Studies were carried out on anaerobic co-digestion of primary and secondary excess sludge obtained from tannery effluent treatment plant. Anaerobic biomass collected from a treatment plant was used as the source of microorganisms. The optimum feed to microorganism ratio was evaluated as 0.7 on the basis of volatile solids reduction cum gas production. Both feed and anaerobic biomass was subjected to ultrasonic pre-treatment in order to enhance the digestion process. Experiments carried out on batch mode showed significant increase in the gas production for pre-treated feed and biomass. Optimum pre-treatment durations were evaluated as 5 minutes for feed and 3 minutes for anaerobic biomass. Heat flow analyses of the anaerobic biomass using isothermal microcalorimetry throw light on different stages of digestion process. The effect of ultrasonic pre-treatment on anaerobic biomass was also substantiated using this technique. The heat energy released by pre-treated and untreated anaerobic biomass was evaluated as 16.3 and 7.6 kJ/kg respectively. Kinetic analysis revealed that the overall rate constant of digestion process increased by 1.5 times due to pre-treatment. However, the initial lag time increased by about 20% for the optimally pre-treated sample compared to untreated sample. Modified Gompertz equation was used to model and the parameters were evaluated. The significance of this work lies on energy production (bio gas) and at the same time increasing the maintenance metabolism rate thereby minimizing excess sludge biomass generation.