---
title: "Percentage Removal"
canonical: "https://help.innovyze.com/space/infodrainage2021v1/16548093/Percentage%20Removal"
format: markdown
---
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: [Percentage Removal With](#PercentageRemoval-PercentageRemovalWith), or [Percentage Removal Without](#PercentageRemoval-PercentageRemovalWithout) background concentration.
The sub-methods outlined below use the following variables:
*C* = pollutant concentration
*C**out* = pollutant concentration in the outflow
*C**in* = pollutant concentration in the inflow
*PercentageRemoval* = the specified percentage by which the concentration in the outflow is reduced
## > Macro (anchor)
Percentage Removal Without Background Concentration
The concentration in the outflow is calculated as the resident concentration, reduced by the specified percentage.
* C**out = **C**in** * (1 - PercentageRemoval)*
## > Macro (anchor)
Percentage Removal With Background Concentration
Realistically some pollutant concentrations are not reduced to zero. In such cases, users may specify a background (minimum) concentration (*C**back*) 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.
*C**out **= C**back **+ [(C**in **- C**back**) * (1 - PercentageRemoval)]*