Question

A rectangular sedimentation basin is to handle 10 million liters/day of raw water. A detention basin of width to length ratio of 1/3 is proposed to trap all particles larger than 0.04 mm in size. Assuming a relative density of 2.65 for the particles and ${20}^{\circ }$C as the average temperature . The depth of the tank is 3.5 m.

• A. Length of the tank is 68.1 m
• B. The length of the tank is 20.54 m
• C. Length of the tank is 25.64 m
• D. Length of the tank is 69.34 m

Answer 1

The correct option is D Length of the tank is 69.34 m

Hazen's formula

$V_s=418(G-1)d^2\frac{3T+70}{100}$

$V_s=418(2.65-1)(0.04{)}^2\times \frac{3\times 20+70}{100}$

$V_s=1.434mm/sec$

$V_H=\frac{Q}{B\times H}=\frac{V_s\times L}{H}$

$B=\frac{Q}{V_s\times H}$

$B=\frac{0.1157}{3.5\times 1.43\times {10}^{-3}}=23.116$

$L=3B=3\times 23.116$

$L=69.34m$