Internal short-circuit revolution: Unveiling enhanced nitrogen removal efficiency in Mn-rich constructed wetlands

Electron acceptor limitation restricts ammonium removal in constructed wetlands (CWs). This study hypothesized that integrating carbon fiber filaments (CFs) within manganese-rich substrates (CF-CM) could enhance electron transfer and nitrogen transformation. Five lab-scale CWs were established. Results showed that CF-CM significantly improved NH₄⁺-N and total nitrogen removal rates, reaching 8.1 and 13.1 gN/(m²·d), 3.1-fold and 5.4-fold higher than CF-CK, respectively. CFs facilitated internal short-circuiting, increasing anammox bacteria abundance and electron utilization of denitrifying bacteria. The qPCR gene chip testing revealed upregulated napA and narG, with nirS being the most abundant nitrogen transformation gene. And anammox genes (hzsA, hzsB, hzo) at 3.22 × 10⁷ copies·g^-1, 288 times higher than the control. This strategy enhanced regional electron transfer and regulated spatial electron supply-demand relationships, with the core enhancement being the enrichment of anammox bacteria, electroactive bacteria, and denitrifiers in CF-CM systems. One point of view is that integrating internal short-circuits in Mn-rich substrates CWs is an efficient treatment approach for nitrogen removal.