Effect of the sequence ultrasonic operation on anaerobic degradation of sewage sludge

• High-density ultrasound can disintegrate sludge.
• Low-density ultrasound can enhance metabolic activities inside the cells.
• Intermittent ultrasonic treatment was an efficient method to disintegrate and degrade sludge.

In this study, ultrasonic disintegration (UD) was used on excess sludge during the anaerobic degradation (AD) process to improve treatment efficiency. After applying AD for 7 days followed by UD for 15 min, soluble chemical oxygen demand (SCOD) concentration reached 4320.5 mgL−1; this was 1.6 times when UD was applied alone for the same treatment time. Intermittent UD during the AD process was more efficient than pretreating sludge using UD under the same ultrasonic energy consumption. According to the central composite design, the optimized application of intermittent UD was 3 min every 2 days. At the optimized operation levels, SCOD concentration and VS concentration reached 3482.5 and 2690 mgL−1 (VS removal: 41.6%). The morphological characteristics and bacterial community structure of sludge changed significantly, enhancing the efficiency of sludge degradation during the AD process with intermittent UD. The intermittent ultrasonic technology had a significant impact, increasing the sludge anaerobic degradation efficiency and reducing energy consumption in engineering applications.