Estimating Combined Sewer System Percent Capture

Sangameswaran Shyamprasad, Khalid N. Khan and Gary Mercer


One of the criteria within the “Presumption Approach” in EPA’s CSO policy is to provide “…the capture for treatment of no less than 85 percent by volume of the combined sewage collected in the combined sewer system during precipitation events on a system-wide annual average basis.” The percent capture criterion is applicable only to a system generating and conveying combined sewage (i.e., sanitary sewage together with stormwater). Estimating this important system performance evaluation measure can be a relatively straight forward process for a simple combined system with limited number of combined sewer overflows (CSOs). However, as the size and complexity of a collection system increases, computing percent capture process can quickly become a more involved task. Further, when multiple municipalities and/or basins contribute wastewater to a single wastewater treatment plant, percent capture may need to be estimated at both system-wide and basin levels.

This paper presents a tiered approach, whereby the percent capture is estimated for each combined point of connection, for each planning basin (which comprises of multiple points of connections), and finally for the whole system (which includes the various planning basins). While the policy only requires for a certain percent capture criterion to be met on a system-wide level, the availability of percent capture results at the planning basin and point of connection can also be useful for planning appropriate level of control and determining the most benefit for dollar spent.

The CSO policy description of percent capture allows credit for the dry weather flow during a wet weather period as part of the captured flow. Thus, identifying the wet weather periods is a very important step when estimating percent capture. For the analysis discussed in this paper, a wet weather period is defined as “a reported period of time during which the system experienced wet weather flow conditions and the system inflow exceeds the system dry weather inflow by a threshold of five percent or a reported period of time when the system is experiencing an overflow event resulting from wet weather conditions.” Examples of these concepts are presented for a representative calibrated SWMM model.

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