Biomethanation of waste biomass

Abstract

The process of methanogenesis by which organic materials are converted anaerobically into methane-rich biogas by microorganisms has been studied intensively in recent years. The process involves (a) an initial step of enzymic hydrolysis of complex molecules to simple molecules, (b) an acidification step in which the products of hydrolysis such as glu- cose, long chain fatty acids, peptides, amino acids etc. are metabolized to form volatile acids, aldehydes, alcohols, carbon dioxide and hydrogen, and (c) lastly, a methanogenic step in which methane formation from CO2, H2 and acetate is brought about by methanogenic bacteria. In the second step, acidogens are able to store excess reducing equivalents by forming amixture of volatile acids of 3-5 carbon atoms which cannot be utilized by the methanogens. These are converted to acetate and dihydrogen by acetogens. It is generally considered that methanogenic step is rate limiting in the overall process of methane formation. But in the digestion of ligno-cellulosic waste, the rate limiting step is reported to be the hydrolysis of organic solids rather than the methanogenic step. With plant biomasses, a number of physical problems such as shape and size of the feed, bulk volume, feeding, mixing, non-homogeneity of the digestion mixture etc. are involved requiring special study. Kinetics of biomethanation is generally treated on the basis of Monod equation but its inadequacy in dealing with the digestion of complex substrates has been reported. The process of hydrolysis which is likely to be the rate limiting step in the digestion of complex organic waste has to be taken into account in the formulation of an adequate kinetic model for biomethanation. The thesis presents studies on the biomethanation of organic feeds under batch and continuous digestion systems leading to the development of modified batch digestion processes and kinetic models for biomethanation. Part I of the Thesis presents the state of art on the above subject. Part II describes the studies on the...