In the United States the data regarding the cost of operation of sand filters are less complete. At Mt. Vernon, N. Y., with reservoir-water, the cost has averaged about two dollars per million gallons. At Poughkeepsie, N. Y., with the Hudson River water, which is occasionally moderately turbid, the cost for twenty years has averaged three dollars per million gallons. This cost includes the cost of handling ice, and as the average winter temperature is considerably below that suggested for open filters, the expense of this work has been considerable, and has increased considerably the total cost of operation.
At Far Rockaway, L. I., and Red Bank N. J., for iron-removal plants, the cost of operation has hardly been appreciable. The plants are both close to the pumping-stations, and it has been possible to operate them with the labor necessarily engaged at the pumping-station without additional cost, except a very small amount of labor on the sand at Far Rockaway. No computation has been made in these cases of the additional coal required for pumping.
At Lawrence, Mass., the cost of operation for 1895 was as follows:
| Cost of scraping and replacing sand | $3,467 | |
| Cost of care of ice | 2,903 | |
| Total cost of operation | $6,370 | |
| Water filtered, millions of gallons | 1,097 | |
| Cost per million gallons | $5.80 |
The cost of care of ice has been excessive at Lawrence, and it has been repeatedly recommended to cover the filter to avoid this expense. The cost of handling sand has been very greatly increased, because the filter is built in one bed, and all work upon it has to be done during the comparatively short intervals when the filter is not in use, an arrangement which is not at all economical in the use of labor. The cost of operation is thus much higher than it would be had the plant been constructed in several units, each of which could be disconnected for the purpose of being cleaned in the ordinary manner. As against this the first cost of construction was extremely low, and the saving in interest charges should be credited against the increased cost of labor in cleaning.
The cost of operating filters at Ashland, Wis., has been estimated by Mr. William Wheeler at $2.26 per million gallons. This estimate is based upon the performance for the first year that they were in service.
In the operation of mechanical filters one of the largest items of expense is for the coagulant, and the amount of this depends entirely upon the character of the raw water and the thoroughness of the treatment required. The data regarding the other or general costs of operation of mechanical filters are few and unsatisfactory.
I recently made some estimates of cost of clarifying waters of various degrees of turbidity by sand and mechanical filters. These estimates were made for a special set of conditions, and I do not know that they will fit others, but they have at least a suggestive value. The results shown by Fig. 26 include only the cost of operation, and not interest and depreciation charges. These figures, when used for plants in connection with which preliminary treatments are used, should be applied to the turbidity of the water as applied to the filters, and not to the raw water, and the costs of the preliminary processes should be added.
With sand filters the frequency of scraping is nearly proportional to the turbidity; and as scraping represents most of the expenses, the costs of operation are proportional to the turbidity, except the general costs, and the cost of the amount of scraping, which is necessary with even the clearest waters.
With mechanical filters the amount of sulphate of alumina required for clarification increases with the turbidity, and most of the costs of operation increase in the same ratio. The diagram shows the amount of sulphate of alumina in grains per gallon necessary for clarification with different degrees of turbidity.