to increase the useful life of a cesspool. See figure 10-5.

When the first cesspool becomes filled, a second well may be built to take the overflow from the first. In such cases, the first cesspool should operate as a septic tank to collect the settling and floating solids and provide a trapped outlet on the connection leading to the next leaching cesspool. Septic tanks may be placed advantageously ahead of leaching cesspools in larger installations.

Leaching cesspools should not be placed closer together than 20 feet by out-to-out measurement of walls. Leaching cesspools should be used only where the subsoil is porous to a depth of at least 8 or 10 feet and where the ground water is normally below this elevation. When they are located in fine sand, the leaching area can be increased by surrounding the walls with graded gravel.

The number and the size of cesspools required depend on the quantity of sewage and the leaching characteristics of the total exterior percolating area above the ground water table, including bottoms and sidewalls below the maximum-flow lines. The allowable rate of sewage application per square foot per day, based on the recommended leaching test, is given below. Soils that require more than 30 minutes for a fall of 1 inch are

Figure 10-5. - Leaching cesspool.