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
The concentration in the outflow is calculated as the resident concentration, reduced by the specified percentage.
Cout = Cin * (1 - PercentageRemoval)
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)]