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Effectiveness of Fine Screens in Meeting CSO Treatment Requirements for Identifiable Sanitary Materials

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Removal of identifiable sanitary material is a necessary component required to achieve adequate treatment of combined sewer overflows (CSO) by the Michigan Department of Environmental Quality (MDEQ). As part of the demonstrative approach that the city of Detroit is using to develop a Long-Term CSO Control Plan for the Detroit River outfalls, the city constructed two pilot facilities to test the effectiveness of fine screen technology in meeting the requirement for removal of identifiable sanitary material. These facilities are located along the Detroit River nearthe St. Aubin and Leib outfalls. This paper provides a summary of the results of a sanitary material removal evaluation based on a two-year monitoring program conducted at both facilities.

The St. Aubin and Leib screening and disinfection facilities provide flow-through CSO treatment, first requiring flows to pass through fine (approximately 4-millimeter (mm)) screens and then introducing disinfectant (sodium hypochlorite) through induction mixers. The St. Aubin and Leib facilities were designed to fully screen and disinfect flows of 250 and 1550 cubic feet per second (cfs), respectively. These flows are based on the 10-year, 1-hour design storm event of 1.8 inches. The hydraulic design flows of the facilities (370 and 2000 cfs, respectively), represent the upstream capacity of the conveyance system. These flows would receive partial treatment (disinfection and partial screening) prior to discharge to the Detroit River.

Two types of screens were evaluated at the facilities. These included a vertical configuration supplied by ROMAG Pipes & Machines, Ltd., and a horizontal upflow configuration supplied by Copa Limited. Both of these technologies were installed and compared at the Leib facility. At the St. Aubin facility, only the ROMAG vertical screens were installed.

During the two-year evaluation period at each facility, 4-mm mesh sampling nets were installed downstream of the fine screens prior to each overflow event. The sampling nets captured a portion of the total flow, and provided a sample of any material that passed through the facility's fine screens. Following each overflow event, the mass and volume of material captured in each net were measured and the contents were examined and photographed. Video records of screening operation were also acquired during each event and reviewed to provide further datawith regard to screen performance. In addition, visual observations of the discharged effluent to the Detroit River were made. This information was used to assess the capability of the screens to achieve the applicable MDEQ "Criterion for Success" by providing significant removal of identifiable sanitary trash in the facility effluent.

Based on 11 overflow events monitored at the St. Aubin facility and 2 overflow events monitored at the Leib facility, the effectiveness of fine screens in achieving success in the removal of identifiable sanitary material has been demonstrated. Details on the review of screen performance and operation are included in the paper. The results of this work also demonstrated that both screen configurations and manufacturers (vertical and horizontal) performed equally well.
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Document Type: Research Article

Publication date: 2006-01-01

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