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Methane Prodcution - 1st Step Hydrolysis and MicroOrganisms
Methane Prodcution - 1st Step Hydrolysis and MicroOrganisms
http://www.fao.org/docrep/w7241e/w7241e0f.htm
Methane Prodcution - 1st Step Hydrolysis and MicroOrganisms
http://www.fao.org/docrep/w7241e/w7241e0f.htm
4.1 Microbial consortia and biological aspects of methane fermentation
Methane fermentation is the consequence of a series of metabolic interactions among various groups of microorganisms. A description of microorganisms involved in methane fermentation, based on an analysis of bacteria isolated from sewage sludge digesters and from the rumen of some animals, is summarized in Fig. 4-1. The first group of microorganisms secrete enzymes which hydrolyze polymeric materials to monomers such as glucose and amino acids, which are subsequently converted to higher volatile fatty acids, H2 and acetic acid (Fig. 4-1; stage 1). In the second stage, hydrogen-producing acetogenic bacteria convert the higher volatile fatty acids e.g., propionic and butyric acids, produced, to H2, CO2, and acetic acid. Finally, the third group, methanogenic bacteria convert H2, CO2, and acetate, to CH4 and CO2.
..........Polymeric materials such as lipids, proteins, and carbohydrates are primarily hydrolyzed by extracellular, hydrolases, excreted by microbes present in Stage 1 (Fig. 4-1). Hydrolytic enzymes, (lipases, proteases, cellulases, amylases, etc.) hydrolyze their respective polymers into smaller molecules, primarily monomeric units, which are then consumed by microbes. In methane fermentation of waste waters containing high concentrations of organic polymers, the hydrolytic activity relevant to each polymer is of paramount significance, in that polymer hydrolysis may become a rate-limiting step for the production of simpler bacterial substrates to be used in subsequent degradation steps.
Polysaccharides such as cellulose, starch, and pectin are hydrolyzed by cellulases, amylases, and pectinases. The majority of microbial cellulases are composed of three species: (a) endo-(3-l,4-glucanases; (b) exo-p-l,4-glucanases; (c) cellobiase or p-glucosidase. These three enzymes act synergistically on cellulose effectively hydrolyzing its crystal structure, to produce glucose. Microbial hydrolysis of raw starch to glucose requires amylolytic activity, which consist of 5 amylase species: (a) a-amylases that endocleave a±1-4 bonds; (b) p-amylases that exocleave a±1-4 bonds; (c) amyloglucosidases that exocleave a±l-4 and a±l-6 bonds; (d) debranching enzymes that act on a±l-6 bonds; (e) maltase that acts on maltose liberating glucose. Pectins are degraded by pectinases, including pectinesterases and depolymerases. Xylans are degraded with a²-endo-xylanase and a²-xylosidase to produce xylose.
Thus, in hydrolysis and acidogenesis (Fig. 4-1; Stage 1), sugars, amino acids, and fatty acids produced by microbial degradation of biopolymers are successively metabolised by fermentation endo-products such as lactate, propionate, acetate, and ethanol by other enzymatic activities which vary tremendously with microbial species.