The performance and microbiology of a biotrickling filter (BTF) treating methyl mercaptan, toluene, alpha-pinene and hexane at the mg m(-3) level was studied at empty bed residence times (EBRT) of 50, 30, 11 and 7 s. Removal efficiencies (REs) higher than 95% were observed for MeSH, toluene and alphapinene even at 11 s, while hexane REs exceeded 70%. At 7s, an irreversible damage of the microbial activity due to the accumulation of toxic metabolites resulted in a decrease of REs. The addition of silicone stabilized process performance but only re-inoculation allowed achieving a complete removal of MeSH, toluene and alpha-pinene, and hexane REs of 80%. The high KLa values (ranging from 38 +/- 4 to 90 +/- 11 h(-1)) explained the good BTF performance at such low EBRTs. A high bacterial diversity, along with a vertical distribution of the bacterial communities was observed, the main phyla being Proteobacteria, Actinobacteria, Nitrospira, Chlorojlexi and Cemmatimonadertes. (C) 2012 Elsevier Ltd. All rights reserved.