The subject of nitration, and the action, construction, and connection of filters should be thoroughly understood by the plumber and by the architect as well.

At first thought this subject may seem to be somewhat outside of the plumber's province, but this is not true, for it is a matter which is very often associated with the procuring of a pure water supply, and also with the proper disposal of sewage. The filtration of water supplies is a matter more especially pertaining to city work, while the filtration of sewage, as taken up in this work, applies chiefly to country work, or at least to those plumbing systems which have not the advantage of disposal into a public sewage system.

Filtration depends upon the action of certain forms of bacteria, which attack the impurities of the water or sewage, reducing them to other and purer forms. In the purification of drinking water, the action is ordinarily performed by bacteria which exist in countless numbers in sand, charcoal, stone, and various other porous substances. This form of bacteria depends entirely upon the presence of oxygen for the performance of their work. In the purification of sewage, however, it must first come under the action of an entirely different class of bacteria, which exist in vast numbers in the sewage itself, and which multiply enormously in tanks or other receptacles for sewage, when arranged in proper manner.

These bacteria have the ability to reduce animal and vegetable matter to liquids. These liquids are then further purified in the manner described above, the two purifying processes enabling the transformation of sewage into water of such purity that it may be used as drinking water with perfect safety.

In the purification of water supplies there are two methods of filtration, that by pressure and that by gravity. The former clarifies the water and renders it suitable for most commercial purposes and for all household purposes, except as drinking water.

Fig. 232.   Domestic Filter.

Fig. 232. - Domestic Filter.

Filters used for commercial and municipal purposes are in general of two classes, sand filters and mechanical filters, and for domestic purposes the same classes of filters are used, constructed in somewhat modified forms.

In Fig. 232 is shown a pressure filter for domestic purposes, depending upon the use of sand. Running through the center of the cylinder is a shaft provided with projecting arms. In cleaning the filtering material, the revolving of the arms breaks up the mass, thus enabling foreign substances, sediment, etc., to free themselves, and then be washed out through the waste by a reverse flow of water.

Another form of pressure filter is to be seen in Fig. 233, bone black in this case being the filtering medium.

Fig. 234 shows still another form of pressure filter, the filtering material used being sand and charcoal, mixed in with which is a quantity of stilts of irregular shape and made of burnt fire clay. In cleaning this filter, a reverse flow of water forces these stilts through the mass of filtering material, breaking it up into small particles, and freeing foreign substances, which pass off through the waste. Thus it will be seen, that in pressure filters, of which there are a great variety on the market, various forms of filtering substances are made use of, as well as various methods of cleaning, the latter being of great importance to the proper action of any filter. The value of various filtering materials and cleaning methods, and systems in use for delivering filtered water supplies in buildings, is taken up by the author in a previous work entitled "Modern Plumbing Illustrated," on page 217.

A general idea of the connections for a pressure filter for use in residences, manufacturing plants, etc., may be obtained from Fig. 235.

While the pressure filter is of importance under many conditions, and is of special value in large city buildings, certain kinds of manufacturing establishments, etc., it does not generally purify impure water sufficiently to insure safety in its use as drinking water.

As the country grows and increases in population, the subject of obtaining pure water supplies constantly demands greater and closer attention, for as our population spreads out and occupies more territory, the danger of the contamination of once pure supplies of water continually increases.

Fig. 233.   Pressure Filter; Use of Bone, Black.

Fig. 233. - Pressure Filter; Use of Bone, Black.

Fig. 234.   Filter for Domestic or Commercial Use.

Fig. 234. - Filter for Domestic or Commercial Use.

The filtration of city water supplies by means of public filtration plants has been of untold benefit to the residents of many cities, but the danger from the use of contaminated water is not entirely overcome by this means, for inefficiency, carelessness, and accident in the operation of such plants may easily endanger the health of an entire city.

Fig. 235.   Pressure Filtering System for Residence Use, etc.

Fig. 235. - Pressure Filtering System for Residence Use, etc.

That there is cause for extreme caution in procuring and maintaining a pure supply of water may be seen when it is considered that many of our most dreaded diseases are often directly traceable to the use of contaminated water.

Typhoid fever and diphtheria are chief among these diseases, and the former especially is in a great majority of cases due to impurities in the water supply.

Many cases may be recalled of epidemics caused by a single case of pollution. For instance, several years ago, 1,200 cases of typhoid fever were traced to the use of an outhouse by a typhoid patient, the drainage from this house having made its way into a water course which served as the source of supply for the section in question. It may be said that even perfect filtration, that is, as near perfect as it is practicable to make it, does not entirely safeguard the users against the typhoid germ, as a small percentage, at least, always pass through the filter.