Introduction fermentation biogas and biohydrogen production from food waste

Food processing waste has significant potential to pollute land, air, and water because of its high chemical oxygen demand (COD) and sheer volume. The COD concentration can be 90 000 mg/L or more, which is more than 100 times greater than common domestic sewage. It may also have a moderately high salt or acidity content and it might be contaminated with pathogens. Generally, food processing waste does not contain significant amounts of toxic chemicals.

The fact that food waste is generally organic and non-toxic means it lends itself to biological treatment. Biological wastewater treatment is primarily used to remove dissolved and colloidal organic substances in a wastewater stream. Organic substances in water decay due to the presence of microorganisms that are naturally occurring. Several biological processes have been utilized (Mawson, 1994; Gonzalez Siso, 1996). In aerobic treatment, organic materials are converted into oxidized end products, which are mostly carbon dioxide and new bacterial mass (Schugerl, 1997). The anaerobic wastewater treatment process converts organic materials primarily into methane, a fuel that can yield a net energy gain from process operations and carbon dioxide (McCarty and Smith, 1986).

During anaerobic fermentation, hydrogen is produced. In a normally functioning anaerobic digester, it is rapidly consumed by hydrogen-utilizing methanogens as they reduce carbon dioxide to methane (Gujer and Zehnder, 1983; Huang et al., 2000). For that reason, hydrogen gas had been considered only as a process control index and an indicator of organic shock loading because it is rarely detected unless methanogenesis is disturbed by external environments, despite significant amounts of hydrogen being produced in reality (Archer et al., 1986; Huang et al., 2000). Hydrogen is considered a promising alternative clean energy source, which produces no greenhouse gases and is more economical than methane at less than stoichiometric yields, if it is captured during anaerobic fermentation. To extract hydrogen effectively from an anaerobic reactor requires special procedures to block out the co-metabolic chains (Adams and Stiefel, 1998), this will be discussed further in the chapter.

Most of the research and development on hydrogen production from organic materials has focused on the use of photosynthetic and fermentative bacteria. The latter is preferred, because it does not rely on the availability of a solar conversion process with a large surface area and transparency of the media (Zaborsky, 1998). An advantage of the anaerobic fermentative route is the fact that hydrogen can be produced directly from organic wastewater as raw material. This has considerable potential as an environmentally friendly process that does not consume fossil fuels (Billings, 1991; Nandi and Sengupta, 1998).

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