Mechanism Study Of Conditioning Dewatering And Flow Behavior For Municipal Sludge

Sewage sludge is the main by-product of urban and industrial wastewatertreatment plant. With the speeding up of urbanization, discharge amount ofwastewater increases dramatically, and then serious environmental problems areappeared. Standards about disposal ways for sludge in China require that solidscontent of filter cake is larger than40wt%. Based on the idea of equal attention toenvironmental protection and economy, it is the problem to be solved that how toimprove processing capacity of sludge, and how to reduce moisture content of filtercake in order to satisfy subsequent processing requirements, such as landfill,incineration.Mechanical dewatering methods with simpler operation and lower cost, havebeen praised by most of sewage treatment plants. However, nature of the sludge(composed by lots of zoogloea, microorganisms and bound water), determined that itis difficult to dewater without any pretreatment.In this paper, the aims were to reducemoisture content of filter cake and improve processing capacity of sludge. Themechanisms that flocculation conditioning and thermal hydrolysis pretreatmentimprove sludge dewaterability were studied, and the effect of thermal hydrolysis onsludge rheology was analyzed.The main work is as follows:（1）Eight parameters (such as filtration rate, cake solids content and net solidsyield) as evaluation indexes of sludge dewaterability were come forward. Uniformdesign was employed to design filtration and expression dewatering experiments,and equations on independent variables (PAC dose, lime dose, filtration pressure andexpression pressure) and dependent variables (such as solids content of filter cake,net solids yield.) were established. Results show that lime had the most significantimpact on each dependent variable, such as filtration rate, cake solids content and netsolids yield.Sludge dewaterability could improve with the increase of lime dosage.As the lime doseage reaches50%(lime mass ratio to dry solids mass in sludge),maximum net solids yield was obtained. Effect of PAC on cake dry solids content wasnegligible. The optimum dose of PAC for the filtration stage was10%(PAC massratio to dry solids mass in sludge), which was in accordance with that for the expression stage and for the whole dewatering process. In addition, filtration pressurehad little effect on filtration rate, while handling capacity of sludge increased athigher filtration pressure. Furthermore, higher expression pressure could contributeto the mobility of particles in filter cake, and then drier cake and higher expressionrate were obtained. However, when the expression pressure exceeded3.5MPa, theexpression rate increased slowly in this study.（2）Mechanism of filtration and expression dewatering on municipal sludgeconditioned by composite flocculation was studied. Results showed compositeflocculation conditioning increased permeability of filter cake, improving creepability of particles in filter cake, and reduced elasticity and viscosity of filter cake.Meanwhile, bonding strength between water and particle surface was weakend, andthen removal rate of bound water and capillary water increased, which is beneficialto expression dewatering.（3）Effect of thermal hydrolysis on sludge physical characteristics was studied.Based on rheological theory, equations describing rheological properties of sludgewere established. Bound water content, particle size and non-Newtonian fluidcharacteristics of sludge, decreased with the thermal hydrolysis temperature.Sludgefloc became more compact, and the fractal dimension of floc increased. However,when the thermal hydrolysis temperature reaches170℃, the decreased trend isgradually. For sludge with hydrolysis at170℃, as the solids content is lower than150g/l, the relationship between shear stress and shear rate could be expressed byNewtonian fluid model, while relationships (between sludge viscosity and sludgeconcentration, between sludge viscosity and temperature) were satisfiedexponential equation and Arrhenius equation, respectively. Correlation betweenviscoelasticity parameters and frequency were described by logarithmic equations,and evolution of viscosity over time can be simulated by one order thixotropickinetic equation.（4）Based on expression theory, impact of thermal hydrolysis temperature onsludge characteristics was carried out, and proposed threshold value of thermalhydrolysis temperature. The lower threshold value was120℃, and the higher was170℃. After thermal hydrolysis, reduction of bound water content andorganic matter content in sludge solids improved sludge dewaterability. Dissolutionof suspended organic solids and hydrolysis of macromolecular components, led todecreased particle softness and lower compression coefficient (from1.1to0.7), which improved sludge filterability. Filtrate amount significantly increased by nearly7times during60min, and a much compacter and less elastic filter cake was built up.Consequently, for expression stage, contribution of primary consolidation stagedecreased. Creep factor increased by nearly12times, suggesting that the mobility ofparticles in filter cake became easier, and the contribution of secondary consolidationstage also increased. Due to the reduced bound water content, the contribution oftertiary consolidation stage decreased.The results of this study provide theoretical basis for pretreatments of chemicalflocculation and thermal hydrolysis, and provide reference for design of sludgereduction process. It has important theoretical significance and application value forsolving problem of sludge dewatering.