Deep Sand Bed ......Hydrogen Sulfide question................

[pmrossetti]

In the dsb info I've read there seems to be 3 basic layers, top-aerobic-oxygenated, middle-anoxic-low oxygen, bottom-anaerobic-no oxygen.

top reduces ammonia, nitrite to nitrates, middle reduces nitrates to nitrogen gas(bubbles), bottom produces the hydrogen sulfide because no oxygen.

OK-- let's say for the example that the bed is 3 inches deep. 1" aerobic,
1"anoxic, 1"anaerobic.

Question---if we eliminate the bottom inch, do we eliminate the hydrogen sulfide, since the top 2 layers have a good amount of oxygen in them?

Hope I expressed my question ok.

thanks.

 

[plantastic]

The thinner the sand bed is, the less likely it will harbor the conditions that allow sulfate producing bacteria to exist. However, it is not just the depth of the bed that controls this. The amount of flow that is present in the tank, and over the sandbed also contributes to the development of the types of bacteria that reside in the sand. As well as the amount of organic matter that has to be processed. Although you want good, brisk flow over the sand, you don't want it to be constantly shifting either.

Hope that makes sense.

Scott

 

[disc1]

I doubt a 3" sandbed would have an anaerobic layer. Most people talk about DSB being 6 inches or more.

 

[pmrossetti]

I doubt a 3" sandbed would have an anaerobic layer. Most people talk about DSB being 6 inches or more.

the 3" was just for example. say it's 6". eliminate the anaerobic bottom 2" and no HS? because the top 4" are aerobic and anoxic (oxygenated) no HS?

 

[Randy Holmes-Farley]

Hydrogen sulfide won't form unless you bury organic matter under the sand. It is not a matter of inches only.

 

[Paul B]

Hydrogen sulfide won't form unless you bury organic matter under the sand. It is not a matter of inches only.

Randy, won't organic matter eventually make it's way under the sand in the form of dead bacteria, pods and worms?
I know my very first DSB crashed in a few weeks. In those days I used beach sand that was much too fine, I know that.
In the tide pools where I collect, when I dig down a couple of inches, there is hydrogen sulfide everywhere.
I am just asking because you know way more about this stuff than I ever will.

 

[Randy Holmes-Farley]

A while back I removed some deep (6+") beds of fine oolitic aragonite from a couple of refugia, and there was no sign of hydrogen sulfide anywhere. They had been in use for years. Maybe we (or at least I) don't have the sorts of creatures that are mixing things up so much, but storms and such may do some burying as well.

 

[pmrossetti]

so HS can occur in a couple inches?

i thought it had to be in a no oxygen layer?

 

[disc1]

What do you mean by remove the bottom layer then? Do you mean start with a 2" shallower bed? Then it would not be a DSB and you wouldn't have any anaerobic layer. Do you mean taking out the bottom 2" of an established bed? That would be difficult.

 

[pmrossetti]

yes, start with a 2" shallower bed. this would eliminate the bottom anaerobic layer where the HS forms.

you'd be left with aerobic 2" for ammonia, nitrite processing , and anoxic 2" for nitrate processing. no anaerobic layer for HS to form.

It wouldn't form in these two 2" layers (4") because they're oxygenated?

 

[disc1]

Yeah, that's called a not a deep sand bed. It's what you have if you don't have a deep sand bed.

Well I guess it depends. If you start with a 12" sandbed and take off the bottom 4" you would still have 8" and that would make all three layers 2.6". But to start with 6" and go to 4", yes you are probably not going to really have a DSB with an anaerobic layer with 4". If you start with 3", then there's no reason to take away 1" because 3" is not deep to begin with.

 

[pmrossetti]

Yeah, that's called a not a deep sand bed. It's what you have if you don't have a deep sand bed.

Well I guess it depends. If you start with a 12" sandbed and take off the bottom 4" you would still have 8" and that would make all three layers 2.6". But to start with 6" and go to 4", yes you are probably not going to really have a DSB with an anaerobic layer with 4". If you start with 3", then there's no reason to take away 1" because 3" is not deep to begin with.

??????????

 

[pmrossetti]

Maybe I can ask it this way.

By knowing a particular grain size, can it be determined what depth is needed to have only aerobic and anoxic layers?

There would be no anaerobic later.

 

[disc1]

The layers in the sand aren't so discrete. It will depend on a lot of things, especially on how well the sand gets stirred up by critters. I think most all sandbeds will have some anoxic areas. Especially under the rocks and such. But if you stay shallower than the recommendations for deep sand beds, you shouldn't get much anaerobic action.

 

[jenglish]

IMO the dangers oh Hydrogen sulfide are generally exaggerated. That is not to say it is not possible but I have seen manyh DSBs, some undisturbed for years and never seen an actual case of it. I deep clean mine so I know it isn't a problem. Really in the 4-5" range I doubt it is a problem at all.

Sometimes the terms anaerobic, hypoxic, and anoxic get used interchangably. As I understand it; it is the functional anearobes in the hypoxic zones you want and the true anaerobes in anoxic zones that create hydrogen sulfide.

 

[pmrossetti]

I did see a video where the guy said his 3-4" dsb went HS. He stiirred the sand by the glass to make it look better and a week later when he did that again he smelled rotten egg.

 

[Randy Holmes-Farley]

I agree that the concerns are often overblown, and I also agree that one cannot completely eliminate the concern with any sand bed that is deep enough to become hypoxic.

I discuss such issues in more depth here:


Hydrogen Sulfide and the Reef Aquarium
http://reefkeeping.com/issues/2005-12/rhf/index.php