Differences in Ammonia Self-Purification between Simulated and Actual Rivers under Different Flow Velocities

This study investigated the degradation of NH4+-N in river channels under varying flow velocities, using both a simulated river channel (SRC) and a practical river channel (PRC) to treat discharge from an urban wastewater treatment plant (WWTP). The results showed that NH4+-N degradation follows first-order kinetics, with the degradation coefficient (KNH4+-N) increasing as flow velocity increases. By analyzing the microbial and physicochemical pathways of ammonia nitrogen degradation, the contribution differences between the two can be effectively highlighted. In the SRC, at an NH4+-N concentration of 2.41±0.48 mg·L−1, KNH4+-N rose from 0.10 day−1 (0.01 m·s−1) to 0.26 day−1 (0.02 m·s−1). In the PRC, at an NH4+-N concentration of 1.86±0.18 mg·L−1, KNH4+-N increased from 0.19 day−1 (0.10 m·s−1) to 0.28 day−1 (0.18 m·s−1). Ammonia-oxidizing bacteria (AOB), such as Nitrosomonas, were detected with counts increasing at higher flow velocities, while other AOB and nitrite-oxidizing bacteria (NOB) were not detected. This study confirmed that the degradation of NH4+-N in rivers is determined by the influence of flow velocity on the balance of ammonia particles and free ammonia in water, which significantly affects the removal of NH4+-N. The maximum removal efficiency in the study was 35.36%. In conclusion, the findings of this study highlight the significant role of flow velocity in enhancing NH4+-N degradation in river channels, with higher velocities promoting both faster degradation rates and greater ammonia-oxidizing bacteria abundance, ultimately improving the efficiency of NH4+-N removal from urban wastewater discharges.