My Plenum

[Uncle99]

For as long as I have been running SW tanks, more than 25 years, in addition to LR, I always run a DSB over a plenum area. Two powerheads pull the water from underneath the DSB, in the free water area, allowing water to "fall" through the sand bed,to be cleaned, (so I am assuming) then up the stacks and out to the DT. I have no room for a sump.

I have rebuilt my DT every five years or so and never seemed to have a problem with using this method in addition to LR, one HOB for carbon, one HOB for GFO, four wavemakers,and a skimmer.

But with all that I read, a plenum is a totally outdated process that appears now to not provide any value in the system.

I had originally thought that the plenum flow under a DSB created a large surface area for micros to live, AND, process wastes, something like a sump, but in the bottom of the tank.

I have been told that this method to too old, outdated, and has not been employed for more than 20 years. While it may not work, it does not appear to cause any problems.

If it has no value, should I just remove it in the next rebuild?

Comments?

 

[DaveK]

A traditional plenum is a sealed area under the gravel, and doesn't have powerheads pulling water from beneath the sandbed. In theory, this dead water area hosts anaerobic bacteria and they remove nitrates.

What you have amounts to an undergravel filter. By drawing water through the sandbed, you get aerobic bacteria. These are very good at reducing ammonia to nitrite to nitrate.

Personally, I'd consider both methods to be obsolete by today's standards. Yet, there are quite a few people that use them. For example @Paul B 's tank uses undergravel filters and foe could dispute the success he's had.

Like anything else, plenums and undergravel filters have their own sets of possible problems. In a plenum, with out any circulation, the water under the sandbed can get really bad over time, and if it gets out in mass amounts into the main water, can cause problems. Undergravel filters can cause problems because they can become massive dirt traps, and get clogged creating anaerobic areas. This is why you need to "muck out" the sandbed once a year or so.

As to what filtration system you should use today, this all comes back to the size of the tank, and what your trying to do with it. In larger tanks, my personal preference would be a berlin type sump with a large powerful skimmer, and optionally a refugium or algal scrubber. Also, optional would be various reactors for carbon, gfo, and so on. On smaller tanks I'd use a HOB filter and HOB skimmer. Optionally you could add additional reactors for gfo or carbon.

From that point, you could add all sorts of other additions, however, once you have all the basic items, the additions generally give only a marginal improvement.

Keep in mind that your live rock is a big part of the biological filtration system, and if your using enough of that, you really don't need too much else in that department.

As for the sandbed, you really can't go wrong here. If you don't have anything that needs to "dig in" you can use any amount from no sandbed to a 4 inch plus deep sandbed. If you have something that does enter the sandbed, you'll need it at least deep enough for that animal.

 

[Uncle99]

Thanks DaveK!

When I move to the next rebuild, I guess it's finally time to get up to date.
I do like the RED SEA rimless packages, sump area below, something to works towards.

When you say "muck out" the sand bed, is that vacuum the sandbed or something else?

My sand bed comes in two layers. The top 3" separated by a screen and this area I vacuum weekly, the bottom stays undisturbed. When I've rebuilt, both areas seem rather "clean" when pulled apart after 4-5 years.

 

[DaveK]

Yes, I'm talking about vacuuming the sand bed. On a typical deep sand bed you only want to go down about an inch or so. Since your using the deep sand bed as an undergravel filter, you can go as deep as you like.

It sounds like your taking care of your sandbed, so I don't see an issue there.

 

[Uncle99]

Thank you Sir!
We will run this system for another couple of years, then start a new rebuild and switch to sump, that will be a new learning curve for me.