## 3 Results and Discussion
### Effect of Hydraulic Residence Time and Porous Media  
Previous studies have shown that hydraulic residence time affects the efficiency of CW units in removing various pollutants [36]. In this study, the effect of HRT on pollutant removal was examined by comparing the removal efficiency of the W1 FG-R and W3 FG-R units, which have the same construction characteristics but differ in HRT (8 and 10 days, respectively) (Table 2). In addition, the two units were in the same area and operated simultaneously under the same climatic conditions. The average removal rates for each unit and the \(t\)-test results are presented in Table 6. The W3 FG-R unit with an HRT of 10d exhibited higher average removal rates for all pollutants, indicating that longer HRTs increase both OM and nitrogen removal. Statistical analysis showed that the COD removal at an HRT of 10 days is statistically significantly higher (\(p<0.05\)) than that of 8 days. HRT values longer than ten days may further improve unit performance. According to previous studies, a longer HRT contributes to the creation of microbial communities and at the same time increases the contact time of pollutants with the biofilm, enhancing the performance of the CW [41; 49; 52; 53]. [PERSON] et al. [54] reported removal efficiencies of 55%, 42%, and 41% for BOD, COD, and TKN, respectively, at an HRT of 10 days in HSF CWs, which are comparable to those of the present study. By increasing the HRT to 28 days, the BOD, COD, and TKN removal efficiencies increased to 71%, 58%, and 46%, respectively, while decreasing the HRT to 5 days resulted in reductions to 44%, 33%, and 20%, respectively. [PERSON] and [PERSON] [40] also reported comparable BOD and NH\({}_{4}\)-N removal efficiencies of 35.7% and 38.3%, respectively, in HSF CWs at an HRT of 12.5 days. Furthermore, similar results to the BOD, COD, and NH\({}_{4}\)-N removal efficiencies of the present study are reported in the literature [55; 56]. These results show that HRT plays an important role in pollutant removal. With longer HRT values (e.g., 28 days vs. 8 days), more thorough biological, chemical, and physical treatment processes are possible. Since ammonium is oxidized to nitrate in aerobic zones, and nitrate is reduced to nitrogen gas in anaerobic conditions, an extended HRT improves nitrification-denitrification cycles [49]. Furthermore, because microbial populations have more time to decompose complex organic substances, a prolonged HRT promotes the decomposition of organic compounds [53; 57]. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{5}{c}{**One-Way ANOVA**} & \multicolumn{3}{c}{**Post Hoc (Tukey HSD) Test**} \\ \hline
**Parameter** & **F** & \(\mathbf{p}\) & **CW (l)–CW (j)** & **Mean Difference** (l–j)** & \(\mathbf{p}\) \\ \hline BOD & 25.106 & 0.001 & W2 FG-U–W3 FG-R & \(-\)15.51 & 0.001 * \\  & & & W2 FG-U–W4 FG-C & \(-\)23.26 & 0.001 * \\  & & & W3 FG-R–W4 FG-C & \(-\)7.74 & 0.061 \\ COD & 54.412 & 0.001 & W2 FG-U–W3 FG-R & \(-\)24.23 & 0.001 * \\  & & & W2 FG-U–W4 FG-C & \(-\)29.20 & 0.001 * \\  & & & W3 FG-R–W4 FG-C & \(-\)4.96 & 0.209 \\ TKN & 3.548 & 0.032 & W2 FG-U–W3 FG-R & \(-\)8.93 & 0.063 \\  & & & W2 FG-U–W4 FG-C & \(-\)9.26 & 0.045 * \\  & & & W3 FG-R–W4 FG-C & \(-\)0.32 & 0.996 \\ NH\({}_{4}\)-N & 3.722 & 0.027 & W2 FG-U–W3 FG-R & \(-\)8.59 & 0.071 \\  & & & W2 FG-U–W4 FG-C & \(-\)9.52 & 0.040 * \\  & & & W3 FG-R–W4 FG-C & \(-\)0.93 & 0.969 \\ \hline \hline \end{tabular}  
* The mean difference is significant at the 0.05 level.  
\end{table} 
Table 5: One-way ANOVA and Tukey HSD test results for comparison of pollutant removal efficiencies of W2 FG-U, W3 FG-R, and W4 FG-C units. The effect of porous media on pollutant removal was examined by comparing the removal efficiency of the W3 FG-R and W5 FGZ-R units. Both units were planted with common reed and had the same operational characteristics, but the porous media differed. In the W3 FG-R unit, the porous medium was FG, while in the W5 FGZ-R unit, the media were composed of 75% FG and 25% FZ (Table 2).