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UBC Theses and Dissertations

Two-phase methanation of lactose in biofilm reactors Yu, Jian


Anaerobic methanation of lactose, the main component of cheese whey, is an attractive alternative for the disposal of this wastewater because of its added benefit of recovering energy as methane. Two advances in this technology have been the separation of the bacteria in two reactors according to their substrates and the retention of the anaerobic bacteria in the form of bacterial biofilms on solid supports. The mass transfer rate of the substrates within the active biofilms must be known for the determination of an optimum biofilm thickness. A prerequisite for the investigation of substrate mass transfer rate in an active biofilm is the knowledge of intrinsic kinetics of the substrate utilization by the embedded bacterial cells. Start-up of symbiotic methanogenic biofilms was also investigated on inert supports. In recycled reactors, mesophilic (35 °C) acid-producing bacteria or methane-producing bacteria, cultured with lactose at a pH of 4.6 for the former or cultured with mixed acids (acetate, propionate and butyrate) at a pH of 7.1 for the latter , attached onto PVC sheets, forming thin acidogenic or methanogenic biofilms, respectively. After the external mass transfer resistance had been eliminated by increasing the recycle rate and the internal mass transfer resistance was minimized by using thin biofilms, the intrinsic kinetics of lactose acidogenesis and methanogenesis of organic acids were investigated in the newly formed acidogenic and methanogenic biofilms, respectively. The lactose digestion rate as well as the production of two main products, acetate and butyrate, can be described by Michaelis-Menten equations. The production of propionate, as a minor product, was depressed in the culture environment. The digestion of acetate could also be modelled by a Michaelis-Menten equation while the dissimilation of propionate and butyrate was affected by propionate concentration, the propionate digestion being promoted but the butyrate digestion being inhibited at high propionate concentrations. Two models have been proposed for their utilization. Substrate mass transfer in the active biofilms was investigated with a diffusion cell. After symmetric biofilms formed on the two membrane filters of the cell, the substrate concentrations on the biofilm surfaces and inside the cell were measured at steady state. The effective diffusivities of substrates in the active biofilms were estimated by numerically solving the diffusion-reaction equations using the intrinsic kinetics and the substrate concentrations as the boundary conditions. The effective diffusivity of lactose in an acidogenic biofilm was about 65.3 % of its diffusivity in water, and the diffusivities of acetate, propionate and butyrate in a methanogenic biofilm were reduced to about 30.2 % of the values in water. Comparing the fractional void volumes in the two types of biofilms showed that the methanogenic biofilm, which grew more slowly, had a more tortuous structure of channels than did the acidogenic biofilm. Studies on the build-up of symbiotic methanogenic biofilms were conducted using supports of wood, ceramic rings, PVC and stainless steel, which gave a range of water contact angle from 0° to 99.7°. The accumulation of acetate-, propionate- and butyrate-degrading bacteria on the supports were monitored by measuring the substrate utilization rates of each bacterial group in a standard batch culture which was seeded by the supports with the attached biofilms. The three types of bacteria had different preference to a hydrophilic support surface, butyrate degrader > acetate degrader > propionate degrader Based on the analysis of the process of biofilm formation, a model has been proposed. The parameters which depict the attachment of free cells onto clean surfaces were found to have a linear relationship with the water contact angles of the support surfaces.

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