Wastewater conveyance systems are designed to pipe raw wastewater from a municipality to a wastewater treatment facility. Due to the long distances traveled and the presence of obstacles such as rivers and other waterways, the desired pipe slope requirements might not be met, as in the case of an inverted siphon. The challenge is maintaining adequate flow velocities to transport wastewater without the settling of solids. Most design criteria identify this minimum velocity to be 2 ft/s. Air entrainment is a risk with an inverted siphon, which can reduce the fluid density and constrict flow, reducing flow velocities, thus complicating the issue of settling of solids. To simulate the complex processes of air entrainment and movement in an inverted siphon, ANSYS CFX, a commercial software package for computational fluid dynamics (CFD) was used. The siphon contained two branches, an east and west branch. Simulated velocities for the proposed design of the west overflow branch were well below the 2 ft/s threshold. It was also found that the proposed design allowed for air entrainment in the west branch downslope. To alleviate the air entrained in the west branch, a vent was modeled at the knee of the downslope. This allowed the air to evacuate the system via an upstream manhole. It was also found that moving the location of the 66x42 reducer from the downstream end of the west branch siphon to downstream of the wye junction on the main line improved flow distribution between the east and west branches (from 99:1 percent split to a 91:9 percent split) and increased water velocities in the west branch.
