Which method best removes fine and dissolved solids that are not easily removed by sedimentation or mechanical filtration?

• A. Buffering system
• B. Rotating biological contractor
• C. Foam fractionation
• D. None of the above

Answer: (C)

Fine and dissolved solids that don’t settle out or get trapped by filters are often dissolved organic compounds and colloids. Foam fractionation uses small air bubbles to grab these surface-active substances as they rise with the foam. The dissolved materials adsorb onto the bubble surfaces, the bubbles coalesce into a foam layer, and this foam is skimmed off. By continually removing this foam, the water is stripped of those fine, hard-to-remove solids, improving clarity and overall water quality. This direct mechanism targets dissolved and colloidal material that sedimentation and mechanical filtration miss, making it the most effective choice for this purpose.

A buffering system just controls pH and won’t remove solids. A rotating biological contractor relies on microbial breakdown of waste but isn’t designed to remove dissolved solids in the same targeted way as foam fractionation.