| The quantity of sewage sludge as the by-product of wastewater treatment plants has been generated in an ever increasing amount due to the development of industry and increase of population.Sludge treatment and disposal has become one of the most important tasks for environmental protection in order to prevent consequent pollution and make effective use of sewage sludge.There are three sludge disposal methods currently used,which is recycling in agrieulture,land filling and incineration.Each of them needs the drying technology.How to dry the municipal sludge economically and efficiently has become an important topic in the science field.The thin layer drying behavior of sewage sludge is experimentally investigated in an indirect drying process and the mathematical modeling by using thin layer drying models is performed.The logarithmic model is found to show better predictions than the other models. In this period,moisture transfer from sewage sludge is described by applying the Fick's diffusion model.The effective diffusivity coefficient of moisture transfer varies from 8.486×10-10m2/s to 4.386×109m2/s over the temperature 80～150℃.In addition,the temperature dependence of the effective diffusivity coefficient is expressed by Arrhenius type relationship.The activation energy for the moisture diffusion of sludge drying is found to be 29.56 kJ/mol.A great of water vapor,which carries pollutant,is produced during sludge drying,so this thesis focuses on the analysis of condense water of thin layer drying.Through investigating different temperature,drying time,thickness of sludge and some water-quality indexes,such as pH,COD,TOC,TN,TP,NH4+-N,NO3--N and NO2--N after being settled for a few time, laws on how the pollutant changes during sludge drying on different conditions can be summarized as follows:(1) The condense water,which is formed by volatilizing during thin layer sludge drying, forms buffer solution which shows neutral,with pH between 6 and 8.(2) During sludge drying,the concentration of pollutants in the condense water increases as the drying temperature increases,especially the increasing is the most significant between 140℃and 150℃.(3) During sludge drying,the concentration of pollutants in the condense water is increased with the drying time increase.(4) During the same drying time,the thinner the drying layer of sludge is,the higher the deposition concentration of pollutants is;during the same degree of drying,the deposition concentration of pollutants increases with the thickness increasing.(5) The pollution level of condense water increases after the sludge is laid dried,and the higher the temperature is,the greater the degree of pollution is.Lower temperature is more suitable for the preservation of the sludge.(6) The condense water produced by sludge drying is organic wastewater over the legal limit,and it must be treated before being discharged. |
