Undergravel filtration discussion

[MarkD40]

The following is my response to a private message sent to me. Please feel free to evaluate my response and offer criticism.

1 wrote on 03/09/2006 05:12 PM:
hi mark just read through some of your threeds im very impressed you know your stuff anyway my 100 gln reef is 2yrs old i would say largish stocking level 10 fish mostly small [1 large tang 5"] lots of soft corals sump with mud 5gln water changes a week [now increasing to 10 glns] about Ã"šÃ‚£100 worth fugi rock tank kept very clean sand vaced every week my nitrates are off the clock i never know becouse i was useing a hagen test kit whats your fish levels how do you keep 0 nitrates and what advice can you other me thanks mick hope you dont mind me writeing to you im new to rs and not sure of protocals

Hi Bolton!
The reason I have very low nitrates IMO are the following:

1. I have a very low bioload. I have only 4 fish. 3 - 4 inch Tangs and a 2.5 inch six line wrasse, and many mushroom corals.

2. I have a 110 gallon display tank with 80-100 pounds of live rock. My display has only about 1/2 inch of sand on the bottom with none underneath the rock.

I have a 75 gallon sump with an 12 inch X 48 inch undergravel filter over a plenum made of PVC pipe under 4-5 inches of sand. The remaining 6 inches by 48 inches of the sump has sand down to the glass with no undergravel or plenum. I also have about 80 pounds of LR in the sump suspended on egg crate so the rock does not sit on the sand.

You should understand that when I set up my tank 7 years ago the conventional wisdom at that time was exactly how my tank is set up. Undergravel filter over a plenum was the way to go. Now all the "experts" at RC will tell you that UG filters lead to higher nitrates. This makes absolutely no sense! X amount of ammonia will be converted to a corresponding amount of nitrate. No more, no less! How quickly that occurs depends on the amount of bacteria present to make the conversion. UG filters and wet/dry filters are VERY efficient at rapidly removing ammonia from the system and converting it to less toxic nitrate. Wet/dry is by "definition" an oxygen rich environment and so it will not remove nitrate. So by itself wet/dry filtration is an incomplete form of biologic filtration.

If you also have a deep sand bed and plenty of live rock you will have the anaerobes needed for the anaerobic conversion of nitrate to nitrogen gas. If your system has the ability to keep up with nitrate production you will have 0 nitrate. If your system does not have enough capacity your nitrate will gradually increase and so water changes are the only way to remove nitrate.

I believe that a deep sand bed over an undergravel filter over a plenum is the best of both worlds. Aerobic bacteria in the upper layers of sand convert ammonia-nitrite-nitrate as the water is pulled through by the UG filter. This process removes oxygen fom the water and the deeper the water goes the more anoxic it becomes. As the water is pulled into the deeper layers of sand facultative anerobes and then strict anaerobes convert the nitrate into nitrogen gas. The water then passes up the tube and is forcefully ejected into the sump re-oxygenating the water.

I have used an oxygen test kit and tested my water. The water in the sump was saturated with oxygen when corrected for temperature and sg.. Water collected coming from below UG filter had no measurable oxygen! That being the case, why isn't that more efficient than a DSB that functions by diffusion only? By pulling water through the sand you are exposing the bacteria to many more molecules of "food" over a given time, which I believe leads to an increased number of bacteria = more efficient and more rapid complete breakdown of nitrogenous waste.

I have asked repeatedly what is wrong with my logic on RC because I was thinking about removing my UG filter after repeatedly reading about how it leads to higher nitrates (I don't get that logic as I have stated above) No one was able to give me a satisfactory response that made any real sense, so I have decided to leave my UG in place.

Perhaps with a marked increase in biolode in my system I would start having higher nitrates, but I believe that the system would adapt, and adapt more quickly to an increased load than a plain DSB. I am not implying that a DSB does not work, only saying that a DSB over an UG filter and plenum is superior.


3. I have protein skimmer that is geared to a tank (water volume) twice my present set-up running 24/7. I also have a 1 gallon nu-clear canister filter with a 50 micron pleated filter in line as the last filtration before the water is returned to the display.

Suggestions:

I think you need to increase the amount of live rock. I don't know how much LR 100 pounds will buy but it is probably not very much.

I think you have too many fish.

I am going to post this on RC under New to the Hobby so that you can get other comments form people more knowledgeable than me. Good Luck!

 

[BurntOutReefer]

nice accurate response

 

[MarkD40]

Thanks Reefer!

 

[bertoni]

Re: Undergravel filtration discussion

<a href=showthread.php?s=&postid=6924299#post6924299 target=_blank>Originally posted</a> by MarkD40
The following is my response to a private message sent to me. Please feel free to evaluate my response and offer criticism.



Now all the "experts" at RC will tell you that UG filters lead to higher nitrates. This makes absolutely no sense! X amount of ammonia will be converted to a corresponding amount of nitrate. No more, no less!

That's true as far as it goes, but not complete. The reason bio-balls, UGF setups, etc, are thought to increase nitrate levels in some cases has to do with inhibiting denitrification. The chemistry forum has more details. Here's one reference:

http://www.reefcentral.com/forums/showthread.php?s=&threadid=498053&highlight=proximity

If you also have a deep sand bed and plenty of live rock you will have the anaerobes needed for the anaerobic conversion of nitrate to nitrogen gas.

That doesn't work for all cases. As noted, live rock doesn't seem to function as well for denitrification with bio-filters in place, and I don't see how a DSB can take up large amounts of dissolved nitrate, either. Macroalgae can.

I believe that a deep sand bed over an undergravel filter over a plenum is the best of both worlds.

There's an article in AdvancedReefkeeping.com that talks about some tests of plenum-based systems. They were unable to measure any actual benefit. I suspect that removing your plenum would have no long-term effect one way or the other on your system.

 

[MarkD40]

Thanks Johathan.

I was thinking of removing my UG a month or so ago but could not get anyone to tell me why I should. I am updating my 7 year old tank with new lights, pumps etc. and I wanted to incorporate the lastest wisdom on reef tanks.

The article you referred me to states that denitrification is negligable. But that was for a small sample of sea bottom. Multiply that times the huge surface area of the sea bottom and negligable becomes significant. Bacteria are stupid and eat and multiply. If molecules they eat are available they will eat them. I can not imagine why wet/dry filtration would inhibit denitrification. It makes no biologic sense to me. If nitrate is released into the water column it will be consumed by the bacteria, and macroalgae. If bioballs somehow retain nitrate then that nitrate is isolated and has no ability to be measured or consumed. At some point this nitrate retention would saturate the bioballs and no more would be retained and would therefore be released back into the WC where it would be available for consumption. In other words, the system would eventually reach equilibrium given a "static" bioload. The amount of nitrate released by the bioballs would then be proportional to the amount of ammonia released into the system and would not be greater. You can't make something out of nothing.

 

[bertoni]

The hypothesis is that the nitrate needs to be created close to the denitrifying organisms, because no sufficient transport mechanisms exists to move the nitrate to the denitrifying zone otherwise. For a simple example of how that can happen, some bacteria and archaeans live in a 2-cel layer on surfaces, with the inner surface anaerobic and performing denitrification. Such a structure would be very susceptible to the competition of bio-balls, which don't seem to support denitrification.

The whole sentence from the article was:

The coupled N and O isotope measurements also indicate that there is no significant gross efflux of 15N-depleted nitrate from nitrification, leading to the conclusion that nitrification is closely coupled to denitrification, even in the bioturbated sediments of the SMB.

which doesn't state that denitrification is negligible, rather that little nitrate escapes unprocessed from the denitrifying processes.

This issue comes up over and over again, so a search will turn up a lot of information, and I can dig up some references from the literature, if you want. These might be a good starting point:

Phosphorus and nitrogen in coral reef sediments, Barrie Entsch, Kevin G Boto, Robin G Sim, John T Wellington, Australian Institute of Marine Science, Limn. Oceanogr. 1983.

Nitrogen efflux from the sediments of a subtropical bay and the potential contribution to macroalgal nutrient requirements,
Journal of Experimental Marine Biology and Ecology

Volume 252, Issue 2
20 September 2000
Pages 159-180 (available on-line)

Microbial nitrogen transformations in unconsolidated coral reef sediments, Douglas G. Capone, Susan E. Dunham, Sarah G. Horrigan, Linda E. Duguay, Marine Ecology Progress Sieres, Vol 80, 75-88, 1992.

This one is really useful, IMO:

Organism-Sediment Relations on the Muddy Sea Floor, Donald C Rhoads, Oceanogr. Mar. Biol. Ann. Rev., 1974.

 

[MarkD40]

Thanks Johnathan. I will check out those references and get back to you if I am not convinced! For the moment let us forget bio balls/wet/dry and talk about DSB's.

"The hypothesis is that the nitrate needs to be created close to the denitrifying organisms, because no sufficient transport mechanisms exist to move the nitrate to the denitrifying zone otherwise."

How Close? This is my point exactly. I have read that in a DSB the zone between aerobic and anaerobic bacteria is only a few millimeters, with facultative anaerobes sharing space with aerobes in the transition zone between oxygen rich and anoxic zones.

If the handoff of the nitrate molecule to the anaerobic bacteria occurs by diffusion only, the transition zone could not be very thick and would get saturated quickly with nitrate molecules waiting their turn to be eaten, and the end products moving to the surface to be released into the water column. Traffic moving in both directions. The speed of this turnover would be the rate limiting step for denitrification.

With a DSB over an undergravel filter over a plenum (with the UG pulling the water down through the sand), you have one way traffic through the sand bed. As the nitrate is produced by the aerobic layers it is "pulled" into the deeper layers where first facultative and then anaerobic bacteria can break down the nitrate. The nitrogen gas is then pulled all the way through the sand and ejected back into the water column. This pulling of water through the sand would make the transition zone much thicker and would therefore support billions of more bacteria working their little hearts out processing nitrogen in it's various forms. A denitrification superhighway with one-way traffic.

"There's an article in AdvancedReefkeeping.com that talks about some tests of plenum-based systems. They were unable to measure any actual benefit. I suspect that removing your plenum would have no long-term effect one way or the other on your system."

Was the system driven by reverse undergravel filtration? If so then I would readily agree that there would be minimal additional benefit because you have oxygen rich water on both the top and bottom of the sand bed. In order to have an anoxic zone for denitrification you would need a much thicker sand bed. This would decrease flow and reduce the speed limit on the superhighway, reducing efficiency enough to show no measurable difference compared with a DSB in my opinion. In my system the sand is only 4-5 inches deep and the water at the bottom of my sand bed has no measurable oxygen!

Interesting discussion but Time for BED! Mark

 

[MarkD40]

!

 

[Swanwillow]

when/if I get my big tank (200+ gallons!!!), after reading the 35 year old tank thread-I'll be doing a reverse flow UG filtration system...

I think the reason yours and others haven't turned into nitrate factories is the fact that ya'll don't have a crap load of fish-low bioloads. IF you had the higher bioload, you would have a nitrate factory just because of the reasons you stated: it can only process so much mitrates, and bacteria is the limiting factor. (in small speak)

Many people forget that bacteria can be a limiting factor in the whole cycling process. Bacteria only multiplies x amount/hour. lets say, they can split once every 5 minutes. start with one bacterium, and you get... oh... 4096 ish bacterium. probably not enough to cover a pinhead, in an hour. yes, in the next hour, that multiplies, again and again and again. limiting factors: bacteria only live for a day or so... so those will have to be replaced also.

so where am I going? the bacteria can only process certain amounts of nitrates, and once you reach the equilibrium, thats the end, folks... if you add 'just one more fish' then you may go over the amount of bacteria avalible for processing the nitrates, and BAM-nitrate factory./.. coupled with feeding, etc etc.

the reason I'm going with the revers UG is becuase heck-I think 35 years using one is sucess, as long as my bioload stays within a decent parameter!!! but the flow has to be VERY low. Paul B is the one with that system, and he explains it very well in his thread about his tank...

 

[MarkD40]

I agree swan, but if you add more fish your bacteria culture wil expand in response to the increased ammonia released. While this ability is not unlimited, I feel that my system allows for a larger colony of nitrifying bacteria to develop. You would see a brief spike in nitrate but over time the bacteria would multiply to take advantage of the increased food supply. As far as bacteria living only a day or so that is not correct. Bacteria can live for very long periods of time as long as they have food to eat.

 

[MarkD40]

I am hoping that barryhc will lend his insite into this thread as he seems to have good knowldge about plenum systems.

 

[Swanwillow]

come now, I'm agreeing with you... kinda, in an odd way

I don't know the full life expectancy of the nitrifying bacteria in our tanks, going back to biology courses in college, a few days is what many bacteria live... but they reproduce so quickly it doesn't make a difference...

 

[Paul B]

when/if I get my big tank (200+ gallons!!!), after reading the 35 year old tank thread-I'll be doing a reverse flow UG filtration system...

Swan, don't get a RUGF.
I want to be the only one with a 35 year old reef. :lol:

:beer:
Paul

 

[WaterKeeper]

I know I wouldn't trust anyone that old.

Barrry is not so much talking about UGF as "wasting plenums" his current thread can be found Here.

 

[bertoni]

<a href=showthread.php?s=&postid=6930087#post6930087 target=_blank>Originally posted</a> by MarkD40
"The hypothesis is that the nitrate needs to be created close to the denitrifying organisms, because no sufficient transport mechanisms exist to move the nitrate to the denitrifying zone otherwise."

How Close?

I wish I knew. I'm not sure that there's much data on this point. In the case of a 2-cell layer, though, the distance is, well, a couple of cell membranes.

If the handoff of the nitrate molecule to the anaerobic bacteria occurs by diffusion only, the transition zone could not be very thick and would get saturated quickly with nitrate molecules waiting their turn to be eaten, and the end products moving to the surface to be released into the water column.

For DSBs, the transition zone is discussed in the Rhoads paper, if I picked the right reference there). If not, I'll go through the papers again. Diffusion is unimportant in a DSB. Nutrient movement is done by the infauna almost entirely.

With a DSB over an undergravel filter over a plenum (with the UG pulling the water down through the sand), you have one way traffic through the sand bed. As the nitrate is produced by the aerobic layers it is "pulled" into the deeper layers where first facultative and then anaerobic bacteria can break down the nitrate. The nitrogen gas is then pulled all the way through the sand and ejected back into the water column. This pulling of water through the sand would make the transition zone much thicker and would therefore support billions of more bacteria working their little hearts out processing nitrogen in it's various forms. A denitrification superhighway with one-way traffic.

That's an interesting hypothesis, but it doesn't seem to work in reality. Some UGF systems do work, but the longer-running systems are actually prefiltered with sponges and reverse flow.

Was the system driven by reverse undergravel filtration?

In aquarium terms, a plenum means no undergravel filter.

I'm not surprised that your system has anoxic conditions under the sand, but I don't see how that guarantees a priori that denitrification will proceed. In practice, these systems seem to clog rather quickly with debris when located in a display tank.

 

[MarkD40]

Johnathan

I think we can agree that denitrification is occuring someplace. My nitrates are very low, usually 0. Whether it is occuring primarily in my LR, or the sand, is moot. It does occur. In my opinion I don't see how anything is happening in a static plenum system. What moves the partricipating molecules down into the deeper layers and then back up into the water column? Where does the energy come from for this directed movement?

I suppose Brownian movement could account for some of the movement, but this is so random the molecules would go up down and sideways. It would take, I would think, a very long time for any significant migration to occur and would be highly inefficient.

This probably explains why they found denitrification in the sea bed to be negligable.

It seems that this is an area that needs some serious study. I am begining to feel that many of the theories that are accepted as dogma by us reefers may have some truth but are incomplete. These theories may have started out as opinion and were repeated so often they are now accepted, when nobody can point to a specific scientific study that proves what they are talking about.

I have neither the time nor inclination to do the study myself, but it seems that it would be a pretty easy one to set up. Have several tanks set up with different configurations and cycle them all the same way. You could then add equal amounts of ammonia to each system and do longitudinal qualitative and quantitative measurement of the products of the denitrification process. This should give you the data needed to determine which system is the most efficient.

As you can tell I am frustrated by this whole discussion because I am being told things that fly in the face of common sense. Denitrification and nitrogen fixation are essential for life and it occurs everywhere on this planet. I just can't accept that the conditions necessary for it to occur need to be as precise as we are all making it out to be!

 

[bertoni]

<a href=showthread.php?s=&postid=6932882#post6932882 target=_blank>Originally posted</a> by MarkD40
I think we can agree that denitrification is occuring someplace. My nitrates are very low, usually 0.

The bioload seems very low in your tank, and it's not uncommon to find low nitrate levels. Your UGF might be support a small level of denitrification somewhere, and perhaps in your case the total impact is positive, but getting data would be difficult. Most systems that use standard UGF systems (high flow, unfiltered, standard flow direction) seem to have issues with nitrate levels.

Whether it is occuring primarily in my LR, or the sand, is moot. It does occur. In my opinion I don't see how anything is happening in a static plenum system.

I don't think a static plenum does anything, so we are in agreement there.

I just can't accept that the conditions necessary for it to occur need to be as precise as we are all making it out to be!

I suspect that there are many possible paths for denitrification. One of the more useful ones for our systems seems to be algal uptake. For more natural systems, I think some of the papers I have discuss the actual flow of nitrogen, and should help with that issue, if you want to do the reading.

The experiment you suggest could turn up some useful data, but there don't seem to be any volunteers, and there's no money in aquarium science, so I think we're out of luck.

 

[bertoni]

I went back and read the proximity thread more carefully, which I haven't done in a while. The total rate of denitrification of the sediment is said to be negligible, which I had long since forgotten. That's actually consistent with the relative rates of uptake observed for a macroalga vs a DSB in my experience and that of the chemistry forum moderator. I'll have to check my references again on nitrogen flow in the reefs to see what the pathways are thought to be, if anyone knows.

This book is old, but is supposed to have a lot of good data in it:

Biological Oceanographic Processes, 3rd edition. T R Parsons, M Takahashi, B Hargrave, Pergamon Press, 1984, reprinted with corrections, 1988, 1990.

I haven't gotten to work on it yet because I'm busy with an invertebrate zoology book.

In general, reefs get an enormous ocean flow over them, and that moves more nutrients past them than can be consumed. I think the Charpy reference goes into that topic. So nutrient flow in the reef is likely a small portion of the ocean flow effect.

 

[Paul B]

Coral reefs occupy a very tiny almost insignificant part of the sea. They are only in a thin band near the equator and only a little more than a hundred feet deep.
Any nutrients that pass by a reef or are created by a reef will be diluted by the vast quantities of water most of which is much too deep to grow algae. The surface of the sea is filled with plankton and singular celled algae's. The algae is the major consumer of nutrients which becomes incorporated into animal tissue via plankton. The seafloor does not have to process nitrogen.

We as aquarists do not have the good fortune to have an ocean to dispose of our fish wastes or the associated diatoms to feed it to.

Just a little rant.
Have a great day.
Paul

 

[Swanwillow]

<a href=showthread.php?s=&postid=6932027#post6932027 target=_blank>Originally posted</a> by Paul B
Swan, don't get a RUGF.
I want to be the only one with a 35 year old reef. :lol:

:beer:
Paul

too bad

I figure if I start when I get my house finished, and remodle the spare bathroom for a fish room, I'll be in my 60's when its as old as your tank is now...

which means, yours would be around 70, and there would be no contest... maybe they'll let ya take it to the nursing home, right?


but, back on topic: I love this discussion... I have to go through and re-read it all, and warp my mind to it again