What Is an Inverted Drain in a Sewer System?

Inverted Siphon

• Inverted siphons are more often seen in large city sewer systems, but they should be used with caution. These U-shaped dips in a drain pipe are pressurized to keep waste moving through the line. Installed to avoid obstructions in the path of the drain, they can have their problems. For best results, a series of parallel inverted siphons needs to be installed. A drain for rainwater flowing into the inverted siphon will help to flush solid particles through the line.

Inverted Siphon Precautions

• In "Construction Plumbing: Level 2," Niels Wieffering retells a cautionary story about improperly installed inverted siphon drains in a sewer in Guadalajara, Mexico. An inverted siphon was used in the sewer drain line. While solid and liquid waste was pushed up through the base of the U-shaped inverted siphon, sewer gases still built up. A gas pipeline made of steel and a copper and zinc water line were installed alongside the inverted siphon. Humidity created condensation on the outside of the pipes. This created a reaction between the copper, zinc and steel of the water and gas pipes, producing a hole in the gas pipe that leaked into the sewer drain. The buildup of petroleum gas and sewer gases only needed a stray spark to explode, which is exactly what happened. The final toll of the incident was 206 people killed and more than 500 injured from the buildup of gases in an inverted siphon sewer drain.

P-Traps

• The drains under a sink or toilet feed into the sewer system. Notice that these drains have a sharp bend downward before the line comes back up and flows into the home's waste drain. When viewed from the side, this bend resembles a letter "P," giving this pipe its name of P-trap. These traps play a critical role in protecting your family. The odd shape of the P-trap holds water in the base of the curve. This water creates a barrier to sewer gases that could come back up through the sewer system.

Inverted Y-Branch

• A Y-branch joins two pipes in a drainage system together. The stem feeds into the longer vertical portion of the branch at a 45 degree angle. When the branch is held so the stem's opening is pointing down, the Y-branch is inverted. You cannot use an inverted Y-branch for drainage of waste because the solids and liquids cannot flow upward through the branch. Inverted Y-branches are only used in sewer drain lines to create a joint between the waste line and the vent pipe for venting gases because gases will naturally rise.