Mechanics of the Household / A Course of Study Devoted to Domestic Machinery and Household Mechanical Appliances

by E. S. Keene

—In places where the use of the septic tank alone is not possible, it sometimes happens that the natural conditions are such as will permit the effluent to be drained directly into the soil. With such a condition, the effluent goes into a filter bed composed of gravel or other loose material, where it undergoes still further bacterial action and if the process is complete, the water which comes from the filter bed is clear and odorless. Under good conditions it is clear sparkling water and contains but a small amount of impurities.

Septic tanks are made in many forms but that illustrated in Figs. 151 and 152 is commonly used. In Fig. 151 the tank is shown in position to receive the sewage from the house drain, where it is to undergo the first treatment and then to be conducted to a filter bed made of porous tile, set in loose soil. The tank is shown in detail in Fig. 152. It is a cemented brick cistern with an opening to the surface that contains a double cover as a protection during cold weather. A brick partition divides the tank into spaces G and H, that contain volumes that are to each other as 1 to 2. The tank is of such size as will hold a volume of sewage equal to 24 hours’ use; that is, it is expected that any portion of sewage will remain in the tank for that length of time. The sewage enters at A, in such a way as will give the least disturbance of the liquid of the tank. An opening C allows the liquid to pass from H into G, where any additional sewage entering H will displace an equal amount in G, which will pass out at B to the filter bed. The partition D is high enough so that the scum that forms on the surface will not pass directly into the space G. The entrance and exit pipes are made of vitrified sewer tile with the openings below the surface.

Fig. 151.—Sectional view of a septic tank, connected with a sand-bed filter; for the disposal of sewage from a residence.

As the sewage enters the tank A, a considerable portion will sink to the bottom, while some will float to the top where a thick scum will gather. By far the greatest portion of solids will be readily dissolved in the water and the remainder will be still further reduced to liquid form by bacterial solution. The process of disintegration that goes on evolves a considerable amount of carbon dioxide and ammonia which filters through the scum. The process that now goes on in the tank is that of liquefying the organic matter and changing it from organic to the inorganic state.

The bacteriologist recognizes in the process of sewage disintegration the work of two classes of bacteria, the aerobic or those bacteria that work by reason of air and do their work only in its presence and the anaerobic or those that work in the absence of air. In the action of the sewage-disposal plant both kinds of bacteria are at work. If, in the final stage where the sewage passes into the filter, air can be carried into the earth the action will be hastened.

Fig. 152.—Section of the septic tank in Fig. 151 showing details of construction.

It is evident that, since the sewage entering the tank is almost entirely dissolved, under ideal action this system would give very little trouble, but actually as the sewage enters the tank the disturbance caused by the incoming water forces some of the undigested matter into the outlet and being carried into the filter bed it will be deposited at the first opportunity. This will cause the filter bed to fill up with undigested sewage at the point nearest the entrance, and in course of time it will stop the pipe because of this accumulation.

To avoid such an occurrence, tanks have been built in which an automatic siphon discharges the effluent whenever a certain quantity has collected. Such a tank is shown in Fig. 153. With this arrangement, the sewage enters the first tank at A, and passes into the second tank at B. At S is shown an automatic siphon, so made that when the effluent has collected to the height of the water line, the siphon automatically discharges the contents of the tank. This is known as a dosage tank because periodically a dose of the effluent is discharged into the filter bed. The volume discharged is sufficient to fill the greater portion of the bed, and force out the air in the loose soil. As the water filters from the bed the air is drawn in to take its place and gives the bacteria which work in the presence of air an opportunity to do their work. The work done by this filter bed is first to filter out any suspended matter carried in the effluent which will lodge on the surface of the filter material and then to undergo the slow process of integration, and to permit the oxidation of the dissolved sewage. If this matter is deposited faster than it disintegrates then the filter will fill up and finally refuse to work.