Figure 2. Mound with pressure distribution system.
Figure 3. Indication of distribution failure in the mound system.
Figure 2. Mound with pressure distribution system.
Figure 3. Indication of distribution failure in the mound system.
Pressure distribution networks with laterals and manifolds are used to provide relatively uniform distribution of wastewater effluent over the entire distribution area simultaneously during each dose (Figure 2). Uniform distribution of the septic tank effluent is especially important in mound systems. Uneven distribution of septic tank effluent can result in localized overloading and short-circuiting through the mound. Surface seepage in one area of a mound may be caused by uneven distribution (Figure 3).
Pressure distribution systems are carefully designed so that the volume of septic tank effluent flowing out of each hole of the distribution pipe is nearly identical. The pipe diameters and hole diameters must be carefully sized to achieve uniform distribution.
A perforated lateral begins at the manifold and ends with a drain hole and turn up. In a pressure distribution system, 1 to 1 ½ inch diameter pipes are used as laterals to distribute the wastewater. The 4-inch perforated pipe, used in conventional soil absorption systems, is not suitable because it is too large and the holes are not appropriately sized and spaced to provide even effluent distribution.
Schedule 40 PVC pipe and fittings are used in pressure distribution systems. Holes are drilled perpendicular to the pipe in a straight line along the top of the pipe. A sharp drill bit will drill a more uniform perforation than a dull drill. Any burrs or rough edges must be removed from the holes so they do not collect debris and clog. Slide a rod or small diameter pipe along the inside of the lateral pipe to remove burrs. Upon installation, the pipe must be clean and clear of debris and PVC cuttings that can clog holes. During construction, protect the ends of pipes to keep rodents and their food and nesting material out of pipes. Duct tape works well for this.
The steps presented herein can be used to design pressure distribution systems where each lateral is at the same elevation. Developing a pressure distribution system for a series of laterals at different elevations is more involved and is not presented in this bulletin.
The pipe diameter, hole diameter and hole spacing are determined for each pressure distribution system. The sizing of the distribution pipe network is presented in the next five steps along with an example.
Establish dimensions of mound from estimated daily flow rate and soil conditions (Figure 4) described in Ohio Extension Bulletin 813, Mound Systems for On-Site Wastewater Treatment: Siting, Design, and Construction in Ohio. Copies of Bulletin 813 are available from Ohio county Extension offices.
| Design Example Dimension of the distribution area: 4 ft wide × 90 ft long |
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| Figure 4. Distribution area for mound system. |
| Selected Parameter (1) Width of distribution area: 4 ft (2) Length of distribution area: 90 ft |
Determine the length of laterals and the distance between laterals (Figure 5). All lateral lines are to be on the same elevation to avoid uneven distribution. The lateral length is measured from the distribution manifold to the end of the lateral. A center manifold is preferred because it minimizes pipe length. For a center manifold it is approximately half the length of the distribution area. Figure 6 illustrates some possible manifold positions. The goal is to keep lateral lines short. Consider split manifolds shown in Figure 6 to keep lines short on long mounds.
| Design Example Length of laterals = Distribution area length / 2 Length of laterals = 90 ft / 2 = 45 ft Distance between lateral = 2 ft Total lateral number = 4 with center manifold connection |
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| Figure 5. Network configuration for distribution area. |
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| Figure 6. Alternative manifold designs for distribution system. |
| Selected Parameter (3) Length of laterals: 45 ft (4) Distance between laterals: 2 ft with center manifold (5) Total lateral number: 4 |
Determine hole spacing and number of holes, which can be calculated by maximum coverage area per hole. For example, 6 ft2/hole is recommended for sand infiltration for all pressure distribution application. Hole spacing should be less than 4 feet. The higher the hole density, the more uniform the wastewater distribution. The holes should be staggered between adjacent laterals, if possible (Figure 7). A hole should always be drilled on the bottom at the end of the pipe to facilitate draining after a dose (Figure 7).
Select hole diameter. For mound systems meeting siting criteria in Bulletin 813, ¼-inch holes are sufficient. Larger holes require larger pipe diameters and pumps. Smaller holes may clog. Use shields over each hole when using gravel. For example, shield with a 4-inch perforated pipe placed in a gravel bed.
| Design Example Required hole numbers = Total distribution area, ft2 (width, ft × length, ft) / Maximum coverage area per each hole (6 ft2/hole) Total distribution area, ft 2 (width, ft × length, ft) = 4 ft × 90 ft = 360 ft2 Required hole numbers = 360 ft2 / (6 ft2/hole) = 60 holes Hole spacing, ft = Lateral length, ft / Number of holes in each lateral Number of holes in each lateral = Required number of holes / Lateral numbers = 60 holes / 4 = 15 holes Hole spacing, ft = 45 ft / 15 = 3 ft Therefore: Checking: 3 ft hole spacing is less than 4 ft, so OK! |
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| Figure 7. Hole spacing in distribution system. |
| Selected Parameter (6) Hole diameter: ¼ inch (7) Total number of holes: 60 holes (8) Number of holes per lateral: 15 holes (9) Hole spacing: 3 ft |
Determine lateral diameter. The lateral diameter selection is based on the hole size, hole spacing and lateral length. Graphs, such as the one in Figure 8, have been developed to help in selecting minimum lateral diameters between 1 and 1 ½ inch. Select the pipe diameter indicated between the lines on the graph.
| Design Example Diameter from Figure 8 = 1.5 inch based on hole spacing 3 ft and 45 ft lateral length |
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| Figure 8. Minimum lateral diameters of plastic pipe for lateral lengths versus spacing for ¼ inch hole diameter (after Otis, 1982). |
| Selected Parameter (10) Lateral diameter: 1.5 inch |
Select a pressure head to be maintained at the end of each lateral. The head should be between 2 feet and 4 feet. Using the selected head and the hole diameter, the flow rate per hole can be determined from Table 1. By multiplying the flow rate per hole by the number of holes in the lateral, the lateral flow rate can be calculated.
| Design Example Flow rate per hole for ¼ inch at head 4 ft from Table 1 = 1.47 gpm/hole Numbers of holes per lateral = 15 holes Flow rate per lateral = Flow rate per hole × Numbers of holes per lateral = 1.47 gpm/hole × 15 holes = 22.1 gpm |
Table 1. Flow rate per hole for 1/4 inch holes and various network pressures (after Otis et al. 1978).
| Head (feet) | Pressure (psi) | Gallon per minute (gpm) |
|---|---|---|
| 1 | 0.434 | 0.74 |
| 2 | 0.867 | 1.04 |
| 3 | 1.301 | 1.28 |
| 4 | 1.734 | 1.47 |
Selected Parameter
(11) Head at end of lateral: 4 ft
(12) Flow rate per hole: 1.47 gpm
(13) Flow rate per lateral: 22.1 gpm