The dewaterability of the digested sludge and soluble residual organics in the anaerobic digested sludge was also characterized to evaluate the impact of the treatment of feed sludge on the characteristics of digested sludge. The anaerobic biodegradability of feed sewage sludge was higher at an solids retention times of 20 days with its longer substrate–microbe contact time. The dewaterability of the digested sludge decreased due to the treatments of feed sludge. Molecular weight distribution of organics in the digested sludge was measured with calibrated size exclusion chromatography. Excitation emission matrix fluorescence spectroscopy analysis was conducted to measure the fluorescence intensity of fluorescent compounds in the supernatant of digested sludge from the reactors. Soluble polymeric substances were found to accumulate in the anaerobic digested sludge following anaerobic digestion of ultrasonication (ULS)-ozone-treated feed sludge. The biodegradability of fulvic acid -like substances was improved due to the ULS-ozone and ULS + alkaline treatments of feed sludge.