Breakpoint chlorination in drinking-water treatment During breakpoint chlorination, chlorine is added in an amount sufficient to achieve which primary objective in the bulk water?

Introduction / Context:
Breakpoint chlorination is a standard practice in potable water treatment. As chlorine is incrementally dosed, it reacts with reduced species in the water before a free residual appears. Recognizing what is consumed prior to the “breakpoint” is key to selecting the correct dosage and interpreting residual-chlorine curves.

Given Data / Assumptions:

• Source water may contain ammonia (NH3/NH4+), organic nitrogen, and other oxidisable organics.
• Chlorination proceeds under typical plant pH and contact time.
• We refer to classical breakpoint behavior in bulk water, not specialized pathogen inactivation goals.

Concept / Approach:
Initially, chlorine reacts with reducing agents and forms combined chlorine species (e.g., monochloramine, dichloramine). Continued addition oxidizes chloramines and other organics until these demands are met. At the “breakpoint,” further addition results in a rise of free chlorine residual. Thus, the key objective up to breakpoint is satisfying the demand imposed by ammonia and readily oxidisable organic matter.

Step-by-Step Solution:
1) First dose: chlorine consumed by ferrous iron, sulfide, and easily oxidised organics; residual remains near zero.2) Intermediate dose: ammonia reacts → formation then oxidation of chloramines; combined residual peaks then declines.3) At breakpoint dose: oxidisable nitrogenous and organic demands are satisfied.4) Beyond breakpoint: free chlorine residual increases linearly with dose, indicating disinfection reserve.

Verification / Alternative check:
Residual vs. dose curves show the characteristic “valley” at breakpoint, after which free chlorine appears—confirming the objective has been met.

Why Other Options Are Wrong:
Giardia cyst inactivation needs much higher CT and is not the defining breakpoint objective; inorganic matter reactions occur but the primary limiting demand is ammonia/organics; filter bacterial growth control is a downstream effect, not the breakpoint purpose.

Common Pitfalls:
Equating breakpoint dose with universal pathogen kill; ignoring pH/temperature effects on chloramine speciation; misreading combined vs. free residuals.

Final Answer:
react with any ammonia and readily oxidisable organic matter