Rural Hygiene

by Henry N. Ogden

As a matter of economy of operation, it has been found desirable to take out from the sewage before the treatment already described as much of the solid matter as may be reasonably done, and for this purpose sedimentation is made use of. Most of the solids in sewage are slightly heavier than water, so that if they be allowed to stand in the water for a short length of time, they will settle to the bottom of the tank and allow the liquid above to pass on, considerably clarified. It has been found worth while to do this, since all three processes described are interfered with if the solids taken out by sedimentation are allowed to be deposited either upon the surface of the ground, giving rise to odors as well as to objectionable appearances, or onto the surface of the sand beds, which they clog up, or in the three-inch tile drain, which may be filled in a short time.

It has been further found by experience that if these sedimentation tanks are made large, really larger than necessary for sedimentation, in some way a large proportion of the matter accumulating in the tank will disappear, so that the amount of sediment to be taken out of the tank is not as large as might be expected. In fact it is usual for such tanks to run one or two years without cleaning, although the amount of solids shown by chemical analysis to have been removed from the sewage would fill the tank twice over.

It has been found that a tank, in order to do successful work in separating solids and in eliminating as much as possible of the sediment, needs to be of a capacity to equal about one day's flow of the sewage, and this is a good basis for computation. Here, again, the fact that the sewage from a single house is considerably fresher than the sewage from a city must be remembered, since, while many cities build tanks holding only one third or one fourth of their daily flow with good results, in the case of a single house this is not possible, and the tanks, if built at all, ought to hold at least the full day's flow. Ten persons, at 25 gallons each, furnish 250 gallons per day or 33 cubic feet. The tank, then, must be large enough to hold this volume, and suitable proportions generally require that the tank be at least 5 times as long as wide. A certain allowance must always be made for deposit in the bottom and for the accumulation of scum on the top, so that an extra foot or more of depth is desirable. The tank, then, to furnish the required 33 feet, might be made 3 feet wide, 3 feet deep, and 5 feet long, and probably in no case would a tank much smaller than this be used.

Fig 72.—Section of a septic tank with syphon chamber.

There are two or three details of tank construction which may be suggested, although almost any kind of tank will answer the purpose. It is desirable in order that the surface scum may not be disturbed, and in order that the inflowing sewage may distribute itself as uniformly as possible across the tank, to attach an elbow to the entering pipe so that the sewage enters about halfway between the top and bottom of the tank (see Fig. 72). Similarly, at the outlet or weir an elbow should be provided because it is not desirable to allow the floating matter of the surface to be carried onto the bed, and a pipe taking off liquid, open halfway between top and bottom, will carry away but little of either the surface scum or bottom sediment. Such a tank must be built of concrete or masonry or timber, although the latter is not to be recommended because of its short life. The walls of an ordinary tank may be built 6 inches thick at the top and 12 inches to 18 inches thick at the bottom, the latter being necessary if the depth is over 8 feet. The tank should have 6 inches of concrete on the bottom, and the roof may be made of flagstone or of concrete slabs in which some wire mesh has been buried.

It is not necessary to ventilate this tank, although it is desirable to have perhaps a foot of air-space between the water level and the roof of the tank. During the first few months of its operation such a tank is very likely to smell badly, and, if ventilators are provided, the presence of the tank will be well known by the odors sent off. After the tank has been in operation two or three months these odors gradually disappear, due presumably to the fact that the surface of the water in the tank has become coated with a thick blanket through which odors cannot penetrate. On the other hand, there have been a few cases recorded where the production of gas in a septic tank was so great that an explosion occurred, tearing off the roof and otherwise doing considerable damage.

The full plant, therefore, will consist of the settling tank, receiving the raw sewage from the house and discharging it into a small tank holding about one hour's flow and containing the automatic syphon apparatus for intermittent discharge. This dosing tank must provide for one hour's flow at the maximum rate of flow, and should hold about one fourth of the total daily flow. Then the ground area, either natural or artificial, which receives the intermittent discharge from the dosing tank, completes the installation (see Fig. 73).

Fig. 73.—Plan of sewage disposal for single house with details of receiving tank.