## 3 Results and Discussion
### Operation and Performance of the CW with the Studied Substrates
During Phase I, the effluent ammonium nitrogen values exhibited greater stability, ranging from 31.6 to 47.2 mg/L, in contrast to Phase II, where values fluctuated between 43.1 and 66.1 mg/L, indicating a tendency towards nutrient production in this latter phase. The low efficiency of ammonium nitrogen removal can be attributed to the brief emptying time of the wetland (1 h), which likely limited natural aeration. This is reflected in the recorded average dissolved oxygen levels of (0.77 \(\pm\) 0.58) mg/L in Phase I and (1.60 \(\pm\) 0.15) mg/L in Phase II, conditions that are insufficient to support the nitrification process. Nitrification, mediated by ammonia-oxidizing bacteria such as _Nitrosspira_, _Nitrosomonas_, and _Nitrosococcus_, and nitrite-oxidizing bacteria such as _Nitrosspira_, _Nitrobacter_, and _Nitrooccus_, was likely inhibited, hindering the conversion of ammonia to nitrate [32]. Therefore, the average influent nitrate concentration (3.40 \(\pm\) 0.50 mg/L), which was relatively low to begin with, was entirely removed in both operational phases. This complete removal is likely due to absorption by macrophytes, although denitifying bacteria may also convert nitrate to gaseous nitrogen in anoxic areas within the bed [32]. In the 3 rd, 4 th, 5 th, and 6 th cycles of the vegetated phase, it was observed that the effluent ammonium nitrogen concentrations were higher than the influent concentrations. This anomaly may be attributed to several factors, including potential accumulation or desorption of N-NH\({}_{4}\)\({}^{+}\) within the system during these cycles, fluctuations in system performance, or variations in the biological activity of the vegetated bed. Additionally, temporary imbalances in nutrient uptake or microbial activity may have contributed to these unexpected results. Figure 3a illustrates the variations in ammonium nitrogen and phosphorus concentrations across the cycles of Phases I and II. For phosphorus, the average removal efficiencies were (35.2 \(\pm\) 4.6%) in Phase I and (40.2 \(\pm\) 15.4)% in Phase II (Figure 3b). The vegetated phase achieved a maximum phosphorus removal efficiency of 70.4% in its first cycle. The effluent phosphorus concentrations in the non-vegetated phase ranged from 18.2 to 27.3 mg/L, with a mean of (22.4 \(\pm\) 3.0) mg/L. In contrast, the vegetated phase had a lower average effluent phosphorus concentration of (20.8 \(\pm\) 4.9) mg/L, ranging from 27.2 to 11.5 mg/L. The system demonstrated exceptional performance in phosphorus removal. Typical phosphorus removal efficiencies in vertical wetlands are under 20%, which aligns with the 10% to 30% range reported by [PERSON] and [PERSON] [33; 34] for secondary biological systems. The enhanced removal efficiency observed in this study may be attributed to the inclusion of marine shells and bricks in the filter bed, which likely contribute to improved phosphorus retention due to the composition of these materials. Bivalve shells, which contain significant amounts of CaO (51.0-54.7%), are known for their ability to retain phosphorus, achieving removal rates of nearly 90.0% [35; 36]. However, in this study, phosphorus removal cannot be solely attributed to adsorption by the substrates. The total mass of phosphorus removed was 2.08 mg, whereas the adsorption capacities of the shells and bricks are 0.001 mg/g and 0.002 mg/g, respectively [7]. Giventhese adsorption capacities, the maximum phosphorus that could theoretically be removed by the shells and bricks would be 0.015 g and 0.022 g, respectively. This indicates that the observed phosphorus removal exceeds what could be accounted for by adsorption alone. Thus, with the layers of shell and brick alone, a maximum of 73.5 mg/g of support material would have been retained, a value that exceeds the phosphorus adsorption capacities of 0.015 g and 0.022 g for shells and bricks, respectively. This suggests that phosphorus removal was likely due to microbial activity, plant uptake, or the formation of phosphorus precipitates through biochemical reactions in the medium [37].