An anaerobic digestion process with ozonation was studied by conducting pilot-scaleexperiments for on-site reduction in withdrawal sludge quantities while improving the biogas recovery, using a mixture of primary and secondary municipal sewage sludge. High VSS digestion efficiencies of ca. 80% and corresponding much methane production were achieved at reactor SS concentration of 6.5%. An empirical first-order kinetic model reasonably predicted the VSS degradation rates in the steady state conditions. The model suggested that about 61-70% of VSS fraction in the sludge was biodegradable nature and their specific degradation rate highly depended on the sludge source. The specific degradation rate in the model was 0.243 day-1 (primary sludge), 0.101 day-1 (excess sludge) and 0.019 day-1 (ozonated anaerobically digested sludge). In contrast of highly VSS degradation, solubilization of inorganic compounds was very limited and accordingly high fraction of inorganic solids, mainly acid insoluble inorganics and iron compounds, were found to accumulate in the reactors. The part of heavy inorganic solids could be removed by centrifuging and thus concentrated into the sludge to be withdrawn. The sludge having low VSS/SS ratio gave less moisture content of dewatered sludge cake. A linier relationship was found between VSS/SS ratio and the moisture content.
