Solubilization of excess activated sludge by self-digestion

A new approach to the solubilization of excess activated sludge by self-digestion was studied to reduce the amount of sludge produced in the activated-sludge-treatment process. Excess sludge was effectively made soluble by anaerobic incubation at 60°C after the addition of NaOH at a final concentration of 0.01 N. Under these conditions, approximately 40% of the sludge became soluble within 4 days. During self-digestion, solids in the sludge were thickened by flotation, and a clear liquor was obtained. This process was evaluated with a bench scale apparatus. The system consisted of an upflow column as the solubilizer. The resulting solubilization rate was approximately 20% at the hydraulic retention time of 24 h. This was comparable to the value obtained in batch experiments under identical conditions.