Dear Shakeel Ahmed Talpur I'm not an expert in this field enough to give you a qualified answer to your interesting question. However, there are various useful articles about this topic available. The following relevant literature references might help you in your analysis:
Quantitative Microbial Risk Assessment of Drinking Water Quality to Predict the Risk of Waterborne Diseases in Primary-School Children
Article Quantitative Microbial Risk Assessment of Drinking Water Qua...
and
Microbial Risk Assessment of Pathogens in Water
Chapter Microbial Risk Assessment of Pathogens in Water
The good thing is that both papers have been posted by the authors as public full texts on RG. Thus you can freely download them as pdf files.
I hope this helps. Good luck with your work and best wishes!