Reduction of Wastewater Sludge and Leaching of Heavy Metals by Ultrasonic Treatment

In this study, wastewater sludge was collected at a chemical plant in Ulsan, Korea, and the effect of ultrasonic treatment on sludge reduction and heavy metal leaching was investigated. In lab-scale experiments, the amount of sludge decreased with increasing ultrasonic irradiation intensity (100~600 W) and time (0.5~30 min). A pilot-scale ultrasonic reactor made in this study showed the highest performance (i.e., 46.2% of sludge reduction) in a batch mode operation. These results demonstrated that the ultrasonic treatment was useful for the reduction of sludge. On the other hand, the levels of heavy metals (Cd, Pb, Cu, and Zn) in supernatants of the sludge samples after the ultrasonic treatment substantially increased, addressing the need for proper management of toxic chemicals after dehydration of sonicated sludge. The reason for the sludge reduction and heavy metal leaching after the ultrasonic treatment was the disintegration of sludge flocs and cell walls. Therefore, an advanced ultrasonic technology considering both sludge reduction and leaching of toxic chemicals should be developed for practical applications.ope