The distribution piping system includes the supply pipe, which carries effluent from the septic tank to the pressure distribution system, and the laterals, which actually distribute the effluent. One important variable is the volume necessary to fill the pipe, which is the amount of effluent needed to charge the system before even distribution can occur. The dose volume must be at least five times the amount needed to pressurize the system. So this is the minimum amount of effluent that can be pumped and still result in equal distribution. This comes into play when a strategy of more frequent and smaller doses is employed to provide better treatment. There are limitations as to how small a dose can be provided.

Another important consideration in sizing the supply line is the volume of the drainback to the pump tank. In cold climates allowing drainback is necessary to avoid freezing problems. The piping needs to be properly bedded and laid on a slope that ensures effluent will drain back when the pump turns off. Even in warmer climates it is not advisable to use a check valve to stop the drainback. If a check valve is used, bacteria in the pipe will grow between doses, creating solids that can plug the holes (orifices) in the pressure laterals which will require more frequent maintenance.

Drainback cannot be too large because the pump has to move effluent that has drained back more than once, making the system operate less efficiently. The size and type of pipe (usually PVC) will determine the friction loss in the pipe and needs to be considered for the overall pressure requirement when sizing the pump. In general larger pipes have less friction loss, which can result in the ability to use a smaller pump; this is usually offset by more material costs when using larger pipes. Typically supply pipes and manifolds used in sewage treatment mounds are 2-inch-diameter PVC piping.

Manifolds connect the laterals and distribute the effluent to each lateral with ell or tee fittings. The manifold should be connected to the supply pipe with the connection sloped toward the supply pipe so it is part of the delivery system and drains back to the tank. Manifolds must be installed level, so it is important that they are properly bedded. If the manifold is not level, effluent will flow by gravity to the lowest elevations during the pipe filling and draining cycles as the pump turns on and off.

The size of the connections will affect system flow. If Schedule 40 PVC is used the connections to the laterals should be reducing tees. If the installer is going to drill and tap the pipe for connections Schedule 80 PVC should be used. Schedule 40 will not maintain the necessary strength if it is drilled and tapped. All burrs should be removed and the whole network flushed through clean-outs before the system is used. This removes any leftover material that can later plug the distribution orifices in the pressure laterals.

About the Author
​Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

This article is part of a series:

• Why Use Pressure Distribution?
• What You Need to Know About Dosing Tanks
• How to Select the Best Pump for a Pressure Distribution System
• Here’s Your Supply Line and Manifold Master Class
• Tips for Laying Out Your Laterals