## 2 Materials and Methods
### System Configuration and Operation  
Five pilot-scale HSF CW units were used for the experiment (Figure 1). The units were placed outside the Ecological Engineering and Technology Laboratory (location: 41\({}^{\circ}\)08\({}^{\prime}\)47\({}^{\prime\prime}\) N, 24\({}^{\circ}\)55\({}^{\prime}\)09\({}^{\prime\prime}\) E). Each CW was rectangular, with dimensions of 1.10 m long, 0.45 m wide, and 0.60 m deep, and was filled with porous material to a thickness of 45 cm. The porous media used were (a) fine gravel (code name: FG) with D\({}_{50}\) = 8 mm, originating from igneous rocks and taken from the bed of a local river, and (b) fine-grained zeolite (code name: FZ) with D\({}_{50}\) = 8 mm, the main component being clinoptiolite. The proven effectiveness of zeolite and fine gravel to improve pollution removal served as the basis for their usage as fill materials. In addition to supporting plant root growth, zeolite and gravel also promote microbial colonization, which is critical for biodegradation processes. Because of its high cation exchange capacity, especially for ammonium adsorption, which improves nitrogen removal, zeolite was chosen and added to one unit at a rate of 25% by volume [34]. The plants used for planting the CWs were common reed (_Phragmites australis_) and cattail (_Typha latifolia_), with code names R and C, respectively, collected from watercourses in the area. The plant species were selected due to their proven effectiveness in wetland treatment systems. According to [PERSON] et al. [35], both species support microbial communities \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**Parameters**} & \multicolumn{2}{l}{**Landfill Age**} & \multicolumn{2}{l}{**Present Study**} \\ \cline{2-5}  & **Young** & **Middle-Aged** & **Old** & \\ \hline Age (years) & \textless{}10 years & 10–20 years & \textgreater{}20 years & \\ pH & 3–6 & 6–7 & \textgreater{}7.5 & 8.24 [3 (\(\pm\)0.67) \\ \({}^{1}\) EC (mS/cm) & 15–41.5 & 6–14 & & 28.5 (\(\pm\)5.2) \\ BOD (mg/L) & 10,000–25,000 & 1000–40000 & 50–1000 & 685 (\(\pm\)102) \\ COD (mg/L) & 15,000–40,000 & 10,000–20,000 & 1000–5000 & 9344 (\(\pm\)1956) \\ BOD/COD & 0.6–0.7 & 0.1–0.2 & \textless{}0.1 & 0.07 (\(\pm\)0.005) \\ \({}^{2}\) TKN (mg/L) & 1500–4500 & 400–800 & 75–300 & 762 (\(\pm\)151) \\ NH\({}_{4}\)-N (mg/L) & 1500–4250 & 250–700 & 50–200 & 579 (\(\pm\)97) \\ \hline \hline \end{tabular} \({}^{1}\) EC = electrical conductivity; \({}^{2}\) TKN = total Kjeldahl nitrogen; \({}^{3}\) (\(\pm\)n) = standard deviation.  
\end{table} 
Table 1: Characteristics of landfill leachate depending on landfill age and comparison with the leachate of the present study [10; 12; 32; 33]. engaged in pollutants degradation, improve oxygen transmission in the rhizosphere, and demonstrate excellent flexibility to changing water conditions. Both plants were tolerant to a high EC of 14-16 mS/cm and exhibited good nutrient (N and P) uptake in CWs [36]. To ensure viability for large-scale applications, their selection was also based on their local availability. The design and operational characteristics of the CW units are presented in Table 2. In the CW units with code names W1 FG-R, W2 FG-U, W3 FG-R, and W4 FG-C the porous media was FG, and in the unit codenamed W5 FGZ-R, it was FG 75% and FZ 25% (by volume). Furthermore, the CW units W1 FG-R, W3 FG-R, and W5 FGZ-R were planted with _Phragmites australis_, and the unit W4 FG-C was planted with _Typla latifolia_. The W2 FG-U unit was used as a control unit and therefore remained unplanted.