There is a constantly growing demand for filtered water supplies for city buildings of nearly all classes, the demand increasing as the country grows in population, and as a consequence hitherto pure supplies of water become polluted.

There are two forms of filtration, that which clears the water of all mechanical impurities, such as rust, sediment, etc., and that which not only clarifies the water, but frees it of all germ life and renders it free from the danger of producing such diseases as typhoid fever. For commercial purposes, for the bath room, etc., the first-named form of filtration is sufficient, but for drinking and culinary purposes, the latter form should be required. It is a mistaken idea that the ordinary filtration plant which filters the supply for an entire building in every case purifies the water of disease germs. The water coming through such apparatus is certainly rendered purer as far as inorganic matter is concerned, but a filter working under pressure cannot deliver water so free from the more dreaded disease germs as the filter which operates by the gravity of the water passing through it.

An ideal provision for the supply of filtered water would include the installation of a pressure filter on the main supply of the building, to clarify and purify the entire supply for the building outside of the supply for drinking purposes, and the installation of a gravity filter supplying a separate system of piping for drinking and culinary purposes. In place of the latter, a form of filter of excellent construction, described as follows, may be used. The common form of the filter referred to is made in different sizes for domestic use, filtering enough water during the twenty-four hours of the day to provide a liberal supply of drinking water.

The apparatus is briefly as follows: Connection by means of block-tin pipe is made to the supply pipe, the water being conveyed to a sheet-metal tank hung on the wall, inside of which, and attached to a collecting device, are unglazed porcelain tubes filled with bone-black or animal charcoal. The water is admitted to the tank through a ball cock, which admits it only as fast as drawn. The water, by means of its own gravity only, filters through the tubes and their contents, and flows into the collector to which the tubes are connected by rubber connectors. From the collector the filtered water runs down into a glass globe attached to the bottom of the tank, from which the water may be drawn as required.

In most of the large cities will be found companies operating this and other domestic filters, who inspect, clean, and sterilize the apparatus each month. Upon the periodical attention given to filters depends their satisfactory operation. If no attention is given them, after a time the tubes clog up and refuse to filter, or if filtration continues it is under very unsafe conditions, as all water passing through must come in contact with the thick covering of sediment and impurities collected on the outside of the tubes. This same style of filter in a modified form, can be made to produce any amount of pure water desired per day, and is made use of extensively for providing the drinking supply of hotels, restaurants, hospitals, and other institutions which desire nothing but a pure quality of drinking water.

On large work, the empty tubes are placed in large copper tanks, supplied through a ball cock, and the water after filtering through the tubes is conveyed from the collectors into other smaller tanks filled with animal charcoal. The double nitration is done entirely by gravity, and produces a perfectly pure water. The animal charcoal is placed in separate tanks on large work, simply to economize labor in cleaning. If the water delivered to niters of this class first passes through the house pressure filter, much of the heavier matter in suspension, sediment, etc., will be taken out, rendering less frequent attention to the gravity filter necessary.

Pressure filters are of various form and make, using many different materials for the filtering medium. When stone or porcelain of a fine quality is used as the filtering medium, a very large percentage of the germ impurities may be removed. A very important feature of all pressure filters is the matter of frequent cleansing, which is absolutely essential. Certain makes of pressure filters depend upon large masses of bone-black for their filtering material, and experimental tests show this to be one of the most effective filtering mediums.

One form of bone-black filter consists of two separate cylinders filled with bone-black, but so connected that by the use of a device known as a manipulator, the entire filter may be switched off from the house it supplies; or the water supply may be divided and sent through each cylinder equally; or the water may be sent through each cylinder in succession, thus filtering the same water twice; or the water may be filtered through either cylinder alone without effecting in any way the supply of filtered water to the building supplied. Thus in this filter, as in the one previously described, each cylinder may be washed by filtered water from the other, and while the entire filter is thus being cleaned, the supply to the house is not cut off or affected in any way.

Experiment has shown that the effectiveness of a filtering medium depends directly upon the amount of air space contained between its particles. This is the reason that porous stone, porcelain, and such materials do such excellent filtering. Sand contains a great deal of air also, but it is claimed that bone-black contains nearly twice as much as sand, owing to the packing together of the latter. The action of filtration depends upon the action of infinite numbers of bacteria which live and multiply in the air spaces of the filtering medium. These bacteria must have air in order to perform their work, and air will not penetrate in sufficient quantity through sand to feed them at a depth of more than three or four feet. Air will penetrate much more thoroughly through bone-black, it is claimed, and therefore this material is preferable for filter use.

The bone-black filter described above is cleaned by forcing compressed air into the mass of bone-black, thus breaking it up into particles, after which the flow of filtered water is sent through the material, thoroughly cleansing it, and carrying it off into the waste.

In the use of sand in pressure filters, it is necessary to use a coagulating agent, owing to the closeness with which the sand packs. For this purpose alum is generally used, and its action is to coagulate the sediment and other impurities of the water into such large masses that they cannot pass through the sand. While the use of alum is not ordinarily harmful, it is not desirable, and it makes the water hard, which is undesirable for many manufacturing purposes.

A great many forms of pressure filters are now made, most of them using either sand, bone-black, porcelain or stone as the filtering medium, and being provided with a variety of apparatus and methods for cleansing.

There are three methods of providing a storage of filtered water, each having advantages of its own.

Storage by means of the closed overhead tank is mostly used. The delivery pipe from filter to tank also answers as the down supply for the building, thus effecting a saving in pipe. An air vent at the top allows air to pass into and out of the tank, but prevents overflow. In this system no impurities can reach the water, which is not true of the open-tank system.

Storage by the open gravity tank is often the most convenient to install in houses already provided with an attic tank.

The open gravity tank is used when the filtered supply must be forced into it by a pump.

The pressure, or compression system is also much used. Only pressure tanks should be used for this work, as others will not hold air sufficiently well to produce the desired compression.

The pressure tank is placed close to the filter into which the latter delivers filtered water, and from the tank the house supply is taken, under pressure. When the tank is filled to its full capacity with water under air compression, the compression stops the action of the filter until water is drawn. The chief drawback to the use of this system is the use of tanks of too small capacity to provide a sufficient reserve supply of filtered water.