Preliminary treatment of sewage — which operations occur at this stage? Identify the correct grouping of unit operations typically provided before primary treatment.

Introduction / Context:
Preliminary treatment protects downstream units by removing materials that would cause wear, clogging, or operational instability. It typically includes screening, grit removal, and oil/grease separation before sedimentation or biological steps. Recognizing these components is essential for design and troubleshooting of wastewater facilities.

Given Data / Assumptions:

• Sewage contains rags, plastics, sand/grit, floatables (oils, grease), and organic matter.
• Objective: protect pumps, clarifiers, and biological reactors.
• Conventional municipal plant layout.

Concept / Approach:

Each listed operation targets a specific contaminant class: screening for coarse floatables, grit chambers for mineral particles with high settling velocities, and skimming for oils/grease that float. Grouping these together describes the preliminary train accurately.

Step-by-Step Solution:

Verification / Alternative check:

Standard process flow diagrams in design manuals show screens → grit chambers → skimming prior to primary sedimentation.

Why Other Options Are Wrong:

(a), (b), and (c) each describe only part of preliminary treatment; the comprehensive correct choice is (d).

Common Pitfalls:

Confusing preliminary with primary treatment (sedimentation), or placing oil skimming only in primary clarifiers; overlooking maintenance needs of screens and grit systems.

Final Answer:

All the above.