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Unread 11/30/2013, 12:40 PM   #1
121
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Silica sand depth

I have nine 3 liter containers that I'm going to fill with silica sand. These will be going in my sump for denitrification.

How deep should I fill them for denitrification?

If you're reading this TMZ, I'd love your input as you have advanced knowledge regarding this. I was going to send you a PM but I thought it best to share this info on the forum.

I've read on the 'DSB in a bucket' thread that 4" min and 8" max is the way to go. Anything over or under is a waste. What are your thoughts on this?

What depth IYO is the best for this to function at it's best?

The silica sand is sugar sized. I plan to give them extreme flow to help with diffusion/denitrification.
I thought about making holes in the bottom of each container and lining each with women's tights to stop the sand from falling through.
The idea was to have the top and bottom of each container oxygenated so only the middle was hypoxic. I figured I would double the denitrification area and cut out the anoxic zone at the bottom. I'm not sure if this is necessary though, as the correct depth would cancel out the anoxic zone that creates hydrogen sulfide pockets.

When I place these in my sump, I'm not going to touch them again for a few years. The reason why they're in separate containers is so I can number them to keep tabs on their age. If a problem arises (old tank syndrome) I can just pull out the oldest one for cleaning without releasing anything into the system.


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Unread 11/30/2013, 10:47 PM   #2
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6" minimum is what i recommend. HS gas is not very soluble in water but even in small quantities can give fish a drunk swimming effect and kill over 20ppm. i have A test strip for HS i use every now and then as i use sulfur denitrator and in its initial setup there are chances to have HS gas get into the water system...


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Unread 12/01/2013, 12:32 AM   #3
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I wouldn't expect the sand to do much denitrification, personally. Diffusion is irrelevant here: only bulk flow can move enough nutrients into the sand to make even a 1" bed useful. Is there some specific problem you're trying to solve?


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Unread 12/01/2013, 06:10 PM   #4
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Quote:
Originally Posted by bnumair View Post
6" minimum is what i recommend. HS gas is not very soluble in water but even in small quantities can give fish a drunk swimming effect and kill over 20ppm. i have A test strip for HS i use every now and then as i use sulfur denitrator and in its initial setup there are chances to have HS gas get into the water system...
Thanks for the info, I'll order some HS strips. Are you sure about the 6" depth for sugar sized sand?
I put some sand in see-through pots that are 4" deep for a test. They've been in there for 4 weeks. I can see bubbles forming that are trapped under the sand. Today I noticed the sand turning black on some parts of the pots. The HS is forming 1" below the surface and stops after another 1". The remaining 2" underneath isn't black. Very odd. I remember TMZ stating that denitrfication occurs in very shallow areas. Even in bacterial films on the aquarium glass. IIRC, if you crack a large piece of LR open, you'll see black HS. What I'm seeing might not be anything to worry about.

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I wouldn't expect the sand to do much denitrification, personally.
I don't understand as sandbeds carry out most of the denitrification. LR performs in the same way as sand, but we have more control with sand.


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Diffusion is irrelevant here: only bulk flow can move enough nutrients into the sand to make even a 1" bed useful.
That I also don't currently understand. The flow affects the diffusion within the sand. The more flow, the more exchange, the more diffusion. I'm referring the Anthony Calfo's 'DSB in a bucket technique, and not a DSB with critters. .

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Is there some specific problem you're trying to solve?
Elevated nitrates.


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Unread 12/01/2013, 06:59 PM   #5
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Before you go and put in a marine litter box, have you tried feeding less? Are you running a skimmer big enough for the tank? By the way there is no such thing as "old tank syndrome" its really neglected or dirty tank syndrome.


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Unread 12/01/2013, 09:40 PM   #6
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A lot of people, including me, have had deep sand beds. I liked mine for the creatures in it, but as far as anyone can tell from experience, the filtration effects are fairly small. Lots of people run without a sandbed at all, for example.

I'm using diffusion in the technical sense, as the result of random motion on the atomic level. I'd use "dispersal" for the effect that I think you're trying to describe.


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Unread 12/03/2013, 09:45 AM   #7
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Ok, so why would part of the sand turn black in the presence of HS? If it's a gas, why does it color the substrate (as apposed to just forming see-through pockets of HS gas)?


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Unread 12/03/2013, 10:00 AM   #8
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Originally Posted by 121 View Post
Ok, so why would part of the sand turn black in the presence of HS? If it's a gas, why does it color the substrate (as apposed to just forming see-through pockets of HS gas)?
The black color may be caused inpart by presence of bacteria coming into contact with the H2S gas. H2S itself is a colorless gas.


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Unread 12/03/2013, 10:54 AM   #9
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I wouldn't expect the sand to do much denitrification, personally. Diffusion is irrelevant here: only bulk flow can move enough nutrients into the sand to make even a 1" bed useful.

I agree with Jonathan 's assessment; most of the denitrificatin in a sand bed occurs in the the first half inch or less because the bacteria performing this function need nutrients( organic carbon,dissolved nitrogen and phosphorus) .

The bacteria performing dentirification are facultative;they use free oxygen and nitrate for oxygen when free oxygen is limited. They are heterotrophs that need , organic carbon, phosphate and nitrogen. A deep bed without some force to move nutrients down into it does very little. Diffusion a molecular force is too weak and slow.

The old deep buckett threads just don't account for the nature of the bacteria expected to do the nitrate removal. These bacteria use the oxygen in an area assimilating some nitrogen and phosphate as they grow and then switch to nitrate for oxygen when free oxygen is exhausted. At that stage the nitrate is reduced to N,some of which binds with other N forming N2 gas which bubbles out of the tank further reducing nitrate.These bacteria create low oxygen areas and use nitrate in the upper layers of the bed, live rock porous media or even in their own mulm but need to be in an area where the nitrogen phosphate and organic carbon are available to them.

Th black stains noted in the above posts are probably sulfides, residue from sulfate reducing bactrerial activity. When there is some organic carbon present in an areas where both free oxygen and nitrate have been exhausted by the denitrifying bacteria, sulfate reducing bacteria move in to consume the organic carbon using sulfate( SO4 for oxygen. This process produces sulfides and toxic hydrogen sulfide gas as by products.

Sand is useful for critters that need it and can be aesthetically pleasing when it's kept clean.Some enjoy observing various sand critters and reactions in the sand too. It also provides a lot of surface area for bacteria to colonize but it's is not needed at any significant depth for nitrification and is mostly sterile down deep unles nutreints are moving down ther via a viable population of sand criiters ,channeling or some other force. Anoxic areas ( no oxygen and no nitrate) with organic material in them are alos a conern vis a vie sulfate reducing bacteria.


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Unread 12/03/2013, 12:53 PM   #10
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Quote:
Originally Posted by 121 View Post
Ok, so why would part of the sand turn black in the presence of HS? If it's a gas, why does it color the substrate (as apposed to just forming see-through pockets of HS gas)?
You mean H2S? The black color isn't the H2S. H2S is a fairly strong reducing agent. It forms metal sulfides with various metal ions in your tank water. Those sulfides are usually black.


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Unread 12/06/2013, 10:24 AM   #11
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Quote:
Originally Posted by tmz View Post
'I wouldn't expect the sand to do much denitrification, personally. Diffusion is irrelevant here: only bulk flow can move enough nutrients into the sand to make even a 1" bed useful'

I agree with Jonathan 's assessment; most of the denitrificatin in a sand bed occurs in the the first half inch or less because the bacteria performing this function need nutrients( organic carbon,dissolved nitrogen and phosphorus) .
Right, that might explain why two of the silica sand pots turned black only 3/4 of an inch below the surface. That currently tells me that only the first 1" of sugar sized silica can use NO3 as an electron acceptor before it runs out and switches to SO4.
The rest of the pots have no black areas but the two that have are identical. I'm assuming the two affected pots might have had more organic matter trapped in them prior to adding them to the sump.

So the question is, would a mature 1" sugar sized silica sand bed have the same denitrification (effective hypoxic zone) as a 6" one with the same foot print?

I would say 99% reading this would say the 6" bed would be for efficient, but it seems that the 1" is all that's required.


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Unread 12/06/2013, 11:10 AM   #12
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There was a much debated study several years ago that demonstrated depth beyond an inch made no difference in denitrification.

I would also guess the balck spots 3/4 inches down are sufides and NO3 processing is stopping or at least slowing at that level. However, if you are observing that through the glass it could be left over cyanobacteria or some form of micro alge growing on the edges near the light. Smell it if you can. If it smells like sewer gas/ rotten eggs it's from sufate reduction.


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Unread 12/06/2013, 11:36 AM   #13
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I'll try and get a photo later when my cams charged, but it's definitely H2S in the plastic pot.
My next project involves making multi level 1" sand beds stacked on top of each other (or vertically side by side) with a 1/2" gap between them for flow. I could build them up to near the top of my sump (like 10 layers?) giving me an enormous surface area addition of active denitrification without wasting any space with dormant sand surfaces. I'm sure this must have been tried before. Have you ever heard of this idea?

The major thing is whether to go for 1/2" beds or 1" beds. With the 1/2" I can double the surface area with 20 beds stacked high or vertically. With 1" I could only probably do about 10 high.

I think I'll go with 1" to account for the unknown, but the 1/2" would be twice as effective if it had the same denitrification ability.


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Unread 12/06/2013, 11:47 AM   #14
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Am I missing something? You're going to put SILICA sand in your tank? You can expect to see a diatom bloom if you do this, especially if your nutrients are already high.


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Unread 12/06/2013, 12:05 PM   #15
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The stack is an interesting idea. I've not seen anyone try that. Might be tough to clean ut any detritus accumulations depending on the design.


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Unread 12/06/2013, 12:14 PM   #16
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Quote:
Originally Posted by McPuff View Post
Am I missing something? You're going to put SILICA sand in your tank? You can expect to see a diatom bloom if you do this, especially if your nutrients are already high.
It might for a short while and not very much. Some diatoms are a good thing ,imo.This thread has more on silca sand and silicate:

http://www.reefcentral.com/forums/sh...ht=silica+sand


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Unread 12/06/2013, 12:42 PM   #17
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Quote:
Originally Posted by 121 View Post
I'll try and get a photo later when my cams charged, but it's definitely H2S in the plastic pot.
Can you smell it? Lots of things will make the sand black, but the smell is unmistakable.

Have you been able to see any drop in nitrate? That I think would be the bigger question before gong forward.


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Unread 12/06/2013, 12:44 PM   #18
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Lots of people have used silica sand successfully in tanks. In any case, diatoms can't live on silica alone. If adding silicate causes a bloom, the general nutrient load must be high enough to sustain the growth.

The active depth of a sandbed will depend on the infauna, or lack thereof. If the sandbed has sufficient animals of the right type, just about any depth can be productive, but for our tanks, 6" might be the practical limit. In the ocean, burrowing animals can move water a meter or more into the sand in some places, but those animals aren't going to survive in our tanks.


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Unread 12/06/2013, 12:58 PM   #19
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Quote:
Originally Posted by tmz View Post
The stack is an interesting idea. I've not seen anyone try that. Might be tough to clean ut any detritus accumulations depending on the design.
Take a 1" tall tray of sand. Glue a perforated screen over it to stop sugar sized sand falling through. You could literally turn that upside down and have no detritus collecting in it. Now you have multi stacked beds that never need siphoning.

If you put two of them back to back and stand that new tray vertically, you'll have a tiny foot print with a maxed out foot print.
You could stack these 1/2" apart, side by side, fitting a ridiculous amount of active 1" sand in a sump.

If you wanted to remove a specific tray (say to upgrade the tray or change tanks), it would be as quick as pulling a book from a shelf.



Last edited by 121; 12/06/2013 at 01:06 PM.
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Unread 12/06/2013, 01:13 PM   #20
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Can you smell it? Lots of things will make the sand black, but the smell is unmistakable.

Have you been able to see any drop in nitrate? That I think would be the bigger question before gong forward.
I smelt a slight egg smell in my sump a few days ago when the windows were shut.
My new ULR Hanna P Checker has now arrived so I can remove N03 within the next week or so using vodka without an unmonitored phosphorus depletion/crash. I have trisodium phosphate on hand for that. My phosphate is really low (0.03) and that's why carbon dosing hasn't worked so far. There's no fish in this system so nothing is threatened by my currently elevated NO3 issue.

Edit:Oh and thanks for kicking me up the a*s about learning molar mass



Last edited by 121; 12/06/2013 at 01:19 PM.
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