Abstract
Ultrasonic treatment and thermal treatment at low temperature were employed together to analyze and compare the effect of temperature on ultrasonic sludge hydrolysis. Waste activated sludge was more susceptible to ultrasound than anaerobic sludge and primary sludge. In ultrasonic treatment of waste activated sludge for 1 hour, ΔSCOD/(−ΔVSS) ratio decreased from 2.40 to 0.44, indicating that high COD components were solubilized faster than the low COD components. Ultrasonic treatment increased the temperature significantly and the heat effect on sludge hydrolysis was not negligible. Primary sludge was more susceptible to heat than waste activated sludge. A sequential treatment of heat and ultrasound of primary sludge showed that hydrolysis efficiency was more affected by the ultrasonic power than the temperature and the time duration. In case of waste activated sludge, the overall hydrolysis efficiency increased with the temperature up to 50°C, and it remained almost constant at higher temperature. From the results the contribution of shear force by cavitation bubbles decreased at higher temperature. The effects of shear and heat in ultrasonic sludge treatment need to be analyzed separately for the optimum sludge pretreatment.
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