Backgrounds and Objectives: Large quantities of sludge are produced in biological wastewater treatment. Because this sludge is highly rotten, it should be stabilized before its disposal. Aerobic and anaerobic digestion is widely considered as stabilization techniques. Because of high retention time and sludge dewatering difficulties, reduction in retention time, operation and maintenance should be given into consideration. Ultrasonic process increases the enzymatic activity, so decreases the hydrolysis time, a limiting factor in digestion process, and contributes to the decrease of the detention time. The objective of this investigation is to determine the effect of ultrasound in improving dewatering and stabilization of aerobic and anaerobic digested sludge. In addition, the impact of ultrasonic treatment on improvement of sludge dewatering and aerobic and anaerobic digestion is compared.
Materials and Methods: In this survey, samples of aerobic and anaerobic digestion were collected from local full-scale Garb Town and Tehran South wastewater treatment plant, respectively. The grab samples were collected for 4 month from July to October 2010. Total numbers of 20 samples were collected biweekly for each type of digestion. Each sample was sonicated for 15, 30, 60, and 90 min under 35 and 131 kHz frequencies separately. Total solids, volatile solids , pH, temperature , total COD, dissolved COD and settle able solids were measured. Ultrasound bath of the solution in a 300 mL glass reactor was performed as a bath reactor with power of 500 W.
Result: The results showed that the application of ultrasonic wave increased dissolved COD and temperature and decreased volatile solid, pH and settle able solids. Application of ultrasonic wave with frequency of 131 kHz decreased the VS and increased the dewatering of sludge more effective than the 35 kHz frequency and the highest performance was at 15 min of time and 131 kHz of frequency. Also sonication method showed better efficiency for anaerobic sludge samples compared to the aerobic sludge samples.                 
Conclusion: The results obtained showed that digestion and dewatering properties of sludge improved by ultrasonic application. Therefore it can be used as an alternative method for the sludge treatment.