Advances in Water Treatment and Enviromental Management

178 WATER TREATMENTsmall suspended objects being able to pass the filter pore. Fig. 5 and Fig. 6 show two differenttypes of statistical deposits of 2000 solid particles performed using our original simulationprogram. In each case, the suspension is assumed to flow from the left to the right.The first case models a filter fouling due to a suspension flow where particle-particle and particlewalladhesion forces are much greater than hydrodynamical forces carrying the particles away.Here a suspended particle making contact with the filter surface or with a particle of the aggregateis immediately stopped and captured. Thus N r equals 0 and the given value for θ i is about 60°.The deposit structure is dendritic and the porosity has a mean value about 51 %.The second simulation presents a very compact aggregation we could observe experimentallyif, on one hand, the effect of parallel shear flow was efficient even inside the deposit in orderto reorganize particle arrangement and, on the other hand, the wall was very attractive inorder to prevent aggregated particles from being carried into the flow again. To take intoaccount the importance of tangential flow, N r is chosen equal to 3. and θ i and θ v have a verysmall value. The 2D mean porosity is then equal to 29 % and the mean thickness is obviouslylower than the one obtained in the first case (see Fig. 6).It now appears to be necessary to investigate the whole range of possible deposits by varyingthe most important model parameters.Influence of incidence angle αThis parameter is used to specify the relative magnitude of the parallel shear flow over theradial suction flow. Radial variations of concentration are plotted in Fig. 7 for a large range ofα. It is seen that aggregates become more and more compact from 10° to 30°, after this pointthey keep approxikatly the same structure. This is also demonstrated by the continuouslydecreasing porosity shown in Fig. 8.Fig. 7: Vertical concentration variationswith θ i =1°θ v =1º, N r =1Fig. 8: 2D mean porosityvariationsInfluence of incident sticking angle θ iThe importance of hydrodynamical entrainment force over adhesion force is determined bythe numerical value given to this parameter. The vertical concentration diagram shown inFig.9 provides an adequat classification of the solid particle deposits obtained when varyingθ i from 5° to 60°. Of course the small values lead to a very compact structure and the meanporosity increases quickly if the sticking angle increases (see Fig. 8).Fig. 9: Vertical concentration variations with a=20°, θ v =1°, Nr=0