Experimental Study On Thin-Layer Drying Behavior Of Municipal Sewage Sludge With Various Salt Contents

With the rapid development of economy,the production of high saline wastewater is considerably increasing.A large number of high salinity sludge is produced from the biological treatment of high salinity wastewater.High salinity sludge contains amounts of salts,nutrients,heavy metals,pathogens and organic pollutants,which will cause serious environmental problems if sludge cannot be treated properly.The sludge drying process not only protects the ecological environment,but also promotes the resource utilization in the field of energy.In present work,the drying behaviors of saline sewage sludge were investigated based on a lab-scale far-IR dryer and ultrasound-assisted convection set up.The effects of ultrasonic power levers,drying temperature,salt concentration and freezing/thawing treatment on the drying kinetics of the samples were determined based on kinetics theory.And evolutions of moisture ratio,drying rate,crack pattern and precipitation of salt in the sewage sludge samples in drying process were performed.Based on the experimental research and kinetics theory,the influences of radiation source temperature（130-200 0C）and freezing/thawing treatment on the far-infrared radiation drying kinetics of sewage sludge(0,0.0214,0.178,0.265,0.438 and 0.731 mol·kg^-1 NaCl)thin layer were investigated.The results indicated that the average drying rate was significantly affected by radiation source temperature and salt concentration.The drying rates decreased with increasing the salt content of the sewage sludge.At 130-200 ℃,the average drying rates of conventional sludge with 0.0214 mol·kg^-1 NaCl in the whole process decrease by 4.4%compared to salt-free sludge.The average drying rates of hypersaline sludge(0.178-0.731 mol·kg^-1 NaCl)in the whole process decrease by 10.1%compared to the conventional sludge.The whole drying process includes four periods,i.e.a short initial period,constant rate period,the first falling rate period and the second falling rate period.With the radiation temperature growing from 130 ℃ to 200 ℃,the average effective moisture diffusivity for salt-free sludge in the falling rate stage was 1.34×10-8 m2·S-1,the conventional sludge was 1.28×10-8 m2·s-1 and hypersaline sludge 1.20×10-8 m 2·s-1.After the freezing and thawing treatment,the drying rates of the conventional sludge and the hypersaline sludge increased by 3.5%and 4.5%,respectively.In addition,the Modified Midilli model had the best ability in describing the moisture ratio change of all sewage sludge samples.In the ultrasonic-assisted hot air convection drying,the effects of hot air temperature（80-130 ℃）,ultrasound powers（0,30,90,150 W）and freezing/thawing treatment on the drying kinetics of sewage sludge were discussed.The drying time of saline sludge containing 0.178 mol·kg^-1 NaCl with the ultrasound-assist decreased by about 8.0%compared with the convective drying alone.The average effective moisture diffusivities for salt-free sludge at ultrasound powers of 150W increased by about 86.4%during the falling rate period when drying temperature ranged from 80 to 130 ℃,while the effective moisture diffusivities for the conventional sludge and hypersaline sludge increased by about 88.7%and 93.6%during the falling rate period,respectively.Compared with conventional sludge,the average effective moisture diffusivities of salt-free sludge increased by 3.5%in the falling rate stage between 80 and 130 ℃.While the average effective moisture diffusivities of high saline sludge samples decreased by 4.5%during the falling rate period in contrast with the sludge with 0.0214 mol·kg^-1 NaCl.The values of the apparent activation energy in the falling rate period for high saline sludge is more than that for conventional sludge,while that for salt-free sludge was the smallest ones.The drying performance of saline sludge sample can be improved by freezing and thawing treatment.The sldge structure was affected by the salt contents,increasing the salt content in the samples can lead to fewer cracks and more salt precipitations on the surface of the sewage sludge samples.