Response to Temperature Transitions and Organic Loading Rates in Mesophilic Biogas Digesters

The effect of changes in temperature and organic loading rates (OLRs) on mesophilic biogas production was evaluated. Laboratory-scale digesters (2 l) were inoculated with biomass from a full-scale sludge-based digester operated at 34 ºC. The temperature was increased incrementally from 34 ºC to 37 ºC and 40 ºC, and the OLR was changed from 2.0, to 3.0, and 4.5 g volatile solids (VS) l-1 d-1. Methane (CH4) production positively correlated with minor temperature differences and increased by 6-11% as the temperature increased from 34 ºC to 37 ºC or 40 ºC. Increases in the OLR were effective up to 3.0 g VS l-1 d-1, and the CH4 yield at this rate increased by 10% compared with that at 2.0 g VS l-1 d-1. However, a further increase of the OLR inhibited the reactor performance. Bacteroidetes, Firmicutes and Chloroflexi were the predominant bacterial phyla. The relative abundance of syntrophic bacteria declined with an increase in OLR. Candidatus Methanofastidiosum was predominant initially, although Methanosaeta and Methanobacterium were predominant in later phases. The results demonstrate the significance of minor temperature differences and the OLR as key tools for optimizing mesophilic biogas production in full-scale reactors.