Study On Hydraulics Characteristics Of SMBR By CFD Simulated

Membrane Bioreactor(MBR)is an efficient wastewater treatment technology through membrane-enhanced biological reaction.It has high quality of water treatment,small footprint and less residual sludge.It has been widely used in the field of wastewater treatment engineering field.The current MBR design method and the commonly used biokinetics basis are assumed to be fully mixed within the MBR internal gas and liquid-solids,while ignoring the actual hydraulics in the reactor.However,these hydrodynamic characteristics,such as gas phase and liquid velocity distribution and turbulent kinetic energy distribution,have a significant effect on optimizing mass transfer effect,alleviate membrane fouling and improving system energy utilization efficiency.However,it is nearly impossible to characterize these hydrodynamic features in the MBR system by experimental monitoring due to the complexity of the gas-liquid-solid three-phase turbulence in the reactor and the inability to determine the representative monitoring points within the membrane module.Therefore,to study the hydraulic characteristics within the MBR by means of theoretical simulation,which has been found out,the computational fluid dynamics(CFD)method,as an efficient fluid simulation tool,can accurately simulate and quantify the flow field with high precision and apply it to the study of the hydraulic characteristics in the MBR system.It is expected to study the system’s membrane pollution prevention and operation parameter decision from the micro level,and put forward the basis for the optimal design of the new MBR.This research adopted FLUENT software in a CFD method,to make the engineering application of the most submerged hollow fiber membrane bioreactor(SMBR)in present as the research object,to conduct gas-liquid two-phase flow simulation.System expanded on the three major parameters(membrane diameter,aeration rate and bubble size)to see the effect of hydraulic characteristics in the reactor,such as the influence of gas and liquid velocity,gas holdup and the turbulent kinetic energy and the shear stress at the membrane surface.The influence of hydraulic characteristics on the membrane fouling was analyzed by simulating the different flow field under single membrane module in the reactor.Then,by analyzing the distribution of different flow fields in the nine membrane module,the hydraulic characteristics and the interaction between the hollow fiber membrane were investigated on membrane fouling.The reliability of the simulation results was verified by the method of document verification.The main conclusions were as follows:(1)The hydraulic characteristics of single membrane and nine membrane module reactors were investigated under the conditions of 1mm,3mm and 5mm.The results show that with the increase of the diameter of the filament,the gas velocity in the upper part of the reactor increases obviously,and the liquid velocity increases gradually with the gas velocity,and the turbulent kinetic energy on both sides of the membrane module increases obviously.It was shown that the membrane module in the upper part of the reactor can also be effectively washed with water and scavenging the membrane on condition of crude membrane.(2)The hydrodynamic characteristics of the SMBR system were investigated under the conditions of aeration rate of 0.1 m/s,0.3 m/s and 0.5 m/s,respectively.In the reactor with single membrane module and nine membrane module,the nonuniformity of gas and liquid velocity increases with the enlargement of aeration rate,and a high speed region exceeding 1 m/s appears in the reactor.Due to the use of wall model,the membrane module as an obstacle,making the changes of liquid phase velocity with the gas velocity more obvious.Nine module which as a larger obstacle,making the speed distribution within the reactor not even more uniform than the single membrane reactor,and increase speed of unilateral components.The gas holdup increased at the bottom of the reactor,but not with the height of the reactor.The gas holdup in the upper part of the reactor was almost zero,indicating that the aerobic microorganism mainly completes the organic matter at the bottom of the reactor degradation.The turbulent kinetic energy increased with the increase of the aeration velocity,and the turbulent kinetic energy was the maximum at aeration speed of 0.5m/s,whether it was a single membrane or nine membrane modules.(3)The hydraulic characteristics of the SMBR system were investigated in the case of bubble diameters of 1 mm,3 mm and 5 mm,respectively.In the single membrane module and the nine membrane module,with the increase of the bubble diameter,the gas phase velocity and the liquid velocity increase in the reactor,and the high speed region was higher than 1.1 m/s in the reactor,which made the reactor gas and water mixture enhanced.However,the change in gas holdup in the reactor was not significant,especially when the bubble diameter was 3 mm and 5 mm,the gas holdup distribution was similar.The bubble generated by the large bubbles is larger than the small bubbles,so that the turbulence energy was the largest under the bubble with diameter of 5mm.And the turbulent kinetic energy on both sides of the membrane module was significantly reduced as the height increases due to the proximity of the nine membrane module assemblies closer to the reactor walls.(4)The comparison of the flow field distributions of the single membrane and the nine membrane module under the same parameters shows that the flow field of the reactor was higher due to the larger volume of the nine membrane modules in the reactor and the closer to the reactor wall uneven,the flow field area at a velocity of more than 1.1 m/s appears closer to the reactor wall.The gap between the filaments of the nine membrane module was 4mm,allowing 1mm of bubbles to enter the module,creating a clear flow field in the lower part of the module.However,due to the interaction between the membrane module,the gas and liquid phases can’t be as evenly distributed,which made the composition of the film has the different degree of contamination.(5)In the nine membrane module,the gas phase and liquid phase velocity in the vicinity of the cross section of the side membrane were significantly higher than that of the middle membrane cross section and the cross section of the module,and the turbulent energy was also the largest,indicating that the outer part of the membrane module was the largest.The results showed that the shearing force of the membrane was different,the shear force of the side membrane was the largest,and the corner film was the second,and the center position was the smallest.But the shear force was concentrated in the lower part of the membrane,indicating that the upper part of the membrane of the membrane module was more prone to pollution.