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Water Lecture 3 A. Disinfection B. Water Safety Plans - page 23 / 47

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Free Chlorine - Background and History

Considered to be first used in 1905 in London

But, electrochemically generated chlorine from brine (NaCl) was first used in water treatment the late 1800s

Reactions for free chlorine formation:

Cl2 (g) + H2O <=> HOCl + H+ + Cl-

HOCl <=> H+ + OCl-

Chemical forms of free chlorine: Cl2 (gas), NaOCl (liquid), or Ca(OCl)2 (solid)

Has been the “disinfectant of choice” in US until recently.

recommended maximum residual concentration of free chlorine < 5 mg/L (by US EPA)

Concerns about the toxicity of free chlorine disinfection by-products (trihalomethanes and other chlorinated organics)

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