Study On Ultrasound Disintegration Techniques For Sewage Sludge

Sewage sludge, containing a mass of organics, is the products of wastewater treatment plant, it's necessary to deal with sludge to avoid "secondary pollution", the main works of sludge treatment include "stabilization process" which removes the organics and "dewatering process" which reduces the volume of sludge, currently, "anaerobic digestion" is the most widely used digestion method, but "hydrolysis" is a rate-limiting step during digestion treatment. This is because the sludge flocs and cell wall is vary steady and it's difficult to break them by biological treatment, so sludge pretreatment by disintegration is significant.Ultrasound disintegration destroy the cell wall of sludge and release the intracellular materials to the aqueous phase, in addition, ultrasound promotes the dewatering capability by helping the coagulation of sludge particulates. Compare to other disintegration techniques, ultrasound disintegration has some merits such as it won't generates secondary toxic compounds,compact equipments and installations,low cost and so on.This paper research the ultrasound disintegration process by experimental analysis. Using the popular frequency of 20kHZ in industry application, and the power are 120W,240W,400W,720W and 1000W respectively. The results show that: (1) 720W is the best power choice integrating the disintegration effect and energy cost. After 60min disintegration time with the power of 720W, SCOD of sludge has increased from 192mg/L to 6186mg/L,disintegration degree reach 60%. The effect is obvious. (2)When energy cost E_V reached 6000kJ/L, the changing of disintegration effect is no longer obviously. In addition, under the same energy cost, the higher intensity of ultrasound would get a better disintegration effect. (3)After ultrasound disintegration with the power of 120W and the disintegration time of 60min, the turbidity of the supernatant after centrifugation decrease from 39.3NTU to 27.8NTU. the percentage of the supernatant after centrifugation also decrease from 0.25 to 0.1833, under other treatment conditions, this two data increase excepted 240W within 10min. This implied that the dewatering capability of sludge was increasing. So we should choose right disintegration conditions according to the purpose of treatment.The primary cause of ultrasound disintegration is acoustic cavitation, this paper builds the dynamic model of cavitation bubble. The simulation results show that the cavitation intensity is decreasing as the ultrasound frequency increases. Along with the increase of sound pressure ,the cavitation intensity is also increasing. But when sound pressure increase to a certain value, the cavitation intensity would fall off.The comparison between experimental analysis and simulation results shows that there is a consistency in the influence to disintegration effect and cavitation intensity with different ultrasound power. These results prove the fact that acoustic cavitation is the main reason for ultrasound disintegration. The experimental and simulation results will provide theoretical bases to choose operating conditions in the process of ultrasound disintegration.