A new hybrid biological reactor (HBR) using powders as a bio-carrier was developed, and compared to conventional activated sludge (AS) with increasing substrate organic loading rate (OLR) from 1.0 to 4.0 kg COD/m(3) per day. The HBRs employed different powders of bentonite and clinoptilolite, respectively, and its concentration in an aeration basin was sustained at 500 and 4000 mg/l, separately. Biomass concentrations in the HBRs were greatly increased by biofilm formation attached on the powders, which were linearly augmented with OLR. The HBRs operated under 4000 mg/l of minerals concentration highly improved removal efficiencies of organic compounds as well as organic removal rates, although no significant advancement was found in the HBRs run under 500 mg/l, compared to conventional AS. This was probably attributed to high biomass concentration (the maximum value of 7100 mg MLVSS/l) by biofilms formed on the minerals. The efficiencies of total Kjeldahl nitrogen (TKN) removal and nitrification were decreased with increasing substrate COD to TKN (C/N) ratio. However, the HBR with 4000 mg/l of clinoptilolite concentration showed higher nitrification efficiency rather than that in other reactors, little depending on substrate C/N ratio varied. It could be concluded that high ammonium exchange capacity of the clinoptilolite enabled nitrfiers to colonize favourably in the biofilms on this mineral, which increased nitrification. (C) 2002 Elsevier Science Ltd. All rights reserved.