Study On Mass Transfer Mechanism And Media In Biological Treatment Process For Municipal Wastewater

With the rapid development of economy, the increasing pollution of water resource has become a notable social problem that not only restrains the economic growth but also poses threat to people's health. Thus, protection of water resource has already aroused great concern from our government and all people. But there are two obstructive factors of protecting water resource and wastewater treatment, which hamper the industrial development of environmental protection constantly. One is the excessive investment for infrastructure construction and high operating charge,the other is low efficiency and instability of the treated effect. Therefore, to research a kind of sewage treatment technology with high efficiency and low energy consumption is the key to solve the problem of polluted water condition thoroughly.As core technology, biological reaction treatment is widely used in wastewater treatment engineering. With innovations and improvements on its oxygen supply modes, operating conditions, reactor structures, etc. in the past over 80 years' research and practice, a lot of novel methods and processes was developed. But in routine practices, there are many problems such as large investment, high operating cost, unstable treated effect, and low efficiency etc. lead to aggravating pollution of our water resource, which becomes more limited. Those problems have been the most serious obstacles to industrial development of environmental protection, and restrict the economic growth as well as threaten the people's health. And those are also the important subjects demanding urgent solutions to the engineers of municipal wastewater engineering. There are more serious problems of protectingthe water resource calling for the tremendous concern of people, government and related national departments.Nowadays, the problems existing in the field of municipal sewage treatment had caused some towns and factories cannot afford to build up wastewater treatment plants or the operating expenses. The real reasons of this situation are vague understanding to the phases mass transfer mechanism and unreasonable control to the biological reacting process, the core step of wastewater disposal flow. Low efficiency of mass transfer in biological reactors leads to over-long retention time that increases the designed cubage of treatment works and makes huge waste investment of construction. Simultaneously, low efficiency of mass transfer results in low efficiency of aerators, high power consumption loss, increasing operating cost, insufficient and late oxygen and organism loading demand to the microorganism in reactors, which lower biologic activity and affect the subsequent treated water quality. Thus, on the theoretical basis of mass transfer mechanism of multiphase hydrodynamic system, from the view of enhance mass transfer efficiency, biological reacting mass-transfer mechanism was discussed, and novel high efficient media for mass transfer and equipment was researched and developed, which are not only the key points to solving the difficult problem existed in urban wastewater treatment engineering, but also developing trends of biological treatment technology for wastewater. Therefore, "research and exploitation of mass transfer mechanism and its media for biological wastewater treatment"(04-2-058) was authorized as key scientific and technological project by Ministry of Construction, P.R.C.Biomembrane method treatment is one of the most important and widely-used technology in municipal wastewater treatment. Compared with other methods, biomembrane method shows many distinct features. In this paper, on the basis of referencing plentiful data and investigating extensively, different medias were compared with various characters of mass transfer, the mass transfer mechanism of aerobic biological treatment for municipal sewage was discussed, dynamical rule ofmass transfer controlling reaction was analyzed, and dynamics model and parameter for mass transfer action that can apply to guiding routine practice were inferred and summarized. Novel processes and technology as well as a kind of equipment with higher efficient were studied and exploited.The multiphase mass transfer mechanism of sewage biotreatment process system was thoroughly researched. The analysis result showed that the whole mass transfer process could be divided into two phases: internal and external mass transfer. The external one was controlling step and oxygen transfer in gas-liquid phase and organic substrate transfer in liquid-solid phase were key steps of external mass transfer. The major effective factors to the above two pivotal mass transfer process are ki — the liquid-side mass-transfer coefficient, a — unit contacting surface areas of gas-liquid phase; and the amount and size ^ of vortex in mixture turbulent flow.Based on a bran-new view of hydrodynamic mass transfer mechanism of multiphase system, mass transfer mechanism of wastewater biotreatment was deeply analyzed and discussed. The result showed that the mass transfer process in biological reactor could be divided into two steps: macroscopical mass-transfer and submicroscopic one. The principal mechanism of macroscopical mass-transfer was not Fick Diffusion caused by Brownian Movement but cooperative effect of convectional and turbulent diffusion in overflow. Submicroscopic mass-transfer, an important factor to the whole efficiency, based on the mechanism of centrifugal inertia effect of tiny vortex. The shear force of turbulent flow was the dominant factor.Based on a series of theoretical analysis and experimental study, theory that providing ideal mass-transfer conditions and accelerating macroscopical and submicroscopic mass-transfer were the most crucial parts to improve the whole efficiency of bioreactors was putted forward. According to the mass-transfer mechanism of wastewater biotreatment, the ideals conditions in the reactors were as follows. On the premise of not increasing extra energy consumption, making highdensity micro-eddy leaded to violent turbulence and sheared force which decreased the bubble size and speeded rebirth of the biomembrane surface, enabled the small bubble spread around, increased contacting surface areas of gas-liquid phase, and formed turbulent flow in which mass transfer is higher than usual.A new kind of media Model with high efficient mass-transfer named SCMT-1 (Self-Circle & Mass-Transfer) was exploited in this research. The novel packing materials was characterized by wearable point, compression resistance, corrosion-resistant, good performance in machinery, innocuity, no-dissolve appearing, rough surface, facility for biological membrane formation, larger specific surface area, anti-shortcut, and anti-jam etc. Compared with traditional padding, its outstanding feature was the novel structure that maked full use of the energy supplied by aeration, shaped forceful turbulence by mixture's irregular rotation, causes strong overfall shear force, cutted down the bubble size and increase frontal specific surface areas of gas-liquid phase, promoted the rebirthing of biomembrane, and maked higher disperse and efficient mass transfer turbulent flow in the reactors.Mixture wastewater were full of micro-vortexes with high scale and strength under the using of Model SCMT-1. Centrifugal inertia effect of micro-vortex in turbulent flow was fully embodied. The rate of small bubble and organic substrate relative to biomembrane was increased. Those ideal conditions for mass transfer were created, higher velocity of macroscopical and submicroscopic mass-transfer was stimulated, reacting period was decreased, and the of biological reaction was promoted.Contrastive experiments were conducted between traditional polypropylene stage ring and and treatment efficiency of using SCMT-1 was obviously higher than that of using the other one. For instance, at the same conditions SCMT-1 needed lower air/water ratio, or costed shorter residence time, the CODcr and NKU+-N removal rate were 10% and 20% higher respectively than conventional packing materials.Tests of clean water aeration-oxygenation performance showed that dissolved oxygen saturation value and oxygen transfer rate in packing reactor were higher than it without medias under the conditions of different aeration flux. Reasons for this performance were the air flow effect to medias increase the turbulence in the reactor, decreased the bubble size, maked more uniform of air-distribution and enlarged the frontal surface of gas-liquid transfer. Consequently, gas oxygen phase was easier transfer to liquid phase than reactors without carriers. In addition, under the concentration difference of gas and liquid oxygen, packing materials accelerated oxygen transfer rate and efficiency; dissolve oxygen in water saturated immensely; more aeration flux resulted in more quickly the oxygen transfer that fillers promotes, and more dissolve oxygen in water. But this unstable over-saturate situation was adequate to demand the microbial oxygen utilization for the dynamic balance caused by continuous aeration.Based on the experimental data of static dynamics and theoretical calculation, combined with proper simplification and curve fitting, an empirical formula of static dynamics in reactors was concluded. The following empirical equation was obtained for treated pollutants quality as a function of initial sewage quality, reacting periods and aeration flux.C _ C -0.0053V 4"44o — o0ewhere: So------ initial organic loading concentration, mg/L;S------ remaining organic loading concentration after t reaction periods,mg/L;/------ reacting time, min;q — the number flux of aeration, m3/h.The studied results of this project will be applied to municipal engineering, environmental engineering and related water treatment projects, especially municipal sewage treatment, recovery and reuse engineering. After spreadingapplication of this research results, some difficult problems of wastewater treatment will be solved, the level of environmental pollution control will be improve, more technological insurance will be provided for protecting the national environmental resources and ameliorating the ecological environment will. Hence, the topic results are not only of concernful value in academic theory, but also of great significance to practice and the society.