Research On Gas-Liquid Flow And Mass Transfer In Micro-Aeration Oxidation Ditch

Employing the microporous aerator and underwater propeller in micro-aeration oxidation ditch,the aeration and plug mix of the mixture of sludge and sewage were completed independently,which is beneficial to control the flow velocity of liquid phase and the distribution of dissolved oxygen concentration.Aeration and plug flow must be operated in a coordinated manner.The energy-extensive consumption and the poor effluent quality could be caused by the inappropriate coordinated operation between plug flow and aeration.Most studies focused on the pollutants removal efficiency,operating conditions and biochemical reaction principle of the micro-aeration oxidation ditch,and were limited in the gas-liquid flow characteristics and its influence on mass transfer performance of the micro-aeration oxidation ditch.The gas-liquid flow and mass transfer performance were analyzed in the complex flow conditions of micro-aeration oxidation ditch to optimize the coordinated operation scheme of oxidation ditch,which had important significance and research value for the design and operation of actual sewage treatment in oxidation ditch.In this paper,by the micro-aeration oxidation ditch as the research object,the bubble characteristics and bubble plume movement behavior were comprehensively analyzed using high-speed photography and image processing technique.According to the theoretical analysis of gas-liquid two-phase flow,the hydrodynamic and mass transfer performance of the perfect mixed gas-liquid system and the deflection of bubble plume and Coanda effect in crossflow were systematically studied.Combined the synergistic effect of the mass transfer performance and the bubble plume behavior,a novel design method of micro-aeration oxidation ditch was present,by which a decelerated micro-aeration oxidation ditch was established.The actual sewage treatment effect was analyzed,and the process operation parameters were further optimized.It is provided solutions for the effluent quality and energy-saving of sewage treatment process in the micro-aeration oxidation ditch.The main experimental and theoretical work of this paper is summarized as follow:(1)The experimental apparatus of perfect mixing system was built and the micro-aeration experiment was systematically carried out.The distribution of bubble diameter,hydrodynamic characteristics and mass transfer coefficient were obtained and the gas-liquid mass transfer mechanism was analyzed under different gas phase conditions and liquid phase properties in perfect mixing system.The study shows that the hydrodynamic parameters and mass transfer coefficient of gas-liquid system increased with the increase of superficial gas velocity.With the increase of weight percentage of liquid phase,the bubble diameter increased and the distribution of bubble diameter widened,and the hydrodynamic parameters and mass transfer coefficient of gas-liquid system decreased.By the theoretical analysis in further,it is indicated that the gas-liquid mass transfer was dependent on the rheology of non-Newtonian fluid and the mobility of gas-liquid interface.Based on the classical mass transfer theory,combination of the rheological parameters of liquid phase,a dimensionless correlation was established to predict the gas-liquid mass transfer rate in perfect mixing system for Newtonian fluid and non-Newtonian fluid.(2)The flume device was built to form the uniform and stable crossflow for the experimental study of movement of bubble plume in environmental fluid.The motion and spread of bubble plume in crossflows were obtained by using high speed photography and image processing technique.The research shows that the inclination trajectory of bubble plume approximated linearly in crossflow.Compared with Newtonian fluid system,the deflection of bubble plume in non-Newtonian fluid system changed significantly.With the increase of weight percentage of liquid phase,the inclination angle of bubble plume increased obviously.The inclination angle of plume decreased slightly with the increase of superficial gas velocity.The motion integral equation of plume in crossflow condition was modified by considering the force exerted on the bubble plume and rheological properties of liquid phase.The theoretical correlations were provided for predicting the motion trajectory of bubble plume in crossflows.The width of bubble plume core region in gas-liquid system spread linearly along the direction of plume motion,while the spreading rate of bubble plume changed in segment in non-Newtonian fluid.Ajusting the distance between two aerators,crossflow velocity and superficial gas velocity in this experiment,the merging characteristics and inclination trajectories of double plumes were observed respectively and the Coanda effect of double bubble plumes was systematically analyzed.The optimal distance between two aerators was determined by analyzing the Coanda effect of double plumes.(3)The decelerated micro-aeration oxidation ditch was designed and built for comprehensive study of the gas-liquid mass transfer performance in perfect mixing system and the motion behavior of bubble plume in crossflow.The velocity distribution of liquid phase,oxygen transfer performance and motion behavior of bubble plume in oxidation ditch were measured under different air flow rates,aerator arrangement modes and plug flow intensity respectively.The oxygen transport performance and mass transfer effect was enhanced by the increase of air flow rate,but the aeration efficiency decreased signifcantly.The influence of the aerator arrangement mode on the oxygen transfer process was obvious.Adopting the uniform aerator arrangement mode,the oxygen transport performance was optimal.With the increase of plug flow intensity,the aeration performance increased and aeration efficiency decreased for the enlarging energy consumption.The actual sewage treatment experiment was carried out to investigate the influences of hydraulic retention time and dissolved oxygen concentration on the sewage treatment effect.The removal effect of nitrogen and carbon was analyzed to optimize the operating parameters of micro-aeration oxidation ditch.The energy consumption of the sewage treatment system in oxidation ditch was investigated and the optimal coordinated operation scheme of the delecrated oxidation ditch was determined for actual sewage treatment.