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.