In wastewater treatment trains, “advanced treatment” (tertiary or higher) is generally employed to: (Select the best description of its purpose beyond primary and secondary treatment.)

Introduction:
Advanced (tertiary) treatment targets contaminants not adequately removed by primary clarification and secondary biological processes. This question tests recognition of the goal of higher-level polishing steps in a treatment plant.

Given Data / Assumptions:

• Primary removes coarse and settleable solids; secondary reduces BOD and some nutrients.
• Advanced steps include nutrient removal, filtration, adsorption, and advanced oxidation.
• Effluent quality requirements drive process selection.

Concept / Approach:
“Additional objectionable substances” covers nutrients (N, P), fine suspended solids, refractory organics, microconstituents, and pathogens—depending on permit limits. Technologies include nitrification–denitrification, biological/chemical phosphorus removal, membranes, activated carbon, and UV/ozone.

Step-by-Step Solution:
Map each treatment stage to its primary function. Recognize that advanced treatment is beyond bulk BOD/solids removal. Identify tertiary objectives: nutrient polishing, turbidity reduction, microcontaminants. Choose the option that most generally and accurately captures these aims.

Verification / Alternative check:
Discharge permits often require total nitrogen/total phosphorus caps or very low turbidity/pathogen levels achievable only with advanced processes.

Why Other Options Are Wrong:

• Remove coarse/settleable solids: Handled in preliminary/primary stages.
• Reduce BOD: Principal role of secondary biological treatment.

Common Pitfalls:
Equating “advanced” with any treatment; it specifically denotes polishing beyond conventional secondary.

Final Answer:
remove additional objectionable substances is the purpose of advanced treatment.