Rain, collected from the roof and stored in tanks is a major source of drinking water and since it is generally pleasant and of excellent quality there is no reason for this not to continue.
There has always been an understanding that the water tank would have its own resident population of “bugs” but that regular drinkers of the water would be able to cope with them. However most people drinking tank water recognise that visitors used to treated water would often develop a “funny tummy” from the tank supply.
Well it is worse than that. The bacteria population in many water tanks can be very high. The almost invariably includes coliforms, sourced form faecal matter (from birds, possums, rats etc), particularly E. Coli which should not be present in drinking water at all.
In addition cysts such as Giardia and Cryptosporidium also infect birds and mammals so they can also be found in rain water tanks.
The Salmonella STM160 strain ins spread by birds, contaminates rain water supplies and is a source of human infection. This is worse than a funny tummy and the hospitalisation rate is high and 15%, it is a wide spread and genuine problem.