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Combined Sewer Overflow (CSO) Treatment through fine screening coupled with Engineered Procure Construct (EPC) construction delivery method have proven to be an economical solution for the City of Lima, Ohio, to obtain compliance with Federal EPA regulations on CSO control, and allows for new development upstream of the combined system.

Accumulation of floatable materials along riverbanks is a very visible type of river pollution. The fine screening of combined sewer overflows (CSOs) was beneficial to Lima and downstream communities by removing the objectionable materials that currently discharge from Lima's CSOs. These CSOs discharge near the vicinity of the City's river walk. The screens capture fine solids and floatable materials in the combined sewage that are greater than 4- millimeters (0.15-inch) (such as Styrofoam cups, cigarette butts, etc.) during a storm discharge event and hold these solids within the sewerage system for diversion and treatment at the wastewater treatment plant. Other US cities that have reduced pollution in their waterways have experienced energized development. The City of Lima's expectation is that their investment in pollution reduction will result in increased economic development, improved recreational areas, and community pride.

The type of CSO screens utilized in this project is relatively new to the United States market. Fine screening equipment has 4-millimeter (0.15-inch) openings to not only trap floatables but also provide CSO treatment. For Lima's CSO solution, horizontal fine screens were retrofitted into five existing 20-year-old chambers to provide an innovative and economical solution.

The screen does not screen solids in the traditional sense by removing the collected waste, but rather acts as a barrier to solids, retaining solids on the dirty water side of the screen and diverting them to the main wastewater treatment plant for removal once the storm flow subsides.

Excessive flow entering the screening chamber will pass through the horizontal screen and discharge over a new fixed spill weir and discharge into the Ottawa River through the existing river gates. Floatable materials are trapped by the screen move laterally along the face of the screen via a series of mechanical combs to the transverse end section where they can be directed to the sanitary sewer line for ultimate removal at the wastewater treatment plant.

As a direct result of the consultant work undertaken by MWH and the City of Lima in reviewing the Short and Long Terms goals for the CSOs and combined sewerage system, the City of Lima selected a Design Build Construction procurement method to execute the project. Appointing MWH as the Design Build Contractor, the City of Lima used a reimbursable form of contract with a Target Cost providing for a sharing of cost overruns or cost savings relative to the Target Cost between the City and MWH. The City of Lima's experience with the Engineer-Procure-Construct (EPC) method was favorable, with prompt delivery of the project resulting in a 30 percent schedule reduction and 10 percent cost savings.

More than 90 percent of the entire CSO volume receives treatment for a storm event with a recurrence interval of four months. The CSO screens will assure that the City of Lima can continue to provide sewer services for new developments located upstream of the Combined Sewer Overflow locations.
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Document Type: Research Article

Publication date: 2002-01-01

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