Journal of Biotechnology 111 (2004) 297–309 Anaerobic bio-hydrogen production from ethanol fermentation: the role of pH Moon H. Hwang a , Nam J. Jang a , Seung H. Hyun b , In S. Kim a,∗ a Department of Environmental Science and Engineering, Bio-Environmental Engineering Lab (BEEL), Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea b Department of Civil & Environmental Engineering, University of Central Florida (UCF), P.O. Box 162450, Orlando, FL 32816-2450, USA Received 6 October 2003; received in revised form 16 March 2004; accepted 1 April 2004 Abstract Hydrogen was produced by an ethanol–acetate fermentation at pH of 5.0 ± 0.2 and HRT of 3 days. The yield of hydrogen was 100–200 ml g Glu -1 with a hydrogen content of 25–40%. This fluctuation in the hydrogen yield was attributed to the formation of propionate and the activity of hydrogen utilizing methanogens. The change in the operational pH for the inhibition of this methanogenic activity induced a change in the main fermentation pathway. In this study, the main products were butyrate, ethanol and propionate, in the pH ranges 4.0–4.5, 4.5–5.0 and 5.0–6.0, respectively. However, the activity of all the microorganisms was inhibited below pH 4.0. Therefore, pH 4.0 was regarded as the operational limit for the anaerobic bio-hydrogen production process. These results indicate that the pH plays an important role in determining the type of anaerobic fermentation pathway in anaerobic bio-hydrogen processes. © 2004 Elsevier B.V. All rights reserved. Keywords: Anaerobic bio-hydrogen production; pH effect; Ethanol–acetate fermentation; Methanogenic activity 1. Introduction The anaerobic process has been developed for the efficient treatment of waste and high organic wastew- ater. The one of advantages of the anaerobic process is the recovery of the useful matters such as solvents, VFA and methane. In general, the recovery of these useful matters in the anaerobic process has been fo- cused on methane only, which is the final product in ∗ Corresponding author. Tel.: +82-62-970-2436; fax: +82-62-970-2434. E-mail address: iskim@gist.ac.kr (I.S. Kim). the anaerobic process. However, the production and utilization of methane has the several problems such as the green house effect and the storage of methane. Conversely, hydrogen, whose recovery from anaerobic fermentation was suggested by several researchers, has been considered as ideal fuel, because residues are not produced in its utilization, except H 2 O(Fang and Liu, 2001; Zoetemeyer et al., 1982). In addition, the development of two phase anaerobic process (TPAP), which is operated separating acidogenesis (fermenta- tion) and methanogenesis, has suggested the potential to produce hydrogen and methane simultaneously. However, research on the anaerobic bio-hydrogen 0168-1656/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jbiotec.2004.04.024