| With the acceleration of the urbanization process in China,the municipal excess sludge has increased rapidly.The excess sludge contains a large number of pathogenic microorganisms,heavy metals and nitrogen and phosphorus nutrients,which will cause serious secondary pollution if not disposed properly.Therefore,it is of great significance to develop economical and efficient disposal methods to meet the requirements of“four conversions”?those are stabilization,resource reuse,harmlessness and reduction?.However,a large number of microbial cells existing in the sludge reduce its biodegradation rate,which limits the application of economical and efficient disposal methods such as anaerobic digestion.In order to break the microbial cells in the sludge and improve the efficiency of anaerobic digestion and degradation,the research on sludge pretreatment technology has been widely concerned.Ultrasonic pretreatment can effectively break the microbial cells in the sludge,which is an economical and environmentally friendly method of sludge pretreatment.Based on ultrasonic mechanism,this paper explores the basic characteristics and changing regularities of sludge pretreatment based on the ultrasonic cavitation effect through theoretical simulation and experimental verification.The main research contents and conclusions are as follows:?1?Based on the Rayleigh-Plesset theoretical model,the motion of ultrasonic cavitation bubble is derived.Matlab is used to iterate through different parameters?ultrasonic frequency,sound pressure amplitude,initial radius of cavitation bubble and sludge viscosity?to solve the motion equation of cavitation bubble,and simulate and analyze the influence of these influencing factors on the ultrasonic cavitation effect.At the same time,Fluent is used to further express a series of cavitation processes of cavitation bubbles from expansion to contraction,and finally to collapse,which makes the simulation more visual.The results show that the ultrasonic cavitation effect is stronger at low ultrasonic frequency and high ultrasonic power,and the ultrasonic cavitation effect is the strongest when the ultrasonic frequency is 20kHz and the ultrasonic power is 1000 W.When the initial radius of the cavitation bubble and sludge viscosity are increased,the amplitude of cavitation bubble will be weakened and the cavitation effect will be weakened.When the initial radius is 5×10-6m and the sludge viscosity is in the range of 1.0×10-33 Pa·s to 11×10-33 Pa·s,the effect of ultrasonic cavitation is the best.?2?The experimental device is built by ourselves,and the thermal probe is used to verify the accuracy of numerical simulation.The results show that the cavitation effect is the strongest when the ultrasonic frequency is 20 kHz under other conditions unchanged,which is consistent with the numerical simulation results.However,the ultrasonic cavitation effect doesn't become stronger when the ultrasonic power is higher,which is different from the simulation results.The reason for this is that the numerical simulation does not take into account the influence of other factors.?3?Under the optimal process conditions,the ultrasonic cracking experiments are carried out on the excess sludge.The sludge disintegration effect is studied by observing the microscopic and particle size changes of the sludge and biochemical analysis.The results show that the effect of temperature on sludge disintegration is small and can be ignored.In actual sludge disintegration experiment,the intensity of the ultrasonic power and the length of the ultrasonic time are important factors affecting the degree of excess sludge disintegration by the ultrasonic wave.With the increase of ultrasonic power and ultrasonic time,the indexes in sludge increase obviously.When the ultrasonic power is 900 W,the SCOD value of the sludge supernatant increases significantly,and after 30 minutes of ultrasonic treatment,the SCOD value of the sludge supernatant after disintegration increases from 96 mg/L to a maximum of 4070 mg/L.The protein dissolution value increases from 112.26 mg/L to 258.52 mg/L;the polysaccharide dissolution value increases from 49.95 mg/L to186.87 mg/L;and the reduction rate of TSS increases from 13.6%to 54.2%.It shows that the ultrasonic treatment of sludge can realize sludge resource reuse and stabilization.In conclusion,ultrasonication is an effective method of sludge disintegration.The simulation and experimental results of this project can provide guidance and theoretical basis for the later study of ultrasonic cavitation conditions,and also provide relevant practical knowledge for the design of chemical reactors for ultrasonic sludge treatment in the future.Figure[33]Table[15]Reference[105]... |
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