“A Comparison of Air Flow in Four Types of Sewage Drop Structures”

Dropping sewage from near surface collection systems into deep tunnels involves a number of challenges, including the odor control, air entrainment, solids management, energy dissipation, surge control, and access for personnel and equipment. The use of drop structures to handle large flows will become more prevalent over the next several years due to the large number of Combined Sewer Overflow (CSO) tunnel projects on the horizon. Numerous types of drop structures have been used to convey sewage to tunnels, with varying degrees of success. Several recent projects have demonstrated that not all drop structures perform equally well. While numerous models (both mathematical and physical) have been used to analyze drop structures, there is relatively little data that compares the performance of each type. Much of the previous work has focused primarily on hydraulics, and comparatively little emphasis has been placed on the pneumatics of drop structures. Both full-scale prototypes and laboratory-scale models suggest that the flow regimes in drop structures can be extremely complex and variable. Further, the true nature of two-phase flow, which occurs in many drop structures, is not well understood or modeled in many instances. Using both qualitative and quasi-quantitative criteria, this paper compares the relative performance of four types of sewage drop structures:• The plunge drop (sometimes called the elbow drop),• The helicoidal ramp,• The vortex drop and• The baffle drop (sometimes called the cascade drop).In comparing the four types of drop structures, the paper draws on several case studies and lessons learned from others.