Determining Settling Velocity for a Tank 1

[RJB32482]

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.

 

[RWF7437]

Darcy's Law

 

[bimr]

A sedimentation tank is much more than just a tank with an overflow. The flow pattern throughout the sedimentation tank is carefully designed to minimize turbulence. The tank would also have some type of solids removal device. One would doubt that someone with limited experience in hydraulics would be able to design a sedimentation 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://www.water.siemens.com/SiteCo...irex_Products/Brochures/EN-RIMFLO-BR-0606.pdf

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

http://www.csupomona.edu/~fjjanger/ce431/431_06.doc

http://www.ajdesigner.com/phpstokeslaw/stokes_law_terminal_velocity.php

If you do not do a hydraulic design of the entire sedimentation tank, you are probably wasting your time.