Research On Extrusion Molding Of Sewage Sludge And Its Drying Characteristics

Huge amount of sewage sludge has been generated as the by-product of the waterwater treatment plants (WWTP) due to the rapid accelerated process of urbanization. Four conventional sewage sludge disposal methods including landfill, agriculture, incineration and composting are widely employed. In those disposal methods, the low moisture content is the prerequisite. Reducing the moisture content of sewage sludge is the essential step for sludge reduction, stabilization, harmlessness and resources recycling. Therefore, it is necessary to carry on the research of sewage sludge drying.The extruding moulding experiments on the sewage sludge were carried out. The relationship between the diameters of the sludge strips and the moisture content of the sewage sludge was found. Based on best extruding moulding results, the diameter of the sludge strips was6mm, when the moisture content is above75wt%, or else the diameter4mm. The hanging length of sludge strips with53.92wt%moisture content was longest, and it was found that the sticky phase of sewage sludge was about53.92wt%of moisture content. The stickiness of the dewatered sludge was ascribed to sludge floe structure and extracellular polymeric substances (EPS).The isothermal drying experiments were conducted on the halogen moisture analyzer at the constant drying temperature of90℃,105℃and120℃. The results showed that the higher the drying temperature was, the smaller the diameter of sludge strips, the greater the drying rate, and the shorter drying time. The greater initial moisture content of the sludge was, the longer the drying time. The relationship between the maximum drying rate and sludge strip diameter was as follows:Vn=0.4642*d-1.0362. A lot of cracks generated in the sludge drying process were related to the drying temperature, the diameter of sludge strips and the initial moisture content of sludge. The increase of drying temperature and the initial moisture content and the decrease of the diameter of sludge strips were favorable for crack generation. Shrinkage change of the sludge strips was enhanced with the increase of the drying temperature and initial moisture content of sludge.The results obtained during the convective drying process was that the higher the temperature of the drying medium was, the greater air flow rate and the smaller the diameter of the sludge, and the faster the drying rate. The observation was verified by univariate multifactor analysis and the significant level of the three factors on drying rate was the diameter of the sludge strips> the temperature of drying medium> air flow rate. Page model was the most suitable model for fitting the data of convective drying of sludge strips. In addition, the Fick diffusion model was employed to describe the moisture transfer from the sludge strips during convective drying process. The values of effective diffusion coefficient were in the range of2.918×108to4.085×108m2/s at the temperatures of90～105℃. Based on Arrhenius equation which describing the relationship between the drying temperature and the effective diffusion coefficient, the activation energy of moisture diffusion of the sludge strips convective drying was found to be13.413kJ/mol.