Multiplc-scalc Analysis And CFD Simulation In Biological Aerated Filters

In this paper, gas-liquid two-phase fluid dynamics and pollutant transportprocess were studied in the biological aerated filter, which involved the mechanicstheory of two-phase flow fluid dynamics, mass transfer of oxygen species andcontaminant component in porous media and biodegradation reactions within thebiofilm.Considering gas-liquid two-phase Darcy flow in porous media which wasdescribed by Slattery et al. and single-phase non-Darcy flow in porous media wasdescribed by Whitaker, we firstly derived mass conservation and Navier-Stokesequations about gas-liquid two-phase non-Darcy flow in porous media undermacro-scale.Based on computational fluid dynamics Euler multiphase model, gas-liquidtwo-phase co-current upflow through packed bed was investigated. Modified Siefkenmodel, Reed model, Lipinski model and Attou k-fluid model were applied,respectively; the gas-particle, liquid-particle and gas-liquid momenturn exchangecoefficients were implemented to CFD three-dimensional numerical simulation usinguser defined functions. By comparison with the experimental data, we found that:modified Siefken model was the best one to predict the pressure drop of gas-liquidtwo-phase upflow through porous media which was packed with2-8mm sphere atnormal temperature and pressure. The CFD numerical simulated values of pressuredrop by modified Siefken model were consistent with experimental results.Considering control equations and corresponding boundary conditions from thecell scale to the biofilm scale which was derived by Whitaker et al.and Orgogozo’smethods which was upgraded from biofilm-scale to the macro-scale model aboutsingle-phase flow in porous media with biofilm. We analyzed the multi-scalephenomena of the time scales and spatial scales in biological aerated filter with localvolume averaging theory. We derived the governing equations of the biofilm scalewhich was upgraded from the cell scale and the Darcy scale which was upgraded frombiofilm scale with up-scaling method. We firstly deduced the reaction equations ofgas-liquid-biofilm-solid phase four-phase in biological aerated filterAccording to the thickness of the biofilm and liquid film, conmbining themechanism of diffusion-reaction which was described by Rittmann and Atkinson’s effectiveness coefficient mathematical expression, We established a one-dimensionaldiffusion-reaction pseudo-analytical model of organic compounds, Ammonia anddissolved oxygen, and the parameters were selected reasonably according to themodel.We not only established species transport equations including oxygen in the gasphase, organic matter, Ammonia and dissolved oxygen in the liquid phase and biofilmphase, but also established the mass conservation equation and momentum equationabout gas and liquid phase. We simulated the Biostyr biological aerated filter in thesewage treatment plant is located in the southern city of Rome. Eulerian model andspecies transport model were used to simulate three-dimensional flow in biologicalaerated filter. The mathematical relationship between the biofilm scale andmacro-scale parameters were added to Fluent software using user defined functions.We found that the COD and ammonia nitrogen values which were simulated wereconsistent with the measured values, which verify the correctness of simulation. Wesimulated the dissolved oxygen in the liquid, oxygen in the gas phase, effectivenesscoefficients of COD and ammonia distributed along the filter. Finally, we studied theempty bed contact time, gas-water ratio and influent concentration etc. affected thefiltration performance of the biofilter.