## 4 Discussion  
### CW Performance  
When testing the potential of CWs for the removal of persistent or emergent contaminants, one must assure that those contaminants do not affect the removal of conventional water parameters (e.g., organic matter and nutrients), which represents the main function of CWs. In the present study CWs microcosms maintained, in general, their function despite the presence of either a mixture of NP and OP or PCP. In fact, NP and OP showed a negative impact on COD removal in an initial period, but the system was able to recover. PCP on the other hand did not showed any effect. [PERSON] et al. (2019) also observed a decreased in COD removal when 4-clorophenol (4-CP) was present in the wastewater, also only in an initial stage after which the microbial community adapted to the presence of the contaminant. CWs with vertical flow system can achieve high COD removal due to the high oxygen transfer from macrophye roots into the rhizosphere through the wetland system ([PERSON] et al., 2019), which can be potentiated with water recirculation as done in the current study, as higher amount of oxygen can stimulated microorganisms of the system to degrade organic matter ([PERSON] et al., 2019). COD removals were variable but were similar to those reported in other studies (e.g., ([PERSON] et al., 2019; [PERSON] et al., 2019)), in which removals depended on influent COD concentrations. COD concentration in influent varied along each experiment and between experiments. In fact, the composition  
Figure 11: Relative abundance of the specialist genera of bacteria in plant roots bed substrate samples, (**a**) in control CWs and (**b**) in doped CWs collected at the end of the experiment (after 8 weeks). Systems irrigated for eight weeks with Ribeira de Joane water not doped (control) or doped with PCP. Only genera with abundance > 1% are shown. of Ribeira de Joane stream water varied along time, being also affected by some heavy rains which lead to a higher flow and a dilution effect. COD concentrations variation in CWs effluents can also be related with system performance, namely with other oxygen-consuming reactions such as nitrification of ammonia and/or plants life cycle [42]. The main mechanisms for nitrogen removal in CWs are plant uptake, mineralization, nitrification, denitrification, volatilization, adsorption and sedimentation [43]. Not all CWs system types can perform high nitrogen removal efficiency, for instance, VFCWs are advantage for nitrification [43]. In this study, accordingly, NH4\({}^{+}\) and NO2\({}^{-}\) removals were high, reaching values above 85% in both experiments, similar to other studies (e.g., [43]). Despite some removal variability, no relationship with influent concentrations was observed. On the other hand, removals of NO3\({}^{-}\) varied between experiments, being higher than 85% in experiment#1 and between 60 and 80% in experiment #2 with high variability. In the case of VFCWs, denitrification process can happen in anaerobic microsites and during the loading phase of the CWs substrate bed [44]. The process is also significantly affected by temperature, carbon source, loading rate as well as dissolved oxygen [44]. NO3\({}^{-}\) influent concentrations varied along time and were lower in experiment#2, along also with lower COD levels. Low carbon amount can restrict the process of denitrification [44]. This can explain the difference between experiments. In both experiments, the presence of target contaminants (NP plus OP and PCP) did not interfered in nitrogen removal, although other studies reported that some pollutants (e.g., PAHs) can inhibited the abundance and activity of nitriifying bacteria decreasing NH4\({}^{+}\) removal [45]. In the present study, PCP seemed to slightly promote NO3\({}^{-}\) removal in the initial period. PCP can be an extra carbon source, as observed in another study where PAHs promoted denitrification [45].