AWWA MTC64621

A model was developed, based on computational fluid dynamics (CFD) for the description of the complete flow field inside a membrane capillary, driven in inside-out mode. Particle or floc transport and deposition can be described by superimposing the calculation of forces and torques equilibria on the previously modelled fluid flow field. The model is used to give an overview of deposition behavior of flocs of certain size depending on operation condition, i. e. dead-end or slow cross-flow. The results can be used to derive appropriate operation and cleaning procedures. It is shown that the choice of dead-end or cross-flow operation is more significant if small particles or floc aggregates have to be filtered by the membrane. Small particles will be deposited more or less homogeneously along the membrane wall after some significant distance to the capillary inlet, letting the first membrane area, here approximately 30% unused for floc deposition. A slow cross-flow could be used to transport small flocs out of the capillary which entered the capillary cross section area in the neighborhood of the axis. Larger flocs will be "collected" in one resulting equilibrium trajectory and are transported to the rear end of the capillary, independent of their starting radial position at the inlet and operation condition. It can be further assumed that floc growth occurs during transport along this equilibrium trajectory. This will lead to transport of larger flocs out of the capillary lumen in cross-flow operation, avoiding excessive floc deposition and clogging of the membrane. Includes 24 references, table, figures.