Tugba Keskin and Patrick C. Hallenbeck

10.1 Introduction

The sustainability of economic growth and the ecology of the environment are under threat by rising petroleum prices and global warming. Much research is going into finding alternative reliable and effective energy sources. Among the energy sources under development, hydrogen is recognized as the most promising alternative to fossil fuels, and it is assumed that it will play a major role in the future energy supply because it is recyclable, energy efficient and clean energy carrier [1]. Hydrogen can be produced from fossil fuels by steam reforming or thermal cracking of natural gas, partial oxidation of hydrocarbons, coal gasifica­tion or pyrolysis. A second option is to produce hydrogen from water by elec­trolysis, photolysis, thermochemical processes or thermolysis. A third option is to produce hydrogen by biological processes such as biophotolysis of water by algae and cyanobacteria, photofermentation of organic substrates, or dark fermentation of organic substrates [2]. Among the various options for hydrogen production, biological production would appear to be the most efficient due to its low energy demand compared with physical and thermochemical processes, and due to its ability to use organic wastes. When biological hydrogen production is coupled with the treatment of organic wastes two main problems, the reduction of pollution from the uncontrolled degradation of waste, and the production of a clean fuel, can be solved [3].

T. Keskin • P. C. Hallenbeck (H)

Departement de Microbiologie et Immunologie, Universite de Montreal, CP 6128 Succursale Centre-ville, Montreal, QC H3C 3J7, Canada e-mail: patrick. hallenbeck@umontreal. ca

T. Keskin

Environmental Biotechnology and Bioenergy Laboratory, Bioengineering Department, Ege University, 35100 Bornova, Izmir, Turkey

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_10, © Springer-Verlag Berlin Heidelberg 2012

Dark fermentation processes are well-known biohydrogen production methods. Acidogenic bacteria like Enterobacter, Bacillus and Clostridium are the main groups of hydrogen producing bacteria which convert organic substrates (e. g. glucose and sucrose) into soluble metabolites, i. e. volatile fatty acids (VFAs) and alcohols, as well as hydrogen. On the other hand dark fermentation by mixed anaerobic consortia is assumed to be more economical since this process does not incur sterilization costs, and hydrogen can be produced continuously since there is no direct light requirement. Proper pretreatment method allows the hydrogen producing species to dominate in the mixed culture, leading to utilization of carbohydrates with the formation of hydrogen and VFAs, as well as biomass, growth which lowers the experimental yields below theoretical values [4].

Photofermentation is hydrogen production from organic acids in the presence of photoheterotrophic bacteria under illumination with visible light. Purple non-sulfur bacteria are the main hydrogen producing organisms capable of photofermenta­tion. The bottleneck of photofermentation systems is the source of organic acids used as substrate. Pure organic acids are too expensive for the practical sustainable production of hydrogen. Organic wastes can be inexpensive, but they are not pure and can have a variable composition, affecting systems operation. Organic wastes like starch and cellulose cannot be metabolized by photofermentative bacteria. Using dark fermentation as a first step to convert waste and complex products into organic acids, and then using the produced organic acids for photofermentation can increase the maximum yield of hydrogen production from 4 to 12 mol of H2/mol glucose theoretically [5]. Dark fermentation is an incomplete oxidation of substrate, therefore converting the remaining organic acids by photofermentation can improve overall hydrogen yields. For an economically viable process, it is very important to combine dark fermentation with photofermentation to obtain high yields of hydrogen production.