The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for estimating direct runoff or infiltration from rainfall-excess. The method is simple to use and requires basic descriptive inputs that are converted to numeric values for estimation of direct runoff volume from the watershed. CN that is descriptive of runoff potential of watershed which is widely preferred by hydrologists, engineers and watershed managers as an independent simple watershed model, as well as the runoff estimation component in many complex watershed models such as AGNPS, and SWAT. Recently, Mishra et al. and Ojha illustrated the SCS-CN analogy in metal partitioning and water quality modeling of River Bank Filtration (RBF) system, respectively. An example of such an engineering index or parameter is the Curve Number (CN), there also exists a need for an improved alternative to the exponentially decaying function describing a first-order process, which is commonly assumed. An example is the concept of the Soil Conservation Service Curve Number method (Soil Conservation Service 1956), which is a generalization of the first-order rainfall-runoff process. These works indicate the potential of CN approach in different wastewater treatment systems. Therefore, one of the objectives of the present work is to explore the use of CN approach in describing the removal of various water quality parameters. The paper begins with a background on CN approach and develops a link between kinetic coefficient, residence time, and CN.

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