# undergravel as plenum



## danrs (Jan 21, 2011)

I have an undergravel filter in my 75gal freshwater. Can I just unhook the air lines and leave it in place as a plenum?


----------



## beaslbob (May 29, 2009)

danrs said:


> I have an undergravel filter in my 75gal freshwater. Can I just unhook the air lines and leave it in place as a plenum?


sure.

not that it would make much difference. One test in marine tank showed no difference in nitrates between tanks with plenums and without.

Might try reverse flow if you haven't already.


my .02


----------



## Gizmo (Dec 6, 2010)

N00b question - what is a plenum and what is its purpose?


----------



## beaslbob (May 29, 2009)

[email protected] said:


> N00b question - what is a plenum and what is its purpose?


From an on line dictionary:

ple·num (plnm, plnm)
n. pl. ple·nums or ple·na (pln, pln) 
1. An assembly or meeting with all members present.
2. A condition, space, or enclosure in which air or other gas is at a pressure greater than that of the outside atmosphere.
3. The condition of being full; fullness.
4. A space completely filled with matter.


4th kinda applies here.

in marine tanks you would have the substrate on a plate some distance (say 1/2" or so) above the tank bottom. So the area between the plate and bottom glass was filled with water. That was the plenum and the idea was to form an anaerobic area to reduce nitrates.


my .02


----------



## Gizmo (Dec 6, 2010)

beaslbob said:


> That was the plenum and the idea was to form an anaerobic area to reduce nitrates.


I've heard of anaerobic bacteria generating hydrogen sulfide gas, so I thought the idea was to get rid of that bacteria. Am I completely misguided, or would that be why you said to use reverse flow?


----------



## roacan (Dec 25, 2010)

I think he is trying a Jaubert system or Monaco system where anerobic bacteria would consume nitrates.

Advanced Aquarist Feature Article
Jaubert Live Sand Filtration

There was a study in marine tanks (I can't find it right now) where they say this does not make a difference and the depth of the sand is what makes a difference.


----------



## roacan (Dec 25, 2010)

Found it!

Feature Article: An Experimental Comparison of Sandbed and Plenum-Based Systems. Part 1: Controlled lab dosing experiments | Advanced Aquarist's Online Magazine

Feature Article: An Experimental Comparison of Sandbed and Plenum-Based Systems: Part 2: Live Animal Experiments | Advanced Aquarist's Online Magazine


----------



## danrs (Jan 21, 2011)

Actually, I was wondering if it would do any damage to just leave the undergravel filter down there and disconnect the pumps from it. 

Also, should the return tubes be pulled off?

Thanks, for these helpful references per the plenum.


----------



## roacan (Dec 25, 2010)

danrs said:


> Actually, I was wondering if it would do any damage to just leave the undergravel filter down there and disconnect the pumps from it.
> 
> Also, should the return tubes be pulled off?
> 
> Thanks, for these helpful references per the plenum.


Oopss! *blue sorry, I thought you were trying to do the plenum thing. I was just researching that and also fluidized bed filters.


----------



## beaslbob (May 29, 2009)

[email protected] said:


> I've heard of anaerobic bacteria generating hydrogen sulfide gas, so I thought the idea was to get rid of that bacteria. Am I completely misguided, or would that be why you said to use reverse flow?


You could be right on the anaerobic. Some say deap sea beds (sand) actually use anaoxic or low oxygen bacteria to reduce nitrates to nitrogen gas. And if those "go bad" you do get sulfides sometimes but even more common and important the bacteria can actually reduce the nitrates to ammonia.

Of course that is just what I have read and subject to my ignorance.

The reverse flow is kinda an improvement on the standard under gravel filter(UGF). The idea is you 1) filter the crud, and 2) any crud that pass through are trapped outta sight under the substrate. Plus you can always flush it out be using the lift tubes to drain the water during water changes.

although UGFs are not popular and much maligned for reef tank Paul_B has ran on for 40 years on a reef tank. 

my .02


----------

