Water pollution effects: The presence of non biodegradable surfactants such as alkyl benzene sulphonate (ABS) in a polluted stream most directly causes which visible or operational problem?

Introduction:
Surfactants reduce surface tension and change how bubbles form and persist. Non biodegradable detergents such as alkyl benzene sulphonate, abbreviated as ABS, were historically problematic in wastewater because they resisted biological breakdown and created conspicuous foaming. Understanding this signature effect helps diagnose pollutant types during field observations.

Given Data / Assumptions:

• ABS present in polluted surface water or wastewater.
• Aeration from turbulence or cascading flow exists.
• No advanced treatment steps removing surfactants.

Concept / Approach:
Surfactant molecules stabilize air water interfaces. Under agitation, bubbles coalesce and are prevented from collapsing rapidly, forming long lasting foam blankets. Biodegradable linear alkylbenzene sulfonates replaced early ABS to mitigate this issue. Oxygen depletion may occur indirectly due to high organic loads, but the most characteristic, immediate symptom of ABS is persistent foam formation that accumulates downstream of weirs, aerators, or outfalls.

Step-by-Step Solution:
Identify the pollutant class: non biodegradable surfactant.Recall the primary physical manifestation: stable foam.Relate field conditions: turbulence enhances foam visibility.Therefore, persistent foam is the most direct effect.

Verification / Alternative check:
Historical case studies show excessive foaming in rivers receiving detergent laden effluents before regulatory changes mandated biodegradable formulations.

Why Other Options Are Wrong:
Fire and explosion hazards are not typical for dilute surfactants. Dissolved oxygen depletion may arise from broader organic loading, not uniquely from ABS. Immediate fish kills require acute toxicity or severe hypoxia rather than surfactant presence alone.

Common Pitfalls:
Attributing any foam to ABS. Natural foams can occur from organic matter; chemical analysis is needed for confirmation.

Final Answer:
Persistent foam on the water surface