GATE CE 2024 Set 2 — Question 42
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Environmental Engineering → Drinking Water Treatment → Filtration
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Question
A hypothetical multimedia filter, consisting of anthracite particles (specific gravity: ), silica sand (specific gravity: ), and ilmenite sand (specific gravity: ), is to be designed for treating water/wastewater. After backwashing, 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 The CORRECT option fulfilling the diameter requirements for this filter media is
(i) Slow discrete settling (Stoke’s law is applicable)
(ii) All particles are spherical
(iii) Diameter of silica sand particles is The CORRECT option fulfilling the diameter requirements for this filter media is
Correct answer
(A) diameter of anthracite particles is slightly less than 0.35 mm and diameter of ilmenite particles is slightly greater than 0.141 mm.
Solution
According to Stoke's Law, settling velocity . For the layers to form as specified (anthracite top, silica middle, ilmenite bottom), the settling velocities must follow: .
.
.
Thus, and . Option (A) matches these conditions.
1.For anthracite to be above silica:
.
2.For ilmenite to be below silica:
.
Thus, and . Option (A) matches these conditions.
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