## 3 Results and Discussion
### Phenanthrene Degradation Efficiency of the FTWs and CWs  
The comparative efficiency of the FTWs and CWs for the biotereatment of phenanthrene-contaminated water was assessed by analyzing the treated water for different physico-chemical parameters. There was a significantly higher reduction in the COD, BOD, and phenanthrene concentration in the water treated by the FTWs and CWs when compared with the controls (C1 and C1*) (Figures 5-7). There was an 81% reduction in the COD and a 78.5% reduction in the BOD of the water treated by the CWs (T1); there was a 79.7% reduction in the COD and a 76% reduction in the BOD of the water treated by the FTWs (T1*). The treatments that contained only bacterial strains (T2 and T2*) also showed a higher reduction in the water quality parameters (COD and BOD) when compared with the controls (C1 and C1*), but their efficiency was lower than that observed in the water treated by FTWs and CWs augmented with the bacterial consortium. The performance of the CWs was greater (non-significant) than the FTWs in the occurrence and absence of bacterial augmentations. In this study, the decrease in the BOD, COD, and phenanthrene concentration in the water treated by CWs (T1 and T3) was greater (non-significant) than that of the water treated by the FTWs (T1* and T3*). Approximately, a 92% reduction in the COD, an 89% reduction in the BOD, and a 93% reduction in the phenanthrene concentration were observed in the CWs augmented with the bacterial strains (T3); similarly, a 91% reduction in the COD, an 88% reduction in the BOD, and a 92% reduction in the phenanthrene concentration were observed in the FTWs augmented with the bacterial strains (T3*) (Figures 6 and 7). In an earlier study, a 99.9% removal of phenanthrene was observed in water treated by a vertical-flow CW [10]. In another study, a comparatively smaller (85.5%) removal of phenanthrene was observed in CWs than was observed in this study [31]. Similarly, in another study, a comparatively smaller (79.91%) removal of phenanthrene was observed [32] than was obtained in this study. Usually, plants possess natural mechanisms to detoxify pollutants present in their surrounding environment [4, 7, 20, 27, 28, 33, 34]. Plants degrade complex organic compounds by processes known as phytodegradation [3, 35, 36, 37]. This could be the basis that even the unaided vegetation considerably reduced the phenanthrene concentration and other pollution parameters in the polluted water. _Phragmites australis_ allowed the bacterial strains to proliferate in the endo- and rhizosphere, which generally upsurged the plant health and treatment proficiency of the wetland systems [38]. Figure 4: Hydrocarbon (phenanthrene) removal by nine different bacterial strains. Figure 5: COD reduction in CW and FTW vegetated with _Phragmites australis_. Figure 6: BOD reduction in CWs and FTWs vegetated with _Phragmites australis_. In this study, the bacterial species inoculated in the FTWs and CWs possessed the genes responsible for hydrocarbon degradation and PGP activities, which helped the plants to improve their growth in the presence of contaminants [26]. The decrease in the COD and BOD could be associated with enzymatic activities of the bacteria that take part in the breakdown and transformation of organics into simpler substances that are used by the plants as nutrients [39]. Both parameters are vital water quality indicators, and their decline is an indication of the cleaning of contaminated water. Lastly, the gravel and sand in the CWs also acted as a filter and reduced the pollutants from the water through absorption and adsorption [28,40,41], which might also have contributed to the treatment process. In this study, apart from a decrease in the aforementioned parameters, a similar trend of reduction was found in the other tested parameters; i.e., the pH and EC (Table 1). The decrease in pH showed that the denitrification and nitrification processes occurred well within the treatment system [33].