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. ~Table 29--Average Results.~ Columns: A - Group. B - Number of filters. C - Number of days of service. D - Million gallons filtered. E - Cost of labor per treatment. F - Sand removed in cubic yards. G - Sand removed in cubic yards. H - Cost of labor. I - Bacteria per cu. cm. in effluent. J - Turbidity in effluent. =========================================+===============+============== | Per Million | | Per Run: | Gallons | | | Filtered | I | J -----+-----+-----+-------+-------+-------+-------+-------+ | A | B | C | D | E | F | G | H | | -----+-----+-----+-------+-------+-------+-------+-------+-------+------ _A_ | 5 | 82 | 221.2 |$68.44 | 215 | 1.11 |$0.309 | 13 | 1 _B_ | 9 | 36 | 101.4 | 29.25 | 84 | 0.83 | 0.288 | 16 | 1 _C_ | 5 | 21 | 60.0 | 10.92 | 24 | 0.40 | 0.182 | 18 | 1 _D_ | 10 | 32 | 86.0 | 20.10 | 46 | 0.54 | 0.234 | 22 | 1 =====+=====+=====+=======+=======+=======+=======+=======+=======+====== _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.