Diminishing the area of the filter and rendering it more rapid in its operation has attracted a good deal of attention in the last few years. I have visited a plant of this kind at Columbus, Ohio, which has a capacity of 30,000,000 gallons daily. The rapid filtration is preceded always by a chemical treatment, which flocculates and precipitates the undesirable matters in the water and also attacks and destroys a large part of the living bacterial organisms and renders the subsequent filtration more easy and more rapid. Naturally the sand grains forming the filter bed of the rapid filtration are much coarser than in the slow filtration, and this renders the previous precipitation necessary in order that the effluent water may be bright, clear, and free of undesirable matters.

One great advantage of the rapid sand filtration is in the arrangement of the machinery in such a way as to force water back through the filter when it is desired to be cleaned, which loosens the coagulant from the sand and carries it off with the wash water to any desirable exit. The cost of cleaning the filter is reduced to a minimum both of expense and of time. The largest rapid sand filtration plant in the country is that at New Orleans, which has a daily capacity of 40,000,000 gallons.