If the positively charged adsorbed species are in the form of isolated regions, then a form of “electrostatic patch” attraction may be important. It has to be noted that the PCN model does not cover bulk hydroxide precipitation sweep-floc coagulation. There is no doubt that, at the correct dosage, charge neutralization by the adsorbed hydrolysis products and hydroxide precipitate can cause negatively charged particlestobecomedestabilizedandhencetocoagulate. When charge-neutralization is the predominant destabilization mechanism, then there should be a stoichiometric relationship between the particle concentration and the optimal coagulant dosage.10 At low particle concentrations, only low coagulant dosages should be required. Under such conditions, coagulation rates can be very low, thereby causing problems in watertreatment.Anotherpracticaldifficultyisthatthe optimal coagulant dosage range can be quite narrow, whichmeansthatfairlyprecisedosingcontrolisneeded. These difficulties can be overcome by using higher coagulant dosages, for which extensive hydroxide precipitation occurs, giving rise to sweep-floc coagulation. 1.3.ThePhenomenologyofFlocSedimentation. It is well-known that it is just as important to achieve good settleability parameters in the physicochemical treatment of wastewaters as it is to attain a high level of decontamination. These parameters will determine the dimensions of the required equipment and hence thecostsoftheinstallation.Thesecondpartofthestudy analyzes the influence of the different operating variables on the following settleability parameters of the system: sediment volumetric percentage, settling velocity, sludge volume index, and residual conductivity in the clarified water. The phenomenon of the sedimentation of particles that have formed or are in the process of forming flocs is different from that of discrete particles. As they descend, flocs merge with each other by adsorption or coalescence. As they increase in size, their settling velocityincreases.Inasuspension,themannerinwhich the particles settle depends on the concentration of the suspension and the characteristics of the particles. Fitch11 describes the different types of sedimentation: clarification,flocculentsedimentation,zonalsettlingand compression or compaction. In the sedimentation of a concentrated suspension of solids, the particles are so close to one another that there is interference between their respective velocity fields. Furthermore, as the liquid is displaced upward, it acts to brake the descent of the particles.12