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Assessment of Grit Collection Data for the Refinement of a Long Term Combined Sewer Overflow Control Plan City of Omaha, Nebraska

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The City of Omaha (the City) is currently in the process of implementing their approved Final Long Term Control Plan (LTCP) to substantially reduce wet-weather overflows from the City's combined sewer system (CSS) service area as required by Federal and State of Nebraska regulations. The Final LTCP was submitted to the Nebraska Department of Environmental Quality (NDEQ) on September 25, 2009; and was based on the refinement of the Substantively Complete Long Term Control Plan (SCLTCP) that was submitted to the NDEQ in October 2007. The SCLTCP included proposed controls at each of the combined sewer overflow (CSO) outfalls. The Final LTCP was approved by the NDEQ on February 10, 2010.

The operation and maintenance the facilities proposed under the LTCP, which include a 12.5 foot diameter, 30,700 foot long conveyance/storage tunnel, and two retention treatment basins (RTBs), will result in the capture and handling of significant quantities of grit that would otherwise overflow to the receiving waters. During the completion of the LTCP, one of the refinement tasks was to assess grit removal and handling needs for the City. This was an important step in the further development of the LTCP, and involved the assessment of whether the recommended grit management technology, grit pits, is appropriate for the needs of the City; and whether the conceptual sizing requirements of the grit pits are appropriate. The selection of inappropriate technology, or an appropriate technology sized unsuitably would likely diminish the performance of the facilities and increase operation and maintenance costs.

The proposed grit pits are rectangular 12 ft × 12 ft structures located downstream of diversion structures, and upstream of tunnel drop shafts, RTBs and storage tanks. The bottom depth of the grit pits would be at least ten ft below the invert of wet wells for pump stations serving associated RTBs and storage tanks; and at least ten ft below the invert of influent conduits to tunnel down shafts. The grit pits are intended to remove large inorganic solids to protect downstream screening and pumping facilities; and to minimize the difficulty of facilities cleaning operations.

The approach taken was to analyze grit collection data provided by the City for existing facilities to develop an understanding of the quality of the available data with a view towards determining grit quantities associated with various rainfall events, and seasonal variations. The results of the analysis were then used to assess the adequacy of the proposed grit pits. The data consisted of the number of grit loads removed from each existing City grit facility during the years 2004 through 2007.

The results of the data analysis and assessment of the grit pit technology were:

No clear correlation was identified between total rainfall and grit collection as total grit loads.

The grit data indicate a slight seasonal trend of increased grit collection in the spring months of March, April and May.

The grit data suggest that existing facilities are removing material that is classified as medium sand and smaller.

Based on analyses, and conservative application of literature values for grit quantities, the general sizing of the grit pits needed to be increased from what was assumed in the SCLTCP for the applications envisioned.

The need for larger and additional grit pits would increase capital costs, but the indications are that the application of grit pits is appropriate from an engineering standpoint.

Keywords: CSO; Combined sewer overflow; Long Term Control Plan; grit; retention treatment basin

Document Type: Research Article


Publication date: January 1, 2010

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