Direction: A plain sedimentation tank with a length of 20 m, width of 10 m and a depth of 3 m is used in a water treatment plant to treat 4 million litres of water per day (4 MLD). The average temperature of water is 20°C. The dynamic viscosity of water is 1.002 × 10–3 N.s/m² at 20°C. Density of water is 998.2 kg/M³. Average specific gravity of particles is 2.65.
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What is the minimum diameter of the particle which can be removed with 100% efficiency in the above sedimentation tank?
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- 11.8 × 10–3 mm
- 16.0 × 10–3 mm
- 50 × 10–3 mm
- 160 × 10–3 mm
Correct Option: B
For 100% removal, settling velocity = SOR
| Settling velocity, Vs = | (S - 1)d² | |
| 18 v |
| = | (S - 1)d² | ||
| 18 | μ |
| ∴ d = √ | ||
| g.ρ(S - 1) |
| Vs = SOR = 20 m³/m²/day = | = 0.2315 mm/s | |
| 24 × 60 × 60 |
| d = √ | = 1.606 × 10–5m = 16 × 10–3mm. | |
| 9.81 × (2.65 - 1) × 1000 |
