| Sludge is a kind of microorganism used for sewage treatment,but its moisture content is high,which does not meet the requirements of subsequent treatment.Therefore,sludge dehydration is an important pretreatment link.Dewatering of freeze-thaw cycle sludge can not only reduce the moisture content of sludge and treatment cost,but also dry sludge produced can be used for fertilizer,fuel and other fields of reuse.Applying straw to the freeze-thaw cycle of conditioning sludge can improve the dewatering efficiency of sludge,realize the resource utilization of straw and sludge.Based on the sludge freeze-thaw cycle experiment,in this work,straw was mixed into the sludge to explore the effect of straw conditioning on the heat and mass transfer characteristics of the sludge freeze-thaw cycle,and the effect of freezing temperature,freeze-thaw time,number of freeze-thaw cycles and straw mixing ratio on the water content of the sludge freezethaw cycle was analyzed.It is proved that the freeze thaw cycle of straw conditioning sludge can not only reduce the moisture content of sludge,but also realize the resource utilization of straw and sludge.Then the polynomial response surface model was used to further optimize the factors affecting the sludge dewatering performance,and the influence of the interaction between various factors on the change of moisture content in the process of sludge freeze-thaw cycle was analyzed.It was found that the straw mixing ratio had a significant influence on the change of sludge moisture content,and the sludge moisture content decreased with the increase.The interaction between straw blending ratio and freeze-thaw time had the best effect on sludge moisture content.The influence of straw conditioning on the heat and mass transfer characteristics of sludge during freeze-thaw cycle was characterized and analyzed,including sludge dewatering performance analysis,sludge flocculation particle size analysis,thermogravimetric analysis and scanning electron microscope(SEM)analysis.The results show that:with the increasing of straw dosage,the required channels in the dewatering process will gradually increase.Coupled with the synergistic effect of freeze-thaw cycle,the particle size of sludge decreases,the structure of sludge floc is seriously damaged,large pores appear,the specific resistance of sludge decreases gradually,and the dewatering efficiency of sludge is improved.Moreover,due to the conditioning of straw,the maximum weight loss rate of sludge is increased,which makes the sample burning stage more intense,and the weight loss temperature of sludge pyrolysis is also increased correspondingly,indicating that the freezing and thawing cycle of straw conditioning is of great significance for the later application of sludge in fertilizer,fuel and other fields.The ANSYS FLUENT software was used to carry out the numerical simulation calculation of the changes of the internal temperature and humidity of the sludge under the experimental conditions,and the comparison and analysis with the experimental results were carried out to verify the degree of coincidence between the numerical simulation and the experimental results.Moreover,by observing the temperature and humidity cloud map inside the sludge,the heat and mass transfer characteristics of the sludge in the process of dehydration can be clearly seen.This model can be used to simulate the dehydration process of the sludge,and has guiding significance for the dehydration process of the residual sludge by freeze-thaw pretreatment method. |
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