Correct answer: (C) Grit is Type I;primary settling is Type II.; (D) Secondary settling is Type III;primary settling is Type II.
Explanation
Grit particles settle discretely, Type I. Primary wastewater solids typically flocculate while settling, Type II. Concentrated activated sludge in secondary clarification exhibits hindered/zone settling, Type III. Therefore **C, D** give the intended classifications. Concentration and location within a clarifier can introduce additional settling regimes.
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