----------+--------------------------------------------- | Chemical Required, Grains per Gallon. +----------+----------+----------+------------ Suspended | Raw | Subsided | Subsided |Minimum for Matter, |Water for |Water for |Water for |Raw Water Parts in | Sand | Sand |Mechanical| for 100,000. | Filters. | Filters. | Filters. |Mechanical |Cincinnati|Cincinnati|Cincinnati| Filters. | Report, | Report, | Report, | |Page 290. |Page 290. |Page 341. | Pittsburg. ----------+----------+----------+----------+------------ 1.0 | 0 | 0 | 0.75 | 0.40 2.5 | 0 | 0 | 1.25 | 0.50 5.0 | 0 | 0 | 1.50 | 0.70 7.5 | 0 | 1.30 | 1.95 | 0.90 10.0 | 1.50 | 1.60 | 2.20 | 1.00 12.5 | 1.60 | 1.80 | 2.45 | 1.15 15.0 | 1.70 | 2.00 | 2.65 | 1.30 17.5 | 1.80 | 2.10 | 2.85 | 1.40 20.0 | 1.95 | 2.20 | 3.00 | 1.60 30.0 | 2.25 | 2.45 | 3.80 | 2.00 40.0 | 2.50 | 2.75 | 4.40 | 2.50 50.0 | 2.80 | | | 60.0 | 3.05 | | | 75.0 | 3.40 | | | 100.0 | 4.00 | | | 120.0 | 4.75 | | | ----------+----------+----------+----------+------------ Mr. Fuller’s results seem to show that a greater amount of coagulant is required for the preparation of water for mechanical filters than is necessary in connection with sand filters. The results with sand filters indicate that settled waters and raw waters containing equal amounts of suspended matters are about equally difficult to treat. The results at Pittsburg indicate that the raw waters required much smaller quantities of coagulant for given amounts of suspended matters than was the case with subsided waters at Cincinnati, the results agreeing more closely with the amounts required to prepare raw water for sand filters at Cincinnati. AMOUNT OF COAGULANT REQUIRED TO REMOVE COLOR. The information upon this point is, unfortunately, very inadequate. In some experiments made by Mr. E. B. Weston at Providence in 1893 with a mechanical filter,[38] with quantities of sulphate of alumina averaging 0.6 or 0.7 of a grain per gallon, the removal of color was usually from 70 to 90 per cent. The standard used for the measurement of color is not stated, and there is no statement of the basis of the scale, consequently no means of determining the absolute color of the raw water upon standards commonly used.