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 2-20 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 s i z e. DESIGN Design  guidance  for  wastewater  systems  is contained   in   Domestic   Wastewater   Control, MIL-HDBK-1005/8.