Optimization of low-intensity ultrasonic irradiation for low-strength sewage treatment in anaerobic baffled reactor

The anaerobic treatment of sewage has many advantages, such as low energy consumption, sludge reduction, and bioenergy recovery. However, low microbial activity has severely restricted the application of anaerobic treatment of low-strength sewage. In this study, an ultrasound enhancing anaerobic reactor (ABRU) was established and anaerobic sludge was irradiated once a day by ultrasound with the optimal parameter (ultrasonic density of 0.10 W/mL, irradiation time of 10.2 min, and ultrasonic sludge concentration of 10.0–10.5 g/L) obtained by response surface methodology. ABRU and the contrast anaerobic reactor (ABRC) without ultrasound were operated in parallel for anaerobic process. The highest chemical oxygen demand removal rate of ABRU was 93.6%, which was 5.8–11.3% higher than that of ABRC. Compared with ABRC, the dehydrogenase and coenzyme F420 contents of ABRU increased by 46.6% and 38.2%, respectively, implying that sludge bioactivity was significantly enhanced. The results of extracellular polymeric substances (EPS) demonstrated that the low-intensity ultrasonic irradiation increased the content of EPS by 5.7%, and the protein content increased significantly, which contributed to the increase in organic matter removal. The observation of the morphology of sludge showed that the sludge flocs were dispersed by ultrasonic irradiation, leading to enhanced sludge-sewage mass transfer and a decrease in the dissolved organic matter effluent concentration. Therefore, the aforementioned results indicate that anaerobic treatment of low-strength sewage can be enhanced by low-intensity ultrasound.