## 4 Discussion
radiata_ leachate were removed to a lesser extent than other organic material when COD load was increased. Similarly, the UV\({}_{\rm 280 nm}\)/COD ratio shows that the CW units were able to mineralize the lignin compounds as much as organic matter, measured as COD, i.e., the UV\({}_{\rm 280 nm}\)/COD ratios for the influent and effluent were 0.003 and 0.005, respectively. Additionally, there was an increase in the UV\({}_{\rm 254 nm}\) parameter, which correlates with the aromatic compounds. The UV\({}_{\rm 254 nm}\)/COD ratio was increased in the range of 0.130-0.148 during the CW treatment [12]. The UV\({}_{\rm 254 nm}\)/UV\({}_{\rm 280 nm}\) ratio is used as an indicator of lignin-derived compounds presence in wastewaters, whereas low values indicate a stronger presence of these compounds. During the treatment process, the UV\({}_{\rm 254 nm}\)/UV\({}_{\rm 280 nm}\) ratio at the OLR of 1.10 g COD\({}_{\rm s}\)/m\({}^{2}\)/day was 1.13 and 1.12 for the influent and effluent, respectively. Similarly, [PERSON] [34] showed that the UV\({}_{\rm 254 nm}\)/UV\({}_{\rm 272 nm}\) ratio did not undergo a significant change (ranging between 1.10-1.13). These results led to the conclusion that the residual COD consisted of lignin compounds, which were also the major aromatic species in these leachates [12]. The partial organic matter removal could be attributed to the _P. radiata_ and _E. globulus_ leachates composition. Lignin and total phenolic compounds have been indicated as recalcitrant substrates in biological treatment systems (aerobic and anoxic), due to their molecular weight being higher than 10,000 Da [12; 21; 35]. Also, lignin is responsible for the brown color of these leachates. Approximately 10% of the residual COD consisted of microbially altered organics, while the rest were organics initially present in the wastewater. Similarly, [PERSON] et al. [35] and [PERSON] et al. [36] suggested that anoxic environments enhance the phenolic groups in wastewaters with high phenolic compounds concentrations as a result of biological removal of methoxy groups from the aromatic ring structure. Moreover, it has been indicated that color formation is correlated to the anoxic conditions and the availability of readily biodegradable organic constituents during the wastewater treatment process [24]. The high levels of these compounds applied in the CW probably created a toxic environment that affected the development of the microbial community, especially in the planted systems where an enhanced aerobic environment is usually established. High toxicity can be caused not only by increased phenols concentration, but also due to increased generation of intermediates during phenols transformation, as elsewhere reported [25]. In this way, other studies suggest a tertiary treatment stage after the biological treatment stage for specific compounds and recalcitrant organic matter [5; 37; 38; 39]. Therefore, according to [PERSON] and [PERSON] [37], using ornamental plants like _Zantedeschia aetihiopica_ in subsurface flow constructed wetlands remove nitrogen, chemical oxygen demand, and nonylphenol ethoxylate surfactants. On the other hand, _Eichlornia crassipes_ has been studied as tertiary treatment stage [39]. Results showed that organic matter and color were removed at a maximum rate of 75% and 25%, respectively, by _Eichlornia crassipes_. On the other hand, tertiary treatment via physical processes [35; 40] or advanced oxidation processes [9; 41; 42] can reduce the recalcitrant compounds by adsorption and destruction, respectively [43].