What is the physical treatment where solids settle out of wastewater called?

• A. Secondary sewage treatment
• B. Tertiary sewage treatment
• C. Final chemical treatment
• D. Primary sewage treatment

Answer: (D)

The main idea here is the physical removal of settleable solids from wastewater. Primary sewage treatment does exactly that: wastewater flows into large settling tanks where gravity lets heavier solids sink to the bottom as sludge and lighter materials rise to the surface as scum, which can be removed. This step focuses on physically separating solids from the water, reducing the amount of suspended material before any biological or chemical treatment happens.

This is why it’s the best answer: it describes a process driven by gravity to settle solids, rather than biological degradation or chemical polishing. Secondary treatment relies on microorganisms to break down dissolved organic matter, and tertiary treatment provides further polishing, often with additional filtration or chemical processes. A final chemical treatment isn’t a standard stage in this sequence.