Operational uses of chlorine in wastewater treatment Chlorine is sometimes applied within activated sludge systems primarily to:

Introduction / Context:
Filamentous bulking is a common operational problem in activated sludge plants, characterized by poor settleability and high sludge volume index. Selective chlorination is one process control technique used to suppress filamentous organisms and restore good floc structure.

Given Data / Assumptions:

• Conventional activated sludge process with secondary clarifiers.
• Bulking driven by excessive growth of filamentous bacteria.
• Chlorine application targeted to return activated sludge (RAS) or mixed liquor under controlled doses.

Concept / Approach:
Low-dose chlorine selectively damages filamentous organisms protruding from flocs more than the floc-formers embedded within. By dosing RAS lines or contact points, operators reduce filament density and improve settling without sterilizing the biomass.

Step-by-Step Solution:
1) Diagnose bulking via microscopic examination and SVI trends.2) Apply controlled chlorination to RAS to selectively target filaments.3) Monitor effluent quality and clarifier blanket to confirm improved settling.4) Adjust aeration, F/M ratio, and nutrients to prevent recurrence.

Verification / Alternative check:
Plants report transient increases in residual chlorine at targeted points along with improved sludge compaction and lower SVI after successful anti-bulking chlorination.

Why Other Options Are Wrong:
Chlorine does not promote flocculation avoidance (a); it reduces biological activity rather than increasing it (b); grease separation (d) is better tackled by primary treatment and skimming rather than chlorination.

Common Pitfalls:
Over-chlorination harming nitrifiers; ignoring root causes like low DO, nutrient imbalance, or septicity; failure to monitor residuals to avoid effluent toxicity.

Final Answer:
avoid bulking of activated sludge