## 2 Materials and Methods
### System Configuration and Operation
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**CW Unit** & **Porous Media** & **Plant Species** & \begin{tabular}{l} **HRT** \\ **(Days)** \\ \end{tabular} & \
\begin{tabular}{l} **Q\({}_{\text{in}}\)** \\ **(L/Day)** \\ \end{tabular} &
\begin{tabular}{l} **HLR** \\ **(mm/Day)** \\ \end{tabular} \\ \hline W1 FG-R & FG & _Phragmites australis_ & 8 & 9.5 & 19.4 \\ W2 FG-U & FG & None & 10 & 7.6 & 15.3 \\ W3 FG-R & FG & _Phragmites australis_ & 10 & 7.6 & 15.3 \\ W4 FG-C & FG & _Typla latifolia_ & 10 & 7.6 & 15.3 \\ W5 FGZ-R & 75\% FG and 25\% FZ & _Phragmites australis_ & 10 & 7.6 & 15.3 \\ \hline \hline \end{tabular}  
* FG = fine gravel, FZ = fine zeolite, HRT= hydraulic residence time, Q\({}_{\text{in}}\) = influent rate, HLR = hydraulic loading rate.
* \end{table} 
Table 2: Design characteristics and operational conditions of pilot-scale CWs. Figure 1: (**a**) View of the five pilot-scale HSF CWs and (**b**) schematic description of CW units. The operation of the wetlands began in June 2017 and continued until June 2018. The landfill leachate was transferred from the landfill to the laboratory using a tank of 1.0 m\({}^{3}\). Because the electrical conductivity of the leachate was high (Table 1), and to prevent any adverse effects on the plants, the charging of the CW units was performed with diluted leachate [37]. Dilution was performed with tap water in a tank of 250 L, so that the electrical conductivity was approximately 7 mS/cm. In the tap water, the mean EC value was about 0.55 mS/cm, and the average concentration (mg/L) of the main ions was Ca: 90; Na: 28; Cl: 42; SO\({}_{4}\): 38; and Mg: 14. The residual chlorine was 0.2 mg/L. In the landfill leachate, the mean value of EC was 28.5 mS/cm (Table 1), and the typical average concentration (mg/L) of the main ions was Ca: 240-2330; Na: 85-3800; Cl: 47-2400; SO\({}_{4}\): 20-730; and Mg: 4-780 [33]. It is clear that the EC value and ion concentrations in tap water were much lower than those in the leachate. Therefore, adding water to dilute the leachates is considered unlikely to affect the results. During the first operational period of the CW units (approximately one month), the units were charged with diluted wastewater in which the EC was less than 7 mS/cm to avoid any possible shock to the plants and allow for acclimatization. Normal charging began in early July and was performed daily. The units were loaded twice daily (approximately every 12 h) with equal amounts of leachate. The total daily volume of loading leachate was 9.5 L/d for the W1 FG-R unit and 7.6 L/d for the rest of the units, thus, achieving a hydraulic residence time (HRT) of 8 and 10 days and a hydraulic loading rate (HLR) of 19.4 mm/d and 15.3 mm/d, respectively (Table 2). The selection of HRT was made considering previous studies related to the removal of organic matter and nutrients from domestic wastewater and landfill leachate [38; 39; 40]. Longer HRTs facilitate greater nitrification-denitrification and organic degradation by extending the period that leachate, microbes, and substrate are in contact. The selected values achieve a balance between performance and practicality, because overly lengthy HRTs may result in operational inefficiencies, larger land needs, and a higher capital cost [41]. With this CW unit configuration, it is possible to evaluate the effect of the presence and type of plants, the existence of zeolite in the substrate, and the applied HRT on the removal of pollutants, by comparing units W2 FG-U, W3 FG-R, and W4 FG-C with each other, W3 FG-R with W5 FGZ-R, and W1 FG-R with W3 FG-R, respectively. A meteorological station (model: ELE MM900/950) was installed outside the laboratory for monitoring and measuring meteorological parameters. Air temperature, humidity, precipitation depth, and wind speed and direction were recorded on-site, at 5 min intervals.