. The sanitation of cities. medium fine screen-ing. The Hamburg sewage screen plant hasscreens with A inch clear openings. Altonauses coarse screens for part of its sewage dis-charge, but for the major part provides no treat-ment whatever, depending entirely on water puri-fication for the safety of its water supply. Out of the English practice two types of filtershave been developed for water treatment. Theseare usually named from their most salient fea-tures, slow sand filters, and rapid sand filters. Theearlier installations were all slow sand filters. A slow sand water filter consists of a
. The sanitation of cities. medium fine screen-ing. The Hamburg sewage screen plant hasscreens with A inch clear openings. Altonauses coarse screens for part of its sewage dis-charge, but for the major part provides no treat-ment whatever, depending entirely on water puri-fication for the safety of its water supply. Out of the English practice two types of filtershave been developed for water treatment. Theseare usually named from their most salient fea-tures, slow sand filters, and rapid sand filters. Theearlier installations were all slow sand filters. A slow sand water filter consists of a watertightbasin, the floor of which is provided with under-drains, with a covering of about one foot of coarsegravel, over which are placed several feet of cleansand. To prevent freezing, a roof usually carriedon groined arches is placed over the basin. 47 Zbc Sanitation of Cities The drains referred to, consist of a main col-lector with lateral branches at regular intervals—graded stone or coarse gravel is usually placed. PARIS, FRANCE A park-like development of the irrigated field from the sewage treatment plant by broad irrigation on the bottom, sufficiently deep to cover thelateral drains. Over this, the sand, which is thefiltering material, is placed to a depth of three orfour feet. When ready for service, water is permitted toenter the filter to a depth over the sand of aboutthree feet, and percolates through the sand at adaily rate of about ten cubic feet per square footof bed, affording a daily flow of about 3,000,000gallons per acre of surface. But it is not untilafter several weeks of operation that the filterdoes its best work. By this time, a gelatinous filmhas formed on the surface of the sand, derived 48 ZTbe purification of TPlHater Supplies from the organic matter and sediment present inthe unfiltered water, which gives it its highest effi-ciency. This film, however, becomes in time sothick that it clogs the filter, and the operationmust be interrupted for its
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