Use of In-Line Inflatable Dams for CSO Control

The Detroit Water and Sewerage Department (DWSD) recently installed thirteen (13) in-line in-system storage devices (ISDs) in large combined sewers in Dearborn, Detroit, Hamtramck and Highland Park, Michigan. The ISDs are designed to store combined sewage in the sewers during wet weather, especially during smaller storms that do not utilize the full transport capacity of the sewers. The ISDs are inflatable dams that are controlled based on upstream and downstream wastewater levels and dam pressure.The ISD project is one element of the Long Term Combined Sewer Overflow (CSO) Program for DWSD, consistent with the CSO Control Policy of the U.S. EPA (April 1994). The ISDs maximize the use of the collection system for storage which is one of the “Nine Minimum Controls” listed in the EPA Policy.Control schemes for the ISDs were initially developed and tested using Transient Analysis Program (TAP) models of the ISDs and sewers. An important design criterion for the ISD project was that the number of level sensors required for control of the inflatable dams be minimized. During the design of the ISD project, it was determined that twenty-two (22) level sensors were required for proper control of the ISDs.Seven (7) of the ISDs are standalone with upstream and downstream level sensors at each ISD site. Four (4) ISDs are in series along the First Hamilton sewer. Two (2) ISDs are in series in the Joy and Wetherby sewers. The two (2) most downstream ISDs in these series have both upstream and downstream level sensors at the ISD sites. The four (4) upstream ISDs are in the backwater zone of a downstream ISD and have only upstream level sensors. These ISDs use the upstream level sensor reading from the downstream ISD for control.A total of twenty-two (22) pairs of level sensors were installed for the ISD project. Ultrasonic and entrapped air level sensors were installed at each level sensor location. The ultrasonic level sensors are used for primary control. The entrapped air level sensors are used for comparison and to indicate disparity and loss of signal accuracy.The ISDs were installed from August 2002 through October 2005. The ISDs have operated from July 2005 to present and the initial control scheme was tested and fined-tuned through review of actual operating data. In April 2006, major revisions to the control scheme were implemented and extensive dry weather testing was done. Recent operating data indicate that the final control scheme is working well and as intended. Data continues to be collected for the ISDs to verify the actual CSO volumes being captured and stored at each ISD location and the reduction in the frequency of CSO.