Liquid–solid separation fundamentals: when the goal is to remove trace suspended solids to produce a clear overflow, which unit process is typically selected as the primary polishing step?

Introduction / Context:
Clarification and filtration are core strategies for liquid–solid separation. When suspended solids are present at low concentration and a clear overflow is required, gravity settling aided by coagulation–flocculation is often the first-choice step before any final polishing filters.

Given Data / Assumptions:

• “Trace” suspended solids implies low solids loading.
• We want a primary unit that can continuously produce clear overflow.
• No special constraint on filter media or cake handling is stated.

Concept / Approach:
A clarifier (sometimes with tube settlers or lamella plates) gives quiescent conditions where flocculated particles settle. Sludge underflow is withdrawn, while clarified liquid overflows. For ultraclear specifications, downstream cartridge/sparkler filters may be added, but the baseline polishing step is clarification.

Step-by-Step Solution:
Match “trace solids” to gravity clarification as the primary continuous process.Recognize classifiers separate solids by size in slurries, not polish clear liquids.Rule out batch cake filters when continuous clear overflow is the main goal.

Verification / Alternative check:
Water and wastewater practice, as well as many process flowsheets, place clarifiers before any final filters to reduce loading, operating cost, and fouling in downstream units.

Why Other Options Are Wrong:
Classifier: hydrodynamic sizing device; not a polishing clarifier.Sparkler and rotary vacuum filters: filtration devices used when cake formation or very fine polishing is required; not the primary method for “trace” removal in continuous flow.

Common Pitfalls:
Confusing clarification with filtration. Both may appear in series, but clarification usually precedes filtration to lower fouling and cost.

Final Answer:
Clarifier