A thermophilic two-stage fermentation process using 10% total solids (TS) food waste was tested at varying organic loading rates (OLRs). The system was configured to produce H-2 and CH4 in conjugation with the chemical oxygen demand (COD), nitrogen removal, and adjustment of the pH by returning sludge as an alkali buffer from the sludge storage tank for denitrification. The pH in the H-2 fermentation reactor was maintained in the range of 5.4-5.7 using sludge recirculation (Q(r)/Q(i) ratio 1). The average H-2 (11.1 I-H-2 l(-1)-fed d(-1)) and CH4 (47.4 I-CH4 l(-1)-fed d(-1)) production rates were achieved at OLRs of 39 (H-2 fermentation reactor) and 4.16 gCOD l(-1) d(-1) (CH, fermentation reactor), respectively. These results suggest that long-term stability of the continuous two-stage process can be successfully achieved by recirculation of high-alkalinity sludge of 6.7-7.5 g l(-1) as CaCO3, without any added external chemical buffer. (C) 2009 Elsevier Ltd. All rights reserved