Ultrasonic reduction of excess sludge from activated sludge system: Energy efficiency improvement via operation optimization

Abstract

This paper studied the influences of operational parameters to improve the energy efficiency during ‘ultrasonic lysis–cryptic growth’ sludge reduction. Subsequent batch reactor with a hydraulic retention time of 8 h was used to treat urban sewage, and ultrasound wave with a specific energy of 20 kWh/kg TS was employed for sludge lysis. A new index, EE, was defined to evaluate the energy efficiency in ultrasonic lysis–cryptic growth sludge reduction system. Results showed that the most important operational parameter was the proportion of sonicated sludge (SP), which determined the energy consumption and significantly impacted the energy efficiency. The upper limit of SP in this study was 30%, beyond which the effluent quality was unacceptable. Higher SP caused heavier sludge reduction but more energy consumption; when SP was 30%, the excess sludge reduction was the greatest (67.6%) and the energy consumption was the highest (0.101 kWh/d). With a given SP, frequent sludge lysis was adverse to sludge reduction. In summary, the recommended conditions for ‘ultrasonic lysis–cryptic growth’ sludge reduction were SP of 15%, lysis frequency of 1 time/day. Under these conditions, the highest energy efficiency of 0.012 kg TS/kWh was achieved.