Study On High Degree Dewatering Mechanism Of Activated Sludge By Application Of Thermal Pressure Filtration

The innocuity treatment and resources utilization of activated sludge is the development direction of activated sludge treatment in the future. The premise of innocuity treatment and resources utilization is economical and effective sludge dewatering. Since activated sludge contains the bound water, it is difficult to make a further decline in moisture content by direct mechanical filtration in activated sludge dewatering. High moisture content of the cake will result in greater energy consumption on their follow-up heat drying. Therefore, in order to break through the limits of solid-liquid separation and reduce the energy consumption of the follow-up heat drying operation, research of improving the technological level of economical and effective sludge dewatering and promoting the resource utilization of it has an important theoretical and practical significance.Based on the thought above, a method of the combination of steam pressure with mechanical membrane squeeze for activated sludge dewatering is proposed in this paper based on synthetically analysis of all kinds of researched results and the environmental protection project in Suzhou City, " research on key technologies of innocuity treatment and resource utilization of the urban domestic sewage sludge, " (Approval No.: N3117925). The Thermal-hydro-mechanical (THM) coupling method is introduced into the study of activated sludge dewatering based on its characteristics of porosity and easily deformation which provides a new idea and method for high degree and economical activated sludge dehydration. The main works of this thesis include the following aspects:1. This paper first analyzes the interaction between the mass, momentum and energy after the addition of steam in the sludge, and viscous dissipation on water power-mechanics in multi-fields along with the changing of energy is taken into account with emphasis. Based on the research progresses of the model teories in multiphyisics fields by domestic and international scholar, the Thermal-hydro-mechanical (THM) coupling mathematics model for activated sludge dewatering are derived.2. According to the Thermal-hydro-mechanical (THM) coupling mathematics model for activated sludge dewatering and boundary conditions (including the boundary conditions and initial conditions), the equations are simplified into one-dimensional model, and then the three nonlinear coupling models in THM are carried on numerical simulation with the Comsol Multiphysics software according to the model test conditions. Results show that: the phenomenons of size effect and stress concentration are emerged in the process of hot filtration which illustrate that the bound water is broken in activated sludge. In addition, sudden change in tensile stress is also emerged according to the compound function of thermal solution and mechanical squeeze which results in larger deformation and collapse phenomena in the sludege cake when considering the thermal-hydro-mechanical coupling function.Studies have shown that pore fluid pressure dissipated gradually with the extension of press time when only using the traditional method of press filter. However, the thickness of cake decreased significantly in the dewatering process with the increasing temperature of the steam added into the sludge when combined with thermal solution and mechanical squeeze. However, when the temperature is over 400 K , the decrease of the moisture content and displacement in the filter cake is very small which shows that the bound water is released in the sludge and the thickness of the sludge decreased under the pressure that eventually goes to a balance condition.3. Finally, in order to further validate the practicability of the theory of the combination of the thermal solution and mechanical squeeze, an environmental experiment of high degree and economical activated sludge dehydration is introduced in this paper which has studied the critical temperature of the steam heating, pressure, dewatering rate and the best dewatering time and made comparative analysis micro-structure of the sludge in the process of dewatering. The results show that: the cake moisture content was significantly lower (58~63%) when combing the method of thermal solution and mechanical squeeze, the critical temperature of the steam heating is between(1350C~1450C), the best dewatering time is between(25~30min), and the pressure is less than (0.6~1.0MPa).This topic through theoretical and experimental studies has demonstrated that this research method is an economical and effective method of high degree sludge dewatering which provides the theoretical and practical foundation for the futher study of the sludge dewatering.