News | April 30, 1998

Intercepting Floatable Material from Combined Sewer Overflows

emoval of floatables is part of the nine minimum controls required under the April 1994 EPA CSO Control Policy. As such, this means that now every overflow site in the U.S. has to be modified to catch floatable material. The most commonly applied solutions currently are the bar screen and the underflow baffle. However, their efficiencies at removing floatables until recently remained to be established.

To establish a good basis of comparison, floatables' characteristics were evaluated from samplings taken at different CSO outfalls in the Montreal area during different events. Results show that up to 80 percent of floatable material could be retained by properly designed bar screens fabricated with 0.25-in. bar spacing. Following those results, a pilot bar screen was designed specifically for CSOs and tested. From the same samplings, hydraulic calculations established the underflow baffle efficiency. From these calculations, a model study was conducted in the Polytechnic Institute of the University of Montreal (École Polytechnique) Hydraulic Laboratory.

Results demonstrated the existence of a critical velocity in the turbulent storm flow that marks the transition between the suspension and floating phenomena. The transition is similar to that between the suspension and settling phenomena for settleable particles. Applied to field situations, the results can be used as an evaluation tool. As such, it shows that the actual performance efficiencies of most existing structures retrofitted with underflow baffles will be quite low. The outcome is that they will allow only floatables with very high buoyancy to float. To achieve reasonable efficiencies with the baffle type of solution, therefore, the existing chambers would have to undergo extensive modifications.

Considering these results, an economic analysis for specific sites may substantially favor the use of bar screens over underflow baffles. The floatables removal performance for other technologies, like vortex separator, are currently under study.

This case study is adapted from a presentation appearing in Goose News, a John Meurnier Inc. newsletter. The newsletter feature is an abstract of a presentation at the New York Water Environment Association, New York City, NY, January, 29, 1997.

The paper was written by Martin Couture, Jostran Lamontagne, and Benoit Gagné, all of John Meunier Inc.; also Oscan Dalkir, Cegeo Technologies; and Claude Marche, École Polytechnique de Montréal (University of Montréal). To obtain the paper, contact: Martin Couture, PE John Meunier Inc. 6290, rue Périnault Montréal (Québec) H4K 1K5.Abstract

<%=company%>, 6290 Perinault, Montreal, Quebec, Canada H4K 1K5. Tel: 514-334-7230; Fax: 514-334-5070.

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