Design of Settling Basin

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Settling Velocity Computation Computation of settling velocity is important to determine the dimension of settling tank.

By equating following forces we get relation for settling velocity of particle

Drag force = (ρ- ρo)πD3/6 Impending gravity force=Cd*( πD2/4)* ρ Vs2/2 which gives

The coefficient Cd is related to Reyonald's number s follows

and

Thus iteration of above three equation is required to get settling velocity of the particle

For D≤0.01 mm, Stroke stated that

Thus we have

Again substituting g=9810 and =1.31 centistrokes at 10 oC we get

where t is temperature other than 10 oC

For D>1 mm, Cd=0.4 hence


DESIGN OF SETTLING ZONE

Capacity of tank, C= L*B*H Detention time, t = L/v where v=horizontal velocity Time for sediment to fall from height H, t'=H/Vs equating two times, Vs=Hv/L=BHv/BL=Q/As Q/As is also called surface overflow rate or surface loading

Efficiency of sediment tank

The particle with settling velocity Vs'<Vs will be removed only if enters inlet below point depth h.

In that case Vs'=hv/L=Q/As*h/H=Vs*h/H

Efficiency is defined as the ratio of removal of this size particle to that with settling velocity Vs.

ie

Some Design criteria:

Velocity of flow 150-300 mm/min SOR 15-30 m3/m2/day L/B 3-5 For circular 60m dia max.

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