Biohydrogen production from palm oil mill effluent (POME) using Thermoanaerobacterium thermosaccharolyticum PSU-2-rich sludge was conducted in a 5 L continuous stirred tank reactor (CSTR). Mixing of effluent is crucial factor for hydrogen yield and net energy gain. Two types of turbines, namely the Rushton turbine and paddle turbine at 10, 50, 100, and 150 rpm were the comparative factors in this study. The operating parameters were 60 degrees C, 24-h hydraulic retention time (HRT), and 55 gCOD L-influent(1) d(1) organic loading rate (OLR). Computational fluid dynamics (CFD) was employed to understand the flow pattern, including dead zones and swirls of effluent in the CSTR. Mixing by Rushton turbine developed lesser dead zone and better mixing swirl than those of paddle turbine. At all mixing speeds, higher hydrogen yield was obtained when a Rushton turbine was applied. The highest hydrogen yield was 6826 mL H-2 L-POME(1)(350 mL H-2 gCOD(1)) at 150 rpm. However, the operating speed to achieve the maximum net energy gain (of 29.95 kJ) was 100 rpm. (C) 2017 Elsevier Ltd. All rights reserved.