Characterization of Propionate Utilizing Acetogens in Phased Anaerobic Sludge Digestion Systems

The impact of phase separation and operational conditions on the bio-kinetic characteristics of propionate oxidizing bacteria (POB) developed in each phase of phased anaerobic sludge digesters with either identical or different temperatures was investigated. The results of this study revealed that the POB which developed in the phased anaerobic digestion systems differed between the phases, indicating dominance of fast-growing species in the heavily loaded 1st-phase and slow-growing species within the 2nd-phase digesters. It was found that the maximum specific growth rate in the mesophilic and thermophilic 1st-phase was 11.6 d−1 and 24.5 d−1, respectively; while those of the mesophilic and thermophilic 2nd -phase digesters were, on average, 5.9 d−1 and 18.5 d−1, respectively. It is recommended that different species of POB be employed in the modeling of phased anaerobic digesters to reflect population shifts on the basis of propionate concentration (and/or organic loading rates). Modeling of acetogenesis in anaerobic sludge digestion would be improved through application of at least two sets of bio-kinetic parameters, allowing the prevalence of one species in each condition. The relationship between POB biokinetic parameters and temperature was found to be inconsistent and the application of Arrhenius-like equations for temperature correction of kinetic coefficients should be applied with caution. On the basis of the bio-kinetic values estimated for the various digesters, achieving low levels of propionate with either thermophilic or short HRT digesters is challenging due to a low affinity of the POB developed in these digesters. Therefore, a mesophilic digester should be employed following either a thermophilic or mesophilic 1st-phase digester to accomplish low concentrations of propionate in the effluent.