What is the purpose of primary and secondary wastewater treatment, and which pollutants are primarily removed at each stage?

• A. Primary treatment removes settleable solids and associated organic matter; secondary treatment removes dissolved organic matter via biological processes and reduces BOD/COD.
• B. Primary treatment removes dissolved minerals; secondary treatment removes suspended solids.
• C. Primary treatment targets nutrients and pathogens; secondary treatment targets heavy metals.
• D. Primary treatment uses chemical precipitation; secondary uses filtration.

Answer: (A)

The main idea is how wastewater is split into what you can physically remove versus what biology can digest, and what pollutants each stage tackles.

Primary treatment focuses on removing settleable solids by letting them settle out by gravity and through screening. This captures the solids and any organic matter attached to them, so the process reduces suspended solids and the associated organic load before the water moves on. It doesn’t target dissolved minerals or dissolved organic compounds; those remain largely in the water.

Secondary treatment then uses biological processes to break down the dissolved organic matter that stayed after primary treatment. Microorganisms aerobically (and sometimes anaerobically) digest the organic compounds, which lowers the biochemical oxygen demand (BOD) and, to a large extent, the chemical oxygen demand (COD) as well. This stage is about reducing the biodegradable portion of pollution, not about removing minerals or heavy metals.

The other options mix up what gets removed where: dissolved minerals aren’t the primary target of first-stage treatment, suspended solids aren’t the main focus of secondary treatment, nutrients or pathogens aren’t the primary targets of primary treatment, heavy metals aren’t specifically handled in secondary treatment, and chemical precipitation or filtration aren’t the defining methods described for these stages.