Even in United States, where water is highly treated before supply to homes, tap water is not always safe. A 2010 report stated that 31 out of 35 cities had carcinogenic material, which came via Industrial pollution.
Emerging countries on the other hand, are far behind in controlling the industrial waste and sewage to mix into rivers and other sources of water.
It is obvious that water, once polluted, cannot be treated completely.
Instead of allowing sewage and waste to first go into all rivers and then treating the same water for drinking, It may be a better to designate some of the rivers and water sources as sources of drinking, thereby not allowing any sewage or pollutants to go into these sources. The pollutants would only be allowed to go into other rivers.