Study On Gas-liquid Two-phase Flow Coupled With Biological Reaction In The Oxidation Ditch With Fine Bubble Diffuser

Oxidation ditches are widely used in waste water treatment because of its simple flow,easy maintenance and effective mixing.At present,the oxidation ditch process has some problems,such as large energy consumption in aeration and poor coordination ability of impellers and aerators,which need to be optimized immediately.Compared with experiment,numerical simulation can greatly save time and economic cost.A coupling model of gas-liquid two-phase flow and biochemical reaction for the oxidation ditch with fine bubble diffuser was established based on the based on the commercial CFD software Fluent in this paper.The processes of oxygen mass transfer and biochemical reaction were simulated.The main research work carried out in this paper are as follows:?1?A gas-liquid two-phase flow model for a bench scale oxidation ditch with fine bubble diffuser under the combination of aeration and horizontal flow was established.The flow field and gas holdup distribution in the oxidation ditch were obtained using this model.Based on the gas-liquid two-phase flow model and Higbie permeation theory,a gas-liquid two-phase flow and oxygen mass transfer model was established.The process of oxygen mass transfer from the bubble to the liquid was simulated.The reliability of the model was tested by the experimental results.?2?The influence of aeration parameters on mass transfer efficiency was investigated using the gas-liquid two phase flow and dissolved oxygen transport model.The result shows,with the decrease of bubble diameter,the oxygen mass transfer coefficient increased from5.83 h^-1 to 15.8 h^-1,indicating that decreasing the bubble diameter can effectively enhance the mass transfer effect.With the increase of aeration flow rate,the oxygen mass transfer coefficient increased from 4.21h^-1 to 14.15h^-1,indicating that the gas-liquid mass transfer effect increased with the increase of aeration flow rate.When the transverse flow velocity of was 0 m/s?0.132 m/s?0.211 m/s respectively,the corresponding oxygen mass transfer coefficients were 7.81h^-1,9.97h^-1 and 11.12h^-1,indicating that the transverse flow was conducive to enhancing the effect of oxygen mass transfer between gas and liquid,and the greater the transverse flow velocity was,the better the oxygen mass effect was.?3?Coupled with the activated sludge model,the gas-liquid two-phase CFD and ASM1coupling model was established in the pilot scale oxidation ditch.Using the coupling model,the degradation process of wastewater was simulated in the pilot scale oxidation ditch,and the distributions of dissolved oxygen and biological components were obtained.The reliability of the model was tested by the experimental results.The influence of operating parameters on treatment effect was investigated using the coupling model of gas-liquid two phase CFD and ASM1.The result shows,Prolonging the hydraulic residence time and increasing the aeration flow rate can increase dissolved oxygen concentration in sewage thus promoting nitrification.After changing the hydraulic residence time,the COD removal efficiency didn't change obviously indicating the pilot scale oxidation ditch has anti-impact load ability for COD.Due to microbial respiration,dissolved oxygen concentration increased nonlinearly with aeration flow rates.