A novel hybrid activated sludge baffled reactor (HASBR), which contained both suspended and attached-growth biomass perfect mixing cells in series, was developed by installing standing and hanging baffles and introducing plastic brushes into a conventional activated sludge (CAS) reactor. It was used for the treatment of domestic wastewater. The effects on the operational performance of developing the suspended and attached-growth biomass and reactor configuration were investigated. The change of the flow regime from complete-mix to plug-flow, and the addition of plastic brushes as a support for biofilm, resulted in considerable improvements in the COD, nitrogen removal efficiency of domestic wastewater and sludge settling properties. In steady state, approximately 98 +/- 2% of the total COD and 98 +/- 2% of the ammonia of the influent were removed in the HASBR, when the influent wastewater concentration was 593 +/- 11 mg COD/L and 43 +/- 5 mg N/L, respectively, at a HRT of 10 h. These results were 93 +/- 3 and 6 +/-3% for the CAS reactor, respectively. Approximately 90 +/- 7% of the total COD was removed in the HASBR, when the influent wastewater concentration was 654 +/- 16 mg COD/L at a 3 h HRT, and in the organic loading rate (OLR) of 5.36 kg COD m(-3) day(-1). The result for the CAS reactor was 60 +/- 3%. Existing CAS plants can be upgraded by changing the reactor configuration and introducing biofilm support media into the aeration tank. (C) 2007 Elsevier B.V. All rights reserved.