The sedimentation tank (primary clarifier) removes which type of solids from sewage?
Choose the correct answer
Dissolved organic matter and nutrients
Settleable suspended solids by gravity (50–65% SS, 30–35% BOD removal)
Pathogenic bacteria by UV
Heavy metals by ion exchange
Correct Answer
B. Settleable suspended solids by gravity (50–65% SS, 30–35% BOD removal)
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Primary sedimentation: removes settleable suspended solids (SS) by gravity settling. Removes 50–65% SS and 30–35% BOD. Horizontal flow velocity kept low (< 0.3 m/s) with detention time 1.5–2.5 hours. Does NOT remove colloidal or dissolved matter (needs biological treatment). Scum (grease) is skimmed from surface.
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