TYPHOID FEVER AND MECHANICAL FILTERS

As is well known, there are two principal methods in use in this country for purifying public water supplies. The older of these is the slow sand filtration method practiced for many years in England, Holland and Germany, and long proved to be an efficient safeguard against water-borne infections. When this system is employed with relatively clear waters the difficulty and expense of operation, due largely to the frequent scraping-off of the surface layer of sand, are not great, but if a very turbid water has to be treated the rapid clogging that results makes the filter practically unworkable. Muddy waters, therefore, are usually clarified by the addition of suitable chemicals, such as aluminum sulphate. Many special forms of apparatus for this purpose have been devised and are in use in different parts of the United States. They all agree in providing for mixing the chemicals with the water in