Figure 4-32. - A wastewater collection system.

Table 4-2. - Minimum Grades to Lay Sewer Lines to Maintain a Minimum Velocity of 2 Feet/Second

FLOW

Wastewater systems use gravity flow to move the wastewater. Sewers should be designed to let the wastewater move at not less than 2 feet per second. Solids in the wastewater may begin to settle, and this can cause the pipe to clog and cause the wastewater to become septic. Speed of more than 8 feet per second may cause scouring and damage to the pipe. Table 4-2 shows the minimum grades to be used in laying sewer lines to provide a speed of at least 2 feet per second.

Inverted Siphon

If a part of the sewer dips below grade to avoid a railway cut, a subway, a stream, or some other object, an inverted siphon should be built in the pipe (fig. 4-33). A speed of at least 3 feet per second is needed