Common agricultural tiles three inches in diameter and costing about two cents per running foot are suitable material for these distribution lines. The sewage enters these distribution lines from a larger pipe, usually six inches in diameter, and a difficult adjustment is presented that each branch tile line shall receive its own proportionate share of the sewage. If only one line of tile is provided, say 200 feet long for 5 members in the family, then all the sewage goes into that line with no question of distribution arising, but if a number of short parallel lines must be used, as shown in the sketch (Fig. 69), the difficulty of subdividing the sewage properly among the different branch lines becomes very great. For that reason the writer prefers to use not more than two lines, with the possibility of delivering the sewage alternately in the one and the other. In this way, the bed not receiving sewage is resting, while the other bed is acting, and also the outlet for the sewage is always definitely known. And particularly in the case of these subsurface tile, the necessity for the intermittent dosing is apparent, since with small, constant trickling discharges the difficulty of distribution through the long length of tile is gradually increased, and usually saturation of the soil occurs from joint to joint, as already described. Therefore it becomes most necessary, in this case, for the best results on the soil not merely to alternate the beds receiving sewage, but also to effect the intermittent discharge onto the beds or through the pipes although the sewage itself may flow very uniformly in volume.
Automatic syphon.
This intermittent discharge is accomplished by constructing on the pipe line from the house and before it reaches the beds an "automatic syphon," as it is called, the operation of which may be described as follows: As the sewage enters the tank containing the syphon and rises outside the syphon-bell, air is compressed between the water surface inside the bell and the water left inside the syphon-leg. With greater and greater height of water outside, this compression inside becomes greater and forces the water in the syphon-leg lower and lower. Finally, the water sinks so low as to allow the compressed air to escape suddenly around this bend, instantly relieving the compression, and the water outside rushing in to fill up the space occupied by the air starts the syphon (see Fig. 70).
Fig. 70.—Section of "Miller" syphon.
This syphon, in size suitable for a single house, costs about $12 delivered, and will always be available to secure an intermittent dosing of the bed or pipe line. Usually the chamber in which this syphon is placed holds about one hour's flow, so that it may be estimated that this syphon will discharge on the bed every sixty minutes. The exact interval of time is not essential nor, perhaps, important, although it may be noted that the coarser the material,—that is, the nearer uniform all the sand particles are to the largest size passing the ten-mesh size,—the smaller must be the dose applied, but the more frequently must the application be made. This has been very thoroughly studied in Massachusetts, and the views of experts on this subject may be found in the report of that Board.
Such an intermittent discharge may be made and often is made by a hand valve leading out from this chamber in institutions or in private houses where some one constantly is available for the purpose. Thus it becomes the duty of the man in charge every hour or perhaps three times a day to pull the valve and allow the sewage to discharge (see Fig. 71). An overflow pipe should always be provided, so that if he forgets to pull the valve, the sewage will still find its way into the system rather than out on the ground.
Fig. 71.—Plan and section of a septic tank with valve.
Sedimentation.