A pilot-scale H(2)-producing anaerobic sequencing batch reactor (ASBR) treating food waste was operated. During the operation, the carbon/nitrogen (C/N) ratio was adjusted from 10 to 30 by changing the composition of the food waste. When the C/N ratio was lower than 20, the H(2) yield was maintained at around 0.5 mol H(2)/mol hexose(added), accounting for 2.3% of energy conversion efficiency contained in food waste to H(2), but it gradually dropped at higher C/N ratios. The low performance was accompanied by increased production of lactate, propionate, and valerate. In order to recover the performance, alkaline shock (pH 12.5 for 1 day) was imposed on the entire mixed liquor in the fermenter. This alkaline shock method was so effective that the H(2) yield significantly increased to over 0.9 mol H(2)/mol hexose(added), and was then stabilized at 0.69 mol H(2)/mol hexose(added). In addition, the settling characteristics of H(2)-producing ASBR, which was separated into three layers, were investigated. Ribonucleic acid (RNA) as well as volatile suspended solid concentrations of each layer were measured to suggest how to enhance the H(2) production in ASBR operation. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.