Study On The Effect Of Flow Field On Membrane Fouling In PAC - UF System

In the membrane application area of drinking water, the focus in the field of membrane separation technology for water treatment is the effective control of membrane fouling, because membrane fouling will result in the increase of membrane operation and water production cost, the reduce of the membrane life. The study, which the process conditions (stirring speed) and the different roughness cake layer effected on the membrane fouling, was investigated by Computational Fluid Dynamics (CFD) method. The results were verified with experimental studies based on the adsorption experiment, filtration experiment and scanning electron microscope test method. Based on the simulation and experimental results, flow field analysis indicators were investigated, such as turbulence intensity, turbulent kinetic energy, cross-flow velocity, speed streamline, membrane shear stress, PAC particle content and so on. Hydraulics condition and cake layer roughness are analyzed.Firstly, in this study the effects of characteristic of flow field which was in the reactor on four different stirring speed conditions(60rpm,100 rpm,150 rpm,200 rpm) have been investigated. Based on the simulation results, it has been observed that the stirring speed value affected the thickness of the cake layer. The higher stirring speed value was, the thinner cake layer thickness was. This resulted in the seriously nonuniform cake layer, which is thicker at the top of the membrane module than that at the bottom. And the effect of retarding membrane fouling was weakened. In the membrane filtration process. When the stirring speed was enhanced, the dead zone in the reactor and PAC volume fraction was reduce, vortex flow and backflow in the reactor was strengthened, PAC particles suspension condition was enhanced. This generated that the solid-liquid mixed ability and PAC particles adsorbed rate was increased. The membrane fouling was controlled.Secondly, in order to verify the accuracy of CFD simulation, PAC particles adsorbed rate experiment and membrane filtration experiment was done, which matched simulation parameters. Based on the experimental results, the flow velocity was increased in the reactor and the external diffusion process of the PAC adsorption was improved because of the increased stirring speed. This resulted in the fast PAC particles adsorbed rate. The membrane shear stress was enhanced. This generated that the water production efficiency was improved in the membrane filtration process. However, the higher speed reduced the rate of the retarding membrane fouling. The results of the experimental studies were in accordance with the simulation results.Furthermore, in this study the effects of external flow field characteristics of cake layer on two different membrane roughness(3μm,6μm) have been investigated. Based on the simulation results, it has been observed that turbulence intensity, cross-flow velocity and membrane shear stress of sunken structure were lower than raised structure. Results showed that PAC particles and organic contaminants were easily inside the sunken structure and adsorbed. When the module was larger, particles accumulation to the cake layer was reduced, which is beneficial to alleviate membrane fouling. However, the membrane roughness of sunken structure had little effect on membrane shear stress. When the module was smaller, the raised structure suffered the bigger shear stress. This generated that the particles and contaminants of cake layer surface were taken away by water. The membrane fouling was retarded. The reactor model was in accordance with experiment, so the CFD results were reliable and credibility strongly.