Dark fermentation of acid hydrolyzed ground wheat starch for bio-hydrogen production by periodic feeding and effluent removal was investigated at different feeding intervals. Ground wheat was acid hydrolyzed at pH = 3 and T = 121 C for 30 min using an autoclave. The resulting sugar solution was subjected to dark fermentation with periodic feeding and effluent removal. The feed solution contained 9 +/- 0.5 g L(-1) total sugar supplemented with some nutrients. Depending on the feeding intervals hydraulic residence time (HRT) was varied between 6 and 60 h. Steady-state daily hydrogen production increased with decreasing HRT. The highest daily hydrogen production (305 ml d(-1)) and volumetric hydrogen production rate (1220 ml H(2) L(-1) d(-1)) were obtained at HRT of 6 h. Hydrogen yield (130 ml H(2) g(-1) total sugar) reached the highest level at HRT = 24 h. Effluent total sugar concentration decreased, biomass concentration and yield increased with increasing HRT indicating more effective sugar fermentation at high HRTs. Dark fermentation end product profile shifted from acetic to butyric acid with increasing HRT. High acetic/butyric acid ratio obtained at low HRTs resulted in high hydrogen yields. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.