In the microbial fermentation of biomass, different waste materials can be used as substrates. A new and unique process has been developed where substrates such as carbohydrates are fermented by a consortium of bacteria; they produce hydro­gen and carbon dioxide. Highly concentrated organic waste water is one of the most abundantly available biomasses that can be exploited for microbial conver­sion into hydrogen (Nath and Das 2003). Municipal solid wastes and digested sewage sludge have the potential to produce large amounts of hydrogen by sup­pressing the production of methane through the introduction of low-voltage elec­tricity into the sludge. The substrate from the acidogenesis of fruit and vegetable market wastes gives higher hydrogen evolution rates (about threefold) compared to synthetic medium. A mixed culture of photosynthetic anaerobic bacteria provides a method of utilization of a variety of resources for biohydrogen production (Miyake et al. 1990).

Hydrogen produced by photosynthetic organisms is one of a range of popular scenarios for renewable energy. Hydrogen can be produced by algae under specific conditions. Three different ways to produce hydrogen have been proposed: direct and indirect photolysis and ATP-driven hydrogen production. Direct photolysis is possible when the resulting hydrogen and oxygen are continuously flushed away.

Photosynthetic water splitting are coupled, results in the simultaneous production of hydrogen and oxygen. This results in major safety risk and costs to separate the hydrogen and oxygen. Major factors affecting the cost of hydrogen production by microalgae are the cost of the huge photobioreactor and the cost of hydrogen storage facilities that guarantee continuous hydrogen supply both during the night or during cloudy periods of the day.

Anaerobic hydrogen production proceeds photofermentatively as well as without the presence of light. Anaerobic bacteria use organic substances as the sole source of electrons and energy, converting them into hydrogen.

Glucose + 2H2O ! 2Acetate + 2CO2 + 4H2 Glucose! Butyrate + 2CO2 + 2H2

The reactions involved in hydrogen production (Equations 5.4 and 5.5) are rapid, and these processes do not require solar radiation, making them useful for treating large quantities of wastewater by using a large fermentor.

Since they cannot utilize light energy, the decomposition of organic substrates is incomplete. Further decomposition of acetic acid is not possible under anaero­bic conditions. Nevertheless, these reactions are still suitable for the initial steps of wastewater treatment and hydrogen production followed by further waste treatment stages.

A new fermentation process that converts valueless organic waste streams into hydrogen-rich gas has been developed by Van Ginkel et al. (2001). The process em­ploys mixed microbial cultures readily available in nature, such as compost, anaer­obic digester sludge, soil, etc., to convert organic wastes into hydrogen-rich gas. The biodegradation efficiencies of the pollutants were examined by changing the hydraulic retention time (HRT) as a main operating variable. An enriched culture of hydrogen-producing bacteria such as Clostridia was obtained by heat treatment, pH control, and HRT control of the treatment system. The biohydrogen fermentation technology could enhance the economic viability of many processes utilizing hy­drogen as a fuel source or as raw material. Figure 5.9 shows the basic components of an anaerobic digestion system.

Anaerobic fermentative microorganisms, cyanobacteria, and algae are suitable in the biological production of hydrogen via hydrogenase due to reversible hydro- genases (Adams 1990). Cyanobacteria and algae can carry out the photoevolution of hydrogen catalyzed by hydrogenases. The reactions are similar to electrolysis involving splitting of water into oxygen and hydrogen (Gaffron 1940).

Biological hydrogen can be generated from plants by biophotolysis of water using microalgae (green algae and cyanobacteria), fermentation of organic com­pounds, and photodecomposition of organic compounds by photosynthetic bacte­ria. To produce hydrogen by fermentation of biomass, a continuous process using a nonsterile substrate with a readily available mixed microflora is needed (Hussy et al. 2005). A successful biological conversion of biomass into hydrogen depends strongly on the processing of raw materials to produce feedstock, which can be fer­mented by the microorganisms.

Hydrogen-producing bacteria (Clostridia) were found to have growth rates about 5 to 10 times higher than that of methane-producing bacteria (Van Ginkel et al. 2001). In a continuous-flow bioreactor system, hydrogen production showed a de­clining trend in the later stages of reactor operation. Based on these findings, it is hypothesized that Clostridia may have gone through a phenomenon known as "de­generation” in which they lose their ability to produce hydrogen. Therefore, inocu­lating fresh mixed cultures may be a feasible way to maintain sustainable hydrogen production. Based on this hypothesis, a two-stage anaerobic reactor has been pro­posed. The first-stage reactor is designed as a hydrogen-producing reactor, whereas the second-stage reactor will be employed to cultivate fresh seed culture to perpetu­ally supply to the first one.