For the removal of toxic and recalcitrant organic substances intermittently appearing in wastewater, bioaugmentation with bacteria having specific degradation ability could be a powerful tool to improve the treatment process. 2,4-Dichlorophenol (2,4-DCP) was chosen as the target recalcitrant substance and a 2,4-DCP degrading special mixed culture was used as bioaugmentation microorganisms. The feasibility and strategies to combine bioaugmentation into a conventional activated sludge (CAS) system in terms of enhancing its efficiency and reliability was investigated. Results showed that for domestic wastewater with multiple chlorophenols, bioaugmentation with a 2,4-DCP degrading culture in a CAS system not only enhanced the removal of 2,4-DCP effectively, but also improved the removal of other chlorophenols such as 4-monochlorophenol (4-MCP) and 2,4,5-trichlorophenol (2,4,5-TCP). A separate bioaugmented bioreactor was combined into the original CAS system at different locations and the effects of the bioaugmentation location on the performance of the combined biotreatment process were studied. Results indicated that the CAS-Bioaug system, in which the bioaugmented bioreactor was set at a location after the original CAS reactor, performed better than the Bioaug-CAS system, in which the bioaugmented bioreactor was placed before the original CAS reactor. Bioaugmentation could be used as an effective and efficient method to improve a CAS process facing sudden toxic pollutant shock loading. (C) 2003 Elsevier Ltd. All rights reserved.