The City of Cincinnati’s (Ohio) Metropolitan Sewer District (MSD) has installed an innovative surge tank design to deal with potential odor, corrosion and organic loading issues associated with “first flush” storm water commingling with the sanitary sewer lines during a storm event.
The MSD has always been a nationally recognized innovator in sanitary wastewater treatment and this is the first-of-its-kind use of venturi aerators in a surge tank concept for in-line “treatment and conditioning” of the sanitary waste stream. This “treatment” concept and project is located at Bayley Place. “Treatment and Conditioning” is achieved by the innovative use of venturi aerators that recirculate the contents of the surge tank to keep the sanitary flow “freshened” by adding high levels of dissolved oxygen and stripping carbon dioxide.
The surge tank has been installed in-line i.e. the sanitary wastewater from the surrounding neighborhood will flow by gravity through the surge tank and then flow into the main gravity sewer lines. Using this treatment concept, low organic loading storm water is allowed to flow into the main sewer system first and the higher strength sanitary flow is retained, treated and conditioned prior to release during low flow or off-peak hours. The surge tank is installed below grade and has a low profile. Once it has been landscaped it will blend in with the local neighborhood.
The MSD will monitor the main gravity sewer line and CSO downstream of the surge tank. When a storm event occurs, the MSD will determine the capacity of the downstream gravity sewers and CSO. At a specific volume and flow level, MSD will shut off the wastewater flow from the Bayley Place surge tank. The surge tank will then normally be emptied during off peak hours or post storm event low flow periods. The flow from the Bayley Place surge tank is shut off by a radio controlled electronically actuated plug valve or by a programmable event timer in a control panel.
As the wastewater fills up in the surge tanks, a float switch triggers the dual venturi aerator system into operation, and the aeration process begins. The purpose of the venturi aerators is to keep the sanitary flow aerobic and prevent the build of malodorous and corrosive hydrogen sulfide gases in the sewer system by adding high levels of dissolved oxygen to form sulfates which are not odorous and not corrosive.
The MSD’s surge tank at Bayley Place has been sized to handle up to four days flow capacity based on historical flow data and has an emergency by-pass to the gravity sewer lines. The MSD will continue to monitor the downstream gravity lines and CSO. Once they have determined there is additional capacity in these lines, they will send a signal to actuate and open the plug valve and allow the sanitary wastestream to flow by gravity back into the main sewer lines. The venturi aerators will continue to recirculate and condition the contents of the tank until the float switch hits the low-level shut off setting.
Using the venturi aerators in the surge tank not only oxidizes the sulfides into sulfates, but also reduces the BOD of the sanitary wastewater. This further eliminates an organic overload at the wastewater treatment plant that would occur if the sanitary wastewater were allowed to sit for four days increasing in BOD and going completely anaerobic.
For additional information on venturi aerators and their many uses for wastewater treatment and conditioning contact Venturi Aeration, Inc. at 603-635-8239.