Combined sewer overflows (CSOs) are the release of raw sewage and stormwater into receiving waterways when the system becomes filled. CSOs constitute a major source of water pollution for 860 municipalities across the United States. Combined sewer systems consist of a network of deep tunnels to move both stormwater and sewage. These systems can be filled rapidly with stormwater during rainfall events, resulting in the entrapment of large amounts of air. When the trapped air is released through vertical drop shafts, geyser eruptions are often produced. These geysers can exceed 30 meters in height and can cause massive flooding events that lead to the pollution of streams, rivers, and lakes. Many combined sewer systems are operated below design capacity to prevent geyser formation during rain events, but this increases the likelihood of CSOs. This research project will develop a mechanistic understanding of geyser formation that can be used to design retrofitting methods for drop shafts that allow the smooth release of air pockets without producing geysers. When geysers are no longer a concern, the full capacity of storm sewer systems can be utilized during rain events and hence CSO discharges will be reduced significantly. An outreach program will educate and train researchers, middle-school and college students from underrepresented groups, sewer system operators, and other stakeholders on the control of geysers and the impact of CSOs on water quality.
The goal of the research project is to develop a fundamental understanding of the two-phase flow physics that triggers geysering in storm sewer systems. The research will utilize integrated high-speed optical imaging, digital particle image velocimetry, and 3-D computational fluid dynamics to model and confirm results from the laboratory-scale reproduction of geysers. Studies will be performed to understand the relationship between geyser eruption, system geometry, and initial flow conditions. A reduced order geyser model will be developed from these relationships and implemented in an open-source software format for modeling flow dynamics in storm sewer systems. Retrofitting strategies to permit the smooth release of air pockets will be preliminarily tested and validated using a 3-D model, and the most promising methods will be tested experimentally and confirmed numerically. The outcomes of this research will be used by scientists, hydraulic engineers, and other practitioners to inform future design and retrofitting strategies to prevent geyser formation in storm sewer systems. Students, regulators, and other stakeholders will learn from the software packages and educational outreach how to prevent geyser formation and reduce CSOs and their associated impacts on the environment.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.