Water Environment Federation (WEF)

Continuous Modeling of Wet Weather Strategies Annual Water Quality Results Support a Municipality’s Decision-Making Process

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Courtesy of Courtesy of Water Environment Federation (WEF)

An extensive water quality modeling effort for the City of Columbus Long Term Control Plan (LTCP) was performed in order to evaluate future scenarios to abate combined sewer overflows. This modeling effort was supported by a massive data collection program and employed three different models to analyze the receiving streams. Once the models were selected and calibrated, continuous annual simulations were run to evaluate and compare the alternatives for final selection by the City. The models projected future bacteria concentrations in the receiving streams and also predicted the dissolved oxygen levels in the stream. The modeling tools supported the City’s selection of a LTCP alternative by providing projections of environmental benefits.

The City of Columbus, Ohio developed a Wet Weather Management Plan (WWMP), which addressed both Combined Sewer Overflow Long Term Control Plan (CSO LTCP) and System Evaluation and Capacity Assurance Plan (SECAP) requirements. Because both plans were being developed at the same time, potential changes in the separate system could also be accounted for in the evaluation and analysis of the combined system. This integration of the plans is important, because despite separate regulatory programs, the City’s combined sewers and sanitary sewers function as integrated components of the overall wastewater collection and treatment system.

One of the CSO LTCP focuses was receiving water modeling, which was complemented by a large characterization effort. This effort spanned over two years and resulted in the collection of over 14,000,000 data points. Of particular importance to the modeling program is the collection of continuous in-situ dissolved oxygen concentrations (DO) at 34 locations spanning across the study area (FIGURE 1). This data was integral in the calibration of the dissolved oxygen water quality model. In addition, numerous dry and wet weather discrete samples were collected from the receiving streams, wastewater treatment plants, combined sewer overflows, storm sewers, and designed sanitary relief points. These data assisted in the determination of the parameters of interest for water quality modeling purposes, and were necessary in the calibration of the models.

The parameters that were of interest for further study and modeling were dissolved oxygen and bacteria. The data revealed that there were no metals issues to be concerned about in the receiving streams; most of the samples collected were under detection limit, and certainly met water quality standards. In addition, it was also determined that nutrient loadings on the rivers were not significantly affected by the sewer system. The sampling also revealed that in general, the DO levels in the receiving streams were met applicable standards. There were exceptions to this observance, but most exceptions occurred during unique dry weather conditions that were not directly related to wet weather activity. However, when projecting future loads on the receiving waters, it is important to simulate a dissolved oxygen response in the streams; for this reason dissolved oxygen was carried into the modeling analysis. Therefore, as is often the case with a CSO LTCP, the main parameters of concern were dissolved oxygen and bacteria. These were the two parameters that were carried through the calibration process.

The modeling program consisted of three main models: a hydrologic model for generating the flows in the receiving streams, a receiving streams hydrodynamic model, and a receiving streams water quality model. In combination, these models provided the tools necessary to determine the affects that various changes in the sewer system would have on the receiving streams. The models were all run to evaluate the system response to a typical year of rainfall. With these models the City hoped to determine the potential impact that each alternative might have in the future with respect to the parameters of concern: dissolved oxygen and bacteria.

The CSO LTCP was broken into two main areas of analysis: the local combined collection system and a transport and treat system. As seen in FIGURE 2 all of the combined collection system drains to a single point, at the Whittier Street Storm Tanks (WSST). In the current system, approximately 85 percent of the annual system-wide CSO discharge occurs at the single WSST CSO. Therefore, in any abatement scenario, the majority of the CSO discharge will have to be addressed at the WSST through some combination of transport, treatment, or storage. For this reason the two components of the combined sewer system were analyzed separately, the local combined sewer system, and the transport and treatment component.

During the modeling effort one collection system model was used to evaluate the impact of all of the combined and separate sewer system upgrades. The output from this model was fed into the water quality model to assess the impact of all of the WWMP improvements (both CSO LTCP and SECAP) on the receiving streams.

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