Practical Methods of Sewage Disposal for Residences, Hotels and Institutions

by Henry N. Ogden; H. Burdett Cleveland

The siphon chamber floor may be placed considerably above the level of the floor of the tank as shown in Figs. [2] and [3], since a sufficiently large quantity of effluent for dosing a filter or a sub-surface irrigation system may be collected in the chamber of reduced depth thus provided. This shallower construction saves excavation and also reduces the operating head or fall, which latter is sometimes hardly equal to the demands of the subsequent treatment. The capacity needed in this chamber for different installations will be given later in the discussion of sewage filters and sub-surface irrigation systems.

Having determined upon the dimensions of the tank and selected the site, the construction is commenced by making the excavation about four feet wider and longer than the outside dimensions of the tank and siphon chamber combined, in order to provide room for setting the forms for placing the concrete, provided concrete is to be used in its construction. With brick walls an additional width and length of two feet is needed.

Fig. [4] gives an illustration of the forms to be used in constructing the walls for concrete tanks, the cut at the left showing a view of the form to be used when the tank is constructed either partly or wholly above the natural ground surface, or below the surface in loose soils, and the cut at the right showing a view of the form to be used when excavation for the tank is made in rock, hardpan, or clay. The top width of the walls should be 8 inches, and the bottom width should be 8 inches plus 1½ inches for each foot of height. Thus, for a wall 6 feet high the bottom width should be 17 inches,—the inside face of the wall having a batter of 1½ inches per foot of height. This batter is necessary, when the tank is constructed below the ground surface, to withstand the lateral earth pressure when the tank is empty. If the tank is to be constructed above the ground surface, the outside wall should be battered and the inside wall made vertical, since the pressure which the wall must withstand is then only from the liquid within the tank. The partition wall between the settling tank and siphon chamber should be 10 or 12 inches thick at the top, depending on its height, and should have a batter on both sides.

Fig. 4.—Forms Used for Building Side Walls for Concrete Tank.

To set up the forms for the concrete walls, stakes 2 inches by 4 inches and about 2½ feet long are first driven on each side of the bottom of the wall, and 6 inches away from the wall as laid out, at intervals of 2 feet. Pieces of scantling, 2 inches by 4 inches and with a length equal to the height of the wall, are then placed in upright position and securely nailed to these stakes. The inner scantling are then inclined and temporarily fastened at the top by a short nailing piece to the outer row so as to leave an opening of 10 inches between each pair of scantling. Additional stakes are then driven from 2 to 4 feet from the wall on each side, as shown in the illustration, and braces 2 inches thick and 3 inches or 4 inches wide are nailed to these stakes and to the upright and inclined scantling. One-inch boards are then lightly nailed to the scantling, as shown, the boards making up the inside face of the form being placed in sections of two feet in order to afford opportunity for thorough tamping of the concrete as the form is being filled. The concrete is then placed between the boarded sides of the form in 6–inch layers and well rammed.

The concrete should be composed of one part by measure of Portland cement to two and a half parts of clean, sharp building sand and five parts of broken stone or clean gravel. The cement and sand should first be thoroughly mixed, while dry, to an even color and then wet and tempered to a soft mortar. The broken stone or gravel, after having first been thoroughly wet, should be spread evenly over the batch of mortar and the mass shoveled over at least three times to insure a thorough coating of the stones with mortar. The concrete thus made may then be placed in the forms in six-inch depths and thoroughly rammed until water covers the surface.

When it is essential that the tank be water-tight, and, in fact, in constructing all tanks, each layer of concrete should be placed between the forms, when possible, before the concrete in the layer previously placed has set. If the work of placing the concrete is of necessity interrupted, before placing another layer the surface of the older concrete should first be sprinkled and swept with a stiff broom and a thin coating of neat cement mortar (containing no sand) should then be washed over the surface of the concrete.

It may be noted that a barrel of Portland cement (equal to four bags) contains 3.8 cubic feet, so for concrete with the proportions of cement, sand, and stone as specified above, for each barrel of cement used there should be used 9.5 cubic feet of loose sand and 19 cubic feet of loose stone; and for each cubic yard of concrete required there will be needed 1.30 barrels (or 5.2 bags) of cement, 0.46 cubic yards of sand, and 0.92 cubic yards of stone if the stone is fairly uniform in size and contains forty-five per cent. of voids. With stone or gravel less uniform in size, less cement and sand is required. The cement and sand, made into mortar, will fill the voids or open spaces in the mass of broken stone. (For further details see Chapter VII.)

As shown in the illustration (Fig. [4]), the foot of each upright and inclined scantling should be placed at the proposed elevation of the floor of the tank, and the boarding should not be carried below this level. Then, if the excavation for the wall has been carried to a level 6 or 8 inches lower than the floor of the tank, the concrete when being placed between the forms will spread under the bottom of the forms, making a footing for the wall on the outside and better insuring a water-tight joint when the floor is laid against the inside foot of the walls.