Part of the treatment process is the addition of chlorine dioxide which is used as a disinfectant. When the chlorine is added it tends to regenerate some of the chlorine dioxide. The higher the levels the more frequent potential for odors.
Chlorine dioxide is a very good disinfectant, but it exists as a gas.
When you turn on your tap, the chlorine dioxide tends to diffuse into the air as the water is running. Studies have not identified any health concerns associated with this. The strongest odor problems are usually associated with the VOCs in new materials such as new carpet, paint or upholstery. The problem will last until the level of VOCs decreases. This can take from several hours to a few weeks, and occasionally up to several months, to dissipate depending on the situation, type of materials, amount of ventilation, etc.