Study On The Performance Of Bubble Plume Mass Transfer And Nitrogen Removal In The Micro-aeration Oxidation Ditch

With the transformation of China’s economic development from quantity to quality,more and more attention has been paid to the management of water environment and the recycling of water.Compared with the traditional oxidation ditch,although the energy consumption is reduced by the separation of the aeration and impeller through the communication of micro-aeration oxidation,the unreasonable operation and control of which will also lead to the increase of the system energy consumption.In this paper,the experimental apparatus of pilot-scale decelerating micro-aeration oxidation ditch was set up to investigate the oxygen mass transfer and wastewater treatment with bubble plume in the micro-aeration oxidation ditch through mass transfer and nitrogen removal.（1）Sodium carboxymethyl cellulose（CMC）aqueous solution with shear-thinning rheological properties was selected to simulate the activated sludge mixed liquor flow,and experiment was carried out on oxygen mass transfer with bubble plume in the pilot-scale oxidation ditch.The effects of air flow rate and liquid velocity on oxygen mass transfer performance and aeration efficiency were investigated in water and CMC aqueous solution.The total volume mass transfer coefficient K_La₂₀,standard oxygen mass transfer efficiency（SOTE）and standard aeration efficiency（SAE）were introduced to investigate the oxygen mass transfer performance in the oxidation ditch.In addition,the modified Froude number Fr and Ohnesorge number Oh were introduced to study the oxygen transfer efficiency through the consideration of the combined influence of aeration,liquid velocity,gas bubble size and physical properties of the liquid phase.The results showed that the variation of air flow rate and liquid velocity affected the changes of K_La₂₀,SOTE and SAE,and which regularity in clear water and CMC aqueous solution were the similar.Both K_La₂₀ and SOTE increased as the increase of liquid velocity.As the aeration flow rate increased,K_La₂₀ increased significantly,but SOTE showed a downward trend.In this experiment,SAE is greatly affected by the change of aeration volume,but less affected by the change of liquid velocity.At the same air flow rate and liquid phase flow rate,the rheological properties of liquid had a significant effect on the mass transfer of bubble plume,K_La₂₀,SOTE and SAE decreased as the increase of CMC concentration.According to the modified Froude number and Ohnesorge number,the coordinated operation of the air flow rate and the impeller could make the operating conditions more efficient.（2）An experimental study on nitrogen removal was carried out in pilot-scale oxidation ditch.In the activated sludge mixed liquor,the operation performance of the oxidation ditch was determined by the concentrations of COD,ammonia nitrogen（NH₄-N）,nitrate nitrogen（NO₃-N）,nitrite nitrogen（NO₂-N）and total nitrogen（TN）at the oxidation ditch outlet.At room temperature（20²1°C）,the wastewater treatment performance of oxidation ditch under different dissolved oxygen（DO）concentrations,inlet wastewater flow rate and aerator arrangements was investigated.In this experiment,the oxygen transfer efficiency of activated sludge mixed liquor was more similar to that of the 0.2 wt.%CMC aqueous solution.By studying the oxygen transfer efficiency under different aerator arrangements,the results show that the oxygen transfer efficiency under the cross arrangement mode is the highest,and the cross>abreast>centralized.At room temperature,the wastewater treatment performance of oxidation ditch was mainly affected by operating conditions,such as dissolved oxygen concentration and inlet wastewater flow rate.The increase of DO concentration was conducive to the degradation of NH₄-N and COD in the oxidation ditch,while the concentration of NO₃-N and TN increased.The increasing of inlet wastewater flow rate would lead to the decrease of wastewater treatment performance.In addition,considering that the oxidation ditch in the low temperature in winter was faced with the problem of low wastewater treatment capacity,the temperature of activated sludge mixed liquor in the oxidation ditch was controlled to be 13¹4°C,and the wastewater treatment performance of the oxidation ditch in the low temperature and low carbon was investigated.The wastewater treatment performance was weaker at low temperature than that at room temperature.At low temperature,the wastewater treatment performance of oxidation ditch was less affected by DO and inlet wastewater flow rate,and was very sensitive to temperature changes.The wastewater treatment performance could be improved by raising the water temperature.