Hydrogen sulfide is a gas that causes the noticeable "rotten egg" smell in feed waters, with a threshold odor level of 0.1 ppm and a noticeable offensive odor at 3-5 ppm. Hydrogen sulfide is readily oxidized to elemental sulfur by oxidants (e.g. air, chlorine or potassium permanganate). Sulfur acts as a colloidal foulant and has a history of not being removed well by conventional multimedia filtration. The preferred RO system design suggests leaving the hydrogen sulfide in its gaseous form, let it pass through the RO into the permeate, and then treat the permeate for its removal.