## 4 Discussion  
The physico-chemical characterization of the _P. radiata_ and _E. globulus_ leachates from the sawmill industry showed BOD\({}_{5}\) concentrations between 100-200 mg/L, COD concentrations between 500-1000 mg/L, and total phenolic compounds between 2000-15,000 mg/L. Moreover, low nutrients concentration was found (1 mg TN/L and 1-4 mg TP/L) and the pH was lower than 5.8 (Table 1). Slightly higher pollutant levels are reported by [PERSON] and [PERSON] [7] for organic matter (i.e., 300-2000 mg BOD\({}_{5}\)/L and 1500-10,000 mg COD/L), tanin and lignin (900-3000 mg/L), but lower phenols levels (1-30 mg/L) and pH (3-5). Besides that, low concentration of nutrients (i.e., 1-3 mg/L de TN and 3-4 mg/L de TP) is also reported for similar leachate sources in other studies [1; 7; 23; 24]. Figure 5: Variations of (**a**) effluent COD and (**b**) effluent total phenolic compounds concentration and respective removal rates in the planted and unplanted CW treating _E. globulus_ leachate. Table 2 and Figures 2-5 indicate that the performance of the CW units treating _P. radiata_ and _E. globulus_ leachates is similar. However, the optimum performance in the planted CW was found at the ORL of 13 g COD/m\({}^{2}\)/day for the _P. radiata_ leachate and at 25 g COD/m\({}^{2}\)/day for the _E. globulus_ leachate (Figures 2-5). Moreover, the performance of the planted CW decreased at OLR higher than 19 g COD/m\({}^{2}\)/day for the _P. radiata_ leachate and 40 g COD/m\({}^{2}\)/day for the _E. globulus_ leachate. Inhibition of the biological processes and their respective insufficient extent due to the high organic matter and specific compounds contents is one of the possible explanations for these results. Preliminary studies showed that resinic acids contained in _P. radiata_ leachate result in bacterial inhibition at 76.7 mg AbA/L for non-acclimated biomass [4; 24]. This study revealed higher removal efficiencies by 10-15% in the unplanted than the planted CW for organic matter and specific phenolic compounds. Though the role of plants in CW still remains a topic of discussion, it is generally believed, and several studies have shown, that the presence of plants contributes to the system's efficiency, with regards to organic matter and phenolic compounds [5; 14; 25; 26; 27]. It is reported that the rhizosphere of the plants contributes to the creation of an aerobic microenvironment (i.e., _P. australis_ transfers 5-12 g O\({}_{2}\)/m\({}^{2}\)/day through its roots) that stimulates the microorganism growth and, thus, the biodegradation of the organic matter and specific compounds contained in the influents [5; 14; 28; 29; 30]. However, other studies have shown significant differences (\(p<0.05\)) for COD and BOD\({}_{5}\) removal between planted and unplanted CW, reporting that organic matter removal can be up to 20% higher in unplanted compared to planted CW [1; 2; 31]. Another study reports higher COD and BOD\({}_{5}\) removal efficiencies of 50% and 60%, respectively, for an HRT of seven days in an unplanted CW treating wood waste leachate, while for long-chain fatty acids and tannin and lignin, the removal efficiencies were 69% and 42%, respectively [10]. No significant effect of plants is also elsewhere reported for various parameters [31; 32]. Most of the organic matter is due to the microbial mechanisms of carbon removal in subsurface flow wetlands [33]. Table 3 shows the aromatic compounds behavior in the effluents of CW units in ratio to effluent COD. This relationship between the aromatic compounds and COD is used as indicator of these recalcitrant compounds removal in comparison to total organic matter removal (COD), as it was proposed by [PERSON] et al. [12]. In the case of _P. radiata_ leachate, for the total operation period (OLR from 12 to 19 g COD/m\({}^{2}\)/day), there was a slight increase in the effluent UV\({}_{\rm 346 nm}\)/COD ratio (0.001-0.002), indicating that the lignocellulosic acids contained in the _P.