Sand Filter Design

[devaxrayz]

Hi All...

I'm curently doing a sand filter design job to separating water from solid. The filter should allow maximum 10 m3/h water flow

browsing from all thread that have been posted, i've found that the Flow of water through the filter (Q) can derived from Darcy's Law which is :

Q (mine is 10m3/h) = k I A
k = sands permeability (3,5 ft/day accord. to references)
I = (height of water above filter surfaces + sands depth)/sands depth
A = Surface Area of the filter

My problem is :
I've found that more lower the sand depth (and consequently more high the water height), more small the surfaced area required.

But how low is the sands depth allowable ??? there is some limit right?? And what factor determine that?

I still doesn't sure with the sands permeability 3,5ft/day since it leads me to big surface area needed. Isn't that to slow? Does anyone know what is the sands permeability? Or maybe i should explore it in my lab...

thanx alot

 

[Montemayor]

devaxrayz:

As you are probably already aware, fluid flow through porous media is not only an interesting and challenging subject, its hell on your brain cells and requires a lot of empirical data as well. Is your application for gravity filtration or pressurized (process-driven) filtration?

By stating "more lower the sand depth (and consequently more high the water height), more small the surfaced area required", do you mean: the higher the sand bed height, the higher the hydrostatic water head required and the smaller the required sand bed's cross-sectional area?

My recommendation is to go to a bench-scale set up, using the identical sand you intend to incorporate and test for the bed's permeability and retention time(s). Of particular interest should be the method or technique that you intend to use to regenerate the sand filter bed. You also have to design a workable bed support system.

From a chemical engineering perspective, I urge you to identify this as analogous to a packed bed and deal with it as such - that is essentially what it is. Do not refer to sand "depths"; you are interested in identifying the bed height, its cross-sectional area, and the pressure drop related to it. You also don't mention the quanity of water being filtered or the elements being removed. Is a slightly-pressured vessel out of the question? Or are you dealing with a pond-sized, external application? In any event, a bench-scale - or better still, a pilot plant - will, in my opinion be a prerequisite for a lot of your design basic data.

Good Luck.

Art Montemayor
Spring, TX

 

[devaxrayz]

Art :

The filter intended to be a temporary, portable filter and only use whenever we need. One type of filter being considered is the sand filter apply on gravity, because we can construct it by our selves and cheap .

Since it will be moveable, the filter should small enough (i've been thinking of barrel shapes like) and the total height of the filter should betweeen 1 to 1,5 meters.

That what i mean in "the more lower the sand bed height, the higher the hydrostatic water head available since the more space available for water and consequently following the formula the smaller cross sectional area needed for the capacity given"

I dont really sure with the water quality (TSS). Since it will be the rust/scale debris coming from our cooling water pipeline we planned to chemical cleaning (you should noticed if you read my previous thread).

Thanx alot