Brian Prestwood
Premium Member
Hey All
In case it wasn't clear, our speaker's comments on sand beds contradicted what many of us, including me, have understood for years. If our speaker is correct, our deep sand beds (remote or in-tank) are doing the opposite of what we intend.
To keep this thread productive, I'm not going to mention our speaker by name. I would appreciate it if no one else does either.
Our goal is convert waste to something that is self exporting. Under the right conditions, dictated by oxygen levels, certain bacteria will convert waste to nitrogen gas which bubbles out of the tank. Of course, this does nothing for phosphorous but I'm not going to get into that now.
Our speaker said when a sand bed is sitting on the tank bottom, or sump bottom, with no plenum there are three layers in it, an aerobic (oxygenated) layer, an anaerobic (low oxygen) layer and an anoxic (no oxygen) layer. This is consistent with I've heard. Our speaker said the thickness of these layers is primarily dependant on the grain size of the media. The courser the grain size the thicker the layers. This too is consistent with what I've always heard.
Our speaker gave an example with 2mm grain size. This is much courser than the sugar fine sand we normally use for deep sand beds. In the 2mm grain example, the first 0.5" is aerobic, the next 0.5" is anaerobic and anything below 1" is anoxic.
This is inconsistent with what I've heard from several sources I considered reputable. They said that with even sugar fine sand the anoxic layer doesn't start until 3"- 4" inches down.
Finally, and this is the biggest contradiction to what I understood, our speaker said it is the anaerobic layer between the aerobic and anoxic layers, that converts waste to nitrogen gas. The bottom anoxic layer converts waste to ammonium (fertilizer). In other words, bacteria below the anoxic layer is converting waste to fertilizer, not nitrogen gas as we intend.
Our speaker said a plenum maximizes the thickness of the anaerobic layer and thus the efficiency of the sand filter by introducing oxygen from below as well as above the sand bed. Our speaker was unsure of the source of the oxygen from below but noted that the oxygen levels tested consitently.
Our speaker recommended a 4" layer of 2mm sand on top of a plenum with a 1/2" layer of water under it is a good design. Our speaker remotes the sand bed so the fauna can't affect it.
If it were't for the fact that several reputable sources recommend deep sand beds, I'd believe I simply mixed up anaerobic and anoxic layer dwelling bacteria.
Comments?
Has anyone found any quantitive analysis on anaerobic, vs anoxic, layer dwelling bacteria producing nitrogen gas?
Has anyone found any quantitive analysis that contradicts our speakers comments plenum oxygen levels?
In case it wasn't clear, our speaker's comments on sand beds contradicted what many of us, including me, have understood for years. If our speaker is correct, our deep sand beds (remote or in-tank) are doing the opposite of what we intend.
To keep this thread productive, I'm not going to mention our speaker by name. I would appreciate it if no one else does either.
Our goal is convert waste to something that is self exporting. Under the right conditions, dictated by oxygen levels, certain bacteria will convert waste to nitrogen gas which bubbles out of the tank. Of course, this does nothing for phosphorous but I'm not going to get into that now.
Our speaker said when a sand bed is sitting on the tank bottom, or sump bottom, with no plenum there are three layers in it, an aerobic (oxygenated) layer, an anaerobic (low oxygen) layer and an anoxic (no oxygen) layer. This is consistent with I've heard. Our speaker said the thickness of these layers is primarily dependant on the grain size of the media. The courser the grain size the thicker the layers. This too is consistent with what I've always heard.
Our speaker gave an example with 2mm grain size. This is much courser than the sugar fine sand we normally use for deep sand beds. In the 2mm grain example, the first 0.5" is aerobic, the next 0.5" is anaerobic and anything below 1" is anoxic.
This is inconsistent with what I've heard from several sources I considered reputable. They said that with even sugar fine sand the anoxic layer doesn't start until 3"- 4" inches down.
Finally, and this is the biggest contradiction to what I understood, our speaker said it is the anaerobic layer between the aerobic and anoxic layers, that converts waste to nitrogen gas. The bottom anoxic layer converts waste to ammonium (fertilizer). In other words, bacteria below the anoxic layer is converting waste to fertilizer, not nitrogen gas as we intend.
Our speaker said a plenum maximizes the thickness of the anaerobic layer and thus the efficiency of the sand filter by introducing oxygen from below as well as above the sand bed. Our speaker was unsure of the source of the oxygen from below but noted that the oxygen levels tested consitently.
Our speaker recommended a 4" layer of 2mm sand on top of a plenum with a 1/2" layer of water under it is a good design. Our speaker remotes the sand bed so the fauna can't affect it.
If it were't for the fact that several reputable sources recommend deep sand beds, I'd believe I simply mixed up anaerobic and anoxic layer dwelling bacteria.
Comments?
Has anyone found any quantitive analysis on anaerobic, vs anoxic, layer dwelling bacteria producing nitrogen gas?
Has anyone found any quantitive analysis that contradicts our speakers comments plenum oxygen levels?
Dr. Rob Toonen and Chris Wee did a controlled study of deep vs. shallow and coarse vs. fine sand grains and their affect on nitrate levels in an aquarium. This was presented at IMAC 2004 and I assume it's available online if you search. In TRA vol. 3 Delbeek and Sprung comment on this study with some thoughts of their own, as well as go into a lengthy discussion of plenums and DSBs. Good reads all.
