<![CDATA[Reliable bioreactor engineering is essential for achieving reproducible performance in anaerobic fermentation, particularly for processes aimed at biohydrogen production. This study aimed to experimentally validate the engineering performance of a laboratory-scale stirred bioreactor developed for anaerobic fermentation of oil palm empty fruit bunch (OPEFB) hydrolysate toward biohydrogen-oriented processing. The bioreactor had a total volume of 3 L and a working volume of 2 L, with a height-to-diameter ratio of 2:1, dual 45° pitched-blade turbine impellers, and a baffled configuration. Engineering validation included gas-tightness, mixing performance, flow-regime assessment, temperature-monitoring accuracy, agitation-motor thermal behaviour, and electrical energy demand. The reactor maintained an internal pressure of 20 psi without detectable leakage. Tracer-based analysis showed a mixing time of 60 s based on the predefined homogeneity criterion of H₀ ≤ 0.05, while Reynolds number analysis indicated operation in the transitional flow regime (Re = 2.5 × 10³). Temperature monitoring showed an accuracy of 97.60–98.92% across the tested temperature range. Intermittent agitation reduced electrical energy consumption by 9.97% compared with continuous agitation. Fermentation verification was subsequently conducted through 72 h batch processing of OPEFB hydrolysate, resulting in 65.96 L of cumulative gas production and 76.9% glucose utilisation. HPLC and GC–MS analyses indicated substantial substrate conversion and the formation of fermentation-derived metabolites, predominantly lactate- and succinate-related compounds, providing evidence of active anaerobic fermentation relevant to biohydrogen-oriented processing. Overall, the validated bioreactor demonstrated stable hydrodynamic and operational performance and provided a reliable laboratory platform for anaerobic fermentation of OPEFB hydrolysate toward biohydrogen production.]]>
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