Sulfur dioxide (SO2) and benzene homologs are frequently present in the off-gas during the process of sewage sludge drying.A laboratory scale biofilter was set up to co-treat SO2 and o-xylene in the present study.SO2 and o-xylene could be removed simultaneously in a single biofilter.Their concentration ratio in the inlet stream influenced the removal efficiencies.It is worth noting that the removal of SO2 could be enhanced when low concentrations of o-xylene were introduced into the biofilter.Pseudomonas sp.,Paenibacillus sp.,and Bacillus sp.were the main functional bacteria groups in the biofilter.Sulfur-oxidizing bacteria (SOB) and o-xylene-degrading bacteria (XB) thrived in the biofilter and their counts as well as their growth rate increased with the increase in amount of SO2 and o-xylene supplied.The microbial populations differed in counts and species due to the properties and components of the compounds being treated in the biofilter.The presence of mixed substrates enhanced the diversity of the microbial population.During the treatment process,bioaerosols including potentially pathogenic bacteria,e.g.,Acinetobacter lwoffii and Aeromonas sp.,were emitted from the biofilter.Further investigation is needed to focus on the potential hazards caused by the bioaerosols emitted from waste gas treatment bioreactors.