Yes. We can substitute the use of potable water by the rainwater without any treatment. The average calculations and applications of rainwater without further treatment includes
For Drinking & Cooking - 10%
For toilet flush - 50%
For laundry - 18%
For cleaning and car wash - 10%
Miscellaneous - 12%
Thus 100 % of the potable water needs can be substituted and conserved by the rainwater