This year in Onsite Installer we will visit all parts of an onsite wastewater treatment system. This month we focus on a component that is extremely important but often overlooked, and that is the piping.

There are several applications for piping in an onsite system. First, there is the building sewer that connects the system to the house. Second, there is piping from the septic tank to another pretreatment device, the pump tank, or the soil absorptions system. Third, there is the supply pipe from a pump tank to the distribution network. Finally there is the distribution piping in the soil treatment area.

PVC dominates

For now, we will focus on the building sewer. In the past, a number of piping materials have been used for this purpose, including clay, Orangeburg and cast iron. These materials have given way to plastic polyvinyl chloride (PVC).

In installing piping, first make sure the materials are not defective or damaged. Pipe with defects such as cracks, or pipe that has been weakened by long-term exposure to sunlight, may no longer be watertight and may invite in root intrusion, leakage, or infiltration of clear water into the system.

For a building sewer, the pipe should be at least 4 inches in diameter to accommodate toilet paper and large solids. Because of the solids, there are maximum and minimum slopes needed to deliver the raw sewage to its destination. The minimum slope ensures that sewage runs down the pipe without slowing down, resulting in clogs. Conversely, if the slope is too great, there is a risk of the water and solids separating, solids accumulating, and the pipe plugging.

For 4-inch PVC piping and a building sewer less than 50 feet long, the minimum slope is 1 inch in 8 feet, or 1/8-inch per foot, and the maximum is 1/4-inch per foot. For sewers longer than 50 feet, the slope should be 1/4-inch per foot.