COAGULANTS WHICH HAVE BEEN USED. In actual work sulphate of alumina is practically the only coagulant which has been employed, excepting the alums, which are practically its equivalent in action, differing only in strength. Nearly all important experiments upon the coagulation of water have been made with sulphate of alumina, and in the further discussion of this subject only this coagulant will be considered. AMOUNT OF COAGULANT REQUIRED TO REMOVE TURBIDITY. In the coagulation of turbid waters a certain definite amount of coagulant must be employed. If less than this amount is used either no precipitate will be formed, or it will not be formed in sufficient bulk to effect the desired results. It is necessary that the precipitate should be sufficient, and that it should be formed practically all at one time. The amount of coagulant necessary to accomplish this purpose is dependent upon the turbidity of the raw water. With practically clear waters sulphate of alumina of the ordinary commercial strength, that is to say, with about 17 per cent soluble oxide of aluminum, used in quantities as small as 0.3 or 0.4 of a grain per gallon, will produce coagulation. As the turbidity increases larger amounts must be employed. A special study was made of this point in connection with the Pittsburg experiments.[37] As an average of these results it was found that two grains per gallon of sulphate of alumina were required to properly coagulate waters having turbidities of 1.00, so that they could be filtered by the Jewell filter, and 2.75 grains were required for the Warren filter. [Illustration: FIG. 20.—AMOUNT OF COAGULANT REQUIRED TO REMOVE TURBIDITY.] Aside from the amount required to produce a precipitate in the clearest waters, the amount of coagulant required was proportional to the turbidity. As an average for the two filters the required quantity was approximately 0.30 of a grain, and in addition 0.02 of a grain for each 0.01 of turbidity. Thus a water having a turbidity of 0.20 requires 0.70 of a grain per gallon; a water having a turbidity of 0.50 requires 1.30 grains; of 1.00, 2.30 grains; of 2.00, 4.30 grains, etc. These are average minimum results. Occasionally clear effluents were produced with smaller quantities of coagulant, while at other times larger quantities were necessary for satisfactory results. The amount of coagulant required for clarification at Cincinnati has been stated by Mr. Fuller in his report. A number of his results are brought together in the following table, to which has also been added a column showing approximately the corresponding results at Pittsburg. ESTIMATED AVERAGE AMOUNTS OF REQUIRED CHEMICAL FOR DIFFERENT GRADES OF WATER.