The rates of filtration used in the experiments have ranged from less than 100 to about 130 million gallons per acre daily. To employ a rate much higher than this involves the use of a much coarser sand, or an increase in the height of water upon the filter to an impracticable extent. There would seem to be no material advantage in the use of lower rates within certain limits, while the cost of filters would be greatly increased.
The sand used in the Warren filters has been crushed quartz. In the Jewell filters a silicious sand from Red Wing, Minn., with rounded grains has been used. These sands are somewhat coarser than are commonly used in sand filters, and the uniformity coefficients are very low. It is necessary to use sand with the very lowest uniformity coefficients to avoid the separation of sand particles according to sizes as mentioned above, and for this reason the sand must be selected with much greater care than is required for sand filters.
PLAN JUST ABOVE COPPER.
SECTION SHOWING FILTER DURING ORDINARY OPERATION.
Fig. 23.—Warren Filter: Pittsburg Experiments. Section No. 1.
PLAN OF AGITATOR, GUTTER CASTINGS, ETC.
SECTION SHOWING FILTER DURING OPERATION OF WASHING.
Fig. 24.—Warren Filter: Pittsburg Experiments. Section No. 2.