Efficient nitrogen removal from mainstream sewage in tidal flow constructed wetlands: Targeted migration and conducive spatial distribution for pollutants removal

Abstract

The tidal flow constructed wetland (TFCW) effectively removes nitrogen from mainstream wastewater but encounters limitations in denitrification, resulting in the accumulation of nitrate and nitrite nitrogen (NOx--N). In this study, we investigated the spatial distribution characteristics of pollutants and migration-transformation processes within the TFCW using the stratified analysis theories proposed. The results indicated that during the influent stage, 82.60 % of ammonia nitrogen (NH4+-N) and 87.77 % of COD were adsorbed by the top and middle layers. In the drain stage's oxygen-rich environment, microorganisms and atmospheric oxygen oxidized NH4+-N to NOx--N, achieving in-situ regeneration of NH4+-N adsorption sites. Furthermore, 61.87 % of the NOx--N was washed into the bottom in the next influent stage, subsequently adsorbed by the biofilm, and finally reduced to gaseous nitrogen using carbon source as the electron donor at the drain stage. Consequently, inconsistent locations of carbon source and NOx--N were identified as limiting factors for denitrification in TFCW. Based on this, the study designed a two-way influent tide flow constructed wetland (TTFCW) by altering the influent pattern. It facilitated targeted carbon source addition for denitrification, resulting in a 3.30-fold improvement in actual carbon source utilization efficiency of TFCW. Ultimately, this approach resulted in a notable increase in NOx--N removal rate by 64.51 %, accompanied by a 28.32 % reduction in accumulation levels. This study fills the research gap in the internal distribution and migration-transformation of pollutants within TFCW, providing methods support for enhancing nitrogen removal in TFCW and ideas for denitrification in mainstream wastewater.