Which statement best describes the difference between primary and secondary wastewater treatment?

• A. Primary treatment adds nutrients to water; secondary removes them.
• B. Primary treatment removes dissolved organics; secondary removes solids.
• C. Primary removes settle solids and floatables; secondary removes dissolved and suspended organic material.
• D. Primary uses biological processes; secondary uses chemical oxidation.

Answer: (C)

The main idea is that primary treatment relies on physical processes to remove solids, while secondary treatment uses biology to break down what remains.

In primary treatment, gravity does the work: wastewater sits in large clarifiers where heavier solids settle to the bottom and fats and oils float to the top to be skimmed off. This step reduces the amount of solid material and floatables, but it doesn’t remove most dissolved organic matter, so many dissolved substances stay in the water.

In secondary treatment, microorganisms are encouraged to grow and consume the organic matter that’s left, including both dissolved and suspended organics. This biological degradation happens in aerated tanks or similar systems and significantly lowers the organic load, producing water with much less dissolved and suspended organic material.

So the difference comes down to physical removal of solids versus biological reduction of organic material.