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