Which statement best describes primary, secondary, and tertiary treatment in water or wastewater?

• A. Primary removes solids by physical means; secondary biodegrades organics; tertiary polishing and disinfection for final quality.
• B. Primary disinfection; secondary filtration; tertiary disposal.
• C. Primary chemical oxidation; secondary aeration; tertiary mixing.
• D. Primary chlorination; secondary adsorption; tertiary crystallization.

Answer: (A)

Treat wastewater or water treatment is done in stages: first remove solids by physical means, then use biology to break down remaining organics, and finally polish the water and disinfect to reach final quality.

In the first stage, solids are separated through physical processes like screening and gravity settlers. This step reduces the amount of solids and the load on the next stage, making biological treatment more efficient.

In the second stage, microorganisms biologically degrade the remaining dissolved and suspended organic matter. This is the main cleanup step for organic pollution, typically done in aerated biological reactors (like activated sludge) or biofilm systems, where oxygen is supplied to support microbial metabolism.

In the final stage, additional treatment removes residual fine particles and pathogens to meet discharge or drinking-water standards. This polishing often includes filtration and disinfection, and may involve nutrient removal in advanced cases.

That sequence—primary solids removal, secondary biological degradation, then tertiary polishing and disinfection—best matches the described approach. The other options mix up which processes belong to which stage or suggest nonstandard steps, so they don’t fit typical water or wastewater treatment practice.