Another method of sewage disposal, in which the matter of filtration bears an important part is to be seen in Fig. 242.

Elsewhere in this work the subject of septic tanks in connection with automatic sewage siphons is taken up.

Fig. 242. - Sub-Soil System of Disposal of Sewage from Septic Tank.

These sewage siphons generally discharge the liquid sewage into an underground system of loose-jointed tile, so arranged as to allow the sewage to discharge over a large area and leech away into the surrounding soil.

The subject of cesspools, septic tanks, sewage siphons, sub-soil irrigation, and kindred subjects are treated in a comprehensive manner by the author in his work "Modern Plumbing Illustrated."

Sometimes it happens that the soil is of such nature that the sewage cannot be disposed of underground. Under these conditions the sewage filter bed, shown in Figs. 243 and 244, may be used to advantage.

Fig. 243. - Side View of Sewage Filter Bed.

The partially purified waste is discharged by the automatic sewage siphon from the septic tank into wooden troughs, running the full length of the filter bed, and provided with outlets at frequent intervals, through which the sewage escapes. When the liquids have filtered through the sand and gravel, they reach the under-drain in a pure state, and from this point may be disposed of in any way that is most practicable.

Fig. 244. - End View of Sewage Filter Bed.

The subject of filtration should also include consideration of rain water and cistern filters. Rain water is considered the purest natural water that we have, and in its use for household purposes, it is necessary to clarify it rather than to actually filter it, for whatever impurities it possesses are generally of a mechanical nature, and held in suspension rather than in solution. Its impurities are gathered to a very slight extent in falling, and afterwards, in its passage to cisterns, it gathers such matter as leaves, twigs, dust, etc.

In the upper illustration of Fig. 245 is shown the construction of a very successful and easily constructed filter for use in clarifying rain water before it enters the cistern.

The method of extracting the impurities of the water before it reaches the cistern, is better than the plan of filtering the water after it has reached it, as it leaves the latter in a much cleaner condition, and less frequent cleaning of the cistern becomes necessary.

In the filter referred to brick or stone is used, the entire interior being made waterproof by means of a cement lining.

A partition divides the filter into two compartments, the inlet chamber and the outlet chamber, this partition terminating sufficiently above the bottom to allow a free flow of water from one compartment to the other.

At the bottom of the inlet chamber fine gravel is filled in to a depth of 8 or 10 inches, above which is laid a layer of coarse gravel to a depth of 6 or 7 inches. Above the coarse gravel broken stone is filled in and the whole topped with a flooring of brick laid with loose joints.

The brick covering is used for holding back the coarsest substances entering with the water, such as sticks and leaves.

In the outlet chamber nothing is used but fine gravel, topped with brick, the latter being used to prevent the washing out of the gravel.

The discharge from the filter to the cistern should terminate an inch or two above the bricks.

The lower illustration in Fig. 245 shows another form of cistern filter operating on somewhat different lines.

In this case, the filter is built inside the cistern in the form of a cylindrical chamber, extending from the bottom of the cistern to a point near its top, the space at the top allowing the entrance of air. This chamber should be from 24 to 30 inches in diameter, of brick laid in Portland cement. The water in the cistern filters slowly through the brick work of the cylindrical chamber.

Fig. 245. - Cistern Filters.

It is considered that filtration through stone or brick, while slow, is very thorough. The slow rate of filtration is offset by the large area of filtering surface which the brick filtering chamber presents. The cistern should be lined throughout with Portland cement. Pig. 246 gives a very good method of construction for a cistern overflow. The overflow is made of tile pipe, and as shown, it is laid along the bottom of the cistern and carried up to the level desired for the overflow, and there connected to the outlet. This form of overflow will carry off much of the sediment and filth accumulating in the cistern, thereby rendering it necessary to make less frequent cleanings of the cistern.

Fig. 246. - Overflow for Cistern.

The same illustration shows a good method in connection with the well pipe of a cistern in the use of a return bend on the end of the pipe.

Fig. 247. - Filtered Rain Water from Well.

A straight pipe ending close to the bottom of the cistern will suck up sediment, while in the use of the return bend the suction draws the water into the pipe in a downward direction.

In Fig. 247 is shown another combination of cistern and filter which may often be used to advantage, especially in locations where rain water must be depended upon as a drinking supply at certain seasons of the year, or at all times. The illustration shows a common cistern, into which is built a filter having an arched surface, and constructed of two courses of brick, with a space of 2 or 3 inches between them. This space is filled with charcoal, which is an excellent filtering material.

In connection with the cistern and filter, a well is used, made as deep as practicable, into which the filtered water is discharged from the filtering chamber. The well is made waterproof. This method allows pure drinking water to be pumped, of the same cool temperature as ordinary well water.