This study compared biological hydrogen production from glucose in two continuously stirred tank reactors (CSTRs) and two integrated biohydrogen reactor clarifier systems (IBRCSs) comprising CSTRs with gravity settlers to decouple the hydraulic retention time (HRT) from solids retention time (SRT). The four systems were operated at organic loading rates of 6.5-42.8 gCOD/L-d, and HRTs of 8-12 h. The SRT was maintained at 2 days in the two IBRCSs. The decoupling of SRT from HRT not only increased glucose conversion from 29-50% in the CSTR to 99.9% in the IBRCSs, but also the volumetric hydrogen production from 0.55-1.8 in the CSTRs to 2.4-9.6 L/L-d. Biomass yields in the two IBRCSs were 0.09 and 0.13 g VSS/g glucose converted, about 50% lower than the CSTR yields of 0.19 and 0.29 g VSS/g glucose converted. Hydrogen yield increased from 0.5-1.0 mol H(2)/mol glucose converted in the CSTR to 2.8 mol H(2)/mol glucose converted in the IBRCSs. The inverse relationship between hydrogen yield and biomass yield observed in this study implies that the hydrogen yield is maximized with the minimization of biomass yield, thus necessitating decoupling of SRT from HRT to ensure sufficient reactor biomass. DGGE analysis confirmed the specificity of the microbial culture in the IBRCSs with the high-hydrogen producing Clostridium species, as compared to the more diverse cultures in the CSTR. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.