Mound System:
Pressure Distribution of Wastewater
Design and Construction in Ohio

Bulletin 829

About manifold and main

Figure 9. Examples of manifold to lateral connections: (a) center connection with tee and (b) ell fitting connection between the manifold and main.

Figure 10. Position of dosing tank relative to septic tank and mound.

The manifold and main pipes connect the mound to the dosing tank. The manifold is piping that connects the laterals and distributes the septic tank effluent to each lateral. The main delivers the septic tank effluent from the dosing tank to the manifold. Manifold and mains are usually PVC pipe with appropriate ell or tee fittings joined with solvent cement.

The connections between the manifold and the laterals, and the manifold and the main, affect the design of the system. Connections can be made at the center of the pipes. In addition, the relative elevation of the connections determines how the system drains. Figure 9 illustrates connection between manifold and laterals connected at the same elevation with tees at center connection as shown in Figure 9 (a), and connection between the manifold and main with ell fitting, as shown in Figure 9 (b), the manifold volume is part of the delivery pipe system because it drains back to the dosing tank.

The main should be sloped back to the dosing tank without the pipe sagging so that it drains back to the tank between doses as shown in Figure 10. This prevents freezing for shallow pipe placement in cold weather and biofilm formation inside of pipe.

The manifold should be the same diameter as the main. The size and position of the manifold and main are determined for each pressure distribution system. The sizing and connections of the manifold and main are presented in steps 6 through 7 along with an example.

Step 6: Manifold length, size and connection

Determine the configuration of connection between the manifold and the laterals, and the manifold and the main (Figure 9). The point of the main/manifold connection determines the length of the manifold. The manifold length is measured from the main/manifold connection to the end of the manifold.

Design Example
   From Figure 9,
   Connection between the manifold and the laterals: center
   Connection between the main and the laterals: tee-to-tee
   Manifold length: 2 ft between the laterals
   Manifold size: the same size as the main

Selected Parameter
   (14) Connection between the manifold and the laterals: center
   (15) Connection between the main and the laterals: tee-to-tee
   (16) Manifold length: 2 ft between the laterals
   (17) Manifold size: the same size as the main

Step 7: Main diameter associated with the system flow rate

Determine the main and the manifold diameter. The main diameter is dependent on the system flow rate. From the head loss values given in Table 2, select a pipe diameter that will have a low head loss at the given flow rate. Note that the head losses listed in Table 2 are for a 100-foot piece of pipe. The manifold is same size as the main.

Design Example
   Flow rate per lateral = Flow rate per hole × Numbers of holes per lateral = 1.47 gpm/hole × 15 holes = 22.1 gpm
   System flow rate = Flow rate per lateral × Number of laterals = 22.1 gpm × 4 laterals = 88.4 gpm
   From Table 2, main pipe size 3 or 4 inch available for 90 gpm (» 88.4 gpm) for this system. The 4-inch diameter pipe size has the lower head loss, therefore, it is selected the 4-inch diameter pipe.
   Main and manifold size: 4 inch

Selected Parameter
   (18) System flow rate: 88.4 gpm
   (19) Main size: 4 inch
   (20) Manifold size: 4 inch

Table 2. Head loss in schedule 40 plastic pipe (after Otis, et al. 1978).