To this end the filters were divided into four groups which, during a period of about six months, were subjected to treatments as follows:
| Group A.— | Filters scraped deep at the end of each run; | |
| Group B.— | Filters scraped light at the end of each run; | |
| Group C.— | Filters raked at the end of each run, until raking failed to bring back the proper capacity; then they were scraped light, and at the end of the next run the raking was resumed; | |
| Group D.— | Light scrapings and rakings alternate at ends of runs. |
The term "deep scraping" means the removal of practically all the discolored sand, in accordance with the usual practice prior to the beginning of these experiments; "light scraping" means the removal of only a thin surface layer of sand. This depth has usually averaged about 3/8 in. "Raking" means the thorough breaking up of the clogged surface of the filter by iron‑toothed rakes, to a depth of about 1 or 2 in.
Results.—A general summary of the results of these experiments is given in [Table 29], which also shows the relative costs of the different methods per million gallons of water filtered. A normal period of 9 months just prior to the beginning of these experiments shows a labor cost (corresponding to that in [Table 29]) of $0.29‑1/4 per million gallons filtered.
Capacity of Filters.—The capacity of the filters under the different methods of treatment are shown in a general way in [Table 29] for days of service and millions of gallons filtered per run. This element by itself is decidedly in favor of the deep scrapings, and least in favor of the repeated rakings.
A clearer conception of the capacities of the filters under these different conditions may be obtained from the four diagrams, [Figure 12], showing, for the four different groups, the average number of days of service of the successive runs. The diagram for Group A shows that the variations in the period of service of the filters scraped each time to clean sand follow a more or less definite curve from year to year. For the period covered by this curve, the tendency seems to be toward a slight decrease in capacity from year to year, as shown by the lower average maximum and minimum in the second year than in the first. Group B shows a sudden decrease in capacity following the first light scrapings and, since that time, a low but quite constant capacity. Group C shows a constantly decreasing capacity with successive rakings. The only significance attaching to the curve after the first raking is the prohibitively low capacity indicated, and the ineffectiveness of the measures taken to restore the capacity after the sixth raking. Group D, after the first raking, shows a prohibitively low and constantly decreasing capacity. The diagrams for C and D indicate a dangerous reduction in capacity if long persisted in. The method followed with Group C may be dismissed with the statement that it is entirely insufficient, and would be of use only in the rarest emergencies.
As far as the question of capacity is concerned, these diagrams indicate that a filter in normal condition may safely be raked once. It is believed that the constantly decreasing capacity shown in Group D is not due so much to the rakings as to the small quantities of sand removed at the alternate scrapings, and therefore it would not be proper to condemn this method of treatment without a further trial in which this defect was remedied. This view seems to be supported by the results of Group B. The low but approximately constant capacity there shown would undoubtedly have been higher if a greater depth of sand had been removed each time.
