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>.
- Ozone disinfection, and removal
- Tolt river supply only: water conditioning by filtering through "granular media." (Cedar river supply is clear enough without this step.)
- UV disinfection
- pH adjustment to avoid corroding pipes
- Flouridation for public health
- Chlorination as a final step as water leaves the treatment plant, and also at some downstream facilities (like a networking repeater; just to maintain chlorine levels that would otherwise have fallen due to distance from the upstream chlorination site)
Tolt: http://www.seattle.gov/utilities/your-services/water/water-s...
Cedar: http://www.seattle.gov/utilities/your-services/water/water-s...
EPA mandates a floor of 0.2 mg/L chlorine for all Surface Water based drinking water supplies at all times[0]. There are additional chlorine requirements depending on what sort of filtration you perform, if any, and how far the first service connection is from the chlorine insertion, in minutes. (They also mandate a safety ceiling of 4 mg/L for all drinking water.) This level is continuously monitored.
Seattle does about 1 mg/L to meet these EPA-imposed requirements.[1]
Chlorine evaporates out of water, so if you don't like the taste, you can just let tap water sit a while. Sunlight helps. Boiling water (e.g., for tea) also removes most of the chlorine.
[0]: (PDF) https://www.epa.gov/dwreginfo/swtr-plain-english-guide
[1]: (PDF, p. 8) https://www.seattle.gov/Documents/Departments/SPU/Services/W...