Water purification has made practical use of the concepts of colloidal chemistry. Coagulation is now governed by pH factors, and the operation and design of mixing basins are in accordance with the principles reviewed in this paper.Utilization of sulfate adsorption has increased plant efficiency by giving a precise value to optimum flocculation, while adsorptive beds are no longer considered as a phase of the rule-of-thumb operation.Since proper coagulation of water is the controlling factor in plant efficiency, it is obvious that better sedimentation and filtration procedures follow in relationship. Therefore, the assumption that efficient purification processes are in accordance with the advancement of colloidal chemical knowledge is correct.