Estimation of substrate effective diffusivities in anaerobic bioparticles

A simple and rapid method to estimate effective diffusivities (D-e) of substrates in bioparticles containing anaerobic sludge is proposed. The method is based on substrate mass balances under unsteady-state conditions utilizing data obtained from 'finite-bath' assays performed in an incubator shaker. The experiments were carried out under conditions of negligible liquid-phase mass transfer resistance and absence of biochemical reactions. The first condition was obtained by the application of high agitation speeds. The biomass was inactivated with ethanol solution 50% (v/v) to reach the second condition. Values of the effective diffusivity (D-e) of glucose, sucrose and acetate were obtained from the temporal variation of substrate concentration in the bulk liquid in assays utilizing alginate gel and polyurethane foam bioparticles. Additionally, the specific methanogenic activity technique was used to evaluate the effectiveness of the method proposed for biomass inactivation. The method used to estimate the substrate diffusivity coefficient was found to provide suitable values of D-e for substrates at very low concentrations. Values of D-e for glucose, sucrose and acetate in alginate gel bioparticles were estimated as 0.58 x 10(-5), 1.06 x 10(-5) and 1.49 x 10(-5) cm(2) s(-1), respectively. The value of D-e for glucose in polyurethane foam particles was found to correspond to the value of glucose diffusion in water at infinite dilution. The method applied for cell inactivation was found to be adequate for intraparticle mass transfer experiments, making the temporary sludge inactivity long enough to avoid interference due to biochemical reactions.