Denitrification, carbon, oxygen etc..

[Ontheway]

Hi,
From several experiments, I understand that carbon is the limiting factor for denitrification in a normally maintained aquarium.
Sometime ago, I run a small (10 liters) standalone methanol denitrator 1/3 way filled with siporaxes to create bed for denitr. bacteria, supplied it with methanol and mixed continuously with a tiny pump placed near to base (so, no surface agitation, but no secure lid either, instead, a loose lid with several holes on it). After maturing (and plenty amount of methanol), it started to remove nitrates under 24 hrs (was about 25 ppm), with a relatively minor amount of methanol addition.
In the above scenario, I clearly see that the container was not completely isolated from air (not air-tight), but worked very well. Then, what happened to the fact that denitrification requires no-oxygen (called anoxic, I think) water? Or is it working in low-oxygen (hypoxic, I guess?) conditions, too? Another fact is, biopellets, they are removing nitrates, with using normal tank water with regular oxygen levels.
It seems as long as you supply an appropriate carbon source, anoxic conditions are not necessary.

For me, the main disadvantage of liquid carbon dosing or pellets is, you need to remove excess bacteria from water. Instead, preserving a crowded bacteria colony in a small space (with help of siporax, or other media) and letting them process nitrates without creating waste, looks more attractive to me.
My question here is, what is the benefit of using air-tight denitrators? Are they good only for sulfur method? Or may be just for accelerating the processes?

 

[Randy Holmes-Farley Verified Community Expert]

If you have a carbon denitrator type of device that does not become low in O2, it will still reduce nitrate because aerobic growth of bacteria using the carbon source will consume both nitrate and phosphate to incorporate into the bacterial tissue mass. If O2 is kept out well enough for it to become low in O2, then the amount of nitrate used will increase because bacteria are also using it as an electron acceptor in place of the O2 (producing N2).

When you dose organic carbon directly to an aquarium, you likely get a mix of effects: aerobic growth in the water column and on surfaces, and some growth in low O2 area in sand, rock pores, deeper in biofilms, etc. I really have no idea how much of each, and it almost certainly varies a lot tank to tank. But since folks dosing this way often find residual phosphate and low nitrate, I expect there is at least some low O2 denitrification.

Biopellets seem to effectively reduce nitrate enough more than phosphate that there are likely some low O2 regions on the surface of the pellets, under a biofilm of additional bacteria.

I've never used a carbon denitrator, but it seems that the trickiest part is getting (and keeping) the flow just right so that it does not become anaerobic and potentially produce hydrogen sulfide, while being slow enough to effectively reduce nitrate.

 

[Ontheway]

Randy thank you for the answer. I think tricky part you mentioning is handled by an orp controller.

Here is my following question;

In an air-tight denitrator, is it possible/wise/meaningfull to use a very small amount of biopellets as carbon source (Thinking it as replacement for 'the scary' sulfur) ?
Idea is placing a Sponge disc over siporax mass in denitrator, and putting some minor amount of biopellet balls, a very thin layer of it, without any biopellet movement. Do you think if it functions as a carbon source for bacteria, if yes, will it cause production of excess bacteria? Denitrator is recirculating, btw.

 

[Randy Holmes-Farley Verified Community Expert]

Why is sulfur scary?

I'm not sure of the advantage of putting biopellets in a denitrator instead of a normal recirculating reactor, but it should work.

 

[Ontheway]

I dont know either, in almost all threads across internet, sulfur reactors are considered dangerous by many people, may be people scaring of it due to H2S production, but it will be produced in any other anoxic case (may be in higher amount in existence of sulfur?). No idea..

What I understand from your text is, in existence of O2, nitrate is used only for body building. In no-oxygen, for both body building and as oxygen source. I am afraid there is another question, can a handful of biopellets, without any movement (so no friction), provide carbon to underlying 3-4 liters of siporax bacteria beds in a denitrator ? Or, in other words, may it replace sulfur balls?

 

[redfishbluefish]

This is my very limited understanding of the differences in biopellets versus denitrifier.

Biopellets require the growing bacteria to be sloughed off the pellets and picked up by your skimmer....to export the nitrates and phosphates out of your system. I would imagine the majority of bacteria to be aerobic, considering it is in a swirling reactor, but I'm sure some anerobic guys sneak in there as well....and it's not important as far the the strain of bacteria...you just need any strain of fast growing bacteria. Any bacteria that consumes the polyester food source will do.

Denitrifier does not attempt to slough off the bacteria (although I have to believe some escapes), but is maintained on a matrix (egg crate) to convert nitrate to nitrogen gas....now leaving your tank. The bacteria is a particular strain of bacteria that consumes methanol as it food source. It is my understanding that this particular strain is not happy amoungst oxygen.

You wouldn't want to put biopellets in with the denitrifier because you will end up contaminating the particular strain of bacteria within the denitrifier.

The commercially avialable denitrifier (sold in the US) that I'm aware of does have a solid top on it, limiting outside air from entering. The filling and empting really doesn't bring in new air....just new water. So I'm not sure why you would have a top with holes. Cut a new piece of acrylic that snuggly fits the top of your container.

 

[Randy Holmes-Farley Verified Community Expert]

I don't consider a sulfur denitrator any riskier than a carbon denitrator, and maybe less so. It depletes alk, but that's the only drawback I'm aware of. Hydrogen sulfide wouldn't typically come from the sulfur.

"What I understand from your text is, in existence of O2, nitrate is used only for body building. In no-oxygen, for both body building and as oxygen source."

Yes, that's correct.

I don't really know how much organic matter is released from biopellets in normal use or as you describe.

 

[Ontheway]

Here is my current denitrator:

Then we are coming to a point, whether biopellets are able to slowly release carbon to water or not. Another way of saying, does it melt in denitrator water (do low ph conditions help them on this?). If it does not, bacteria population will only populate around pellets and will not provide carbon for the rest of denitrator...

 

[Randy Holmes-Farley Verified Community Expert]

Here is my current denitrator:

Then we are coming to a point, whether biopellets are able to slowly release carbon to water or not. Another way of saying, does it melt in denitrator water (do low ph conditions help them on this?). If it does not, bacteria population will only populate around pellets and will not provide carbon for the rest of denitrator...

The organic carbon is mostly released from the pellet surface by enzymatic action due to bacteria. The abiotic hydrolysis is quite slow.

The bacteria try to grab the monomers that are hydrolyzed off the pellet surface, but they surely do not get it all. What fraction they get, however, is not known.

 

[Ontheway]

The organic carbon is mostly released from the pellet surface by enzymatic action due to bacteria. The abiotic hydrolysis is quite slow.

The bacteria try to grab the monomers that are hydrolyzed off the pellet surface, but they surely do not get it all. What fraction they get, however, is not known.

Then what is the situation for sulfur beads? Is it acting in a similar way with pellets, ie, bacteria colonizing on their surface too? I have to admit that I did not fully understand how sulfur feeds bacteria and fills the carbon need...

 

[Randy Holmes-Farley Verified Community Expert]

Then what is the situation for sulfur beads? Is it acting in a similar way with pellets, ie, bacteria colonizing on their surface too? I have to admit that I did not fully understand how sulfur feeds bacteria and fills the carbon need...

They will still need carbon to make organic molecules, probably from CO2 or bicarbonate that they convert into CO2.

I discuss all of the nitrate reducing methods here:

Nitrate in the Reef Aquarium - REEFEDITION
https://www.reef2reef.com/blog/nitrate-in-the-reef-aquarium

from it:

Sulfur Denitrators

In these systems, bacteria use elemental sulfur and produce N2 from the sulfur and nitrate according the following equation (or something similar):

2 H2O + 5 S + 6 NO3– → 3 N2 + 5 SO42- + 4 H+

The production of acid (H+) in this reactor can tend to reduce the aquarium alkalinity. It has also been suggested to pass the effluent of such a reactor through a bed of aragonite to use the acid (H+) produced to dissolve the calcium carbonate, and thereby provide calcium and alkalinity to the aquarium. While that is a fine idea, it doesn’t add much calcium and alkalinity to most aquaria.

To estimate the magnitude of the effect, we start with a liberal estimate of how much nitrate might be removed. Say 10 ppm of nitrate per week.

10 ppm nitrate = 0.16 mmole/L of nitrate

Since 4 moles of H+ are produced for every 6 moles of nitrate consumed, this will produce

0.107 mmoles/L of H+ per week

How much calcium this could produce?

Assume that it takes one proton to dissolve one calcium carbonate:

CaCO3 + H+ → Ca2+ + HCO3–

Clearly, this is a substantial overestimate because much of the acid will be used up driving the pH down to the point where CaCO3 can even begin to dissolve. Consequently, we have an upside limit of 0.107 mmoles of Ca2+per week. Since calcium weighs 40 mg/mmol, that’s 4.3 ppm Ca2+ per week.

For comparison, an aquarist adding 2% of the tank volume in saturated limewater daily is adding on the order of 16 ppm of calcium per day. Consequently, this method may not be especially useful for maintaining calcium. Additionally, the acid produced will have a long term lowering effect on the alkalinity. In fact, it is double dipping on the alkalinity depletion since alkalinity is consumed when the nitrate is produced, and again when it is removed in the denitrator. So if you use a sulfur denitrator, be sure to monitor the alkalinity in the aquarium.

 

[Ontheway]

I am currently testing a Siporax container. Its a small (10 liters) glass tank that sits in the sump and its top is above the sump water level. I filled it with 6 liters of siporax. Its top is fully open and when I stop return pump, sump water level rises and floods the container. Other then that, the water in container remain stagnant. One day I decided to check the pH of that water and it was higher then what I have in aquarium. Time to time I just let the siporax container flood to equalize the pH, and saw that ph level rises back, less then 24 h. Today I checked the ph and alkalinity of both water, tank pH is 7.83, dkh is 8.3; siporax container pH is 8.17 and dkh is 10.2.
Now, I know that denitrification process increases pH (I guess alkalinity also) My question is, if denitrification the only reason of pH increase in the container? To my poor eyes, nitrate levels seem equal to me, in both water.
Currently, my main tank ph levels are a bit on the low side and I am planning to increase it from 7.8-7.95 to 8.1-8.2 area, using this water, with help of a pH controller (if siporax container pH is above 8.1, feed it from tank, let it overflow into sump, to help increase Alk and pH in main tank).

 

[Randy Holmes-Farley Verified Community Expert]

I do not know if it is the only reason. How high is the nitrate?

The denitrification of nitrate in seawater consumes acid and effectively raises alkalinity and pH. The production of CO2 should offset the pH rise, but CO2 may be leaking out with the N2 somehow.

organic + 124 NO3– + 124 H+ → 122 CO2 + 70 N2 + 208 H2O

So a drop in nitrate of 1 mM (62 ppm) will boost alkalinity by 1 mM = 1 meq/l = 2.8 dKH. You saw 1.9 dKH, so the nitrate would have to drop a lot for this explanation to hold.

Does it smell of rotten eggs/hydrogen sulfide?

That process also produces alkalinity:

(CH2O)106(NH3)16(H3PO4) + 53 SO4-- > 56 CO2 + 50 HCO3- + 53 HS- + 16 NH3 + 53 H2O + PO4---

 

[Ontheway]

I just did my best from an inch to smell a clue of a rotten egg, but nothing

Forgot to mention, the siporax container is sitting there for over 2 months, and until last week, its fed continuously with sump water (see the small hose in the video above).
Edit: nitrate level is under 5 ppm in both tank and container. I just cannot decide color to further differentiate.

And here is whats going on, with a fresh video;

 

[Randy Holmes-Farley Verified Community Expert]

Might just be testing error.

 

[Randy Holmes-Farley Verified Community Expert]

I see a CO2 cylinder in the background. Do you use it in a reactor?

I also see the Siporax container is open at the top, and may be outgassing CO2, whereas the tank may be getting CO2 from a reactor. That might explain the pH difference.

 

[Ontheway]

Might just be testing error.

Haha, I dont think so, because whats on the video were repeated at least 5 times in different days, last week. And forgot to add, when I was feeding the container continuously, I experienced a pH decrease in tank, both day and night values lowered. After removal of small hose that feeds the container, tank pH seems slowly recovering.

 

[Ontheway]

I see a CO2 cylinder in the background. Do you use it in a reactor?

I also see the Siporax container is open at the top, and may be outgassing CO2, whereas the tank may be getting CO2 from a reactor. That might explain the pH difference.

No, co2 reactor (or any other reactor) is not in use at the moment.
But, the second paragraph tells me something...
Silencer of skimmer is close to Siporax container. At least, they are in same cabinet, in the end. I think I have to put a hose to silencer to extend it then we will see how things develop, thanks Randy, I wonder how I skip such an obvious point...

 

[Randy Holmes-Farley Verified Community Expert]

Let us know what you find.

 

[Ontheway]

Well.. first, I relaxed skimmer air intake by fully open cabinet doors for 24 hrs. No noticable difference..

I am further experimenting with siporax, and want to share some results.

While siporaxes were sitting in a small container, I decided to follow ph and orp status in it.

After flooding the container, ph & orp were equalized with tank, then, ph decreased first for a couple of hours and then started to increase, steadily (From around 7.8).
I let it climb up to 8.2 while tank water were 7.9 max.
I flooded the container and followed ph again. Same repeated in 12 hours. Then I remember what I read in articles, denitrification results with increase of alkalinity, so ph. This time I measured Alk. of tank and container, tank was 8.3, siporax was 10.2 (siporax ph was slightly over 8)
This repeated few times and finally I connected ph controller and a small pump to flush the container when it reaches to 8.0 ph
This was 5 days ago. Since then, pump works twice about every 24 hours and supplies the tank with alkalinity. Dosage pump is also working and its providing Ca and Alk. also (placed after siporaxes).

Any idea how this happens? Will it continue to provide alkalinity forever?