Water Treatment MCQ with answer and solution

Concept

Disinfection efficacy of chlorine against various waterborne pathogens (bacteria, viruses, and cysts/protozoa).

Step-by-step solution

1. Understand the types of pathogens targeted during water disinfection: bacteria, viruses, and protozoan cysts. 2. Recall the resistance levels of pathogens to free chlorine: bacteria and viruses are readily inactivated by standard chlorination doses. 3. Recognize that protozoan cysts and oocysts (like Cryptosporidium and Giardia) exhibit high resistance to normal chlorine concentrations and require alternative disinfectants (like ozone, UV radiation, or chlorine dioxide) or advanced filtration.

Explanation

Chlorination is highly effective against most bacteria and viruses commonly found in water. However, certain protozoan cysts (such as Giardia lamblia and Cryptosporidium oocysts) possess a thick, protective outer shell that makes them highly resistant to standard doses of chlorine disinfection under normal contact times.

Why the other options are wrong

A: Bacteria are easily destroyed by standard chlorination. B: Viruses are also effectively inactivated by chlorine disinfection. D: Incorrect because option C is correct.

Shortcut

Remember the order of resistance to chlorine: Bacteria < Viruses < Cysts/SpOREs (Cysts are hardest to kill with chlorine).

Common mistake

Assuming chlorine kills all microorganisms equally, leading students to select 'None of the above'.