Effects of high interstitial velocity on nitrification in horizontal flow wetlands for tertiary wastewater treatment

ABSTRACT Nitrification is limited in horizontal flow (HF) wetlands due to prevailing anaerobic conditions in these systems. This study examined how high interstitial velocity affects nitrification performance in HF systems used for tertiary wastewater treatment. Experiments were conducted using planted and unplanted HF wetlands operated in batch recycle mode, functioning as continuous stirred tank reactors across interstitial velocities of 15, 36, 56, and 72 m/d. From the Reynolds (Re) number, these velocities fall within a transition hydraulic flow range. In planted and unplanted cells, the levels of DO (2.7-3.3 mgO2/L and 1.9-2.4 mgO2/L, respectively) and COD (15-27 mg/L and 21-35 mg/L, respectively) differed significantly at different velocities. However, the rate constants for NH4-N in planted (0.24-0.33 d−1) and unplanted cells (0.18-0.31 d−1) differed insignificantly at varying velocities due to enhanced aeration caused by high velocities. Similarly, NO3-N concentrations did not differ significantly between systems, although each system showed notable changes with velocity. Up to interstitial velocities of 36 m/d and 1<Re<4, mass transfer effects on NO3-N formation were significant, beyond which they were insignificant. This suggests that high interstitial velocities 36 m/d enhance nitrification in HF wetlands for tertiary wastewater treatment. Further research is recommended to improve denitrification in these systems. .