A wastewater system (fig. 2-20) consists of the collection of sewer pipes and pumps that are designed

Figure 2-21.An inverted siphon.

to convey domestic and industrial wastes and to transport them to a wastewater treatment plant. The purpose of these systems is to safeguard public health by preventing disease-producing bacteria, viruses, and parasites getting into groundwater or drinking water systems. A description of the various elements and structures used in a wastewater system is as follows:

1. SANITARY SEWER. A sanitary sewer system carries mostly domestic wastes but may carry some industrial waste. These systems are NEVER designed to carry storm water or groundwater. (To convey storm water, groundwater, or other surface water to disposal points, a storm sewer system is designed and constructed separately from the sanitary sewer system.) Sanitary sewer system piping includes the following:

a. BUILDING, OR HOUSE, SEWER. A service-connection pipe that connects an individual building to the wastewater system. These pipes are 4 inches or larger in diameter and are commonly concrete, cast iron, or plastic. The building, or house, sewer is the smallest pipe in a wastewater collection system. All other pipes must be a MINIMUM of 8 inches in diameter.

b. LATERAL SEWER. Piping that receives discharge from house sewers.

c. SUBMAIN, OR BRANCH, SEWER. A pipe that receives waste from two or more lateral sewers.

d. MAIN, OR TRUNK, SEWER. A pipe that takes discharge from two or more submains or from a submain plus laterals.

e. INTERCEPTING SEWER. One that receives wastewater from more than one main, or trunk sewer.

f. RELIEF SEWER. A sewer built to relieve an existing sewer that has an inadequate capacity.

2. LIFT STATION. Most piping in a wastewater system consists of gravity pipes that are designed to flow by gravity action at a rate of not less that 2 feet per second. Where gravity flow is not practical or possible, a lift station, such as the one shown in figure 2-20, is constructed to pump wastewater to a higher level. From the lift station, the wastewater is pumped through a pipe, called a force main, to higher elevation gravity pipes. Unlike gravity piping, force mains always flow complete] y filled and under pressure.

3. INVERTED SIPHON. Another sewer pipe designed to flow full and under pressure is the inverted siphon. These pipes dip below the designed gradient of the gravity pipes and are used to avoid obstacles, such as open-cut railways, subways, and streams. An example of an inverted siphon is shown in figure 2-21. The inverted siphon may have one, two, or more pipes and is designed to flow at a rate of at least 3 feet per second to keep the pipe(s) clear of settleable solids. It should have manholes constructed at both ends for maintenance.

4. MANHOLE. A manhole is a concrete or masonry structure used for inspection and maintenance of sewer lines. Examples of manholes are shown in figure 2-22. The bottom portion of a manhole is usually cylindrical and has an inside diameter of at least 4 feet. The upper portion usually tapers to the street or ground surface and is fitted with a cast-iron cover. For proper sewage flow, the bottom of the manhole slopes toward a built-in charnel that has a depth of three fourths of the diameter of the sewer pipe. For sewers up to approximate] y 60 inches in diameter, manholes are usually spaced 300 to 400 feet apart. They are also required at all locations where sewer lines intersect or where the sewer lines change direction, grade, or pipe size.

Design guidance for wastewater systems is contained in Domestic Wastewater Control, MIL-HDBK-1005/8.

Figure 2-22.Types of manholes

When designing a wastewater system, the design engineer begins by first determining the types and quantities of sewage to rehandled. This is accomplished through a careful study of the area to be served. The design engineer bases his design on the average daily use of water per person in the area to be served. A typical value is 100 gallons per person per day. But, the use of water is not constant. Use is greater in the summer than in the winter and greater during the morning and evening than it is in the middle of the day or at night. Therefore, the average daily flow (based on the average utilization) is multiplied by a peak flow factor to obtain the design flow.

Typical peak flow factors range from 4 to 6 for small areas down to 1.5 to 2.5 for larger areas. An allowance for unavoidable infiltration of surface and subsurface water into the lines is sometimes added to the peak flow to obtain the design flow. A typical infiltration allowance is 500 gallons per inch of pipe diameter per mile of sewer per day. From the types of sewage and the estimated design flow, the engineer can then tentative] y select the types, sizes, slopes, and distances below grade of the piping to be used for the system. 

Upon acceptance of the preliminary designs, final design may begin. During this phase, adjustments to the preliminary design should be made as necessary, based upon additional surveys, soil analysis, or other design factors. The final designs should include a general map of the area that shows the locations of all sewer lines and structures. They also should include detailed plans and profiles of the sewers showing ground elevations, pipe sizes and slopes, and the locations of any appurtenances and structures, such as manholes and lift stations. Construction plans and details are also included for those appurtenances and structures.