International Journal of Hydrogen Energy 30 (2005) 1383 – 1391 www.elsevier.com/locate/ijhydene Semi-continuous solid substrate anaerobic reactors for H 2 production from organic waste: Mesophilic versus thermophilic regime IdaniaValdez-Vazquez a , Elvira Ríos-Leal a , Fernando Esparza-García a , Franco Cecchi b , Héctor M. Poggi-Varaldo a , ∗ a CINVESTAV-IPN, Department Biotechnology and Bioengineering, Environmental Biotechnology R&D Group, P.O. Box 14-740, Mexico D.F. 07000, Mexico b Universitá degli Studi di Verona, Verona, Italy Available online 8 December 2004 Abstract We evaluated the influence of the operation temperature (mesophilic vs. thermophilic regime) of semicontinuous, acidogenic solid substrate anaerobic digestion (A-SSAD) of the organic fraction of municipal solid waste (OFMSW) at lab scale. The H 2 percentage was higher in the thermophilic regime than in the mesophilic operation (58% and 42%, respectively). The H 2 yield of thermophilic A-SSAD was significantly higher than in our mesophilic reactors (360 vs. 165 NmL H 2 /g VS rem ) and other studies reported in the literature (range of 62–180 mL/g VS). Mesophilic A-SSAD showed a yield of 37% of the maximum yield based on 4 mol H 2 /mol hexose, while thermophilic A-SSAD exhibited a yield of 80% of the maximum yield. This result is similar to works with pure cultures of extremophile microorganisms where H 2 yields of 83% of the maximum were reported. We found higher concentrations of acetic acid in the digestates of thermophilic A-SSAD, while butyrate was in higher proportion in mesophilic A-SSAD spent solids. The moderate-to-high yields obtained with the semicontinuous process used in this work are in disagreement with previous reports claiming that batch and semicontinuous processes are less efficient than continuous ones.  2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Acidogenic fermentation; Biological hydrogen production; Organic wastes; Semi-continuous; Temperature 1. Introduction During the past 10 years there has been a renewed interest in new technologies that could supply energy in an environmentally friendly, sustainable way [1]. From an environmental viewpoint, there is an urgent need for ∗ Corresponding author. Tel.: +52 555061 3800x4324; fax: +52 55 5061 7002. E-mail addresses: valdez_idania@yahoo.com (I. Valdez-Vazquez), hectorpoggi2001@yahoo.com (H.M. Poggi-Varaldo). 0360-3199/$30.00  2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.09.016 appropriate management of municipal solid wastes (MSW). Nearly 1600 million tonne/year of MSW are generated worldwide with up to 43% contributed by Asia and Oceania and 28% contributed by North America and the European Union [2]. On average, almost 50% of the MSW of underde- veloped countries consists of a fermentable, biodegradable fraction. The anaerobic digestion of the organic fraction of mu- nicipal solid waste (OFMSW) for generation of methane and a soil amender has received increased interest in the last 15 years [3–9]. Yet, even the use of methane as a fuel could be debatable, due to the production of CO 2 known to