Optimization of magnetic field-assisted ultrasonication for the disintegration of waste activated sludge using Box–Behnken design with response surface methodology

Abstract This study investigated for the first time the feasibility of using a magnetic field for sludge disintegration. Approximately 41.01% disintegration degree (DD) was reached after 30 min at 180 mT magnetic field intensity upon separate magnetic field treatment. Protein and polysaccharide contents significantly increased. This test was optimized using a Box–Behnken design (BBD) with response surface methodology (RSM) to fit the multiple equation of the DD. The maximum DD was 43.75% and the protein and polysaccharide contents increased to 56.71 and 119.44 mg/L, respectively, when the magnetic field strength was 119.69 mT, reaction time was 30.49 min, and pH was 9.82 in the optimization experiment. We then analyzed the effects of ultrasound alone. We are the first to combine magnetic field with ultrasound to disintegrate waste-activated sludge (WAS). The optimum effect was obtained with the application of ultrasound alone at 45 kHz frequency, with a DD of about 58.09%. By contrast, 62.62% DD was reached in combined magnetic field and ultrasound treatment. This combined test was also optimized using BBD with RSM to fit the multiple equation of DD. The maximum DD of 64.59% was achieved when the magnetic field intensity was 197.87 mT, ultrasonic frequency was 42.28 kHz, reaction time was 33.96 min, and pH was 8.90. These results were consistent with those of particle size and electron microscopy analyses. This research proved that a magnetic field can effectively disintegrate WAS and can be combined with other physical techniques such as ultrasound for optimal results.