Research On Indirect Film Drying And Thermal Press Dewatering And Drying Of Municipal Dewatered Sludge

Sewage sludge, also known as biosolid, is remainder left behind as water is cleaned in municipal wastewater treatment. Sludge treated by mechanical dewatering, can be called dewatered sludge, always has moisture content between75%and85%and should be dried for further moisture removal.In this paper, based on moisture distribution researches on municipal sewage sludge with different water content, drying characteristics and principles are experimentally studied, which offer basic theory for development of the rotary drum sludge drier, an innovative indirect drier for sludge with high moisture content.Firstly,the definition, classifications and hazards of sewage sludge are introduced,the treatment and disposal technologies are sorted described, and drying technologies, drying theories, as well as drying equipment, methods and models are summarized in this study.Ultimate analysis and proximate analysis of experimental sludge are obtained.And several different methods are used to analyze moisture distribution of sludge with different water content and conclusions can be drawn that the sticky phase is caused by EPS (Extra Celluar Polymers) in sludge which can adhere to one another since it is saturated hydrophilic colloid.Apparent viscosity of sludge with high water content shows its properties as thixotropic fluid.Halogen drying and indirect drying characteristics of sludge are studied by drying curves, drying rate curves and drying temperature curves.Multifractal theory are adopted for modelling drying rate curves and crack distribution for different thickness of sludge and it can be proved that they all have obvious multifractality.COD(Chemical Oxigen Demand) value of condensate from indirect sludge drying shows that, the higher the temperature is, the higher pollutant concentration is, and also the thicker sludge and longer drying duration may induce higher pollutant concentration.The physical parameters include density,specific heat, effective heat conductivity coefficients and effective mass transfer coefficients are determined based on experimental data. Diffusion models based on the obtained parameters are built for heat and mass transfer of sludge indirect drying and the simulation results are in good agreement with the experimental data.Thermal press dewatering and drying characteristics of sludge are studied by self-made equipment and the results show that coupling between heat and stress can make water remove from dewatered sludge in liquid phase.Particle size analysis shows that sludge treated by thermal press has finer particle size distribution.EPS extracted from sludge shows that nucleic acid concentration increases obviously, which proves that the thermal press can cause partial cell to be ruptured, which make water in sludge separated out and removed in liquid phase.The rotary drum sludge drier are developed based on indirect drying principles and thermal press theories. Key experiments indicate that polyester filter cloth with pore diameter of50μm is better for the drier. Different admixture trials indicate that inorganic additive with similar diameter of sludge can increase the dewatering efficiency.Proximate analysis shows that layering phenomena appear in sludge treated by rotary drum drier and more organic substance in sludge is close to the surfaces of drum and filter cloth.Since anaerobic digested sludge has less organic substances,less sludge adheres to drum and filter cloth, which is more suitable for the sludge drier. Continuous experiments show that rotary drum drier with processing capacity of12tons/day can operated stably.