You do not know a lot about collecting rain water do you.
First, there are water tanks made of a special plastic for drinking water !
Second, yes one is able to install rainwater filters quite easily. Back in Queensland i did it for 30 years and the set up is still in use. Why all the rain water is run into the ground here and lost, beats me ! Despite the amount of rain water that comes down here on Samui, it will not be long before everyone is crying " we won't have enough water left soon" it happens every year.