Continuous operation of a new bioreactor for air pollution control called the foamed emulsion bioreactor (FEBR) has been investigated. The effect of several liquid feeding strategies was explored. The FEBR exhibited high and steady toluene removal performance (removal efficiency of 89%-94%, elimination capacity of 214-226 g/m(3)h at toluene inlet concentration of 1 g/m(3)) for up to 360 h, when 20% of the culture was replaced every 24 h by a nutrient solution containing 4 g/L of potassium nitrate as a nitrogen source. This feeding mode supported a high cell activity measured as INT reduction potential and active cell growth without being subject to nitrogen limitation. In comparison, operating the FEBR with the liquid in a closed loop (i.e., batch) resulted in a significant decrease of both the removal efficiency of toluene and INT reduction activity. Operation with feeding active cells resulted in stable and effective treatment, but would require a significant effort for mass culture preparation. Therefore, the continuous process with periodically feeding nutrients was found to be the most practical and effective operating mode. It also allows for stable operation, as was shown during removal of low concentration of toluene or after pollutant starvation. Throughout the study, INT reduction measurements provided insight into the process. INT reduction activity data proved that under normal operating conditions, the FEBR performance was limited by both the kinetics and by mass transfer. Overall, the results illustrate that engineered gas-phase bioreactors can potentially be more effective than conventional biofilters and biotrickling filters for the treatment of air pollutants such as toluene. (c) 2005 Wiley Periodicals, Inc.