A Precise Experimental Study on Key Dissimilarities between Mesophilic and Thermophilic Anaerobic Digestion of Waste Activated Sludge

Key dissimilarities between thermophilic and mesophilic anaerobic digestion of waste activated sludge (WAS) were experimentally studied in this research. Typical WAS with total solids (TS) concentrations of 30 and 60 g/L were digested anaerobically in a batch digester at mesophilic and thermophilic temperatures. Solids reduction, total COD changes, the production of different volatile fatty acids (VFAs), pH variation, the quality and quantity of the produced biogas, an energy audit, pathogen inactivation and sludge dewaterability during anaerobic digestion were investigated and compared for mesophilic and thermophilic processes in this research. Only the thickest sludge (TS concentration 60 g/L) provided auto-thermality under mesophilic conditions. The mesophilic digestion took place after 32 to 36 days with slightly more methane production and removal of organics than thermophilic digestion. The results showed that there was no significant difference between thermophilic and mesophilic digestion with respect to the gas composition. Among the VFAs (key intermediates), only propionate accumulated during sludge anaerobic digestion at both the mesophilic and thermophilic temperatures. Thermophilic anaerobic digestion imparts improved dewaterability. The required time for sludge pathogen inactivation under mesophilic conditions was more than one month.