Document Type: Original Research Paper
Biotechnology Group, Chemical Engineering Department, Faculty of Engineering,Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran
Department of Biochemical Engineering and Biotechnology, I.I.T., Delhi 110016, India
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.