It depends if you have dry ...

It depends if you have dry or wet weather flow. If it is dry weather flow, you can use approach from numbers of connected inhabitant (per-capita approach as mentioned previously by Rakesh Mehrotra. You can then estimate the average slope and cross section to approximate the dry weather flow. The concentration of contaminant in dry weather flow does not depend on the flow (since there is no dilution from external sources such as rain water) but depends on possible chemical reaction which takes place along the drainage channel especially if it is an open channel. You have to measure it in the WWTP inlet.

If it is wet weather flow, it will be more difficult since you have hydrological parameter inputs. You need to have the rainfall input and rainfall-runoff model. One model I know is EPA-SWMM which is built particularly for stormwater modeling. The concentration of contaminant also depends on dilution rate (amount of storm water flow) if you have wet weather flow.