Question

What is the purpose of adding chlorine or ozone during water treatment?

• A. To remove suspended particles
• B. To neutralize pH levels
• C. To kill bacteria and viruses
• D. To enhance taste and odor

Hint:

• Chlorine and ozone are strong oxidizing agents used as disinfectants in water treatment.
• Their main goal is to inactivate or kill pathogenic microorganisms (bacteria, viruses, protozoa) to prevent waterborne diseases.
• Ozone is generally more potent than chlorine and effective against a broader range of microbes.
• Chlorine provides a residual disinfectant effect, while ozone does not.
• Other water treatment steps handle particle removal (filtration) or pH adjustment.

Answer 1

The Correct Option is C

Chlorine and ozone are powerful oxidizing agents commonly used in water treatment primarily for disinfection. Disinfection is the process of inactivating or killing pathogenic (disease-causing) microorganisms, such as bacteria, viruses, and protozoa, in the water to make it safe for consumption or other uses.

• Chlorine (Cl$_2$, hypochlorite):
  • Mechanism: Chlorine compounds (like hypochlorous acid, HOCl, formed when chlorine dissolves in water) damage microbial cell membranes, enzymes, and nucleic acids, leading to cell death.
  • Advantages: Effective against a broad range of pathogens, provides a residual disinfectant effect in the distribution system (protecting against recontamination).
  • Disadvantages: Can form disinfection byproducts (DBPs) like trihalomethanes (THMs) if organic precursors are present; some pathogens (e.g., Cryptosporidium) are chlorine-resistant.
• Ozone (O$_3$):
  • Mechanism: Ozone is a very strong oxidant that rapidly damages and destroys microbial cell walls and other cellular components.
  • Advantages: More effective than chlorine against a wider range of pathogens, including chlorine-resistant ones like Cryptosporidium and Giardia. Also effective for oxidizing and removing taste, odor, and color compounds, and some organic micropollutants. Reduces DBP formation compared to chlorine.
  • Disadvantages: More expensive, unstable (short half-life), so it does not provide a lasting residual disinfectant in the distribution system (often requires secondary disinfection, e.g., with chloramine).

Option (a) To remove suspended particles: This is typically achieved by processes like coagulation, flocculation, sedimentation, and filtration. Option (b) To neutralize pH levels: pH adjustment uses acids or bases, not primarily chlorine or ozone, though chlorination can slightly affect pH. Option (d) To enhance taste and odor: While ozone is effective at this, and chlorine can sometimes improve or worsen taste/odor, their primary purpose is disinfection. For chlorine, taste/odor can be a negative side effect (chlorinous taste). Therefore, the primary purpose of adding chlorine or ozone is to kill bacteria and viruses (and other pathogens). \[ \boxed{\text{To kill bacteria and viruses}} \]