| To deal with the increasingly serious water polution, many wastewater treatmentplants (WWTPs) have been built. Oxidation ditch process is commonly used due to itsstable operation, convenient management and good treatment performance. However,there are some disadvantages. Its operation energy consumption is high, and the highsludge concentration makes sludge deposition easy to happen, which reduces the validtreatment volume of the process. Therefore, The hydraulic characteristics of oxidationditch will not only affect the distribution of sludge and dissolved oxygen, but also impacton the biochemical reactions and relevant water quality indices such as the concentrationof nitrogen and phosphorus. With the development of computer technology, hydraulicsmodel and water quality model become an important research method that is low-costand highly-efficient for the operation and optimization of WWTPs. In this thesis, CFDtechnology was used to study the distribution of sludge and dissolved oxygen in animproved Carrousel2000process in Pingdingshan Sewage Purification Center based onprevious exploratory research. And the coupling of biochemical model and hydraulicmodel was studied.The following researchs were carried out:(1) Solid-fluid two-phase CFD model of the oxidation ditch was built, then the flowfield and sludge concentration distribution in the ditch was simulated.The modified mixture model was used to study the solid-liquid two-phase flow, atlast the simulation results with the single-phase flow simulation and actual measuredvalues were compared, the results show that compared with single-phase the relativeerror was reduced from8%to5%. After the simulation of solid-liquid flows, based onthe results that the sludge are prone to be settled at the current operation condition, anoptimized scheme was made. Keep energy consumption in the same situation, thehydraulics conditions has been optimized, the sludge volume fraction at the bottom wasreduce from0.260to0.258.(2) Gas-solid-fluid three-phase CFD model of the oxidation ditch was built, then thedissolved oxygen concentration distribution in the ditch was simulated. Based on the solid-liquid two-phase flow simulation, a gas-solid-liquid three-phasesimulation of OD was carried out. Compared with traditional Euler-Euler model, we usedthe UDS to simulate the distribution of oxygen, and the source term and sink term wereset by the UDF, then the simulation data was checked with the measured data. Thesimulation results compared with the measured results, the relatively errors were in11%.(3) A research of CFD model and activated sludge model ASM1in a full-scale ODwas made, and a preliminary simulation of the OD water quality was carried.The treatment condition was simulated using ASM1model, and the simulationresults were verified by the measured results. Then the verified ASM1model wasimported into Fluent by UDF. At the same time, source terms equation was built for eachsubstance and UDS transport equation was used to simulated the distribution of eachcomponent. Though the simulation results are rough, it shows the possibility of thecoupling study of hydraulic model and ASM biochemical model by CFD and lays afoundation for further systematic simulation study for WWTPs.WWTPs were made great contributions to improve China’s urban water environment,but how to improve the operating efficiency on the basis of the existing sewage treatmentplant, reduce energy consumption, and reduce operating costs is important in the nextstageissue. The hydraulics model and the coupling of the activated sludge model carriedout in this paper provides a theoretical basis for the optimized operation of the WWTPs,at the same time, there is a certain reference value for the operation of other WWTPs. |
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