Characteristics of organic, nitrogen and phosphorus species released from ultrasonic treatment of waste activated sludge

Batch ultrasonic treatments (sonication) were performed on two waste activated sludge (WAS) samples, BNR-WAS from the biological nitrogen removal unit and BNPR-WAS from the biological nitrogen and phosphorus removal unit of two Shanghai municipal WWTPs, to determine the effects of sonication time and intensity on the amount and distribution of the organic, N and P species released from the samples. The concentration profiles of COD, TOC fractions in different molecular weight (MW) ranges (<2 kDa, 2-100 kDa, and >100 kDa), TN, organic-N, NH3-N, TP and PO4-P were monitored during the treatment at three sonication intensity levels (0.167, 0.330 and 0.500 W/mL). Species releases increased with sonication time and/or intensity; the release rates were accelerated when the sonication intensity was above a critical level between 0.330 and 0.500 W/mL. After 1 h of treatment, 37.9%, 37.5% and 50.8% of the organic content (measured as COD) of BNR-WAS were released, while the same for BNPR-WAS were 40.9%, 55.3% and 56.9%. It also resulted in the release of 40.9%, 38.7%, and 52.1% of total nitrogen from BNR-WAS, relative to 46.2%, 61.6%, and 70.4% of the same from BNPR-WAS; most released nitrogen were organic-N (65.0% and 84.9%), followed by NH3-N (34.7% and 14.9%) and trace amounts of nitrate and nitrite. More total phosphorus of a higher orthophosphate content was released from BNRP-WAS (>60% release after 1 h of sonication, 80% was PO4-P) than from BNR-WAS (<50% release, 40% was PO4-P). The differences in the releases as well as the molecular weight distribution pattern of the soluble TOC species were due to the different structure and composition of the sludge samples. Sonication is a viable sludge treatment process when it is combined with a phosphorus recovery process to remove most of the released PO4-P so that the supernatant may be returned for further biological treatment.