Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Chemical Engineering


Microbial production of methane from alkaline sweet potato wastes was studied. Assessment of methane production potential was based on total COD of the wastes. A single-stage and a two-stage system were studied. In both systems, to ensure stable operation and high performance, methane fermenters had to be initially seeded with large quantities of methane formers. A 50% inoculum (based on total fermenter volume) was found to be most effective. Methane formers tended to aggregate to form spherical particles which had extremely high settling rates, this eliminated the requirement of cell recycle. They obtained the nutrients required for their growth through lysis of acid-forming cells. Thus complete removal of acid-forming cells from the effluent leaving the first stage acid fermenter of a two-stage system resulted in failure of the second stage methane fermenter. In both single-stage and two-stage systems the rates of gas production was sufficiently fast to induce thorough mixing of the fermenter contents. At low residence times of two and four days the two-stage system achieved significantly higher conversions. Gas production started almost immediately after feeding the methane fermenter of the two-stage system. Therefore, in such systems energy can be stored conveniently in the form of liquid organic acids produced in the acid fermenter. The conversions in the methane fermenter of a two-stage system could be predicted by a model based on Contois' kinetics. The kinetic constant K(,c)Y was obtained as 0.16. The composition of the gas produced in this fermenter could also be predicted from the distribution of the organic acids in the effluent from the acid fermenter. The acid formation stage was studied in a chemostat operated at a fixed residence time of 5.5 hours. The inoculum source was mixed acid formers derived from ruminant fluid. The substrate was glucose in a simple salt media. The highest yield of 0.09 g protein/g glucose consumed was obtained at pH 5.5 and 37(DEGREES)C.