Mass transfer of carbon dioxide in anaerobic reactors under dynamic substrate loading conditions

The objectives of the present investigations were (1) to discuss the dominant factors influencing mass transfer of carbon dioxide (CO2) and methane (CH4) in anaerobic reactors, (2) to survey theoretical approaches to estimate mass transfer coefficients kLa and (3) to provide experimental data in order to verify the proposed kLa-correlation which was then used in mathematical modelling of the anaerobic digestion process. CO2 mass transfer was found to be governed by the liquid phase resistance with the volumetric mass transfer coefficient kLa being a function of the actual biogas production rate. From measurements of the HCO3−-concentration and the pH in the liquid phase, the partial pressure of CO2 in the gas phase and the CO2-production rate under dynamic substrate loading conditions, the proposed correlation between kLa and upflow gas velocity was experimentally verified for three different wastewaters. Measured kLa-values for CO2-mass transfer in anaerobic reactors differed by a factor of 2–3 from steady-state to dynamic loading conditions. The experimental results matched with previously reported values and provided a significantly better simulation of CO2- and CH4-production in anaerobic reactors under dynamic substrate loading conditions.