Comparison of ultrasonic treatment of primary and secondary sludges: Physical properties and chemical properties

Abstract

Excess sludges were by-product of sewage treatment and ultrasonic treatment was an efficient pretreatment of excess sludge. However, the difference of primary and secondary sludges treated by ultrasound unclear. This paper systematically studied the ultrasonic treatment of primary and secondary sludges, including changes of sludge particle size, sludge sedimentation, release of carbon, nitrogen, and phosphorous. These results showed that COD, TN, NH4+-N and TP in supernatant increased by 126.9 %, 76.4 %, 60.8 % and 38.5 %, respectively after ultrasonic treatment of primary sludge; and the secondary sludge was increased by 429.6 %, 66.5 %, 23.8 % and 13.7 %, respectively. After ultrasonic treatment, COD of secondary sludge increased more, and the C/N and C/P were higher. Besides, the sludge particle size was reduced from 48.42 to 9.40 μm for primary sludge, and from 41.20 to 12.46 μm for secondary sludge, respectively. That indicated ultrasound could crack sludge effectively, and the effect of secondary sludge lysis was better. Secondary sludge released more biodegradable substances with ultrasonic treatment. In addition, after ultrasonic treatment, the chemical structure of sludge changed little, but the chemical properties changed strongly, especially for the secondary sludge. That revealed ultrasound had little effect on the chemical structure of primary and secondary sludges. This paper will provide guidance in study of primary and secondary sludge with ultrasonic treatment.