Determining Settling Velocity for a Tank
Determining Settling Velocity for a Tank
(OP)
Hello All,
We are looking at turning our wastewater storage tank into a "settling tank" to capture solids instead of having them be pumped from the tank into "prefilters" before heading into a carbon bed treatment system.
We are going to install a dip tube (water just free falls now from top of tank) to enhance settling. Also we are going to let dip tube be as far away from pump suction as possible. I'm looking for help on two things:
1. When determining what velocity the particles are actually traveling, do I use the inlet flow rate (100 GPM) or the outlet flowrate (160 GPM)? Here's the formula I am using to determine actual velocity
particle velocity = depth of settling zone/retention time
Where retention time = volume of settling zone/flow through tank
Flow through tank is where I need help on what flow to use. I believe its flow exiting the tank.
2. Does anyone have any good resources on determining settling velocities of spherical particles vs. other parameters (viscosity of liquid, particle size, etc.)?
Thanks.
We are looking at turning our wastewater storage tank into a "settling tank" to capture solids instead of having them be pumped from the tank into "prefilters" before heading into a carbon bed treatment system.
We are going to install a dip tube (water just free falls now from top of tank) to enhance settling. Also we are going to let dip tube be as far away from pump suction as possible. I'm looking for help on two things:
1. When determining what velocity the particles are actually traveling, do I use the inlet flow rate (100 GPM) or the outlet flowrate (160 GPM)? Here's the formula I am using to determine actual velocity
particle velocity = depth of settling zone/retention time
Where retention time = volume of settling zone/flow through tank
Flow through tank is where I need help on what flow to use. I believe its flow exiting the tank.
2. Does anyone have any good resources on determining settling velocities of spherical particles vs. other parameters (viscosity of liquid, particle size, etc.)?
Thanks.





RE: Determining Settling Velocity for a Tank
RE: Determining Settling Velocity for a Tank
Inflow = Outflow + Sludge Withdrawal.
The outflow is usually controlled in some manner by using an effluent weir to carefully control the hydraulic flow through the tank and minimize turbulence. The inlet to the tank would also have a flow control device. Take a look at the tank cross-sections in the following brochure:
http:
For a simple sedimentation tank, you would use Stoke's law as you would only have type 1 settling:
ceeserver.cee.cornell.edu/jjb2/cee656/Sediment-lect.doc
www.csupomona.edu/~fjjanger/ce431/431_06.doc
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If you do not do a hydraulic design of the entire sedimentation tank, you are probably wasting your time.