The Detroit Water and Sewerage Department (DWSD) recently installed thirteen (13) in-line insystem 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.
The available in-system storage volume upstream of each ISD ranges from 1.42 to 8.46 million gallons (MG) as shown on Table 1. A typical profile of an inflatable dam is shown in Figure 1. The ISDs are inflatable dams manufactured by Bridgestone Corporation. The inflatable dams consist of top and bottom sheets of reinforced rubber that are laid on a new concrete pad and are clamped together around the perimeter and bolted to the sewer. When deflated, the top sheets lay on the bottom sheets. When inflated, a pocket of air lifts the top sheets to impede the flow of the wastewater in the sewers.
For the smaller storm events, the combined sewers flow partly full. The ISDs are inflatable dams that are programmed to inflate/deflate to store wet weather wastewater flow to the 7/10ths or 8/10ths point of the combined sewer while passing a desired wet weather/dewatering flow rate downstream to regulator chambers and interceptor chambers.
The ISDs are programmed to store as much combined wastewater as possible without raising the upstream hydraulic grade line above the 7/10ths or 8/10ths level. Consequently, during larger, more intense storm events, the dams deflate to pass the peak flow rates and re-inflate after the peak passes to capture combined wastewater.
Seven (7) of the ISDs are stand alone while the other six (6) ISDs work in series with one or more ISDs. The standalone ISDs are: ISD001; ISD003; ISD005, ISD010; ISD011; ISD012; and ISD013. ISD004 is upstream of and within the backwater zone created by ISD002. ISD006 through ISD009 are in series in the First Hamilton sewer with ISD 006 being the most downstream ISD.