drain system such as that shown in Fig. 81 or in Fig. 82 may be installed. This plan is open to some objections, however, and in level sections the best plan may be to omit the sludge-drain pipe entirely and depend on emptying the tank with a pump.

Special features.- In Fig. 82 is clearly seen one special feature of a complete disposal system in which the Cornell sewage switch and the Cornell sewage divider are used (page 1747). By means of these fittings a large quantity of

sewage may be evenly distributed in many comparatively short runs of tile suitable for use in heavy soil, without at the same time having the purifying tile at a level lower than that of the water in the septic tank. This is a most important feature for level regions.

The vent in the inlet pipe is a desirable feature if the tank is at some distance from the house. If the tank is near the house the vent had best be omitted.

Construction of the tank. In the type Y septic tank the inlet is through a 4-inch, vitrified, single, Y-branch fitting, while the outlet is a 3-inch, single, Y-branch. The level of the outlet is so arranged that the branch pipe of the inlet is covered with water, but water does not stand in the inlet pipe itself.

The end of the inlet pipe is closed with a solid plug in order to prevent the escape of foul gases, but if there is need the plug may be removed. for cleaning the pipe. The vented plug in the outlet pipe permits the escape of gases into the soil.

The baffles are made of 1-inch plank slipped into slots made in the walls of the tank. The inlet baffle prevents agitation from incoming sewage and virtually extends the inlet pipe downward to the center of

Outlet

the tank; the outlet baffle prevents large solids from leaving the tank; and the bottom baffle serves to provide at the outlet end of the tank a quiet chamber in which very fine solid particles may settle and thus be prevented from stopping up the soil about the purifying tile.

Solid

plag

Slots in wall to take baffles

Inlet baffle

Vented

plugs

Outlet

The side walls of the tank are 7 inches thick, the ends 6 inches, and the bottom 4 to 6 inches, all being reinforced according to directions in the concrete handbooks. The depth of the tank is standard for all sizes discussed in this lesson, and dimensions are given in Figs. 84 and 85. The in

Inlet

Plan of tank as seen

Sludge

CO

drain

Outlet baffle

from above, cover off.

FIG. 84.- Details of type Y septic tank

side length and width of the tanks depend on the number of persons for whose use it is intended, as shown in the following table:

TABLE I.- INSIDE DIMENSIONS OF TYPE Y SEPTIC TANKS

The sizes of tank in the foregoing table are ample, and the action of the bacteria will be impaired rather than improved if for given numbers of persons the tanks are made larger than is here specified.

The problem of forming the slots in the walls of the type Y septic tank is not a serious one. It is necessary only to nail to the concrete side of the inside form, beveled cleats of the shape shown in Fig. 86, and to locate them on the form according to the dimensions given in Figs. 84 and 85. Care should be taken to have the two cleats for the inlet-baffle

slots both at the inlet end of the form, and the crossed cleats for the other two baffles at the outlet end of the form.

Care must be used in building the corners of the inside form so that it can be easily removed from the finished tank. The construction shown in Fig. 87 is recommended. The ends of the boards forming the sides of the form should be well beveled off, and the boards forming the ends should be set out about 1-inch beyond the ends of the side boards as shown. Nail securely to the 2x4-inch corner pieces all the end boards except those cut to receive the inlet and outlet Y's. These pieces should be left loose.

2

When assembling the form as a whole, nail these 2x4-inch corner pieces to the sides of the form, leaving the nails projecting so that they may be drawn with a hammer after the concrete has set. Before placing the inside form in the pit, paint the outside of it, including the cleats, with a mixture of raw linseed oil and kerosene

T

cross braces and the ends of the form. After this the sides, with their cleats, may be removed, leaving the slots for the baffle boards.

Before putting the tank into regular use, the baffle boards should be inserted according to the detail

shown in Figs. 83 and 84, and the tank should be completely filled with water. The tank should be covered with slabs of reinforced concrete or with paving stones. Wooden planks should not be used, as

they may

rot before

the tank

needs cleaning and permit the earth covering to fall.

The purifying system. The principles governing the design, layout, and construction of a complete underground purifying system have been fully treated at the beginning of this lesson on pages 1725 and 1726.

FIG. 88.-Plan of underground disposal system

A plan of a simple arrangement of tile in two sets, each set consisting