## 3 Results and Discussion
### Overall Performance of Pilot-Scale CW Units
The mean removal efficiencies for TKN were 40.9%, 36.6%, 45.5%, 45.9%, and 50.9%, and for NH\({}_{4}\)-N they were 49%, 40.8%, 49.4%, 50.4%, and 63.8%, in CW units W1 FG-R, W2 FG-U, W3 FG-R, W4 FG-C, and W5 FGZ-R, respectively (Figure 4). For both TKN and NH\({}_{4}\)-N, the highest removal was achieved by the W5 FGZ-R unit, and the lowest removal was achieved by the W2 FG-U unit (unplanted unit). The removal of TKN and NH\({}_{4}\)-N in the pilot-scale CW units is mainly attributed to the following: (a) nitrification-denitrification as aerobic and anaerobic conditions in the CW units prevail, with the DO concentrations ranging from 0.33 to 8.58 mg/L (Table 3), (b) uptake by the plants in the planted units, and (c) adsorption of ammonia nitrogen by the zeolite of the W5 FGZ-R unit. Ammonia volatilization was considered limited, as the mean pH values in the CW units ranged from 0.33 to 8.58 mg/L (Table 3), (d) and (e) adsorption of ammonia nitrogen by the zeolite of the W5 FGZ-R unit. Ammonia volatilization was considered limited, as the mean pH values in the CW units ranged from 0.33 to 8.58 mg/L (Table 3), (e) and (f) adsorption of ammonia nitrogen by the zeolite of the W5 FGZ-R unit. Am  
between 7.53 and 7.91. Research on CWs has shown that nitrogen removal through plant uptake accounts for 10% of total removal, with nitrification and denitrification being the primary removal processes [45]. EU Directive 1991/271/EEC imposes the following general effluent limits from wastewater treatment plants and for the discharge of treated effluent into natural water bodies: BOD 25 mg/L; COD 125 mg/L; TN 15 mg/L. Alternatively, this directive imposes minimum reduction rates, i.e., for BOD 70-90%, for COD 75%, and for TN 70-80% of the inflowing concentration. The average effluent values of all parameters from the CW units were higher than the limit set by the EU. However, large-scale CW systems usually involve three or more treatment stages. The first stage usually involves an anaerobic tank, followed by the remaining stages involving horizontal subsurface flow and vertical flow CWs. The various pollutants are effectively removed as the landfill leachate passes sequentially through the above stages, so that the final effluent meets the requirements of the legislation. The performance of CWs is affected by temperature, precipitation, and seasonal variations, which can affect plant performance, microbial activity, and overall pollutant removal. Microbial activity is almost constant throughout the year in temperate and Mediterranean climates, including the one at the study site, and plant species like _Typha tafifolia_ and _Phragmites australis_ thrive [36]. However, the efficacy of nitrogen removal by nitrification and denitrification may be diminished in colder climates, as microbial decomposition and plant growth slow down considerably ([PERSON] et al., 2013). Longer HRTs or hybrid wetland systems are needed in colder climates to compensate for decreased biological activity, according to studies [41]. Higher temperatures in tropical regions may promote microbial decomposition, but they also raise evapotranspiration, which causes leachate condensation in the wetland system. The age of the landfill, the type of waste, and the regional environmental conditions affect the composition of the leachate. A major problem in mature landfills is the relatively low biodegradability (BOD/COD < 0.1), which is reflected in the leachates used in this investigation. According to [PERSON] and [PERSON] (2009), leachates from newer landfills usually contain more biodegradable organic matter (BOD/COD > 0.5), which can promote microbial decomposition but can also result in excessive biofilm formation and the clogging of CWs. Furthermore, increased levels of heavy metals and persistent organic pollutants in landfills receiving industrial waste may require pretreatment or the use of specialized  
Figure 4: Pollutant (BOD, COD, TKN and NH\({}_{4}\)-N) removal chart for pilot-scale CW units.