Research On The Oxygen Transfer Capacity And α Value Of Fine-bubble Diffusers In Plug-flow Reactor

Under the background of"Double-carbon"national policy,wastewater treatment emission reduction and consumption reduction is one of the technical problems that need to be solved urgently.In wastewater treatment system,different reactor flow pattern have an important effect on the oxygen transfer capacity of aeration equipment.At the same time,theαvalue is an important parameter to characterize the change of oxygen transfer capacity of the diffusers in the actual wastewater treatment system.And the oxygen transfer capacity andαvalue of the aeration equipment determines the economy of the construction and operation of wastewater treatment aeration system.Especially in the design process of the actual wastewater treatment aeration system,due to the lack of scientific to choose theαvalue of the fine-bubble diffusers,resulting in a huge waste of energy consumption in aerobic aeration operation of wastewater treatment.This paper focuses on theαvalue optimization of fine-bubble diffusers in the plug-flow reactor,a research was carried out to investigate the changing law of oxygen transfer capacity of fine-bubble diffusers under plug-flow pattern in clear water and wastewater,respectively.The dimensionless analysis method and Computational fluid dynamics（CFD）technique were used to reveal the oxygen transfer mechanism of fine-bubble diffusers in the plug-flow reactor,from a hydrodynamic point of view.And based on the test results of oxygen transfer capacity of fine-bubble diffusers in the clear water and wastewater,an optimization model ofαvalue was established.This research was aimed to achieve the optimization of aerobic tank design and purposes of the energy saving and consumption reduction.The main findings are as follows:（1）Dynamic plug-flow reactors for oxygen transfer capability test of fine-bubble diffusers in clean water and wastewater were constructed,and the mearsurement methods for the oxygen transfer capability of the fine-bubble diffusers was optimized in plug-flow reactor.The velocity field test results showed that,under the conditions with no aeration and the cross-section velocities were 1.0 cm/s,3.0 cm/s,5.0 cm/s,a stable simulated plug-flow pattern can be formed in the test system.And the results of tracer tests showed that,under experimental conditions with the s cross-section velocity was 3.0 cm/s and the aeration intensity was 50.0 Nm³·（h·m²）^-1 and the aeration density was 8%,the flow patetern of the dynamic plug-flow test system was mainly advection effect,and the reactor has good mixing performance.The"Double-zone"DO probe layout mode was determined.The Damk?hler（Da<0.25）was selected as the evaluation indicator.When the test results do not meet the Da number requirements,the test data needs to be processed using the"Double-zone"theoretical model to improve the accuracy of the results.（2）The results of the study on the effect of the cross-section velocity on the standard total oxygen transfer coefficient(K_La₂₀)of the fine-bubble diffusers in plug-flow reactor showed that,under the conditions of the aeration intensity(37.5 Nm³·（h·m²）^-1～62.5 Nm³·（h·m²）^-1),the increased of the cross-section velocity（from 1.0 cm/s to 5.0 cm/s）,the bottom cross-section velocity of the velocity field in the reactor was increased and the average Sauter diameter of the bubble decreased,and the cross-section velocity on the section of the fine-bubble diffusers’surface was significantly positively correlated with the fine-bubble diffusers K_La₂₀ at the level of P<0.01.（3）The results of the study on the effect of different influencing factors on the fine-bubble diffusers clean water oxygen transfer capacity in plug-flow reactor showed that,dunder the experiment conditions of the cross-section velocity（from 1.0 cm/s to 5.0 cm/s）and aeration intensity(37.5 Nm³·（h·m²）^-1～62.5 Nm³·（h·m²）^-1)and aeration density（from 6%to 10%）,the K_La₂₀ of fine-bubble diffusers(from 3.2 h^-1to 9.8 h^-1)was positively correlated with water temperature,cross-section velocity,aeration density and aeration intensity;and the standard oxygen transfer efficiency SOTE of fine-bubble diffusers（from 3.4%to 5.3%）was positively correlated with cross-section velocity and aeration density,and was negatively correlated with aeration intensity.（4）The results of the study on simulation of the oxygen transfer mechanism of fine-bubble aeration under plug-flow pattern based on the CFD showed that,when the aeration intensity were at the range of 37.5 Nm³·（h·m²）^-1 to 62.5 Nm³·（h·m²）^-1,with the increased of the cross-section velocity（from 1.0 cm/s to 5.0 cm/s）,the angle with the bubble plume and the bottom of the tank gradually decreased（from 64 degrees to 77 degrees）,the maximum bubble retention time were increased from 1.3 s to 1.5 s.When the cross-section velocity was certain,with the aeration intensity increased,the angle with the bubble plume and the bottom of the tank decreased,and the maximum bubble retention time were were reduced to 1.1 s.The increased of aeration density and cross-section velocity can weaken the trend of water flow went down in the upper velocity field of the reactor and improve the velocity of water flow in the bottom flow field,reduced the rise velocity of the bubble plume,and the increased of aeration intensity would lead to the increased of the rise velocity of the bubble plume,and reduced the trend of water flow went down in the upper velocity field of the reactor.（5）The results of the study on change of oxygen transfer capacity of fine-bubble diffusers in plug-flow reactor showed that,theαSOTE of the fine-bubble diffusers（from 1.1%to 4.1%）at different points along the aerobic tank,had positive correlation with the cross-section velocity and the aeration density.When under the high load conditions,theαSOTE of fine-bubble diffusers was positively correlated with aeration intensity;when under low load conditions,theαSOTE of fine-bubble diffusers had negatively correlated with the aeration intensity.From the perspective of the load variation in wastewater treatment process,high aeration intensity and high aeration density were recommended at the influent of the aerobic tank,and low aeration intensity and high aeration density were recommended at the efluent of the aerobic tank.（6）The results of the study on the dynamic push-flow oxygen transfer empirical model of fine-bubble diffusers showed that,compared with the oxygen transfer empirical model of fine-bubble diffusers in clear water conditions,the Reynolds number,Froude number,aeration density,water and air flow velocity ratio of oxygen transfer empirical model of fine-bubble diffusers in actual wastewater had a greater impact on the oxygen transfer capacity of the diffusers.Increasing the turbulence intensity and shear force of the velcity field,and increased the surface renewal rate of the interface of air liquid can effectively improve the oxygen transfer capacity of the fine-bubble diffusers under the condition of activated sludge.（7）In the actual wastewater treatment system,the average optimizedαvalue of the fine-bubble diffusers（from 0.24 to 0.92）at four points along the aerobic tank were 0.58,0.63,0.66,0.69,respectively.Theαvalue of the fine-bubble diffusers was increased with the increase of aeration strength,cross-section flow rate and aeration density.The average optimalαvalue of the fine-bubble diffusers（from 0.35 to 0.92）at four points along the aerobic tank were 0.60,0.65,0.66,0.67,respectively.（8）An oxygen transfer empirical model of fine-bubble diffusers along the aerobic tank was constructed.Based on the known K_La of fine-bubble diffusers and the dimensionless parameters,the optimalαvalue under the corresponding operating conditions can be solved.aαvalues optimization prediction model was constructed which based on aeration density（?）,aeration intensity（q）,cross-section velocity（v）,tank length（l）.Finally,a optimization process ofαvalue of aeration systems for a specific plug-flow aerobic tank was established.