Weird. Here in Germany we have some protected watershed areas on smaller rivers that directly feed a surface reservoir, created in the river valley through a dam.
They get filtered, but there is no chlorine directly. Some chlorine dioxide is used at the end, though. Here's the official description of the utility, translated to english:
- Via a raw water pumping station, the dam water first reaches the micro-screening plant. It removes coarse contaminants over 35 ~ µm in diameter through stainless steel mesh filters. This provides special safety in times of mass algae growth or during floods.
- Subsequently, the raw water is de-stabilized with a flocculant; and turbid matter accumulates to form large flocs.
- In filter stage 1, two filter materials of different coarseness are used to remove the flocs.
- Ozone is then added to disinfect the raw water.
- Filter stage 2 is equipped with activated carbon and frees the raw water from the reaction products of ozonation. Excess ozone reacts to form oxygen and is thus removed from the raw water.
- The further filter stage 3 uses natural limestone material over which the water flows. Here the excess carbonic acid in the water is removed.
Finally, a small protective disinfection with chlorine dioxide takes place before the drinking water leaves the clean water tank in the direction of <city>.