Sewage treatment – fine grit removal Fine grit present in sewage is removed in which unit operation during preliminary treatment?

Introduction / Context:
Preliminary treatment in wastewater plants protects downstream equipment by removing coarse and abrasive materials. Grit causes wear in pumps and reduces digester volume if not intercepted. Depending on particle size and specific gravity, different units are adopted to remove sand and fine mineral matter effectively.

Given Data / Assumptions:

• “Fine grit” refers to smaller, more difficult-to-settle mineral particles than those captured in standard grit chambers.
• Conventional municipal plant; no special high-rate devices assumed.
• Aim is to identify the unit commonly specified for finer grit capture.

Concept / Approach:
Standard horizontal-flow or aerated grit chambers target typical grit sizes but may pass the finer fraction. A detritus tank (sometimes called a fine-grit chamber) is designed with controlled velocities and detention to enhance removal of finer mineral particles. Trickling filters and primary clarifiers serve biological and sedimentation roles for organics, not specific abrasive grit capture, and skimming tanks remove floatables like grease and oil rather than heavy grit.

Step-by-Step Solution:

Verification / Alternative check:
Plant layouts in design texts show detritus tanks downstream of coarse screening and standard grit chambers when fine grit is a concern (e.g., sandy sewers).

Why Other Options Are Wrong:

• Grit chamber: Removes coarser grit; less effective for fine fractions targeted by a detritus tank.
• Trickling filter: Biological treatment, not abrasive solids removal.
• Skimming tank: Removes floatables, not heavy grit.
• Primary clarifier: Settles organics; not optimised for fine gritty minerals.

Common Pitfalls:
Assuming a generic “grit chamber” removes all sizes; in practice, fine grit often requires a dedicated detritus tank or advanced grit removal technology.

Final Answer:
Detritus tank