The utility of raking, as an intermediate treatment between scrapings, seems to have been clearly demonstrated. Its practical effect is to allow a greater quantity of water to be passed between scrapings, thereby saturating--if the term may be used--the surface layer with clay and other fine matter before removing it, instead of taking it off when only a thin surface layer of it has been thus saturated. The large proportion of the total purification that takes place in passing through three reservoirs successively, holding in the aggregate a quantity of water equal to about 7 days' use, is very striking. Taking all the records, the percentage remaining after passing through these reservoirs, is as follows: Sediment for the year, 1909-1910, Table 2....................17% Turbidities, 5-year average, Table 3.........................25% Bacteria, 5-year average, Table 4............................24% Bacteria, selected winter months with high numbers in the raw water...................................20% Bacteria, selected summer months with high numbers in the raw water................................... 2.5% There is considerable seasonal fluctuation in the results of settling and filtering, as is shown in Table 21. ~Table 21--Average Removal of Turbidity and Bacteria by Washington Filters for Whole Period, Arranged by Seasons~. ===========================+========+========+========+========+======== | Winter.| Spring.| Summer.| Fall. | Year. ----------------+----------+--------+--------+--------+--------+-------- Turbidity, in | raw | 135 | 96 | 144 | 42 | 105 parts per | settled | 33 | 28 | 27 | 15 | 26 million: | filtered | 4 | 3 | 1 | 0.5 | 2 ----------------+----------+--------+--------+--------+--------+-------- Percentage | settling | 24 | 29 | 19 | 36 | 25 left from: | filtering| 12 | 10 | 4 | 3 | 8 | both | 3 | 1 | 0.3 | 1 | 2 ----------------+----------+--------+--------+--------+--------+-------- Bacteria per | raw | 16,600 | 4,150 | 4,100 | 1,960 | 6,700 cubic | settled | 6,300 | 980 | 160 | 270 | 1,940 centimeter: | filtered | 149 | 29 | 18 | 22 | 54 ----------------+----------+--------+--------+--------+--------+-------- Percentage | settling | 38 | 24 | 4 | 14 | 29 left from: | filtering| 2.4 | 3.0 | 11.2 | 8.2 | 2.8 | both | 0.90 | 0.79 | 0.44 | 1.12 | 0.81 ================+==========+========+========+========+========+========