A combined ultraviolet photodegradation and biofiltration (UV-BF) process was developed to treat gaseous chlorobenzene. The performance of this process was evaluated under various operating conditions, including different inlet concentrations, residence times, and transient loadings, and compared with a control biofiltration (BF) process. Furthermore. the acute biotoxicities of the photodegradation products, the bioaerosol emissions from biofilters. the biomass accumulation and pressure drop in biofilters were investigated. The experimental results showed that the UV-BF process provided higher removal efficiencies than those of the control BF process over an inlet concentration range of 250-1500 mg m(-3) for residence times of 41-122 s inside the biofilters and 24-81 s inside the UV reactor. After UV pretreatment, removal rates of the subsequent biofilter increased linearly with biofilter inlet loading, even beyond 50 g m(-3) h(-1). Similar inlet loading resulted in a gradual decline of removal rates for the control process due to a substrate inhibition effect. These results suggested that UV pretreatment reduced the inhibitory effects of chlorobenzene on microorganisms inside biofilters. Transient loading conditions were tested by increasing the inlet concentration from 1000 to 2500 mg m(-3) or shifting the gas flow rate from 0.1 to 0.3 m(3) h(-1), which led to reduced outlet concentrations in the UV-BF process compared with those of the control BF process. The standalone UV photodegradation of chlorobenzene can produce products with significant acute biotoxicity. Acute biotoxicities as high as 12 mg-Zn2+ L-1 were measured. Biotoxicity levels were reduced to less than 5 mg-Zn2+ L-1 after the biofilter. Ozone, a by-product produced during the UV photodegradation process, contributed to a reduction in bioaerosol emission from the biofilters and helped to control the biomass, thus slowing down the pressure drop increase in the biofilters. (C) 2009 Elsevier B.V. All rights reserved.