improved plant nutrition, health and disease resistance [34,62].
