Efficient removal of nitrogen from tidal flow constructed wetlands based on the in-situ zeolite regeneration: Measures and mechanisms

Tidal flow constructed wetlands (TFCWs) offer a promising method to treat domestic wastewater in rural areas. However, TFCWs are usually severely restricted by denitrification. This study aimed to explain the nitrogen removal process in TFCWs and provide an effective method to increase nitrogen removal. TFCWs filled zeolite (Z-TFCW) was established in this work, and its pollutants (NH4+-N, NOx−-N, and COD) removal performance, layered effect, microbial community characteristics, the in-situ regeneration process of Z-TFCW, and the two-way influent mechanisms were investigated. The results showed that under the downflow influent, the Z-TFCW effluent NH4+-N, NOx−-N, and COD concentrations were 9.79 ± 0.62 mg·L−1, 24.25 ± 1.53 mg·L−1, and 24.04 ± 2.68 mg·L−1, respectively. In the influent and flood period, NH4+-N was adsorbed rapidly by zeolite. Then, they were released into the biofilm and oxidized to NOx−-N during the drain period. The generated NOx−-N was leached to the bottom layer during the next cycle of influent, and most of them were adsorbed by the bottom biofilm for denitrification. Furthermore, the proportion of COD/NOx−-N in the top, middle, and bottom layers of Z-TFCW was 12.9, 3.1, and 1.0, respectively. Layered effect and microbial community analysis represented that COD limited the denitrification of Z-TFCW. The two-way influent optimized the distribution path of carbon sources for denitrification in Z-TFCW and enhanced nitrogen removal. Therefore, the NOx−-N effluent concentration decreased by 50.31 %.