Page:Popular Science Monthly Volume 92.djvu/477

 A Small Sewage Disposal Plant

The Department of Agriculture tells in a bulletin how to construct a sewage disposal plant for the country place. This article is an extract from that bulletin

��EXPERIENCE has shown that, in a small sewage disposal system, a dark, airtight tank of sufficient capacity and so constructed that sewage may remain in it entirely at rest for a period of from 18 to 24 hours, gives the best results. The solid matter settles out in such a tank and, according to the theory at present ac- cepted, it is partially liquefied, deodor- ized and destroyed by countless numbers of bacteria, which thrive without air.

In such a tank a thick scum forms on the surface of the sewage, which protects the bacteria from the incoming air and is evidence of good bacterial action. The breaking up or disturbance of this scum destroys the bacterial action for the time being and is likely to cause considerable annoyance from bad odors.

The septic tank effects only about 40 per cent purification. The liquefying action in the tank, however, makes it possible to subject the sewage to a final treatment by filtration or distribution

��in a natural soil. This final purification is effected by means of bacteria which work in air. Therefore it is necessary that the sewage should enter the disposal system intermittently so that the sys- tem may be given a chance to air out. If the sewage enters continuously and in such quantities that the system is kept saturated, the filter or disposal area becomes waterlogged and "sewage sick" and ceases to be effective. It is therefore necessary that the final treatment system be of sufficient capacity to dispose of each dose of sewage quickly.

The septic tank for a small sewage- disposal system should ordinarily consist of two chambers. In this type of tank, the sewage is received, settled, and par- tially purified in one chamber, and col- lected and discharged from a second chamber. This type of tank, if properly designed, should operate satisfactorily. The sewage in the settling chamber suffers little disturbance, and the discharge to

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���Fig. 1 : The septic tank, although airtight and supposedly watertight, should be located as far from the house and the well or spring, as local surroundings will permit

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