Combination treatment of ultrasound and ozone for improving solubilization and anaerobic biodegradability of waste activated sludge

The hydrolysis is known to be the rate-limiting step of biological sludge anaerobic degradation. The disruptions of sludge flocs and microbial cell walls by ultrasound combined with ozone treatment (US/O₃) were investigated in laboratory-scale experiments. The results showed that temperature, O₃ dose, US energy density and pH had a positive effect on the disintegration of sludge. The organic substrates were released into the liquor, which induced the increases of soluble chemical oxygen demand (COD_S) and turbidity in the aqueous phase. Accordingly, the biodegradability of sludge was improved. The COD_S increased from 1821 to 2513mg/l after reaction for 30min when NaHCO₃ was added, which indicated that the ozone molecule played a major role in the disintegration of waste activated sludge. The COD_S was 2483mg/l after 60min O₃ treatment followed by 60min US treatment, and it changed into 3040mg/l after 60min US/O₃ treatment, which proved that US/O₃ induced a synergetic effect. The pH-drop of sludge from 6.8 to 5.21 might be attributed to the increase of volatile fatty acid from 61.35 to 111.96mg/l during the US/O₃ treatment process.