Using equal values of different precipitants, applied under the most favorable conditions for each, upon the same sewage, the best results were obtained from ferric sulphate. Nearly as good results were obtained from copperas and lime used together, while lime and alum each gave somewhat inferior effluents.... When lime is used there is always so much lime left in solution that it is doubtful if its use would ever be found satisfactory except in case of an acid sewage.
It is quite impossible to obtain effluents by chemical precipitation which will compare in organic purity with those obtained by intermittent filtration through sand.
It is possible to remove from one-half to two-thirds of the organic matter by precipitation ... and it seems probable that ... a result may be obtained which will effectually prevent a public nuisance.
243. Preparation and Addition of Chemicals.—Lime is not readily soluble in water. Therefore, it is not best to add the lime as a powder to the sewage, but to form a milk of lime, that is, a supersaturated solution containing from 2,000 to 4,000 grains per gallon, although dry slaked lime has sometimes been applied directly. The solution is prepared in tanks in a quantity sufficient for some part of the day’s run, commonly sufficient to last through one shift of 8 or 10 hours. The lime is prepared by placing the amount necessary to fill one storage tank into a slaking tank containing some cold water. Sufficient water is added to keep the solution just at the boiling point, or steam may be added to make it boil. After slaking, it is run into the milk-of-lime solution tank and sufficient water added to bring to the proper strength. The milk of lime is added in measured quantities, being controlled by a variable head on a fixed orifice or weir, so that it may be varied with the amount of sewage flowing through the plant. The amount of lime to be added is determined by titration with phenolphthalein, experience indicating the color to be obtained when the proper amount of lime has been added.
The use of either copperas or alum has been so rare, for the precipitation of sewage, that a description of the methods of handling these chemicals as a sewage precipitant is not warranted. An excellent description of the methods of handling these chemicals in water purification will be found in “Water Purification” by Ellms.
| TABLE 81 | |||||
|---|---|---|---|---|---|
| Results of Chemical Precipitation at Worcester, Massachusetts[[146]] | |||||
| 1900 | 1910 | 1920 | |||
| Amount of sewage treated, million gallons | 4,781 | 5,317 | 8,893 | ||
| Amount of sewage chemically treated, million gallons | 3,650 | 3,574 | 7,300 | ||
| Gallons of wet sludge per million gallons of sewage treated | 4,450 | 4,185 | |||
| Per cent of solids in sludge | 4.42 | 8.20 | 4.64[[147]] | ||
| Tons of solids | 7,294 | 4,182 | 6,431[[147]] | ||
| Pounds of lime added per million gallons of sewage pumped | 999[[148]] | 762[[147]] | 534 | ||
| Per cent of organic matter removed: | |||||
| By albuminoid ammonia: | |||||
| Total | 52.7[[149]] | 58.4 | 51.9 | ||
| Suspended | 90.0[[149]] | 88.7 | 83.6 | ||
| By oxygen consumed: | |||||
| Total | 62.8[[149]] | 61.1 | 62.5 | ||
| Suspended | 86.6[[149]] | 89.7 | 86.2 | ||
244. Results.—The results of Hazen’s experiments indicate that a greater amount of suspended matter can be removed in the same time by chemical precipitation than by plain sedimentation. The percentage of removal of suspended matter may be as high as 80 to 90 per cent with a period of retention of 6 to 8 hours and the addition of a proper amount of chemical. That the method is not always a success is shown by the results of some tests at Canton, Ohio.[[150]] The report states:
... lime treatment removes about 50 per cent of the suspended matter, and in the main about 50 per cent of the organic matter.... These data are instructive as indicating that the addition of lime to the Canton sewage in quantities as previously stated does not materially improve the character of the resulting effluent over and above that which could be produced by plain sedimentation alone.
The plant at Worcester, Mass., is the largest in the United States and information from it is of value. A summary of the results at Worcester for 1900, 1910, and 1920 are shown in Table 81.