Chlorine Demand is exerted by which substances?

• A. Ferrous ions, nitrites, hydrogen sulfide, and various organic molecules exert a demand for oxidizing disinfectants such as chlorine.
• B. Ammonia
• C. Hydrogen peroxide
• D. Sodium chloride

Answer: (A)

Chlorine demand is the chlorine that gets consumed by reactions with substances in water, before any free chlorine can remain to disinfect. It arises from compounds that chlorine can oxidize or react with, acting as reducing agents. Ferrous ions, nitrites, hydrogen sulfide, and various organic molecules fit this role because each is readily oxidized by chlorine, turning into ferric iron, nitrates, sulfates or sulfur, and oxidized organic products, respectively. This consumption reduces the amount of chlorine available for disinfection. Ammonia can also react with chlorine, but that forms chloramines and is typically treated as a separate consideration from the general chlorine demand caused by oxidizable inorganic and organic substances. Sodium chloride and hydrogen peroxide don’t create the same free-chlorine-demand effect in the water-treatment sense.