. The sanitation of cities. usual. The sand grains are coarser,and the bed of sand is cleaned in place, by revers-ing the direction of flow—so that filtered water isforced back through the filter bed. The rawwater is first clarified by the addition of the coagu-lants, and after sedimentation in a tank, is passedto the surface of the bed, over which the coagu-lant causes the rapid formation of a gelatinousfilm, and is allowed to pass downward throughthis and the sand at a rate about forty timesgreater than the slow sand filter would of 100,000,000 to 125,000,000 gallons aday to the
. The sanitation of cities. usual. The sand grains are coarser,and the bed of sand is cleaned in place, by revers-ing the direction of flow—so that filtered water isforced back through the filter bed. The rawwater is first clarified by the addition of the coagu-lants, and after sedimentation in a tank, is passedto the surface of the bed, over which the coagu-lant causes the rapid formation of a gelatinousfilm, and is allowed to pass downward throughthis and the sand at a rate about forty timesgreater than the slow sand filter would of 100,000,000 to 125,000,000 gallons aday to the acre of surface are usual. The filters are usually built of concrete, eachconsisting of a rather small unit; embeddedin the floor of each is the underdrainage system,consisting of perforated pipes or strainer cups, de-signed to allow the filtered water to flow out, butto retain all the sand and effect an even distribu-tion of the wash water, when the direction of flowis reversed for cleaning, by which means the layer 50. 51
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Keywords: ., bookcentury1900, bookdecade1920, bookidcu3192400497, bookyear1921
