| The process of drinking water treatment sludge recycling can improve coagulation performance in low turbidity water treatment; however, there are enrichment problems of toxic and harmful substances during recycling, which increase the risk of drinking water safety. It is necessary to innocuous treat the drinking water treatment sludge before recycling. This is the precondition for the realization of the technology of sludge recycling and the resource of the sludge. Ultrasonic technology can diseffect the pathogens and remove the pollutants in the sludge, so as to provide a safe guarantee for the drinking water treatment sludge recycling.In this paper, the motion of a single cavitation bubble in the ultrasonic field was studied, and the model of the cavitation bubble movement was established to investigate the effect of ultrasonic conditions i.e. ultrasonic frequency, energy density, initial cavitation bubble radius on cavitation effect. Then the visualization of ultrasonic cavitation field under different conditions by aluminum foil corrosion test was realized.At mean time, two sorts of simplified models of diffusion-limited aggregation(DLA) floc breakage were established and simulated. The practical flocs breakage patterns were verified through experimental analysis.The numerical simulation results of cavitation bubble indicated that, when ultrasonic frequency got smaller, the cavitation bubble radius was larger, and the collapse pressure could reach up to 105 kPa, meanwhile, the cavitation effect was enhanced. At high energy density, the cavitation bubble grew for a long time; the movement of cavitation bubble was great, conducive to the cavitation effect. The biggest radius of the bubble could be reached 180 times of initial when the energy density was 7W/m L. The cavitation effect was best when the initial cavitation bubble radius was equal to the resonance radius. The influence of media properties on cavitation is small.The results of measurement of cavitation field in bath reactor showd that, when the frequency was 25 kHz, the ultrasonic has good directivity ability, and the ultrasonic energy concentrated around the axis of the transducer; the antinode was the efficient area of the whole ultrasonic field; higher frequency ultrasound attributed to weaker cavitation. The results in probe eactor indicated that, cavitation effect was enhanced with the increase of energy density. The area of high efficiency zone was concentrated in the vicinity of the probe axis; with the increase of the distance of the aluminum foil and the probe, the area of inefficient zone was increased. The addition of solid particles can improve the cavitation effect, and the low concentration of sludge particles can achieve a better effect.The results of floc breakage showed that When the flocs were treated at low energy density(0.03~3W/mL) and low frequency(25~40kHz) ultrasonication, surface erosion dominated the breakage effect, which was reflected by decreased radius and increased fractal dimension of flocs. When the energy density exceeds 3W/m L or the frequency surpasses 40 k Hz, the large scale fragmentation was the main mechanism, leading to the smaller flocs size and looser structures. The sonication of the frequency of 40 kHz is more beneficial to flocs breakage, with the experimental result that 10minutes' ultrasonication can lead to a dinimutioin rate of 9.8% of the radius and a fractal dimension of 1.394, and the structure of the flocs were more compact.The effect of ultrasonic condition on cavitation was confirmed by this study, and the fracture mode was explored. The research results could provide reference for ultrasonic pretreatment of water purification sludge technology. |
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