Under the Stage 1 Disinfectants and Disinfection Byproducts Rule, which compound has an MCL of 1.0 mg/L?

• A. Chlorite
• B. Chlorine
• C. Chloroform
• D. Chloramine

Answer: (A)

Stage 1 sets enforceable limits on certain disinfectants and disinfection byproducts to control health risks while still ensuring effective water treatment. Chlorite has an MCL of 1.0 mg/L because it forms as a byproduct when chlorine or chloramine reacts with natural organic matter, and keeping its concentration around 1.0 mg/L reduces potential health effects without compromising disinfection too much.

The other substances aren’t at 1.0 mg/L for their MCL under this rule: chlorine and chloramine are regulated by maximum residual disinfectant levels (MRDLs) of about 4.0 mg/L, not MCLs. Chloroform is a disinfection byproduct in the total trihalomethanes group, which has a much lower limit (around 0.08 mg/L) rather than 1.0 mg/L.