Impact of Phased Digestion on Microbial Populations and Kinetics of Methanogens during Sludge Digestion

Various configurations of two-phase digesters with either identical or different temperatures were employed to study the impact of phase separation on the bio-kinetics of the methanogenic populations of each phase. Monod kinetic parameters were estimated for each phase using batch experiments. The results of this study revealed that the methanogen populations which developed in the phased anaerobic digestion systems differed between the phases. The results of this study indicate that two distinct species of acetoclastic methanogens should be employed when modeling phased anaerobic digesters. It was also found that use of temperature correction coefficients for bio-kinetic parameters would only be applicable for the digesters with relatively similar operational conditions (i.e. organic loading or HRT). Thus, modeling of these systems should not employ the same kinetic parameters for both phases as the model predictions for heavily loaded first-phase digesters will be sensitive to the parameters. On the basis of the differing bio-kinetic values of the methanogenic populations it is recommended that a mesophilic digester should be employed after either thermophilic or mesophilic first-phase digesters to achieve low concentrations of acetate in the final effluent. Achieving very low concentrations of acetate with thermophilic digesters (either phased or single-stage) is challenging due to the lower affinity of the thermophilic methanogens for acetate