Question

A town has an existing horizontal flow sedimentation tank with an overflow rate of 17 m³/day/m²_, and it is desirable to remove particles that have settling velocity of 0.1 mm/second. Assuming the tank is an ideal sedimentation tank, the percentage of particle’s removal would be approximately equal to:

1. 35%
2. 50%
3. 75%
4. 85%

Answer 1

Correct Answer - Option 2 : 50%

Concept:

Percentage of particle removal or Efficiency is given by

\({\rm{efficiency}}\;\left( \eta \right) = \frac{{{{\rm{V}}_{\rm{s}}}}}{{{{\rm{V}}_{\rm{o}}}}} \times 100\)

Where

Vs is the settling velocity of the particle

Vo is surface overflow rate

Calculation:

Given,

Overflow rate, Vo = 17 m3/day/m2 = 17 m/day

Settling velocity, Vs = 0.1 mm/sec

\(V_o= \frac{{17 \times 1000}}{{24 \times 60 \times 60}} = 0.196\;mm/sec\)

Percentage particle removal or efficiency, 

\({\rm{efficiency}}\;\left( \eta \right) = \frac{{{{\rm{V}}_{\rm{s}}}}}{{{{\rm{V}}_{\rm{o}}}}} \times 100\)

\(= \frac{{0.1}}{{0.196}} \times 100 = 50\%\)