## 3 Results and Discussion
### Overall Performance of Pilot-Scale CW Units  
Table 4 presents the statistics of pollutants in the influent and effluent of the five pilot-scale CW units. The temporal variation in pollutant removal rates in the pilot-scale CW units during the experimental period is presented in Figure 3. The concentrations of the pollutants studied at the outlets of the five units were lower than those at the inlets, indicating that removal occurred (Table 4). Regarding organic matter (BOD and COD), the mean effluent concentrations for the CW units W1 FG-R, W2 FG-U, W3 FG-R, W4 FG-C, and W5 FGZ-R for BOD were 105.4, 133.5, 102.0, 85.7, and 77.0 mg/L, respectively, and for COD they were 1875, 2446, 1516, 1403, and 1354 mg/L, respectively (Table 4). The average removal of BOD was 45.9%, 32.2%, 47.7%, 55.4%, and 60.4% and of COD was 40.6%, 24.1%, 48.3%, 53.3%, and 56.7% in the CW units W1 FG-R, W2 FG-U, W3 FG-R, W4 FG-C, and W5 FGZ-R, respectively (Figure 4). These results showed that the planted units achieved higher organic matter removal than the unplanted unit. Organic matter (OM) removal in CWs occurs by various processes, such as sedimentation and biodegradation by aerobic and anaerobic microorganisms [41]. The plants provide a surface for the growth of the biofilm and, as mentioned above, provide oxygen for additional oxygenation of the CWs' substrate (Table 3). The moderate removal of OM is probably due to the very low (<0.1; Table 1) BOD/COD ratio of the landfill leachate, indicating that the leachate contains a high concentration of biologically recalcitrant organic substances. The mean effluent concentrations of TKN and NH\({}_{4}\)-N at the five pilot-scale CW units were lower than the mean influent concentration, indicating the removal of these pollutants in the CW units (Table 4). The main mechanisms of nitrogen removal in CWs are nitrification, denitrification, removal as ammonia gas (volatilization), adsorption on porous media, and uptake by plants [36]. Nitrification and denitrification occur under aerobic and anoxic conditions, respectively, converting ammonia nitrogen first to nitrate (nitrification) and finally to nitrogen gas (denitrification). Ammonia volatilization occurs when the pH value is between 8 and 9. Adsorption requires an adsorbent material, such as zeolite, which has cation exchange sites. \begin{table}
\begin{tabular}{l l c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Influent**} & \multicolumn{5}{c}{**Effluent**} \\ \cline{3-8}  & & & **W1 FG-R** & **W2 FG-U** & **W3 FG-R** & **W4 FG-C** & **W5 FGZ-R** \\ \hline \multirow{3}{*}{BOD} & mean & 195.9 & 105.4 & 133.5 & 102.0 & 85.7 & 77.0 \\  & SD (n) 1 & 29.8 (46) & 31.2 (46) & 32.8 (44) & 36.5 (42) & 19.5 (44) & 20.0 (44) \\ (mg/L) & min & 146.0 & 50.7 & 56.3 & 28.0 & 51.5 & 47.5 \\  & max & 300.0 & 180.0 & 200.0 & 197.0 & 130.5 & 131.0 \\ \hline \multirow{3}{*}{COD} & mean & 3115 & 1875 & 2446 & 1516 & 1403 & 1354 \\  & SD (n) & 652 (46) & 418 (45) & 706 (41) & 422 (45) & 318 (43) & 438 (44) \\ (mg/L) & min & 2065 & 1065 & 1065 & 710 & 898 & 900 \\  & max & 5207 & 2795 & 4312 & 2842 & 2085 & 3751 \\ \hline \multirow{3}{*}{TKN} & mean & 345.3 & 204.1 & 220.1 & 202.3 & 201.9 & 172.8 \\  & SD (n) & 60.7 (46) & 75.2 (45) & 74.1 (45) & 72.3 (45) & 87.6 (43) & 65.8 (43) \\ (mg/L) & min & 210.0 & 101.0 & 119.0 & 84.0 & 91.0 & 84.0 \\  & max & 483.0 & 350.0 & 385.0 & 317.3 & 396.7 & 308.0 \\ \hline \multirow{3}{*}{NH\({}_{4}\)-N (mg/L)} & mean & 231.6 & 117.3 & 134.9 & 115.4 & 114.0 & 81.4 \\  & SD (n) & 38.9 (46) & 42.8 (44) & 39.8 (45) & 40.4 (43) & 45.2 (43) & 25.8 (42) \\ (mg/L) & min & 140.0 & 35.0 & 49.0 & 21.0 & 41.0 & 42.0 \\  & max & 308.0 & 219.3 & 266.0 & 210.0 & 196.0 & 137.0 \\ \hline \hline \end{tabular}  
* SD = standard deviation, n = number of measurements.  
\end{table} 
Table 4: Statistics of pollutant concentrations in each pilot-scale CW unit.