## 3 Findings and Discussion
### Effects of the Initial Organic Concentration on the Treatment Efficiency of CW
The results of the research of [PERSON] et al. [32] also support the interpretation that humic substances of all molecular weights can be absorbed. However, until now, the mechanisms through which humic substances interact with the  
Figure 5: The effect of the initial DOC concentration on effluent water quality. root cells and may subsequently affect plant physiology and growth have not been completely clarified yet [30]. In addition, humic substances may also be taken up by microbes. There is strong proof that HA was taken up into mycelia of _Aspergillus niger_[33]. Once taken up by plant tissues, humic materials may directly influence the growth of plants via biochemical mechanisms [34]. [PERSON] [35] also reported microscopic evidence that HA was taken into the shoots of several plant species which were allowed to stand with their cut ends in HA solutions. It is likely that in most cases humic substances enhance microbial growth by serving as a food source [34]. Along with the biodegradation of labile OM by microorganisms, there might be some degradation of less degradable organic matter, which would be decomposed by specialized natural soil bacteria [36]. The larger molecular weight OM was firstly degraded into smaller molecule one by microorganism activity, and then absolutely eliminated gradually. During the experiment, the average removal range of UV-254 was higher than that of DOC, 55.2-80.8% compared to 28.1-71.9%, indicating that there were considerable changes in structure (simple break-up) without causing the complete mineralization of large aromatic structures [14]. In addition, an interesting result found from this study is that while the initial DOC concentrations in three runs increased, the DOC concentrations of effluents maintained nearly similar values, ranging from \(2.880\pm 0.257\), \(2.673\pm 0.219\), \(3.119\pm 0.149\) mg/L. This may imply that CW treatment could have a high tolerance of variation of DOC concentration in the influent water and maintain stable effluent water quality. This result agrees with the conclusion of [PERSON] et al. [37] who indicated that there was a load fluctuation tolerance in CWs. In terms of nitrogen removal in CWs, according to [PERSON] and [PERSON] [12], nitrogen undergoes several processes in wetlands, including ammonification, nitrification, denitrification, adsorption, bacterial degradation, and plant uptake. In this experiment, the concentration of NH\({}_{3}\)-N underwent a significant decrease and a removal efficiency higher than 60% was achieved while that of NO\({}_{3}\)-N increased to a slight extent from 0.956 \(\pm\) 0.312 to 1.109 \(\pm\) 0.255, and form 0.707 \(\pm\) 0.234 to 1.052 \(\pm\) 0.155 for run 2 and run 3, respectively. In addition, it is well-known that biological nitrogen removal depends on successful nitrification (ammonia oxidation to nitrate) and the subsequent denitrification (nitrate reduction to nitrogen gas) [20]. However, in this case, plants in CW also played an important role in eliminating NH\({}_{3}\)-N concentration. As stated by [PERSON] et al. [38], umbrella palms should be considered when maximum NH\({}_{4}\)+ reduction is a treatment goal. That NO\({}_{3}\)-N levels showed no significant difference in the influent and effluent concentrations may be explained by the fact that the HRT in this experiment was not enough for the process of denitrification. It probably occurs in most wetland systems that the removal of nitrogen requires longer HRTs [39].