Settling Tank Depth Design

[Steven - Civil]

I'm working on the design of a rectangular concrete sedimentation basin (settling pond). We're doing a preliminary calculation to see if the cost is feasible for the client to run a full design/construction. This is fairly new to me so I've been doing some research. Essentially, we have a potato wash station and they need to remove 65% of the solids from the water before it is released into another pond for treatment. I've attached a spreadsheet I'm working on to assist with the calculations (source links are on the "source" tab). I've figured out the design overflow rate and particle settling velocities, and eventually was able to iterate on a basin width and length until it worked. The only issue I'm having is that the depth doesn't seem to matter. The particles are falling at some constant velocity, so decreasing the depth reduces the horizontal velocity (thus increasing its relative downward velocity) but increases (by the same factor) the distance at which it has to fall. Therefore it seems the depth doesn't matter. The only thing that matters is the storage volume of solids, but even after 2 weeks between cleanings (at 0.09% concentration by mass), the total depth of solids is only 1/3 of an inch. So I could theoretically make the entire tank about 1 foot deep, have an 10 inch weir, and 1/3 inch of solids storage. That doesn't seem realistic to me; I feel that the tank needs to be deeper because they usually are. Is there something flawed with my reasoning or calcs?

Thanks,
Steven

 

[GeoEnvGuy]

Consider a locally supplied precast septic tank, pick the various sizes and determine how much you remove for each size.

Also consider the size of the solid stokes law does not apply to all sizes. Review sedimentation engineering by the asce to determine how much settlement you can expect for all the settlement types.

Along the same lines consider the filter size used to determine the percent solids, 0.045 or 0.015 mm or other from the lab.

 

[ashtree]

A very shallow tank is more likely to be "upset" by flow rate changes. Although it may satisfy the SOR and solids loading rates most of the water in a very shallow tank will be moving horizontally. If you have a significant flow rate change you risk either scouring out some of the sludge that has settled or near so, or carrying sludge over that has not settled. In a deeper tank some of that applies as well but the stored sludge is a lot further away from the horizontal flow path and the flow in the tank is a lot more complicated. Sludge carry over or scouring is likely to be less of a problem.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Compositepro]

As for depth of tank: the fluid flow is across the top of the settling tank. As long as the particles have settled below the exit of the tank, there is no function of greater depth except to store solids. The details of the inlet and outlet are important to prevent turbulence. Bear in mind that fluid containing suspended solids is greater in density, however slight that may be.