The mention of this and other types of cesspools is not to be taken as a recommendation for their use, except when compulsory and after Careful consideration given to their design. A sparsely settled condition of a locality reduces the harm possible from them; but under the most favorable conditions there is always danger of producing permanent pollution of the soil.

Marshes are sometimes unwisely used as a discharge place for the drain pipe. Isolated low spots covered with loosely piled broken rock to prevent the rooting of plants and to favor bacterial action, have given good service, evaporation and oxidation taking care of the discharge for long periods.

The septic treatment of sewage may be considered a biological rather than a chemical process, as its success is dependent upon presenting conditions which favor the rapid growth of certain bacteria. In the complete reduction of sewage by the septic method, bringing it to a harmless state in the form of nitrates which plant life can assimilate, two forms of bacteria are employed - anaerobic and aerobic. Air and light retard the multiplication of the first of these The second require oxygen, and multiply rapidly in the open air. The tank or receiver proper, is a sort of catch-basin, made in form to favor the requirements for the propagation of anaerobic bacteria, which reduce the sewage to simple compounds. The tank, it appears, should hold the output of about one day's use of the fixtures discharging into it. Light and air should be excluded. Warmth to a degree is essential. Such heat as is common to a pit in the earth, closed at the top, with no unnecessary exposure, together with the heat of waste water and that generated by the action taking place in the sewage itself, is sufficient to favor the process in winter weather of quite severe climates. A temperature of 54° F has been stated to be the minimum permissible in this tank, for little or no septic action can take place at lower temperatures. The waste water of baths and lavatories is not turned into the septic tank merely for the heat it brings, but also to secure dilution of the excrement and matter from other sources, which not infrequently carry too little water to favor the best interests of the process. Both the inlet and the outlet of the tank should be arranged to be below the surface of the contents when the tank is full, so that the scum which generally forms on the surface will not be disturbed by entry or exit of matter. This scum, resembling wet ashes, helps to retain the heat, and excludes light and air from the mass - all favoring the accomplishment of the purpose. The scum may be from a few inches to 15 or 20 inches in thickness, according to conditions and nature of the plant.

The contents leaving this initial receptacle, having therein been reduced from a complex nature to one of simpler chemical compounds, principally nitrites, the completion of the reduction process and the change from nitrites to nitrates are brought about by exposure of the matter to light and air, giving the aerobic micro-organisms a chance to develop. This would be accomplished by simply discharging directly into a stream; but a more rapid action is obtained by interposing an open, shallow bed of broken stone or slag for the liquid to flow through first, so as to break up and bring into contact with the air as large an amount of surface as possible before piping to stream or elsewhere. In this way a more complete reduction is certain before the matter reaches any final source of disposal.

The bacteria necessary to the process are always present in abundance in fresh sewage, and nothing more than the time necessary to their cultivation is required in the simplest provision for operation. The resulting product is described as mainly consisting of a harmless, colorless, odorless, stable liquid. In this process, admission of air to the tank, or lack of sufficient heat or dilution, may result in a putrescent state of the matter, such as is occasionally found in a common cesspool.

As already noted, the septic process is not yet widely used, except for town sewage, where it is rapidly gaining in favor. Here elaborate methods are adopted to favor the aerobic or oxidizing end of the operation, mostly through filters of special design, all aiming to secure absolute stability and harmlessness of the final discharge from the sewage disposal plant.

Fig. 140 illustrates a simple arrangement for the septic treatment of sewage. A is the septic tank proper, where the anaerobic action takes place; and B is the second receptacle, with a bed of broken stone . designed to break up the discharge from A in a way to favor aerobic action. C is the inlet, and D the outlet. Wider experience will doubtless develop much data bearing on the form of apparatus and the latitude of conditions under which particular grades of waste can be most successfully treated. Numberless variations from the arrangements shown are being employed, according to size of plant and composition of waste product. From ten to thirty days are required for the development of the bacteria and their action.

Fig. 140. Simple Tank Arrangement for Septic Treatment of Sewage..

Where the level of the outfall of a sewer for either an individual house or a community is below the level into which the final discharge must be made, it is necessary to use a sewage lift or pump to raise the matter to a point where gravitation will again take care of the flow. These lift pumps may be had suitable for either large or small installations. For sub-cellars or other points below the level of the main drain, surface drainage may be assembled in a well like that shown in Fig. 141; and from there, by means of a cellar drainer operated by steam or water, it may be automatically lifted and discharged into the drain, as shown by the engraving. The well is composed of metal rings about 30 inches in diameter, bolted together. One section is provided with pipe hubs for entry of the surface drain-pipes, and the cap is arranged with manhole opening and cover. If the drain into which the water is discharged is subject to reverse currents from tide or flood water, than a trap, with tide-water valve, arranged as below Main Drain, into which it is Subsequently shown between B and C, is used; otherwise, a simple trapped connection, as indicated by pipe A, leads the discharge water into the sewer, and the work shown from B to C is omitted.