Process optimization of an ultrasonic technology combining a CASS reactor for excess sludge minimization

To reduce excess sludge, the MLSS concentration and reaction zone ratio of an ultrasonic lysis-cryptic growth system combined with a CASS reactor (ULG + C) were investigated. Experimental results demonstrated that a high MLSS concentration was adverse to the removal of pollutants but advantageous to sludge reduction on the condition that the bacterial yield coefficient was inhibited, whereas reaction zone ratio had the opposite effect on pollution removal and sludge reduction. Increasing the two parameters increased energy consumption, suggesting that they should be controlled within a suitable range for better performance of the system. With the help of a conventional activated sludge (CAS) process, microbial activity weakened by the sludge lysis process could be recovered in six CASS operation cycles. In addition, the time needed for substrate degradation with the ULG + C process was 4 h, which is more than twice that for a CAS process, because of secondary flocculation. Based on these findings, the optimum conditions of an MLSS concentration of 2640 mg/L and a reaction zone ratio of 8/28 are strongly suggested for ULG + C systems.