Mamta, If you have access ...

Mamta,

If you have access to water meter data for the Area of Interest, you can calculate the domestic sewer coming out of them based on the MLD that they use, which will just be a portion of the 120 MLD since the other part would be storm water. 

Another possible way  would be to evaluate average daily precipitation, substract calculated (or estimated) percolation and evaporation and you would have an estimate of your storm water, the other portion of the 120 MLD would be your domestic sewer.

I hope that helps.

Miguel