Plain chlorination in water treatment In public health engineering, “plain chlorination” refers to dosing disinfectant without prior coagulation–sedimentation or filtration. It is typically adopted for which types of supplies/situations?

Introduction / Context:
Plain chlorination is a minimal-treatment strategy used in water supply engineering when raw water is already quite clear or when time and infrastructure are limited. It focuses solely on microbial safety by adding chlorine without the preceding clarification steps of coagulation, flocculation, and sedimentation.

Given Data / Assumptions:

• Turbidity threshold for effective primary disinfection is relatively low.
• Clear natural sources and temporary/emergency arrangements often rely on rapid solutions.
• Goal is pathogen inactivation, not removal of colour/turbidity/organics.

Concept / Approach:
Chlorine is highly effective against bacteria and many viruses, provided adequate contact time and a reasonable chlorine demand. High turbidity shields microorganisms, increases chlorine demand, and risks by-product formation; thus plain chlorination is best for low-turbidity sources or urgent deployments where maintaining a residual is the priority.

Step-by-Step Solution:

Verification / Alternative check:
Standard texts recommend turbidity generally below about 20 NTU (historical mg/L as silica convention) for reliable disinfection by chlorination alone; otherwise prior clarification is advised.

Why Other Options Are Wrong:
The individual options A–D are not wrong; they are each subsets of the correct comprehensive scenario, hence E is the correct selection.

Common Pitfalls:
Assuming chlorination alone will handle high turbidity or organic colour; neglecting contact time and residual monitoring; overlooking taste/odour issues in highly coloured waters.

Final Answer:
All the above