Sand filtration and ...

Sand filtration and chlorination should work in most cases unless the river water has toxic chemical pollutants i.e., ammonia, heavy metals, pesticides and/or fertilisers. In the latter case, try rainwater harvesting. The capital costs of sand filtration + chlorination aren't astronomical. A lot depends on how you plan to distribute water as pipelines cost more than the treatment plants themselves. If you are distributing treated water you need a centralised facility but otherwise household or small community sized treatment systems using the same combination are possible at low cost.