Anaerobic digestion (in the absence of oxygen) with anaerobic bacteria or methane fermentation is used worldwide for disposal of domestic, municipal, agricultural, and industrial biomass waste. This reaction generally produces methane and carbon dioxide and it also occurs in the ecosystem and in the digestive tract. As shown in the following reactions (6.1) and (6.2), hydrogen along with acetic and butyric acids can also be produced by dark fermentation processes using anaerobic and facultative anaerobic chemohetrotrophs [118].

C6O6H12 + 2H2O ^ 2CH3COOH + 4H2 (6.1)

C6O6H12 ^ CH3CH2CH2COOH + 2CO2 + 2H2 (6.2)

Different types of waste materials can also be used for hydrogen fermenta­tion. Hydrogen production is highly dependent on the pH, retention time, and gas partial pressure. Generally, hydrogen production increases with retention time. Biogas produced from landfills generally contains methane (about 55%) and carbon dioxide with traces of hydrogen, ethane, and other impurities. The following description of the sequence of biochemical reac­tions that occur to convert complex molecules to methane closely follow the excellent review of Weiland [118].

In general, methane fermentation can be divided into four phases: hydroly­sis, acidogenesis, acetogenesis/dehydrogenation, and methanation. As shown by Weiland [118], the degradation of complex polymers such as polysaccha­rides, proteins, and lipids results in the formation of monomers and oligomers such as sugars, amino acids, and long-chain fatty acids. The individual degra­dation steps are carried out by different consortia of micro-organisms, which place different requirements on the environment [119]. Hydrolyzing and fer­menting micro-organisms are responsible for the initial attack on polymers and monomers and produce mainly acetate, hydrogen, and varying amounts of fatty acids such as propionate and butyrate [118]. Hydrolytic micro-organ­isms excrete hydrolytic enzymes such as cellulose, amylase, lipase, and the like. Thus, a complex consortium of micro-organisms most of which are strict anaerobes such as Bacteriocides, Clostridia, and Bifidobacteria [118] participate in the hydrolysis and fermentation of organic material [118].

The higher volatile fatty acids are converted into acetate and hydrogen by obligate hydrogen-producing acetogenic bacteria. The maintenance of an extremely low partial pressure of hydrogen is very important for the acetogenic and hydrogen-producing bacteria. The present state of knowledge indicates that hydrogen may be a limiting substrate for methanogens [120]. This is because an addition of hydrogen-producing bacteria to the natural biogas-producing con­sortium increases daily biogas production. Studies [118] have shown that only two groups of methanogenic bacteria produce methane from acetate, hydrogen, and carbon dioxide. These bacteria are strictly anaerobes and require a lower radox potential for growth than most other anaerobic bacteria. Only few species are able to degrade acetate into CH4 and CO2, for example, Methanosarcina barkeri, Methanonococcus mazei, and Methanotrix soehngenii, whereas all methanogenic bacteria are able to use hydrogen to form methane [118].

The overall process of methane fermentation can be accomplished in two stages and a balanced anaerobic digestion process demands that in both stages the rates of degradation must be equal in size. If the first degradation step runs too fast, the acid concentration rises and pH drops below 7.0 which inhibits methanogenic bacteria. If the second phase runs too fast, methane production is limited by the hydrolytic stage. Thus the rate-limiting step

depends on the compounds of the substrate used for biogas production. Undissolved compounds such as cellulose, proteins, and fats take several days to crack whereas soluble carbohydrates crack in a few hours. Therefore the process design must be well adapted to the substrate properties for achieving complete degradation without process failure.

Numerous studies have been reported for anaerobic digestion to produce either methane or hydrogen from a variety of waste streams. Some of these studies are summarized in Table 6.7. Maximum gas yields and theoretical methane contents that can be generated from carbohydrates, raw protein, raw fat, and lignin are summarized in Table 6.8 [118]. The biogas generated from landfills generally contains about 50 to 55% methane and the remain­ing composition consists largely of CO2 and traces of water, hydrogen, and other impurities. It is clear from these studies that anaerobic digestion is a very widely used process to generate methane or hydrogen from a variety of organic wastes, both of which are important gaseous biofuels.