Effect of mass transfer on concentration wave propagation during anaerobic digestion of solid waste

A distributed model of anaerobic digestion of solid waste was developed to study effects of mass transfer on the rate of propagation of initiation methanogenic area. The diffusion and advection of volatile fatty acids (VFA) and methanogenic biomass were taken into account in the model of a one-dimensional (1D) reactor. It was considered that VFA inhibits both polymer hydrolysis and acetoclastic methanogenesis. This approach allows to view the bioreactor as an active medium that provokes concentration waves from some area of methanogenic initiation (local VFA depression) to the total reactor volume. The model shows that mass transfer-based acceleration of methane production in the reactor is possible when the intensity of VFA utilization in the methanogenic area is sufficient for a complete digestion of the incoming VFA. Otherwise, initiation methanogenic area will be suppressed by increasing concentration of VFA. The obtained results emphasize the importance of considering spatial heterogeneity of the reaction for the analysis of solid anaerobic digestion in bioreactors and landfills. The digestion of solid waste can be optimized by setting the low rate of mass transfer (mixing or leachate recirculation) during lag-phase of the reaction with subsequent increase in the mass transfer rate in parallel with the propagation of methanogenic population. In this case, the rate of concentration waves substantially increases.