Practical Methods of Sewage Disposal for Residences, Hotels and Institutions

by Henry N. Ogden; H. Burdett Cleveland

A section of a tank with a concrete roof is shown by Fig. [6], together with the temporary form built up inside the tank on which to lay the roof. The form is built by setting 2–inch by 4–inch scantling on wedges along the walls of the tank in pairs 18 inches apart and bracing these at the foot. Boards 1½ inches thick and 10 inches wide are then nailed across the tank to the tops of the scantling, the top edges of the boards being 1 inch below the top of the walls. A false roof is then made of boards nailed lengthwise of the tank to the 10–inch boards, and a layer of concrete 2 inches thick is then placed on the floor thus made, reaching over the top of the walls to the outside edges. Iron rods, ¾ of an inch thick and spaced 1 foot apart, are then placed on the concrete across the tank and reaching to within 1 inch of the outside edges of the walls. More concrete is then placed over the first layer to a total depth of 6 inches or 8 inches, depending on the width of the tank, the concrete being well rammed as it is placed. After the concrete has set, the wedges may be knocked from under the upright scantling and the form taken down and removed through the manhole. The manhole covers and frames, as shown in the illustrations in Chapter III, may be cast at local foundries or purchased through sewer-pipe dealers.

To provide manholes or openings through the roof into the tank and into the siphon chamber, round openings 2 feet in diameter should be cut in the false roof while it is being laid, the distance between the pairs of scantling at this point being made 2 feet. The manhole frames should then be so placed that the flange or base of the frame will be imbedded to a depth of 2 inches in the roof when completed. The manhole at the entrance end of the tank should be located at one side of the entrance pipe and over the valve on the sludge pipe. To provide the necessary opening through the concrete roof below the manhole frame, an eight-sided wooden form, as shown in Fig. [7], with an inner diameter of 2 feet and a height equal to 2½ inches less than the thickness of the roof, is placed over the opening in the false roof. On this wooden form the manhole frame is placed and the concrete laid around the form and over the flanges of the manhole frame. Two of the ¾-inch iron rods should be placed across the tank close to each side of the wooden form after the first 2–inch layer of the concrete roof has been placed.

Fig. 7.—Form for Manhole Opening.

When it is desired to carry the manhole some distance above the level of the top of the roof to provide for a rather deep earth covering for the tank, the eight-sided wooden form may be made deeper as desired, and another larger, similar form built for the outside form of the necessary concrete manhole well. The space between the two forms may then be filled with concrete and the manhole frame set on the octagonal-shaped wall thus formed.

In order to insure a more uniform flow of sewage through the tank and thus reduce the velocity of flow in all portions to a minimum, baffle boards of 2–inch planks should be placed across the tank near the inlet pipe and near the outlet pipe, as shown in Fig. [5]. These boards are set in grooves formed in the concrete by nailing 1–inch by 3–inch strips to the inside form when the tank wall is constructed. These baffles also serve a useful purpose by reducing the disturbance of the scum as the sewage enters the tank and by preventing the escape of scum from the tank.

The boards should extend to a depth of one foot below the inlet and outlet pipes, and should usually be placed 12 to 18 inches from the ends of the tank. Where the effluent from the tank is to be collected in a siphon chamber adjoining the tank, it is preferable to provide a weir or wall between the tank and the siphon chamber. The top of this wall should be one foot below the roof to allow the effluent to flow over this wall from the tank into the siphon chamber. In this case no outlet pipe from the tank is used, and the baffle boards should extend downward 12 inches below the level of the sewage in the tank. These baffle boards should be carried up to a level with the top of the tank walls.

It is advisable to provide an overflow pipe from the siphon chamber which should leave this chamber at an elevation of 3 or 4 inches above that of the inlet pipe to the tank, and which should, by means of an elbow, be extended down outside the chamber to connect with the sewer into which the siphon discharges. This is desirable in order to provide an overflow in case the siphon becomes clogged or fails to operate.

Where a tank must of necessity be located near a residence, any nuisance due to odors may be prevented by inserting a 4–inch galvanized-iron conductor-pipe through the roof of the tank, and carrying this pipe up into the air 20 or 30 feet along a tree trunk or the side of a building.

If a sub-surface irrigation field is to be laid out, the tank should preferably be near the proposed location of the sub-surface irrigation area (see Fig. [26], Chapter IV), although the effluent may be carried to the sub-surface irrigation field from a settling tank located at some distance from such field. Since the sewage enters the tank near the top of the tank and the effluent discharges from the siphon chamber at a considerable distance below the top of the tank, it is of advantage to place the settling tank on sloping ground, if possible, so that one end will be wholly in excavation and the other will be partly above the natural ground surface. This reduces the depth of trenching and provides for more readily distributing the effluent by gravity from the tank through the sub-surface tiling which is laid just below the surface of the ground. The tank must always be higher than the distributing field to allow for the flow of sewage, and it is desirable to have the tank buried in the ground if possible in order to keep the temperature of the sewage as high as possible in winter. These ideal conditions are not always to be attained.