EPA Secondary
Drinking Water Standard
(threshold odor number)

Taste and odor problems in water are often considered together. Although the most common source is rotting vegetation, many taste/odor issues are related to the disinfectants used in water treatment.

Other sources:

  • Chlorides in excess of 500 ppm produce a “salty” taste.

  • Blue green algae, depending on its quantity, can cause “grassy” and “musty” tastes and odors, as well as a “spicy” odor. Algae is also blamed for “fishy, rotten, septic, and medicinal” odors.

  • Hydrogen sulfide gas causes what is commonly called a “rotten egg smell.”

Odor is classed by the EPA as a secondary contaminant. The effects are strictly aesthetic, and no health issues have been identified. There is a classification standard called TON (Threshold Odor Number), a system of measurement based on dilutions. EPA suggests a limit of 3 TON for odors.

Water Treatment for Odor

Activated carbon adsorption is the most standard approach to taste/odor improvement. Oxidation/reduction is also used. Chlorine is only partially effective for odor improvement, and chloramine is not effective at all. Chlorine dioxide and ozone are excellent oxidizers for odor improvement. Hydrogen peroxide is very effective against hydrogen sulfide.

Image source: Wikimedia, author: “s.a.”

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