COST OF FILTRATION.
The cost of filtration of water depends upon the character of the raw water, upon the nature of the plant employed, upon its size, and upon the skill and economy of manipulation. These conditions affect the cost to such an extent as to make any accurate general estimate quite impossible. Nevertheless a little consideration of the subject, although not leading to exact results, may be helpful as furnishing a rough idea of the probable cost before estimates for local conditions are made.
Open sand filters, with masonry walls, with reasonably favorable conditions of construction, and not too small in area, have averaged to cost in the United States within the last few years perhaps about thirty thousand dollars per acre. The relative cost of small plants is somewhat greater, and with embankments instead of masonry walls, the cost is somewhat reduced. The cost is less where natural deposits of sand can be made use of practically in their original condition, and is increased where the filtering materials have to be transported by rail for long distances, or where the sites are difficult to build upon. Covered filters cost about a half more than open filters. Mechanical filters at current prices cost about $20 per square foot of filtering area, to which must be added the cost of foundations and buildings, which perhaps average to cost half as much more, but are dependent upon local conditions and the character of the buildings.
To these figures must be added the costs of pumps, reservoirs, sedimentation-basins, and pipe-connections, which are often greater than the costs of the filters, but which differ so widely in different cases as to make any general estimate impossible.
Filters must be provided sufficient to meet the maximum and not the average consumption. The excess of maximum over average requirements varies greatly in different cities, and depends largely upon reservoir capacities and arrangements.
As a result of a considerable number of estimates made by the author for average American conditions, the cost of installing filters may be taken very roughly as five dollars per inhabitant, but the amounts differ widely in various cases.
The cost of operation of sand filters in England probably averages about one dollar per million gallons of water filtered. The following table shows the costs of operation of the filters of the seven London companies for fifteen years, compiled in the office of Mr. W. B. Bryan, Chief Engineer of the East London Water Company. The results have been computed to dollars per million U. S. gallons, and include the cost of all labor, sand, and supplies for the filters, but do not include any pumping or interest costs.
| COST OF FILTRATION, LONDON WATER COMPANIES. | ||||||||
|---|---|---|---|---|---|---|---|---|
| (Computed from data furnished Wm. B. Bryan, C.E., East London Water Works.) | ||||||||
| Dollars per Million U. S. Gallons. | ||||||||
| Chelsea Co. | East London Co. | Grand Junction Co. | Lambeth Co. | New River Co. | Southwark & Vauxhall Co. | West Middlesex Co. | Average. | |
| 1880-1 | 1.16 | 1.16 | 1.00 | 0.83 | 1.34 | 1.16 | 1.67 | 1.19 |
| 1881-2 | 1.19 | 1.39 | 0.95 | 0.82 | 1.15 | 1.37 | 1.54 | 1.20 |
| 1882-3 | 1.10 | 1.23 | 1.39 | 0.96 | 1.40 | 1.47 | 1.74 | 1.33 |
| 1883-4 | 1.00 | 1.06 | 1.73 | 0.92 | 1.11 | 1.62 | 1.67 | 1.30 |
| 1884-5 | 1.06 | 1.06 | 1.82 | 0.90 | 1.02 | 1.40 | 1.30 | 1.22 |
| 1885-6 | 1.15 | 1.16 | 1.35 | 0.90 | 1.00 | 1.15 | 1.07 | 1.11 |
| 1886-7 | 0.80 | 0.96 | 1.39 | 0.87 | 0.98 | 1.43 | 1.70 | 1.16 |
| 1887-8 | 1.07 | 1.22 | 1.74 | 0.90 | 0.92 | 1.28 | 1.00 | 1.16 |
| 1888-9 | 0.83 | 1.28 | 1.55 | 0.95 | 0.98 | 1.52 | 0.83 | 1.13 |
| 1889-90 | 0.66 | 1.50 | 1.22 | 0.88 | 0.90 | 1.70 | 3.56 | 1.49 |
| 1890-1 | 0.72 | 1.42 | 1.32 | 0.85 | 1.02 | 1.16 | 1.00 | 1.07 |
| 1891-2 | 0.75 | 1.54 | 1.23 | 1.00 | 0.92 | 1.15 | 0.96 | 1.08 |
| 1892-3 | 0.67 | 1.42 | 1.30 | 1.19 | 1.16 | 1.26 | 1.42 | 1.20 |
| 1893-4 | 1.15 | 2.63 | 2.00 | 1.46 | 1.43 | 1.52 | 0.95 | 1.59 |
| 1894-5 | 0.60 | 1.68 | 1.67 | 2.53 | 1.03 | 1.34 | 0.96 | 1.40 |
| Average | 0.93 | 1.38 | 1.44 | 1.06 | 1.09 | 1.37 | 1.43 | 1.24 |
Average of seven companies for 15 years, $1.24 per million gallons. | ||||||||
Variations from year to year are caused by differences in the amounts of ice,and in the quantities of new sand purchased. Wages average about $1.00 perday. At Liverpool for 1896 the cost was $1.08 per million U. S. gallons. | ||||||||
In Germany, with more turbid river-waters, the costs of operation are somewhat higher than the London figures, while at Zürich, where the water is very clear, they are lower.
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.
With the clearest waters the costs of operation on the two systems are substantially equal. With muddy waters, the expense of operating sand filters increases more rapidly than the expense of operating mechanical filters.
TURBIDITY
Fig. 26.—Cost of Operation of Filters.
There is another element which often comes into the comparison, namely, the question of purification from the effects of sewage-pollution. Nearly all rivers used for public water-supplies receive more or less sewage, and in filtering such waters it is regarded as necessary to remove as completely as possible the bacteria.
The quantities of sulphate of alumina required for the clarification of the least turbid waters are not sufficient to give even tolerably good bacterial efficiencies. To secure a reasonably complete removal of bacteria with mechanical filters, the use of a considerable quantity of sulphate of alumina is required. Let us assume that 98 per cent bacterial efficiency is required, and that to produce this efficiency it is necessary to use one grain of coagulant to the gallon. With water requiring less than this quantity of coagulant for clarification this quantity must nevertheless be used, and the costs will be controlled by it, and not by the lower quantities which would suffice for clarification, but would not give the required bacterial efficiency.
I have added this line to the diagram, and this, combined with the upper portion of the line showing cost of clarification, represents the cost of treating waters with mechanical filters, where both bacterial efficiency and clarification are required.
This line, considered as a whole, increases much less rapidly with increasing turbidity than does the corresponding line for sand filters, and the two lines cross each other. With the clearest waters sand filters are cheaper than mechanical filters, and for the muddiest waters they are more expensive. It does not appear from the diagram, but it is also true in each case, that the cheaper system is also the more efficient. Sand filters are more efficient in removing bacteria from clear waters than are mechanical filters, and mechanical filters are more efficient in clarifying very muddy waters than are sand filters.