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. | ||||
|---|---|---|---|---|
| Suspended Matter, Parts in 100,000. | Chemical Required, Grains per Gallon. | |||
| Raw Water for Sand Filters. Cincinnati Report, Page 290. | Subsided Water for Cincinnati Report, Page 290. | Subsided Water for Mechanical Filters. Cincinnati Report, Page 341. | Minimum for Raw Water for Mechanical Filters. 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.
At Westerly, R. I., with a New York filter, the actual quantity of potash alum employed from Oct. 10, 1896, to March 1, 1897, was 1.94 grains per gallon, the amount being regulated to as low a figure as it was possible to use to secure satisfactory decolorization. There is no record of the color of the raw water. A very rough estimate would place it at 0.50 upon the platinum scale. The chemical employed in this case was alum, and two thirds as large a quantity of sulphate of alumina would probably have done corresponding work, had suitable apparatus for applying it been at hand.
At Superior, Wisconsin, the water in the bay coming from the St. Louis River, having a color of 2.40 platinum scale, was treated experimentally with quantities of sulphate of alumina up to 4 grains per gallon, by Mr. R. S. Weston in January, 1899, but even this quantity of coagulant utterly failed to coagulate and decolorize it.