GATE 2023 CE (CE2) – Question 29

Water and Waste Water Quality and Treatment · Unit processes and operations · 1 mark · Multiple select

Which of the following is/are NOT active disinfectant(s) in water treatment?

Show answer and explanation

Correct answer: (D) $Cl^-$ (chloride ion)

Explanation

A disinfectant must be able to kill or inactivate pathogens by oxidation or another active mechanism.

**•OH (hydroxyl radical):** an extremely strong oxidant, used in advanced oxidation processes. Active.
**O₃ (ozone):** a strong oxidant widely used for disinfection. Active.
**OCl⁻ (hypochlorite):** the free chlorine species formed when chlorine or bleach is added to water, a disinfectant (weaker than HOCl). Active.
**Cl⁻ (chloride):** the fully reduced, stable ion. It has no oxidising power and does not disinfect.

The one that is **not** a disinfectant is chloride, option **D**.