Biokinetics and bacterial communities of propionate oxidizing bacteria in phased anaerobic sludge digestion systems

Phased anaerobic digestion is a promising technology and may be a potential source of bioenergy production. Anaerobic digesters are widely used for sewage sludge stabilization and thus a better understanding of the microbial process and kinetics may allow increased volatile solids reduction and methane production through robust process operation. In this study, we analyzed the impact of phase separation and operational conditions on the biokinetic characteristics and communities of bacteria associated with four phased anaerobic digestion systems. In addition to significant differences between bacterial communities associated with different digester operating temperatures, our results also revealed that bacterial communities in the phased anaerobic digestion systems differed between the 1st and 2nd phase digesters and we identified strong community composition correlations with several measured physicochemical parameters. The maximum specific growth rates of propionate oxidizing bacteria (POB) in the mesophilic and thermophilic 1st phases were 11 and 23.7 mgCOD mgCOD(-1) d(-1), respectively, while those of the mesophilic and thermophilic 2nd-phase digesters were 6.7 and 18.6 mgCOD mgCOD(-1) d(-1), respectively. Hence, the biokinetic characteristics of the FOB population were dependent on the digester loading. In addition, we observed that the temperature dependency factor (0) values were higher for the less heavily loaded digesters as compared to the values obtained for the 1st-phase digesters. Our results suggested the appropriate application of two sets of POB biokinetic that reflect the differing growth responses as a function of propionate concentration (and/or organic loading rates). Also, modeling acetogenesis in phased anaerobic sludge digestion systems will be improved considering a population shift in separate phases. On the basis of the biokinetic values estimated in various digesters, high levels of propionate in the thermophilic digesters may be best explained by the establishment of FOB with low affinities (high K-s) for propionate. Achieving low levels of propionate with either thermophilic or short HRT digesters is challenging and a relatively long HRT mesophilic digester should be employed for this purpose. (C) 2012 Elsevier Ltd. All rights reserved.