Enhancement of partial denitrification-anammox pathways in constructed wetlands by plant-based external carbon sources

Insufficient carbon source/electron donor often limits the denitrification efficiency of constructed wetlands (CWs) when treating wastewater characterized by low carbon-to-nitrogen ratios. Hence, the viability of wetland plant-based carbon sources for enhanced nitrogen removal and its mechanisms in horizontal subsurface flow CWs was assessed. Results showed cattails to have a better carbon release capacity than wheat straw and corn cob. Alkali pretreatment of cattails is demonstrated to significantly enhance the release of volatile fatty acids (68.31 ± 1.36 mg/g) dominated by acetic acid to sustain a high denitrification rate (0.62 ± 0.05 mg/(L·h)) over a long duration. External carbon source addition to adjusting C/N ratio to 3.5 achieved the highest removal rates of 75.45%, 91.33% and 76.03%, respectively, for total nitrogen (TN), ammonia-nitrogen (NH4+-N) and nitrate-nitrogen (NO3−-N). High-throughput sequencing showed plant-based external carbon sources to significantly increase the relative abundance of the denitrifying bacteria, as well as the anammox bacteria. Moreover, it elevated the number of nitrate reductase relative to nitrite reductase by 4.65-fold. Plant-based external carbon sources enhanced the partial denitrification-anammox (PDN/AMX) contribution to TN removal by at least 27.29% and improved nitrogen removal efficiency through co-promotion of denitrification and PDN/AMX pathways. Findings from this study support the utilization of plant-based external carbon sources in CWs. Additionally, it provides new insights into enhancement pathways of nitrogen removal with plant-based carbon sources.