Denitrification/Dissolved Organic Carbon as anaerobic fuel source? - The Planted Tank Forum
 
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Old 02-28-2012, 12:00 AM Thread Starter
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Denitrification/Dissolved Organic Carbon as anaerobic fuel source?

I'm going deep in immersing myself in this topic, but haven't yet found an answer so thought I'd ask here to see if anyone else does!

It seems that for anaerobic denitrification to occur, anaerobic bacteria require an organic carbon source to act as an electron donor. So, does Dissolved Organic Carbon (DOC), fulfil this function effectively?

I'm talking about the use of micro-porous filter media, such as Seachem Matrix or Pond Matrix in a canister filter, where aerobic bacteria grow on the surface converting ammonia and nitrites to nitrates, and anaerobic bacteria grow underneath that and within the Matrix pebbles where the flow is very low and where the oxygen has already been consumed by the bacteria on the outer layers. Accordingly, that creates an anaerobic zone where denitrifying bacteria provide themselves with oxygen by stripping the oxygen atoms off nitrate molecules, thus producing nitrogen gas which then off-gasses harmlessly into the air. (This is what marine aquarists use Live Rock for).

However, I've been trying to research this and it seems that the denitrification process is limited if a carbon source is not present. For this reason, some hobbyists with denitrifying filters dose a few milliliters of sugar or vodka into the water to provide the electron donor carbon source. Tetra nitrate minus does this as well, and wastewater treatment plants use a proprietary product called MicroC which is carbohydrate based and which contains methanol, in order to enable denitrification to occur. However, there are apparently some drawbacks to dosing sugar or vodka as well.

Accordingly;
A: Does DOC act as a carbon source for denitrification?
B: What does this do to the DOC molecules then? Destroys them somehow?
C: To enable anaerobic denitrification, should I not use activated carbon in my filter as that removes the DOC's I may need for denitrification?
D: Does the use of a powerful UV sterilizer oxidize the DOC's and thus render them incapable of being used to enable denitrification?
E: What is the best size media to use in a canister filter to achieve the most effective denitrification, the smaller Seachem De*nitrate or the progressively larger Matrix or Pond Matrix?

(P.S. Yes, I should perhaps just increase my light intensity to up my plant growth so I wouldn't need to worry about nitrate build-up. But then I'd have to worry about pruning frequently instead, and I'd nevertheless like to get to the bottom of this particular issue, if anyone happens to know anything else about it hopefully!
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Old 02-28-2012, 12:32 AM
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Hi Mxx,

Here is a thread that has been going on for a while that seems similar to the tact you are on.

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Old 02-28-2012, 04:27 AM Thread Starter
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Quote:
Originally Posted by Seattle_Aquarist View Post
Hi Mxx,

Here is a thread that has been going on for a while that seems similar to the tact you are on.
Not really what I was talking about, but was an interesting read nevertheless! Maybe I should start composting my grass clippings for CO2 production...
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Old 03-01-2012, 10:57 PM Thread Starter
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Bump? In case anyone can answer my original questions?
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Old 03-02-2012, 09:54 AM
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You might want to check out some of the SW reef forums. Various forms of denitrification units have been used for years by them. There are also several commercial products available. I've used them on my reef system at one time.

Activated carbon isn't really an issue. In fact, some types of denitrification filters use carbon as the filter medial because it's extremely porous. UV has no real effect either, because your not using it inside the denitrification unit.

Typical denitrification units process water very slowly, usually the output is measured in drops per minute. Too fast a flow and you don't get complete nitrate reduction and the filter throws off nitrite. Too slow and you get sulphate reduction and that "rotten egg smell".

I found that while they worked, they were a pain to constantly control. Control is via the flow rate and sometimes the amount of "food" supplied. As you point out, this is usually some form of carbohydrate or alcohol (vodka).

My opinion of them is that with all the work and testing involved, "the cure is worse than the disease". Maybe the question to be asked is, what are you trying to do? Obviously, your trying to reduce nitrates, but why the interest in this type of filtration? In a FW system water changes are rather easy, and a 50% water change will give you a 50% reduction in nitrates in the tank.

There is one other alternative you can look into. That is algae turf scrubbing. Yes, I've used that on my SW reef system also. In this method of filtration, you grow algae on a plastic screen, and the algae growth removes the nitrates. I have not used this system on a FW planted tank since usually the plant are very effective with nitrate removal on their own.

Hope this helps a little.
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Old 03-03-2012, 12:00 AM Thread Starter
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Actually looking on reefer forums has been exactly what I was now doing, and got some answers there. These practices are utilized much more on the SW side in any case. And it's quite common even to add organic carbon, with vodka/vinegar/sugar to help fuel denitrification.

But I wasn't actually talking about a denitrifying coil filter, which I read about but which I'm not tempted to try due to their various drawbacks. It's much better to use porous media which is anaerobic internally, while still being aerobic on the outer surface of each piece.

Using anaerobic media is in any case supposed to be a completely maintenance free approach to improving water quality. Which is thus easier than monitoring and maintaining an algae scrubber which plants might in any case hinder the effectiveness of. in planted tanks.

I get stressed out however by the thought of my water quality degrading hour-by-hour in-between water changes... which detracts from my enjoyment of the hobby for odd reasons.

Thanks!
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Old 03-03-2012, 01:29 AM
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Glad to see you have been reading the reef forums about the subject. Yes, a source of organic carbon is typically used. As I pointed out, it's got to be controlled carefully. There have been a lot of reef system crashed because people didn't handle things carefully when it comes to denitrification.

I have tried the old method of using a filter sponge to form the aerobic/anaerobic media. I have not tried the various porous media in that capacity. Personally, I didn't find the filter sponge to be that effective. I would expect much better results from the newer media.

Yea, I've heard all about "maintenance free" filter stuff for years. At this point I a sceptic if that's claimed, but there are many low maintenance methods out there that work well.

I will say that I have been amazed with the amount of nitrates and phosphates a fully planted FW system can remove. I never thought I'd need to dose them.

Some people, especially commercial breeders that are trying to get the fish growth rate as high as possible, make massive daily water changes. It doesn't hurt anything if done correctly, but I find this way too much work.
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Old 03-03-2012, 11:58 AM Thread Starter
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I experimented with dosing organic carbon last night, both in the form of vodka and solid carbohydrates. But I think I overdid it. I wouldn't say it caused the system to crash, but there subtle signs of a hangover today, including some cloudiness. I followed that up by dosing nitrates in bacon form, to see if that would help to break down the excess of ethanol as well, but don't have a test kit to check whether this worked. Clearly I'd better use much smaller quantities if I'm gong to be adding it to my aquarium instead...

How much sponge did you use? I'd seen one example where I think they were using a 12" deep layer of sponge for the length of their sump. And apparently for a deep sand bed, 4" of grain of sugar size sand is not enough, while 6" is too much.

In my current tank if I upped my lighting from its current relatively low intensity to medium intensity then I expect I wouldn't have any nitrates, but I might then need to start dosing things as well and pruning frequently. In my next tank however, I'll use MTS and fertilize the substrate directly, while trying to maintain the water column as lean as possible, including the elimination of measurable nitrates if possible through filtration means.
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Old 03-03-2012, 08:20 PM
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How big a tank are you talking about? Typically, vodka dosing is about 1 ml per day in a 100 gal tank. I highly recommend you get some good test kits if your going to do stuff like this. That's the only way you can see if it's working.

I used a sponge about 4" square, that ran the width of the sump. So it was about 4x4x12.

Deep sand beds are a little different. I use one in my reef today. Typically you use about 4 inches, but the sand is usually a little courser that sugar sand. Sugar sand is very fine, and tends to compact, so while it's ideal for a shallow sandbed, it's doesn't allow the slight circulation needed in a deep sand bed.

While this works great in a SW reef system, I'm not sure how it would translate into a FW planted tank. I use about 3 inches of eco-complete in my planted tank.
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Old 03-03-2012, 08:59 PM Thread Starter
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Let's see, I think it was maybe 6 or 7 rounds between the stiff mojitos and shots, among others, followed by late-night takeaway curry and then a full breakfast fry-up...

Okay, so perhaps my nuances are a bit on the subtle side!

On the DSB I can't say I'm speaking from experience, that and the five inch deep bed was just what I'd read as a recommendation somewhere. Course sand is in what mm grain size?

I'd would think that biological filtration would differ too much between FW and SW apart from our stock having slightly differing requirements, but I could be wrong.
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Old 03-04-2012, 12:14 PM
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Sorry about that. I like sand for a SW deep sand bed to be about 2 - 3 mm.

Biological filtration in FW and SW is a similar process but it's set up differently in an aquarium system. In a SW system typically you'd use a large amount of live rock in the main tank, possibly with a deep sand bed and/or refugium. (Note - A refugium is an external tank plumbed into the main system and typically grows macro algae to remove nitrate and phosphate.. Flow through it is comparatively slow.)

In FW the biological media is usually in the filter and substrata. Also, the live plants are also part of the process.

The goals are slightly different also. In SW reef systems the goal is to keep nitrates and phosphates as close to 0 as possible, to prevent algae outbreaks. Note the extremely high amount of light used in reef systems. In FW planted systems you need a certain amount of nitrates and phosphates to get decent plant growth. This is why various ferts are added.

That's why I'm not sure if my experiences with a deep sand bed in reef systems would be applicable to a FW planted tank. It's not that it wouldn't work, but would it be giving you the results you wanted? Some times too good a job can be done by them.
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Old 03-04-2012, 03:36 PM Thread Starter
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Thanks! I ran the same question in reef forums and here were the answers I received. So maybe we do have some things to learn from across the other side of the shore... Below are two of the more informative responses were (as relates to marine, though similar in underlying biology). End result is that I've decided to start with putting a drop or two of white vinegar in my tank daily, adding the carbon back in, and not worrying about my UV sterilizer circumventing denitrification -

A. I expect that some portion of dosed organic carbon does indeed act to drive denitrification. What portion probably depends on what is dosed and a hundred other properties of the tank system,.

B. The organic molecules can end up as CO2, primarily, or can get incorporated into bacteria tissues.

C. GAC won't remove the usual sources of dosed carbon, such as vinegar or vodka.

D. UV won't do much. A lot of ozone treatment might oxidize some, but not much. The molecules we dose are fairly resistant to ozone.

E. Couldn't say what media is best. Live rock and sand might be more than enough when dosing organic carbon.

and -

Many denitrifying filters utilize some form of organic carbon. Older Aquamedic denitrators, coil denitrators, and the newer Aquaripure denitrators all use some form of organic carbon that is readily mineralized.

So, you've mostly answered your own question, actually:

A). Yes, it is, as you've said, completely dependent upon availability. Inorganic carbon is also used by some bacteria, but that usually isn't an issue in seawater up to an extent.

B.) The carbon is oxidized in the process into CO2

C). In many cases, that probably isn't necessary. In the aquarium, there are usually plenty of sources at the substrate level. In seawater, it is best to have activated carbon to help lower concentrations of organic carbon, considering that many organisms (corals included) make lots of it.

D). Some work has been done to show that ozone use can potentially enhance it by breaking down molecules that are normally much more difficult to metabolize. I think it is somewhat plausible that the same may be true for UV sterilization. There are some papers available on the subject, but I haven't looked into them completely to know that this is the case for sure.

Edit: skipped E, lol. To be honest, I'm not entirely sure on that one, either. I would assume media that have a higher internal surface area would be best. That would serve, up to a certain point, to not only provide as many settling points as possible, but to start limiting oxygen as well. As long as you provide the appropriate external conditions, any porous/higher surface area medium can be used, AFAIK.
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Old 03-04-2012, 04:43 PM
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Mxx, you might search wetland soils, carbon limitation, Denitrification.
Denitrifyers do become reduced Carbon limited.

This has been shown in the FL Everglades where aquatic submersed plants are present, algae and plants tend to provide a fair amount of reduced carbon which the bacteria will use.

Unlike a hyper skimmed reef tank...........plants will remove the NO3 and NH4 directly and fast(same for the marine plants/refugiums), they clean the tank very very well........hardly feed............leading to a carbon limited tank.

Sigee's Freshwater Microbiology
Reddy/DeLaune Biogeochemistry of Wetland Soils

Both are good over view general text on the topic.




Regards,
Tom Barr
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Old 03-04-2012, 04:47 PM
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Quote:
Originally Posted by Mxx View Post
A: Does DOC act as a carbon source for denitrification?
It can, depends on what form.

Quote:
B: What does this do to the DOC molecules then? Destroys them somehow?
Oxidizes them just like when we burn carbs, a form of reduced carbon.
CO2 + energy is the end result.

Quote:
C: To enable anaerobic denitrification, should I not use activated carbon in my filter as that removes the DOC's I may need for denitrification?
Does not matter.

Quote:
D: Does the use of a powerful UV sterilizer oxidize the DOC's and thus render them incapable of being used to enable denitrification?
Not likely, might enhance simpler forms of DOC's that are easier for species to utilize.

Quote:
E: What is the best size media to use in a canister filter to achieve the most effective denitrification, the smaller Seachem De*nitrate or the progressively larger Matrix or Pond Matrix?
Deep sediment in a planted tank does a pretty good in conjunction with health plant roots.

Quote:
(P.S. Yes, I should perhaps just increase my light intensity to up my plant growth so I wouldn't need to worry about nitrate build-up. But then I'd have to worry about pruning frequently instead, and I'd nevertheless like to get to the bottom of this particular issue, if anyone happens to know anything else about it hopefully!
I'd just live with it and do water changes with the rich NO3 tap water.

Less worry, more gardening.




Regards,
Tom Barr
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