text: Another solution is to build a CSO storage facility, such as a tunnel that can store flow from many sewer connections. Because a tunnel can share capacity among several outfalls, it can reduce the total volume of storage that must be provided for a specific number of outfalls. Storage tunnels store combined sewage but do not treat it. When the storm is over, the flows are pumped out of the tunnel and sent to a wastewater treatment plant. One of the main concerns with CSO storage is the length of time it is stored before it is released. Without careful management of this storage period, the water in the CSO storage facility runs the risk of going septic.
  Washington, D.C., is building underground storage capacity as its primary strategy to address CSOs. In 2011, the city began construction on a system of four deep storage tunnels, adjacent to the Anacostia River, that will reduce overflows to the river by 98 percent, and 96 percent system-wide. The system will comprise over  of tunnels with a storage capacity of . The first segment of the tunnel system,  in length, went online in 2018. The remaining segments of the storage system are scheduled for completion in 2023. (The city's overall "Clean Rivers" project, projected to cost $2.6 billion, includes other components, such as reducing stormwater flows.) The South Boston CSO Storage Tunnel is a similar project, completed in 2011.
  Indianapolis, Indiana, is building underground storage capacity in the form of a   diameter deep rock tunnel system which will connect the two existing wastewater treatment plants, and provide collection of discharge water from the various CSO sites located along the White River, Eagle Creek, Fall Creek, Pogue's Run, and Pleasant Run. Citizens Energy Group is managing the efforts to construct the first phases of the work, which includes a  deep Deep Rock Tunnel Connector between the Belmont Wastewater Treatment Plant and the Southport Wastewater Treatment Plant. Additional tunnels will branch under the existing watercourses located in Indianapolis. The planned cost for the project will total $1.9 billion.
  Fort Wayne, Indiana, is constructing a ,  diameter, $180M tunnel under the 3RPORT (Three Rivers Protection and Overflow Reduction Tunnel) to address the myriad CSOs which outfall into the St. Mary's, St. Joseph, and Maumee Rivers. The 3RPORT is approximately  below grade, and is anticipated to enter service in 2023.