Which best describes the primary, secondary, and tertiary wastewater treatment levels?

• A. Primary uses physical processes to remove settleable solids; Secondary uses biological processes to degrade dissolved organics; Tertiary uses advanced techniques to remove nutrients, pathogens, and persistent contaminants.
• B. Primary uses biological processes; Secondary uses chemical precipitation; Tertiary uses evaporation.
• C. Primary uses chemical disinfection; Secondary uses filtration; Tertiary uses direct reuse.
• D. Primary uses nutrient removal; Secondary uses solids removal; Tertiary uses chlorination.

Answer: (A)

Wastewater treatment uses progressive steps, each targeting different types of contaminants and needing more advanced processes as you go. The first stage relies on physical means—screening to remove solids, grit removal, and sedimentation—to pull out settleable solids and floatables. The goal here is to make the flow clearer and reduce the load before biological work, not to scrub out dissolved organics.

In the second stage, biology takes over. Microorganisms break down dissolved organic matter, transforming it into energy, cells, and simpler compounds. This is the essence of secondary treatment, where the bulk of organic pollution (the organics that cause biological oxygen demand) is degraded through activated sludge systems, trickling filters, or similar biotreatment processes.

The third stage looks to remove remaining and hard-to-treat pollutants—nutrients like nitrogen and phosphorus, pathogens, and other persistent contaminants. This tertiary level uses advanced techniques such as chemical precipitation or biological nutrient removal, filtration, disinfection (chlorination or UV), and sometimes membrane processes or adsorption to meet strict discharge standards or enable water reuse.

That description fits the option that assigns primary treatment to physical removal of settleables, secondary treatment to biological degradation of dissolved organics, and tertiary treatment to advanced methods for nutrients, pathogens, and persistent contaminants. Other descriptions swap in methods that aren’t the defining processes of these stages (for example, treating primary with disinfection, or using evaporation as a primary or tertiary method), which makes them inconsistent with how wastewater treatment is actually structured.