A pressure distribution system is designed to spread wastewater uniformly over the drainfield. Such systems need to be designed and installed carefully, so the volume of effluent flowing out of each hole or perforation in the pipe is nearly identical. The pipe diameters and hole diameters must be carefully sized.
There are different kinds of pressure distribution. Until recently, typical systems were the rock or aggregate beds of mound treatment systems or low pressure pipe systems in trenches or beds. In the past decade, drip irrigation systems are becoming more prevalent, and they operate at higher pressures than the other types. Pressure distri-bution systems have five components:
• Lateral pipes with equally spaced holes drilled into the invert of the pipe.
• Manifold and main connecting the laterals.
• Dosing tank to collect septic tank effluent.
• Pump to pressurize the system.
• Controls and power supply to operate the pump.
We’ll cover the first two items here and address the others in future columns.
Piping selection
Pressure distribution systems use small-diameter pipes. The 4-inch perforated pipe used in conventional gravity systems is too large, and the holes are not properly sized or spaced for even distribution. An installer cannot simply go to the pump supply outlet and get pipe with the proper holes. A part of the installation process is to drill the holes to the design specifications.
Schedule 40 pipe and fittings should be used for pressure systems. Holes are drilled perpendicular to the pipe in a straight line along the underside. Any burrs should be removed from the inside of the pipe. You can do this by sliding a rod or smaller-diameter pipe inside the lateral pipe.
The pipe must be kept clean during construction. This means no debris, PVC shavings, or grass or leaves from rodents. The pipe and connections must be able to withstand a pressure of at least 40 psi.
For use in a bed system, such as the rock bed of a mound or a sand bed in sandy soils, all lateral lines need to be at the same elevation. The lateral length is measured from the distribution manifold to the end of the lateral. A center manifold is preferred because it minimizes pipe sizes. Remember: All connections must be tight to prevent leakage and to withstand pressure. We have seen many pressure systems fail because the installer did not glue the pipe connections adequately.
Spacing holes
Holes should be spaced at least every 5 feet. The more holes per lateral, the more uniform the effluent distribution. Hole spacing needs to be balanced with pump capacity adequate to supply the volume of water needed to fill the pipe and deliver effluent through each hole, while meeting the head requirements.
The holes should be staggered between adjacent laterals, if possible. This will provide a more even distribution pattern. The last hole should be drilled in the lateral end cap to provide venting as the pipe fills and to release the effluent into the bed when the pump shuts off.
For systems serving single- family homes, 1/4-inch-diameter holes are recommended. Larger holes require larger pipe diameters and greater pump capacity. As the perforation gets smaller, the maximum allowable length increases, but smaller holes can result in more plugging, which requires more frequent maintenance. The number of holes must not exceed 10 percent of the average pressure head on the perforations.
The lateral diameter should be large enough to keep pressure losses low, but small enough to keep costs down. This translates to using 1- to 2- inch lateral pipe. The most common is probably 1 1/2-inch, but that varies depending on what part of the country you work in. As lateral diameter increases, the maximum allowable length increases, but as the diameter decreases, the velocity increases — and that helps to prevent plugging.
Manifold and mains
The manifold and main pipes connect the soil treatment system to the dosing tank. The manifold connects the laterals and distributes the effluent to each lateral. The main delivers the effluent from the dosing tank to the manifold.
Manifolds and mains should be PVC pipe with the appropriate fittings, tees, and ells. Friction loss should not be greater than 5 percent of the average head of the perforations. The connections between the manifold and the laterals, and to the main, affect the design of the system. Connections can be made from the center or from the ends. The relative elevations of the connections determine how the system drains.
If the manifold and laterals are connected at the same elevation with staggered tees, the manifold volume will drain through the lateral holes. If they are connected at different elevations using tee-to-tee connections, the manifold volume will drain back to the dosing tank.
The manifold pipe should be connected to the main pipe from the pump and sloped toward the main. The main should be sloped back to the dosing tank so that effluent drains back between doses. This is key to prevent freezing in cold climates.
The manifold should be the same diameter or larger as the main pipe. The size and position of the manifold will vary with each installation. The main diameter is dependent on the system flow rate. A typical main diameter is 2 inches.
Check future issues of Onsite Installer for information about dosing tanks, pumps, controls and power supplies.
