Effects of lysed sludge reflux point on ultrasound lysis-cryptic growth in anaerobic/aerobic (A/O) wastewater treatment: Sludge reduction, microbial community, and metabolism

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Ultrasonication allows sludge reduction to be performed in situ during wastewater treatment, and the reflux point of the lysed sludge affects this performance. This study investigated the effects of reflux point (anaerobic stage, carbon/nitrogen (C/N) lowest stage, and aerobic stage) on sludge lysis-cryptic growth in an anaerobic/aerobic reactor and variations in the sludge and microbial community. The best reflux point occurred at the lowest C/N ratio stage, and a 50.96% reduction in excess sludge was achieved. The reflux of the lysed sludge to the aerobic stage reduced nitrogen and phosphorus removal. The reflux of the lysed sludge decreased the average sludge size, reaching 29.2 μm when reflux to the aerobic stage. Scanning electron microscopy showed that the sludge surface was unaffected by the reflux point. The Fourier-transform infrared spectrometry and X-ray photoelectron spectroscopy results showed that the most prominent variation in the intensity of the sludge functional groups occurred when the reflux was at the lowest C/N stage. The amount of extracellular polymeric substances decreased the most during reflux to the anaerobic stage. The sludge microbial communities varied with the reflux point, and the dominant phyla during reflux to the anaerobic, lowest C/N, and aerobic stages were Bacteroidetes, Firmicutes, and Bacteroidetes, respectively. Furthermore, the reflux point did not alter the metabolic pathway of sludge microorganisms but increased the number of enzymes in metabolic pathways.