Simultaneous enhanced ammonia and nitrate removal from secondary effluent in constructed wetlands using a new manganese-containing substrate

Constructed wetlands (CWs) are widely used to treat secondary effluent. However, simultaneously removing ammonia (NH4+-N) and nitrate (NO3--N) is challenging because of insufficient oxygen and carbon sources. In this study, a novel composite material (MPCM) comprising MnO2 and polycaprolactone was developed as a substrate for CWs to enhance the synchronous removal of NH4+-N and NO3--N. The CWs with a higher MPCM content (H-CW), lower MPCM content (L-CW), and controlled CW (C-CW) exhibited average NH4+-N removal efficiencies of 75.69%, 70.49%, and 52.40%, respectively. The 15N isotope tracking technique showed that NH4+-N removal was attributed to anaerobic ammonia oxidation mediated by MnO2 reduction (Mnammox), which accounted for 17.16%-27.24% of the NH4+-N removal in the composite material layers (0-20 cm) of the H-CW and L-CW. The richness of ammonia oxidizers in the upper layers (40-50 cm) of the H-CW and L-CW further facilitated NH4+-N removal. Moreover, the average total nitrogen (TN) removal efficiencies of the H-CW and L-CW were 1.99 and 1.59 times that of C-CW, respectively, owing to enhanced denitrification by MPCM. Furthermore, N2O emissions were reduced by 81.31% and 70.83% in the H-CW and L-CW, respectively. This study provides an effective approach for improving nitrogen removal and reducing N2O emissions during the treatment of secondary effluent by CWs.