Hygiene and public health / by B. Arthur Whitelegge.

  • Arthur Whitelegge
Date:
1894
Catalogue details

Licence: Public Domain Mark

Credit: Hygiene and public health / by B. Arthur Whitelegge. Source: Wellcome Collection.

    80/610 (page 68)
    QlwrJ(^s i^rocesf.—Hard waters may be softened u pon tEelafge scale in reservoirs by adding one ounce of quicklime per hundred gallons, for every degree of temporary hardness. The lime and calcic bicarbonate react 17) form insoluble calcic carbonate, and this carries down with it suspended matters. A similar process may be employed on the domestic scale, using lime or washing-soda, or both. The Porter-Clark process is a modification of Clark's. Instead of waiting for slow subsidence, which takes ten or twelve hours, the precipitated calcic carbonate is removed rapidly by filtration through cloth, under pressure. Filtration aims at straining off suspended matters, and oxidising dissolved organic substances. Mineral salts in solution, such as sodic chloride or calcic sulphate, are at most only partially removed by filtration. As a rule, however, hardness is lessened, together with the nitrites and ammonia, titrates are increased. A sand filter capable of removing suspended matters and some of the organic impurity may be constructed like a miniature filter-bed, in an ordinary flower-pot, with or without the addition of a layer of animal charcoal. Sponge-filters have deservedly fallen into disrepute ; they remove suspended matters, but become foul, and harbour organisms. Carbon is the commonest of all filtering media; and animal charcoal is believed to have a far greater purifying effect than vegetable charcoal. Many efiicient patterns of filters are made essentially of animal charcoal; others, equally good, depend upon silicated carbon, manganous carbon, or carferal (charcoal, iron, and clay). Maignen's filtre rapide has no carbon block, but the water is made to pass through a mixture of powdered charcoal and lime {carbo calcis), supported upon an asbestos cloth ; like