Looking for thoughts on organic carbon dosing and nitrate

[Randy Holmes-Farley Verified Community Expert]

I´m not sure that I follow you. IMO - More DOC - more consumption of O2. In the sediment DOC increase the bacterial growth and consume O2 in the first step. When O2 is gone att x µm in the biofilm/sediment NO3 start to be consumed instead.. Because DOC is the limited growth factor - it will be rapidly consumed in the interface between water and sediment - no need for diffusing further down. However - your are right - you can´t expect a rapid denitrification in these cases but you will have it sooner or later. This is exactly my point - when you after a certain time note a decrease in NO3 (higher than before) - the most likely reason is denitrification not growth

Edit: By the way - in nitrifying biofilms it had been shown that O2 levels are low already after some µm and heterotrophic bacterial growth are ten fold higher - if there is DOC enough - the O2 will disappear fast.

Sincerely Lasse

What I've always been skeptical of is the need to get all of the three (O2, nitrate, and organic) to the right concentrations in the same place to have a lot of denitrification.

Your statement simply asserts it is happening, but why would there be more organic than O2 diffusing into the region you propose for denitrification. O2 is faster diffusing and is present at higher concentration.

Then one also needs nitrate to diffuse in fast enough to get denitrification. It too will diffuse slower than O2. How does that happen? Maybe just when it is far higher in concentration than O2 in the bulk water.

I'm sure some denitrification does happen, I just do not see the molecular level processes that allow it to be a major pathway.

What evidence exists that denitrification is the major pathway for organic consumption and nitrate reduction in an ordinary reef tank?

Your setup is a very different scenario where you pump things through the plenum.

 

[Randy Holmes-Farley Verified Community Expert]

FWIW, the reason I think normal plenums under sand helped denitrification was to allow flow through the sand to proceed more readily based on pressure/flow differences at the sand/water interface.

Places where the bulk water was tending to flow downward toward the sand will slowly push water into the sand, down to the plenum, along the plenum to places where water is tending to flow away from the plenum, then the water rises up through the sand, and back out into the bulk, reduced in O2, nitrate, and organics.

Without the plenum, the resistance to lengthy sideways travel through the sand is going to greatly reduce the possible flow.

 

[Randy Holmes-Farley Verified Community Expert]

Edit: By the way - in nitrifying biofilms it had been shown that O2 levels are low already after some µm and heterotrophic bacterial growth are ten fold higher - if there is DOC enough - the O2 will disappear fast.

Sincerely Lasse

The O2 measurement is presumably correct. But you are extrapolating what amount of organic and nitrate will be present at those depths. I don't see how that happens to a large extent, as noted above. How could there be a lot of free organic left if the higher amount of starting O2 got depleted be metabolizing organics?

 

[Lasse]

Lasse can you provide some references on this. Very interesting.

Just google something like this O2 distribution in nitrifying biofilms - or similare

Your statement simply asserts it is happening, but why would there be more organic than O2 diffusing into the region you propose for denitrification. O2 is faster diffusing and is present at higher concentration.

IMO the diffusion speed/rate is of no concern here because we are not talking about processes that take place at the same time. Without adding DOC - it is limited for bacterial growth - at this point O2 and NO3 is not. You add DOC and O2 will be consumed in the substrate. At a certain point/deep O2 will be limited for aerobic growth - anaerobic conditions occur. The diffusion of O2 can´t fill upp the demand of O2 in the sediment. It will be consumed much faster than it diffuse down. In this scenario is DOC not limited - it will in spite of lower diffusion rate/speed spill over deeper in the sediment. NO3 is there and will be consumed when some of aerobic bacteria switch to anaerobic respiration. The growth rate is much slower than for aerobic condition so there will be DOC leftover as electron donor. NO3 in the sediment is consumed and it will diffuse NO3 from the water down to the deep there denitrification take place. It goes slowly but the O2 is already consumed at the point/deep there the anaerobic processes start. If you stop the DOC dosing O2 will penetrate deeper because its not consumed

The question was why people often report a period of months before DOC dosing affect the NO3 concentrations. My answer is still - it take time before denitrification starts and when you start with high concentrations NO3 - the rather fast decrees after a while can´t have any other explanation than denitrification taking place.


As you - my 5 cents are that the passive denitrification with or without DOC is not a effective way to handle this if you have NO3 problems. This is the reason why I have construct my aquarium like I have done. However - the flow through my sand bed is rather huge (around 100 L/hour). My plenum is aerobic. The denitrification must take place in the middle of sand bed when O2 is consumed. I had suspect that I need a lot lesser flow and anaerobic in the plenum. But the fact is that is first since I start to run the flow rather high and a steady dose of DOC that it has start to work the way it does

Sincerely Lasse

 

[GARRIGA]

Just google something like this O2 distribution in nitrifying biofilms - or similare


IMO the diffusion speed/rate is of no concern here because we are not talking about processes that take place at the same time. Without adding DOC - it is limited for bacterial growth - at this point O2 and NO3 is not. You add DOC and O2 will be consumed in the substrate. At a certain point/deep O2 will be limited for aerobic growth - anaerobic conditions occur. The diffusion of O2 can´t fill upp the demand of O2 in the sediment. It will be consumed much faster than it diffuse down. In this scenario is DOC not limited - it will in spite of lower diffusion rate/speed spill over deeper in the sediment. NO3 is there and will be consumed when some of aerobic bacteria switch to anaerobic respiration. The growth rate is much slower than for aerobic condition so there will be DOC leftover as electron donor. NO3 in the sediment is consumed and it will diffuse NO3 from the water down to the deep there denitrification take place. It goes slowly but the O2 is already consumed at the point/deep there the anaerobic processes start. If you stop the DOC dosing O2 will penetrate deeper because its not consumed

The question was why people often report a period of months before DOC dosing affect the NO3 concentrations. My answer is still - it take time before denitrification starts and when you start with high concentrations NO3 - the rather fast decrees after a while can´t have any other explanation than denitrification taking place.


As you - my 5 cents are that the passive denitrification with or without DOC is not a effective way to handle this if you have NO3 problems. This is the reason why I have construct my aquarium like I have done. However - the flow through my sand bed is rather huge (around 100 L/hour). My plenum is aerobic. The denitrification must take place in the middle of sand bed when O2 is consumed. I had suspect that I need a lot lesser flow and anaerobic in the plenum. But the fact is that is first since I start to run the flow rather high and a steady dose of DOC that it has start to work the way it does

Sincerely Lasse

My personal experience with NoPox being I reduced high nitrates post cycle once phosphates were elevated. This happened quickly. Initial tests showed 160 ppm plus. Have to acknowledge I still had some nitrites since I was still stress testing biological capacity by increasing the dosage of ammonium chloride. Therefore nitrite may have resulted in higher nitrate test results.

My filtration is a modified undergravel plate where water enters at one end and travels through 24 inches of media which is 12” wide by 4” high. Goal was to extend the contact time to allow consumption of oxygen by nitrification then allow denitrification by lack DO yet containing bound oxygen in the form of nitrates.

Early on I suspended NoPox and was able to keep nitrates under 5 ppm but eventually that failed. NoPox was my only method to keep nitrates and phosphates in check. Often I allowed both to get elevated by suspending NoPox to then confirm I can repeatedly reduce both N and P and that was successful.

Last stoppage of NoPox occurred mid February to eliminate the resulting white slime and test using a different dosage schedule that would solve my issue yet avoid that white slime. At the time I had no GHA and both N & P at acceptable levels. Oddly, GHA never returned. Red turf did advance but I find that pleasing and not a fanatic seeking zero algae. That’s not how it happens in nature.

Have not tested my water since suspending NoPox because I’m one that lets the tank tell me when tests are needed. Busy with life otherwise. However, decided to finally setup my XR15 Pro and have been running it on full spectrum and full intensity past several weeks. I now finally have algae growing again although that red turf is still prominent and recall Julian mentioning this often happens as tanks mature although some green turf starting to emerge. Perhaps I should have been testing but will this weekend and will be sending out ICP as I’m curious now how effective this home grown algae will be effective at removing co2 and perhaps heavy metals. Something research says happens with plants as far as consumption of certain heavy metals. Goal is allowing nature be nature.

My assumption was that detritus decomposing would naturally provide the carbon and thereby removing the need to dose but that hadn’t been the case unless post suspension of NoPox the detritus has now decomposed to the point it’s naturally providing what’s needed. No clue and just speaking out loud.

Of importance is the fact I’ve not performed a WC since shortly after setting this test tank up in September of 2021. That singular WC was to bring salinity from 35 ppt to 12 ppt to test the acclimation of Sailfin mollies as my test subjects. There’s been no mechanical filtration other than tossing floss although very infrequently used. Cleaning GAC periodically although I don’t always run that and occasionally swirling a brine shrimp net to remove detritus that hasn’t been drawn into my filtration. Being a small tank I haven’t added a wave maker to suspend the detritus allowing it to get filtered. Of interest. Mostly what I see are uneaten pellets. Not necessarily fish poop. Makes me wonder what preservatives exist in those pellets and seems my Nassarius aren’t fans.

No sump. No socks. No skimmer. Just a modified undergravel plate designed to extend contact time and I feed heavy. 2-4 times daily and more than can be consumed in a couple of minutes to allow the more timid a chance to eat. Gone are the mollies and added damsels which make great test subjects as they aren’t prone to disease or stress from neglect. The dominos bring rather aggressive forces me to be sloppy feeding although being a test of decomposition the purpose has always been to be overly sloppy and stress test the system to evaluate the results.

Best pic I can find to show the design. Plate covers four inches of pumice. Water is drawn from the other side via a Tidal 75. Water entering the front by a small section of just couple of inches front to back. This forces the water to travel through a 20” plus horizontal tunnel. Think of it like a horizontal 20” plus DSB where sand replaced with 1/8” pumice.

 

[Randy Holmes-Farley Verified Community Expert]

IMO the diffusion speed/rate is of no concern here because we are not talking about processes that take place at the same time. Without adding DOC - it is limited for bacterial growth - at this point O2 and NO3 is not. You add DOC and O2 will be consumed in the substrate. At a certain point/deep O2 will be limited for aerobic growth - anaerobic conditions occur. The diffusion of O2 can´t fill upp the demand of O2 in the sediment. It will be consumed much faster than it diffuse down. In this scenario is DOC not limited - it will in spite of lower diffusion rate/speed spill over deeper in the sediment. NO3 is there and will be consumed when some of aerobic bacteria switch to anaerobic respiration. The growth rate is much slower than for aerobic condition so there will be DOC leftover as electron donor. NO3 in the sediment is consumed and it will diffuse NO3 from the water down to the deep there denitrification take place. It goes slowly but the O2 is already consumed at the point/deep there the anaerobic processes start. If you stop the DOC dosing O2 will penetrate deeper because its not consumed

You are asserting something (that the O2 has dropped low enough and that the organic rises high enough and nitrate is present in sufficient quantity in some location down in the biofilm) that may or may not be correct, but which i feel needs some sort of experimental evidence since it goes against my simplistic understanding of how those things get there and what consumes or produces them there.

 

[Lasse]

Its simple in the old model that I work after. No denitrification - no large NO3 reduction

some sort of experimental evidence

I´ll think that my aquarium is an example - I force the process but still a question of diffusion into the active points - here is a simple model - PO4 not limited

​

Sincerely Lasse

 

[Randy Holmes-Farley Verified Community Expert]

Its simple in the old model that I work after. No denitrification - no large NO3 reduction


I´ll think that my aquarium is an example - I force the process but still a question of diffusion into the active points - here is a simple model - PO4 not limited

​

Sincerely Lasse

IMO, your aquarium is a different scenario, pumping water, as I understand it, through the sand. Very different mechanism than diffusion into the sand, no longer relying on relative diffusion rates of different molecules.

I also think it is nearly impossible for a hobbyist to distinguish aerobic consumption of nitrate and organics and O2 from denitrification.

What is the evidence that you have denitrification rather than aerobic metabolism?

 

[biom]

Just google something like this O2 distribution in nitrifying biofilms - or similare

Thank you

You are asserting something (that the O2 has dropped low enough and that the organic rises high enough and nitrate is present in sufficient quantity in some location down in the biofilm) that may or may not be correct, but which i feel needs some sort of experimental evidence since it goes against my simplistic understanding of how those things get there and what consumes or produces them there.

There is no need nitrate to diffuse into the biofilm from surrounding water - they (nitrate) are produced here. Possible scenario: Outermost layer of bacteria is oxidizing ammonium using all oxygen available producing nitrite and nitrate. For the inner layer of bacteria there is no oxygen left so they are using nitrate and nitrite to "breath". But the denitrification process is much slower than nitrification that is why certain amount of nitrate produced is diffusing in the water column. It needs time before biofilm to grow ticker enough to "denitrify" all nitrate produced by the outer layer of bacteria.

 

[Randy Holmes-Farley Verified Community Expert]

Thank you


There is no need nitrate to diffuse into the biofilm from surrounding water - they (nitrate) are produced here. Possible scenario: Outermost layer of bacteria is oxidizing ammonium using all oxygen available producing nitrite and nitrate. For the inner layer of bacteria there is no oxygen left so they are using nitrate and nitrite to "breath". But the denitrification process is much slower than nitrification that is why certain amount of nitrate produced is diffusing in the water column. It needs time before biofilm to grow ticker enough to "denitrify" all nitrate produced by the outer layer of bacteria.

OK, I can accept that scenario, where you are denitrifying nitrate produced nearby, which also serves to lower O2.

.

 

[Lasse]

What is the evidence that you have denitrification rather than aerobic metabolism?

The PO4-P/NO3-N ratio when I start at 25 mg/L NO3 and 0.21 mg/L PO4 - In NO3-N and PO4-P ratio i t will be 25*14/62 to 0,22*32/95 mg/L -> 56:1 as mg/L N/P - as mol around 24:1 in N/P quota (if my PO3 was zero but it is not - and I have use GFO to bring it down to 0.07 mg/L.

Very different mechanism than diffusion into the sand,

IMO - not - it must diffusa into the gravel even if area and contact time is higher - its still the same mechanism

And for the books - I do not believe in the direct nitrification/denitrification in a single microfilm. It has been the wet dream for waste water technicians for decades and the closet they have come is the sequencing batch reactor (SBR) . IMO - one of the must important factors why its difficult to achieve nitrification/denitrication in the same biofilm without sequencing is the demand for DOC in the denitrification. If you add DOC in aerobic water - it will favour heterotrophic bacteria which grow faster than the nitrification bacteria. hence conqurt them out

Sincerely Lasse

 

[Dan_P]

IMO, your aquarium is a different scenario, pumping water, as I understand it, through the sand. Very different mechanism than diffusion into the sand, no longer relying on relative diffusion rates of different molecules.

I also think it is nearly impossible for a hobbyist to distinguish aerobic consumption of nitrate and organics and O2 from denitrification.

What is the evidence that you have denitrification rather than aerobic metabolism?

No experiment yet, but when I do set one up it will be with the @taricha hypothesis in mind.

Organic carbon dosing reaches a level where heterotrophic bacteria locally deplete oxygen to the point where other bacteria switch to nitrate reduction for energy. While diffusion of nitrate and DOC into the biofilm still needs to occur, the distance is small because the aerobic bacteria overlay the nitrate reducers (conjecture). This is the simplest model I have heard the explains the excessive amount of DOC needed to remove nitrate and the apparent “nothing is happening until about 1 mL/gallon vinegar” observation.

I wonder how large a scale this experiment needs to be run. I am hoping around 1 liter.

 

[Lasse]

No experiment yet, but when I do set one up it will be with the @taricha hypothesis in mind.

Organic carbon dosing reaches a level where heterotrophic bacteria locally deplete oxygen to the point where other bacteria switch to nitrate reduction for energy. While diffusion of nitrate and DOC into the biofilm still needs to occur, the distance is small because the aerobic bacteria overlay the nitrate reducers (conjecture). This is the simplest model I have heard the explains the excessive amount of DOC needed to remove nitrate and the apparent “nothing is happening until about 1 mL/gallon vinegar” observation.

I wonder how large a scale this experiment needs to be run. I am hoping around 1 liter.

If you do an experiment like this and start with NH3/NH4 - you need to measure NO2 too. From my years in the waste water industry (and the fish farm industry) - I know that NO2 will build up if the BOD7 (include DOC) is to high in the incoming water. If you start with NO3 in the water - still need NO2 measurements because of it will build up if the denitrification is incomplete.

I´m sorry I need to change my statement that the lag time for some system reduction of NO3 is due to denitrification to complete denitrification. For many tests - NO2 in the water give a false NO3 reading. If not NO2 is measured - we can read NO2 residue and think its NO3.

Sincerely Lasse

 

[Dan_P]

If you do an experiment like this and start with NH3/NH4 - you need to measure NO2 too. From my years in the waste water industry (and the fish farm industry) - I know that NO2 will build up if the BOD7 (include DOC) is to high in the incoming water. If you start with NO3 in the water - still need NO2 measurements because of it will build up if the denitrification is incomplete.

I´m sorry I need to change my statement that the lag time for some system reduction of NO3 is due to denitrification to complete denitrification. For many tests - NO2 in the water give a false NO3 reading. If not NO2 is measured - we can read NO2 residue and think its NO3.

Sincerely Lasse

Thanks Lasse for list of chemical species to monitor. I will also monitor oxygen levels as well, possibly, keeping some vessels a little starved of oxygen to see if nitrate reduction rate increases.

 

[taricha]

I wonder how large a scale this experiment needs to be run. I am hoping around 1 liter.

I've seen these effects in 0.5L and 1L bottles of tank water. (Carbon dose made NO3 drop - until more carbon suddenly made it "rebound" up ... when really it was NO2 generated causing the NO3 test to read misleadingly higher)
If there had been sand/rock in the bottle instead of just tank water, it likely would've taken much less carbon dose to start to see these low O2 mechanics.

 

[GirlGotTanked]

Maybe the mentality is similar to phytoplankton dosing for establishing copepods in a reef tank. We are told to feed a small dose in the beginning so that they do not muck up the tank until the copepod population multiplies. There are other examples like that we pick up in all kinds of scenarios, inside and outside of reef tanking. That's not to say it's a correct way of thinking, but with limited knowledge of biology, people tend to copy and paste past insights to fit the protocol. They say "better safe than sorry". Speaking of which, can you explain briefly the difference between introducing vodka dosing, ambient CO2, or increased carbonate hardness (alkalinity)? These all seem like different forms of carbon, but are they the same? Do they all create the same results?

 

[Lasse]

DOC (Dissolved Organic Carbon) is just organic -> it means that the origin is from living tissue (dead or alive) CO2 and CO3 ar inorganic forms of carbon - it means that origin is of no living matter - like minerals. CO2 is a gas and CO3 an ion.

The respiration from living organisms will result in CO2. CO2 is used by photosynthetic organism in order to produce organic matter. CO3 is used by stony corals in order to form limestone. IMO and IME - the chemo autotroph nitrification organism needs CO3 in the water in order to form organic matter - CO2 in water seems not to matter for them. They maybe need to transform the CO3 to CO2 inside the organism but they do not work with only CO2 in the water IME. I know there is different opinions about this.

Sincerely Lasse

 

[biom]

No experiment yet, but when I do set one up it will be with the @taricha hypothesis in mind.

Organic carbon dosing reaches a level where heterotrophic bacteria locally deplete oxygen to the point where other bacteria switch to nitrate reduction for energy. While diffusion of nitrate and DOC into the biofilm still needs to occur, the distance is small because the aerobic bacteria overlay the nitrate reducers (conjecture). This is the simplest model I have heard the explains the excessive amount of DOC needed to remove nitrate and the apparent “nothing is happening until about 1 mL/gallon vinegar” observation.

I wonder how large a scale this experiment needs to be run. I am hoping around 1 liter.

Sorry I didn't understand what the experiment is exactly?

 

[Dan_P]

I've seen these effects in 0.5L and 1L bottles of tank water. (Carbon dose made NO3 drop - until more carbon suddenly made it "rebound" up ... when really it was NO2 generated causing the NO3 test to read misleadingly higher)
If there had been sand/rock in the bottle instead of just tank water, it likely would've taken much less carbon dose to start to see these low O2 mechanics.

Just wondering about details again. Do you think the nitrate consumption is performed by pelagic bacteria or those associated with particulates in the aquarium?

 

[Dan_P]

Sorry I didn't understand what the experiment is exactly?

The purpose of the experiment is to generate data to better understand the factors influencing carbon dosing in an aquarium. What provoked me into experimenting is the large dose of carbon needed before nitrate concentration starts to decline. An even deeper question is whether nitrate is reduced, assimilated or both. I don’t think that I can address the second question.

Make sense?