KINETIC STUDY OF DEFINED METHANE FERMENTATION OF LACTOSE
Abstract
A defined microbial system consisting of serial fermentations by homo-lactic, homo-acetic and methanogenic bacteria was developed for producing methane from lactose. Bacteria selected for each fermentation step were S. lactis, C. formicoaceticum, and M. mazei, respectively. S. lactis grew homo-fermentatively on lactose with a yield of 0.9 g lactic acid/g lactose at 35(DEGREES)C, pH 7.0. C. formicoaceticum performed homo-acetic fermentation when it was grown on lactate at mesophilic temperature. 0.98g of acetic acid was formed from each gram of lactic acid consumed. M. mazei strain S6 grew on acetate by converting it into CH(,4) and CO(,2). About equimolar CH(,4) was formed from acetate consumed. Kinetic models for these bacteria were developed for batch fermentation at 35(DEGREES)C, pH 7.0. A mathematical model for mixed culture batch fermentation by these bacteria grow in a medium initially containing lactose as the sole substrate was also developed and used to simulate this batch fermentation. Methanogenesis from acetate was the slowest fermentative step while formation of lactate from lactose was the fastest one in this mixed culture fermentation. Accumulation of acid during the fermentation was usually observed. In a balanced growth, interactions among these bacteria included both commensalism and mutualism, i.e. one bacteria lived off the fermentative product of the other while benefiting it by consuming its inhibitory product. Fermentation kinetics of methanogenesis from whey lactose was dependent on environmental conditions when microbial system was not well-defined. A mathematical model for a completely mixed anaerobic filter was developed from the defined methane fermentation kinetics. This model could simulate the methane fermentation of a undefined system. The defined culture was found to be superior to undefined seeding (sewage sludge) in both start-up and steady-state performances. Balanced bacterial population was important to successful methane fermentation, especially for reactor start-up. Steady-state performance was dependent on the organic loading rate and cell density in the bioreactor. A two-staged process with lactic acid as the intermediate product was most suitable to the defined methane fermentation studied.
Degree
Ph.D.
Subject Area
Chemical engineering
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