The multimedia biofilter achieved high and stable removal efficiencies for chemical oxygen demand (COD, 62-98%) and NH4+ (68-98%) without costly aeration. Results revealed that lower CL (less than 13.9 g COD/m(3) d) and ACL (less than 2.8 g NH4+-N/m(3) d) or a C/N ratio exceeding five was required to reduce NO3-N accumulation and NO/N2O emission. Integrated analyses indicated that the coupling of simultaneous nitrification, anammox and denitrification processes (SNAD) were the primary reason accounted for the enhanced NH4+-N treatment performance. NH4+-N removal pathways can be ranked as follows: nitrification (amoA, archaeal) (54.6%) > partial denitrification (nirS, nirK) and anammox (37.8%) > anammox and partial denitrification (narG, napA) (12.6%). Specifically, NH4+-N removal was significantly inhibited by NO2-N accumulation in the system (-21.6% inhibition). Results from stepwise regression analysis suggested that the NH4+ removal rate was collectively controlled by amoA, archaeal, anammox, nirS, nirK, narG and napA. (C) 2015 Elsevier Ltd. All rights reserved.