Vertical microbial stratification tailors simultaneous partial nitrification-denitrification to achieve synergistic biofiltration of trimethylamine and nitric oxide.

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.