S. Venkata Mohan

Abstract The global impact of increasing energy demands, depleting reserves of fossil fuels and increasing pollution loads on the environment due to the utilization of energy produced from fossil fuels have received considerable notice in recent years. Generation of energy from fossil fuels is generally convenient but the deplet­ing reserves and associated global warming are major problems. One potential alternative is a shift from fossil fuel to a hydrogen (H2) based economy. H2 is con­sidered to be a clean energy carrier with high-energy yield (142.35 kJ/g) and upon combustion it produces only water. H2 can be produced by the biological routes of bio-photolysis, photo-fermentation and dark fermentation or by a combination of these processes. Dark fermentation offers the particular advantage of using wastewater as a substrate and mixed culture as catalyst. Wastewater contains high levels of biodegradable organic material with net positive energy. One way to reduce the cost of treatment is to generate bio-energy, such as H2 gas by metabolically utilizing organic matter, at the same time accomplishing treatment. This chapter mainly focuses on the evaluation of fermentative H2-generating processes utilizing wastewater as substrate and mixed culture as biocatalyst. A particular insight was also laid on to discuss the process based on important operating factors involved and to delineate some of the limitations. Various strategies such as multiple process integration, microbial electrolysis, polyhydroxyalkanoate (PHA) production, bioaugmentation, self-immobilization and metabolic engineering were discussed in overcoming some of the limitations in the direction of process enhancement.

Keywords Biohydrogen ■ Anaerobic ■ Dark fermentation ■ Wastewater treatment ■ Acidogenic ■ Pretreatment ■ Bioelectricity ■ Microbial fuel cell (MFC) ■ Microbial electrolysis ■ Bioaugmentation ■ Polyhydroxyalkanoates (PHA) ■ Mixed culture ■ Immobilization

S. Venkata Mohan (B)

Bioengineering and Environmental Centre (BEEC),

Indian Institute of Chemical Technology (IICT), Hyderabad-500007, India e-mail: vmohan_s@yahoo. com; svmohan@iict. res. in

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_7, © Springer Science+Business Media B. V. 2010

1 Introduction

Hydrogen (H2) is a potentially sustainable energy carrier as because it produces only water and has a high energy yield of 122 kJ/g; 2.75 fold greater than that of hydrocarbon fuels and can be made from renewable resources, although at present nearly 90% of H2 is produced from steam reformation of natural gas or light oil fractions at high pressure and high temperatures.

Biological H2 production proceeds through two main pathways: photosynthesis and dark fermentation. Photosynthesis is a light-dependent process, comprised of direct biophotolysis, indirect biophotolysis and photo-fermentation, while anaero­bic fermentation, also known as dark fermentation, is a light-independent catabolic process [1-4]. Photosynthetic microorganisms, such as algae, photosynthetic bac­teria and cyanobacteria manifest H2 production in photosynthetic processes [5-6] while, fermentative microorganisms generate H2 during the acidogenic phase of the anaerobic digestion. Fermentative processes yield comparatively better H2 produc­tion than the photosynthetic process and do not rely on the availability of light. They also utilize a variety of carbon sources such as organic compounds, wastes, wastew­aters or insoluble cellulosic materials, require less energy, are technically much simpler, have lower operating costs and are more stable [3, 7-12]. Fermentative microorganisms also generally have rapid growth rates. Dark fermentation is a practically a more feasible process for the mass production of H2. H2 generation via biological routes is relatively pollutant free, requires low energy inputs and is therefore considered as a potential alternative to the conventional physical/chemical methods used for H2 production. Most of the biological H2 production processes are operated at ambient temperature and pressure, thus are less energy intensive. Research on photo-biological routes of H2 production was initially reported with specific strains and defined medium. Subsequently, dark fermentation gained impor­tance due to its feasibility of utilizing wastewater as a fermentative substrate and mixed cultures as biocatalysts. The process simplicity and efficiency are strong features.