## 3 Results
### Nitrate Reduction Analysis  
The average nitrate values obtained from the tertiary treatment of domestic wastewater using _P. striatiotes_, _S. molesta_, and _E. crassipes_ plants for a duration of two weeks with varying retention times of 6, 12, and 24 h are represented in Figures 12-14, respectively. Figure 12: Bar plot of the average nitrate vs. the sampling period with a 6 h retention time. Figure 13: Bar plot of the average nitrate vs. the sampling Period with a 12 h retention time. From the results presented in Figure 12, it can be observed that the average nitrate concentration of the influent samples from the STP varied throughout the sampling period and fell within the range of 4.36-8.7 mg/L. However, the uptake rate of nitrate by the three test plants increased across the 14-day sampling period. An initial nitrate value of 7.86 \(\pm\) 0.05 mg/L was obtained for the influent sample on the second day of the experiment, which was reduced to 3.6 \(\pm\) 0 mg/L, 4.8 \(\pm\) 0 mg/L, and 4.83 \(\pm\) 0 mg/L by _P. stratiotes_, _S. molesta_, and _E. crassipes_, respectively. On the 14 th day, the nitrate concentration of the influent sample before treatment was 7.81 \(\pm\) 0.02 mg/L, but after the phytoremediation process, the nitrate concentrations of the treated wastewater samples were reduced to 2.3 \(\pm\) 0.1 mg/L by _P. stratiotes_, 3.5 \(\pm\) 0.1 mg/L by _S. molesta_, and 2.2 \(\pm\) 0 by _E. crassipes_. Furthermore, the highest reduction efficiency of 77.55% was recorded for the _P. stratiotes_ treatment systems on the 12 th day. At the same time, the maximum reduction efficiencies of 55.29% and 73.33% were obtained for the _S. molesta_ and _E. crassipes_ treatment systems on the 14 th day of the sampling period. Additionally, the ANOVA test indicated a significant difference (\(p<0.05\)) between the influent and effluent wastewater samples with a 6 h retention time. Based on the results presented in Figure 13, the average nitrate values of the influent samples from the STP varied throughout the duration of the research and ranged from 3.8-10.83 mg/L. According to the analysis that was carried out, the results obtained show that the average nitrate value of the influent wastewater sample obtained on the second day of the experiment was lowered from 3.8 \(\pm\) 0 mg/L to 2.2 \(\pm\) 0 mg/L, 1.0 \(\pm\) 0 mg/L, and 1.43 \(\pm\) 0.05 mg/L by the _P. stratiotes_, _S. molesta_, and _E. crassipes_ plants, respectively. However, as the sampling period continued, a significant decrease in the initial nitrate values of all the influent samples continued onward up to the last day of the sampling period. Similarly, the nitrate concentration of the influent sample on the 14 th day of the phytoremediation process was recorded as 9.52 \(\pm\) 0.10 mg/L, but after the treatment with the test plants, the nitrate concentrations were reduced to 3.01 \(\pm\) 0.02 mg/L, 2.2 \(\pm\) 1 mg/L, and 3.04 \(\pm\) 0.01 mg/L by the _P. stratiotes_, _S. molesta_, and _E. crassipes_ treatment systems, respectively. The highest removal efficiencies of 68.38% and 68.06% were recorded for the _P. stratiotes_ and _E. crassipes_ effluent samples, respectively, on the 14 th day. At the same time, the highest removal efficiency of 82.9% was recorded for the _S. molesta_ effluent samples on the fourth day of the sampling period. Furthermore, a significant variance (\(p<0.05\)) was found between the average values of the influent and the individual effluent wastewater samples. From Figure 14, the average nitrate level of the influent samples obtained from the STP varied throughout the sampling period and ranged from 5.4-20 mg/L. However, it was observed that the three test plants efficiently decreased the initial nitrate concentration throughout the sampling period. Figure 14: Bar plot of the average nitrate vs. the sampling period with a 24 h retention time.