Vertical microbial stratification tailors simultaneous partial nitrification-denitrification to achieve synergistic...

Co-treatments of trimethylamine (TMA) and nitric oxide (NO) in sludge drying off-gas receives little attention. Previously, TMA biofiltrations showed limited nitrogen elimination; microbial collaborations in biofilters were typically attributed to horizontal biofilm stratification. This work elucidated how vertical microbial stratification tailored simultaneous partial nitrification-denitrification (SPND) in three-layer counter-current biotrickling filter for decontamination of TMA and NO (loads: 10.9 gTMA m^-3h^-1, 54.7 gNO m^-3h^-1; empty bed residence time: 30 s; bed volume: 1.6 L layer^-1). Total removal efficiencies of TMA and NO attained 98% and 92%, equivalent to 70% and 46%, 24% and 25%, and 4% and 21% at bottom, middle, and top layers, respectively. Herein, 77% of C and 98% of N mineralized into CO₂ and N₂. From bottom to top along the gas flow, dominant genera shifted from TMA-degrading/denitrifying bacteria to nitrifiers; TMA removal pathway shifted from aerobic-denitrifying degradation to aerobic degradation; NO removal pathway shifted from denitrification to nitrification. Vertical stratification was shaped by C/N gradients. Medium recirculation helped donor-acceptor balances at each layer. Denitrification and nitrification sequentially took up leading roles in SPND along bed height. Nitrifiers partially oxidized NO or TMA-derived NH₃ to NO₂^-, while denitrifiers converted carbonic donors (TMA, its metabolites) and nitrogenic acceptors (NO, NO₂^-) to CO₂ and N₂. Hence, vertical stratification gradually upgraded bioremoval of TMA and NO from primary bioconversions to synergistic thorough decontamination into CO₂ and N₂. This study provides new insights that vertical stratification can act as an important driving force of functional synergy in biofiltration systems.