Primary treatment unit operations: In municipal sewage treatment, the function of a skimming tank is to remove which category of materials from the wastewater surface before downstream processing?

Introduction / Context:
Primary treatment aims to remove easily separable materials from raw sewage to protect downstream biological units. Skimming tanks (or grease traps) specifically target floatable matter that accumulates at the water surface and can foul equipment or cause odors.

Given Data / Assumptions:

• Unit placed upstream of biological treatment and often adjacent to primary clarifiers.
• Influent contains soaps, fats, oils, and grease from kitchens and commercial sources.
• Mechanisms available include gravity separation, scum baffles, and mechanical skimmers.

Concept / Approach:
Oil and fats are less dense than water and tend to agglomerate and float, forming a surface scum. A skimming tank provides quiescent conditions so these materials can collect and be removed mechanically. This improves aesthetics, reduces oxygen demand, and prevents interference with aeration systems and digesters.

Step-by-Step Solution:
Identify the physical property exploited: density difference leading to flotation.Relate to equipment feature: baffles and skimmers that capture floating scum.Choose the option stating “Oil and fatty substances.”

Verification / Alternative check:
Plant process flow diagrams show grease removal stages labeled as skimming or scum removal, validating the intended function.

Why Other Options Are Wrong:
Dissolved solids: require processes like ion exchange or RO; not removed by skimming.Suspended mineral solids and grit: removed in primary clarifiers and grit chambers via settling, not skimming.Dissolved gases: managed by aeration/stripping, not skimming.

Common Pitfalls:
Confusing skimming with sedimentation; one targets floatables, the other settles heavy particles.

Final Answer:
Oil and fatty substances (grease and scum)