Distinguish between primary treatment and secondary treatment in wastewater treatment.

• A. Primary treatment removes settleable solids via physical sedimentation; secondary uses biological processes to degrade dissolved and colloidal organics
• B. Primary uses biological processes; secondary removes solids by sedimentation
• C. Primary involves chemical precipitation; secondary uses filtration
• D. Primary is optional; secondary is required by law for all plants

Answer: (A)

The key idea here is that wastewater treatment uses two distinct stages to tackle different kinds of pollutants. Primary treatment is all about physical separation: large, settleable solids are removed by gravity in a clarifier, and oils, greases, and grit are captured or skimmed out. This is a mainly physical step that reduces the amount of solid material and some organic load, but it doesn’t do much to dissolved substances.

Secondary treatment picks up where primary leaves off by using biological processes. Microorganisms are encouraged to break down the remaining organic matter that is dissolved or present as very fine particles. This is typically done with aerated systems or biofilm reactors, where bacteria metabolize the dissolved and colloidal organics, drastically reducing biological oxygen demand.

So the description that matches is: primary treatment removes settleable solids through physical sedimentation, while secondary treatment uses biological processes to degrade dissolved and colloidal organics. The other ideas—treating primarily with biological means, removing solids by sedimentation in secondary, using chemical precipitation in primary, or labeling one stage as optional—don’t align with how these two stages are conventionally defined.