Study On Sludge Disintegration By Using Combined Methods Of Fluid Shear And Ultrasonic

Sewage treatment can produce a large amount of excess sludge. Excess sludge not only results in severe environment pollution but also has significant resource potential. The excess sludge resource is mainly existed in flocs and bacteria. Sludge disintegration can fully release the excess sludge resource, greatly improve the excess sludge resource recovery efficiency, and simultaneously achieve the excess sludge reduction. Even though sludge disintegration is the necessary pretreatment step for many sludge recovery and sludge reduction processes, the high cost and the high energy consumption limit its widespread application.According to the current studies, excess sludge disintegration process can be divided into three stages as follows:flocs disintegration, cell wall disintegration and cytoplasm outflow. In the first stage, flocs are broken into micro-flocs and individual bacteria. In the second stage, the micro-flocs are broken into elementary particles, and the individual bacteria cell walls in micro-flocs are broken. Finally, the broken cell walls are crushed, the cytoplasm will outflow and the disintegrated matters are dispersed. Based on the viewpoint of choosing relatively effective methods for different sludge disintegration stage, a new method as "fluid shear+ultrasonic+fluid shear" was presented to disintegrate municipal sewage excess sludge. In this study the fluid shear was generated by a high-speed tissue triturator and the ultrasonic was generated by an ultrasonic cell disrupter.In this study, excess sludge was disintegrated by ultrasonic for 1-50 min and by fluid shear for 1-35 min respectively. Soluble chemical oxygen demand, soluble nucleic acid and soluble protein of disintegrated sludge were examined. The change of sludge particle size was measured. The disintegrated sludge character was observed by transmission electron microscope. Peptidoglycan in disintegrated sludge was used to evaluate the disintegration degree of bacterial cell wall. The three-stage theory of sludge disintegration was proved.Two sets of orthogonal experiments were carried out to investigate the impact of ultrasonic time, ultrasonic power, ultrasonic frequency, ultrasonic pulse ratio, and the interaction between these factors on sludge disintegration. The results showed that ultrasonic time was the most significant factor, followed by ultrasonic frequency, ultrasonic pulse ratio, ultrasonic power, the interaction between ultrasonic time and ultrasonic frequency, the interaction between ultrasonic time and ultrasonic power, and the interaction between ultrasonic power and ultrasonic frequency. The optimal parameters were ultrasonic frequency of 20 kHz, ultrasonic pulse ratio of 3:1, and ultrasonic power of 600W, respectively. The effects of the interaction between these factors on excess sludge disintegration were not significant.Seven sludge samples with different concentrations were disintegrated by using ultrasonic and fluid shear methods. The results showed that excess sludge characters had obvious influence on sludge disintegration. Relatively high excess sludge concentration would benefit sludge disintegration.By comparing the excess sludge disintegration effects of ultrasonic and fluid shear under the same energy consumption, fluid shear was proved to have higher efficiency in the initial sludge disintegration stage. By comparing the excess sludge disintegration effects of "ultrasonic+fluid shear" and "fluid shear+ultrasonic", "fluid shear+ ultrasonic" was proved to have higher efficiency than "ultrasonic+fluid shear". The simultaneous disintegration by fluid shear and ultrasonic didn't present higher efficiency than ultrasonic.An orthogonal experiment was performed to investigate the interaction between fluid shear and ultrasonic time. The results showed that ultrasonic time played the most important role in sludge disintegration. An obvious interaction was existed between ultrasonic time and fluid shear time of the first phase. These results illustrated that the fluid shear followed by ultrasonic would increase the excess sludge disintegration effect.The experiments of ultrasonic only, fluid shear only and "fluid shear+ultrasonic+ fluid shear" to disintegrate excess sludge were conducted. The results showed that the combined method presented obvious superiority in sludge disintegrative degree, energy utilization efficiency, and the release of organics such as protein, nucleic acid and peptidoglycan than other two methods.