Chlorination – what does chlorination NOT remove in a water-treatment context? Identify the parameter that is not removed (i.e., not significantly reduced) by routine chlorination of water in conventional treatment practice.

Introduction / Context:
Chlorination is primarily a disinfection step designed to inactivate pathogenic microorganisms. It can also participate in side reactions with certain reduced species, but it is not a substitute for upstream processes that remove organic carbon or oxygen-demanding substances.

Given Data / Assumptions:

• Routine post-filtration chlorination for potable water.
• Reasonable doses used for disinfection rather than chemical oxidation of organics.

Concept / Approach:
BOD represents biodegradable organic load that exerts oxygen demand biologically. Typical chlorination doses do not appreciably reduce BOD; removal of BOD occurs primarily by coagulation/flocculation, sedimentation, and filtration, or by biological processes. Chlorine can react with free ammonia to form chloramines (thereby reducing free ammonia), and it can slightly affect measured DO via chemical reactions, but the aim is not DO removal.

Step-by-Step Solution:
Identify the principal purpose → disinfection, not organic removal.Assess each parameter: free ammonia reacts to form chloramines, organics may be partially oxidized but not reliably “removed,” DO is not a target and only marginally affected, and BOD is not substantially reduced by chlorination.Select BOD as the parameter that chlorination does not remove.

Verification / Alternative check:
Design manuals treat BOD removal upstream of disinfection; disinfectant contact tanks are sized for CT (concentration × time) to achieve microbial inactivation, not for organic oxidation.

Why Other Options Are Wrong:

• (a) Ammonia can be combined by chlorination (breakpoint chlorination concepts).
• (c) Some reactive organics may be oxidized partially, though not a controlled “removal” step.
• (d) DO is not removed as a treatment objective; any change is incidental.

Common Pitfalls:
Expecting chlorination to fix high-organic or high-BOD waters; proper clarification and filtration are required before disinfection.

Final Answer:
Biochemical oxygen demand (BOD)