Question

How can I design a rectangular sedimentation tank?

Inputs: Average Flowrate , Peak Flowrate ,Number of tanks, W:L ratio , Launder width, Sidewall depth ,Bottom slope, MLSS concentration ,Recycle ratio (R)

Outputs: Length , Width ,Surface loading (for Qave and Qpeak) ,Weir type (one-sided, two-sided, 1/3 of length, corrugated) Weir loading (for Qave and Qpeak) ,Solids loading (for Qave and Qpeak) ,Retention time (for Qave and Qpeak) ,Horizontal velocity (for Qave and Qpeak) (if it is rectangular)

Answer 1

Inputs:

Population:- 1,00,000 persons

Rate of water supply (Qpeak) :- 150 lpcd

Detention period: 2 hours

L:B=3:1

Launder width=4 m

Suspended Solid Concentration=1800 mg/l

Lets consider 80% water supply as sewage

Solution:-

A)Volume of tank

Assume Depth=3 m and Length=3*Breadth. Then,

Provide 4 m as launder width in outlet zone. Then,

Total length=31.5+4=35.5 m

B)Retention Time

C)Surface Loading

Surface Loading or Surface Overflow Rate can be calculated as follows-

1)For Qavg

2)For Qpeak

Length of weir= Circumference of the tank=2(L+b)=2(35.5+10.5)=92 m

D)Weir Loading

The sedimentation tank has two effluent troughs across the width that allows the water to flow over both sides of each trough. Let sidewater depth of water be 2.4 m.

Weir Loading can be calculated as-

1)For Qavg

2)For Qpeak

E)Solids loading

F) Horizontal velocity