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For percentage removal, the concentration (C) within the drainage system is given by the routing method. There are two sub-methods available for percentage removal: with, or without background concentration. 

The sub-methods outlined below use the following variables:

C = pollutant concentration

Cout = pollutant concentration in the outflow

Cin = pollutant concentration in the inflow

PercentageRemoval = the specified percentage by which the concentration in the outflow is reduced

Percentage Removal Without Background Concentration

The concentration in the outflow is calculated as the resident concentration, reduced by the specified percentage. 

 Cout = Cin * (1 - PercentageRemoval)

Percentage Removal With Background Concentration

Realistically some pollutant concentrations are not reduced to zero. In such cases, users may specify a background (minimum) concentration (Cback) below which the pollutant concentration will not be reduced.

The method is the same as for Percentage removal without background concentration, except for the inclusion of the background concentration variable. 

Cout =  Cback + [(Cin - Cback) * (1 - PercentageRemoval)]