Effects of ultrasonic lysis frequency on sludge lysis-cryptic growth: Sludge reduction, microbial community, and metabolism

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Ultrasonic irradiation can effectively achieve in-situ sludge reduction, and the frequency of ultrasonic lysis may affect the performance. This study explored the effect of ultrasonic lysis frequencies of 1/d, 2/d, and 1/3d on lysis-cryptic growth of sludge and investigated changes in sludge and microorganisms in detail. The optimal ultrasonic lysis frequency was 1/d, and sludge reduction reached 47.90%. The use of ultrasound did not change the chemical oxygen demand (COD), total nitrogen (TN), and ammonia nitrogen (NH3-N) removal, but decreased the removal rate of total phosphorus (TP); the removal rate of TP was the lowest (37.58%) at an ultrasonic lysis frequency of 2/d. The use of ultrasound decreased the average particle size of sludge, and the minimum sludge particle size was 16.98 μm at an ultrasonic lysis frequency of 2/d. SEM demonstrated that the introduction of ultrasound roughened the sludge surface. FTIR and XPS demonstrated that the most significant change in functional group strength occurred at an ultrasonic lysis frequency of 1/3d. Extracellular polymeric substances (EPS) increased the most at an ultrasonic lysis frequency of 1/d. The introduction of ultrasound did not change the microbial communities of sludge, with Firmicutes, Actinobacteria, and Patescibacteria being the main microbial communities. Furthermore, the introduction of ultrasound did not change the metabolic pathway or key genes of sludge microorganisms but increased key enzymes related to carbon, nitrogen, and phosphorus metabolism.