I worked as an extraction metallurgist in semi desert mines and we used an aluminium based flocculant (alum) to clarify water in dams with rotating stirrers.
Quite an old concept and not expensive.
Obviously it doesn't provide total purity, but it was a lot better than doing nothing.
Apparently even the ancient Egyptians knew about this: https://en.wikipedia.org/wiki/Potassium_alum#In_antiquity