The first municipal filtering system in the United States was installed in Scheneetady, N. Y., in 1879. Some years later Lawrence, Mass., developed further the idea of a clean and healthful drinking water. These two cities used filters in which the main constituents were sand and gravel, and the process both mechanical and biological.

A modern municipal filter usually consists of a large chamber with a cement floor from which pipes lead directly into the consumer water mains. The filter is formed by a layer of hollow tiles covered by a layer of crushed stone and gravel about a foot thick, and graded so that the pebbles form the surface; and above this again is spread from three to four feet of finely grained sand the whole acting as a sieve capable of separating out practically all microbial life. As a result of this mechanical arrangement, biological purification arises spontaneously. The top sand soon is impregnated with all kinds of plant and animal microbes that are usually seen under compound microscopes. These form a solid growth the by-products of which often are poisonous to the sensitive microscopic pathogens or disease makers. Moreover, some of these larger and higher microbes, usually seen under compound microscopes, feed on the smaller and lower ones. Horses and cattle are not susceptible to the typhoid fever and most other water-borne pathogens, and may drink with impunity water that would be fatal to the human consumer.
In the course of time, this top quarter-inch of sand becomes so closely packed with plant and animal life that filtration is impeded. For this reason, the filtering chamber is built in no connecting sections so that the inflow from the reservoir into any particular compartment can be cut off at will. In this way, each compartment in turn can be drained, the matted sand scraped off for about half an inch, and the filtering resumed. The sand scrapings are washed, sterilized, and used again. The index of efficiency and safety of such a combined mechanical and biological filter is very high. Kendall describes the amazement of visitors invited to witness a demonstration of a four-foot filter tank of this kind, when they saw the men in charge of the demonstration drink the clear, palatable filtrate flowing from the bottom of a tank into which foul-smelling sewage had been poured.

There also are several different methods of purifying water chemically. One, not in wide use today, depends on substances, which readily crystallize out and thus carry to the bottom all organic matter, including microbes. The newer methods use chemicals, which, while harmless in minute quantities to human beings, are fatal to microbes, like, bleaching powder, alum, blue vitriol, and permanganate of potash. But the most efficient and inexpensive material for the purpose is liquid chlorine, stored in tanks, which automatically dose the reservoir in the right proportion. From one half to one part of liquid chlorine to one million parts of water is a dilution strong enough to purify water from the common forms of infection. And the process costs about one cent annually for each consumer.

Reduction in deaths from typhoid fever results upon the proper disinfections of the municipal water supply in eight European and American cities which tell a startling story but only half the actual truth. The fact is that for every life saved from typhoid fever death directly by a pure water supply, two or three have been saved from other infections. For many of the water-home compound microscope seen, pathogenic microbes, while not virulent or active themselves, sap the strength of the consumers by persistent parasitism, making them more susceptible to the more vicious pathogens.