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A hypothetical multimedia filter, consisting of anthracite particles (specific gravity: 1.50), silica sand (specific gravity: 2.60), and ilmenite sand (specific gravity: 4.20), is to be designed for treating wastewater. After back washing, the particles should settle forming three layers: coarse anthracite particles at the top of the bed, silica sand in the middle, and small ilmenite sand particles at the bottom of the bed. Assume (i) Slow discrete settling (Stoke's law is applicable) (ii) All particles are spherical (iii) Diameter of silica sand particles is 0.20 mm The CORRECT option fulfilling the diameter requirements for this filter media is

1. diameter of anthracite particles is slightly less than 0.35 mm and diameter of ilmenite particles is slightly greater than 0.41 mm.
2. diameter of anthracite particles is slightly greater than 0.35 mm and diameter of ilmenite particles is slightly less than 0.41 mm.
3. diameter of anthracite particles is slightly less than 0.64 mm and diameter of ilmenite particles is slightly less than 0.10 mm.
4. diameter of anthracite particles is slightly greater than 0.64 mm and diameter of ilmenite particles is slightly less than 0.10 mm.

Correct answer: diameter of anthracite particles is slightly greater than 0.35 mm and diameter of ilmenite particles is slightly less than 0.41 mm.

Solution

The correct option indicates that the anthracite particles must be larger than the silica sand to ensure they settle on top, while the ilmenite particles need to be smaller than the silica sand to settle below it, aligning with the principles of particle settling based on specific gravity.

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