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CONSIDERABLEinterest is being shown in the use of chlorinated copperas (ferrous sulfate) as a coagulant.The work described by Mohlman and Palmer (9) with activated-sludge coagulation and the results obtained by Hedgepeth and Olsen (3) with a highly colored water indicate the value of this material. Enslow (£) has recently presented data relating to the operating economic value of this coagulant and also concerning the reactions involved.Laboratory experiments under simulated plant conditions, as explained in a previous article (4), and using the same method of procedure, show that this reagent is a satisfactory coagulant for turbid water, being comparable to alum in this respect. A compact, easily settling floe is obtained at any pH value above 3.5. This is in sharp contrast to the value found efficient for ferric hydroxide floe (4) when using the usual iron and lime treatment.

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CORRECTIVEtreatment with lime to retard cold water corrosion of pipe lines has been more or less continuously practiced in the Baltimore water supply for the past ten years. The initial control in 1922 (/) was at a pH value of 8.3, but in recent years by utilization of the von Iieyer (4) marble test, together with changing buffer conditions, a satisfactory protective coating has been maintained at a pH of 7.9.

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.

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