Research On Simulation Application Of CFD Technology To MBR Equipment

Membrane Bioreactor(MBR)is a new type of sewage treatment system combining membrane separation process and activated sludge process.MBR has the advantages of strong processing capacity,small floor space,low residual sludge content and stable effluent quality.It is widely used in industrial,agricultural and urban sewage treatment fields.However,membrane fouling has been a key reason for limiting the efficiency of MBR operations and increasing its cost.There is a gas-liquid-solid three-phase hybrid system inside the MBR,and the internal flow field changes are extremely complicated and difficult to characterize by experimental methods.Based on the analysis of the pilot operation of MBR,the numerical simulation of computational fluid dynamics(CFD)is of great significance for the prevention of membrane fouling and the optimization of MBR equipment.Through the analysis of the pilot operation of an integrated MBR equipment in Ningxia Autonomous Region,it was shown that the influent COD(Chemical Oxygen Demand)was mainly between 300-800mg/L,the MLSS(Mixed Liquor Suspended Solids)was about 8000mg/L,the influent flow was 5.0m~3/h,and the ambient temperature was 25°C.By analyzing the process conditions,it was found that TMP(Transmembrane Pressure)variation with time was faster than the design parameters membrane cleaning was much worse than the prospections,and sludge deposition at both ends of the membrane is too much.In this study,the simulation calculations were applied to MBR system by CFD method to improve the new equipment.This study has come to the following conclusions:1.Under the conditions of airflow speed of 0.3m/s,bubble size of1mm,and membrane diameter of 1mm,the flow energy,gas-liquid phase flow rate,gas phase of the inner and outer diameters of the membrane wire with different lengths of 130mm,150mm and 180mm and distribution and other fluid dynamic properties were analyzed.It was found that a relatively large flow velocity inside the shorter membrane wire would lead to a large pressure of the membrane filament,which could shorten the service life of the membrane filament and increases the possibility of fracture.The turbulent flow inside the membrane filament at both ends was strong.The fluid flow rate was high,resulting in more sludge deposits at both ends of the membrane surface.Therefore,properly increasing the membrane module size could make a better gas-liquid mixture outside the membrane wire,so that the activated sludge in the membrane pool could exert better degradation then reduce membrane surface pollution.The gas and liquid at the two ports of the membrane wire had large flow velocity,and the turbulent flow energy was also large leading to a large pressure at the outlet of the membrane wire,so much attention should be paid to the reinforcement of the water outlet when manufacturing the membrane module.2.While the length of membrane wire is 150mm,the bubble size is1mm,the membrane diameter is 1mm,and the hydraulic properties under different gas flow rates of 0.15m/s,0.3m/s and 0.45m/s are analyzed,a stronger airflow is obtained.The high flow rate could enhance the turbulence inside and outside the membrane wire,and the gas-liquid mixing is better.However,excessive aeration will also affect the equipment such as over-aerating the sludge and decreasing the activated sludge in the membrane pool.At the same time,excessive aeration would increase the power consumption of the turbine and increase the operating cost of the MBR.In view of the simulation results,adding a baffle to the new generation of MBR design could reduce the aeration area of the membrane,increase the aeration intensity in the membrane module,and reduce the contamination of the membrane surface.3.Under the condition of 150mm membrane filament,membrane wire diameter of 1mm and inlet gas velocity of 0.3m/s,three bubble diameter of 1mm,3mm and 5mm were analyzed.It was found that the gas phase velocity and the liquid phase velocity in the device increased to different extents with the increasement of bubble diameter,the gas-liquid mixing degree in the reactor was enhanced,and the overall gas content rate was less affected.The turbulent flow caused by large bubbles was higher than that of small bubbles,and the size of the bubble had little effect on the fluid flow inside the membrane wire.Therefore,Change the shape of the aeration disk,and the bubble size could be appropriately increased.