Carbon Dosing A Low NO3/PO4 System

[Dan_P]

A year ago, I became interested in carbon dosing because of the good things I heard about it, and the dire warnings (diminished capability to remove nitrate after dosing stops, encouragement of cyanobacteria and other nuisance organism growth, diminished biodiversity of the bio filter and proliferation of harmful bacteria). I took the plunge and started dosing my fish only system. The nitrate level was generally below 1 ppm and phosphate below 0.1 ppm. There was no nitrate issue. I dosed calcium acetate made from vinegar mixed with just enough solid kalkwasser to bring the pH to 10-11. I spent the year slowly increasing the daily dose, observing the system for a week to a month after each step increase. For this post I report data up to the not very large dose of 0.25 mL per gallon. I eventually achieved 2 mL/gallon, but that data is for another post. During this time, the only detected change was alkalinity consumption. Because the system contains only fish, alkalinity consumption should reflect only the metabolic activity of bacteria and algae, the system’s bio filter.

Alkalinity consumption was determined by measuring the alkalinity about once a week and dividing the difference by the number of elapsed days. I raised the alkalinity with a solution of sodium bicarbonate whenever it fell below 3 meq/L.

The first graph shows a highly variable alkalinity consumption rate before dosing and a much more consistent consumption rate after the initial small dose of acetate. The green line charts the size of the daily acetate dose while the yellow line charts the addition of acetic acid as I transitioned from dosing acetate to dosing straight acetic acid. The bar chart shows the average alkalinity consumption rate for each period between dose increases. The first period is the 164 days before dosing started. The reduction in the initial alkalinity consumption rate occurs with the first small dose. The reduction in consumption is larger than can be explained by the added alkalinity of the daily acetate dose. Eventually, the alkalinity of the acetate dose did seem to raise the system’s alkalinity (positive alkalinity consumption rate). This idea is supported by the observation of the return to a negative consumption rate during the transition from acetate to acetic acid dosing.

This dosing experiment has left me with a list of questions and a long reading list. Some of my questions:

Was the decrease in variability of alkalinity consumption rate a coincidence?
Is alkalinity consumption rate effected by other carbon sources, such as ethanol?
Does this alkalinity consumption effect occur in all marine aquaria?
Will a highly variable alkalinity consumption rate return when dosing is stopped?
Might alkalinity consumption rate be a useful diagnostic for the health of an aquarium bio filter?

With regard to this last question, the average alkalinity consumption rate of 0.1 meq/L/Day corresponds to a 10-20 g daily consumption of biomass based on ammonia nitrogen consumption. I feed the fish about that everyday. Maybe just a coincidence. Here is the reference I used for the calculation

https://www.researchgate.net/public...al_of_ammonia-nitrogen_in_aquaculture_systems

Your thoughts and comments would be welcome.

Dan

 

[jsker]

Lets get you some input here Dan

 

[Randy Holmes-Farley]

Here's my take on the questions:

Was the decrease in variability of alkalinity consumption rate a coincidence?

I don't really see a way that dosing organics impacts the "variability" in alkalinity, but if nitrate is declining, it boosts alkalinity, and that might offset a drop between dosing.

Is alkalinity consumption rate effected by other carbon sources, such as ethanol?

All organics will result in an alk boost IF (and only if) nitrate is declining). A drop of 50 ppm nitrate boosts alk by about 4.5 dKH. That is true almost regardless of why and how it dropped (except for water change).

Does this alkalinity consumption effect occur in all marine aquaria?

As to alk consumption, some organisms may grow faster when there is more food present for them (as organics can add) and that may increase the actual demand for alkalinity. Changes in nutrient levels (which may be a result of organic dosing) also impact calcification rats in some organisms.

Will a highly variable alkalinity consumption rate return when dosing is stopped?

I'm not sure why you observed what you saw, so why and when it might come back is not clear.

Might alkalinity consumption rate be a useful diagnostic for the health of an aquarium bio filter?

Not in my opinion. Calcification rate is impacted by a great many factors, from nutrients to light to flow, etc.

 

[Dan_P]

Here's my take on the questions:

Was the decrease in variability of alkalinity consumption rate a coincidence?

I don't really see a way that dosing organics impacts the "variability" in alkalinity, but if nitrate is declining, it boosts alkalinity, and that might offset a drop between dosing.

OK, thanks. I will try to do a better job at mass balance with regards to nitrate.

 

[Randy Holmes-Farley]

You're welcome.

let us know what you find.

 

[Dan_P]

Is alkalinity consumption rate effected by other carbon sources, such as ethanol?

All organics will result in an alk boost IF (and only if) nitrate is declining). A drop of 50 ppm nitrate boosts alk by about 4.5 dKH. That is true almost regardless of why and how it dropped (except for water change).

Maybe an alkalinity boost and a decreasing consumption rate might be difficult to distinguish. Here is another angle.

What about a shift from autrophic consumption of ammonia to heterotrophic during carbon dosing? That would reduce (but not produce) alkalinity consumption per gram of ammonia nitrogen consumed (it would also reduce nitrate production). Nitrogen ends up being assimilated into biomass before it can be oxidized by autotrophs. I think this shift is the basis of Belgian Anthias’ concern about carbon dosing. When ammonia availabilty declines, the accumulated nitrate is then tapped as a nitrogen source by the now carbon dosed bloated heterotrophic community.

Hard to tease these things out.

 

[Randy Holmes-Farley]

Maybe an alkalinity boost and a decreasing consumption rate might be difficult to distinguish. Here is another angle.

What about a shift from autrophic consumption of ammonia to heterotrophic during carbon dosing? That would reduce (but not produce) alkalinity consumption per gram of ammonia nitrogen consumed (it would also reduce nitrate production). Nitrogen ends up being assimilated into biomass before it can be oxidized by autotrophs. I think this shift is the basis of Belgian Anthias’ concern about carbon dosing. When ammonia availabilty declines, the accumulated nitrate is then tapped as a nitrogen source by the now carbon dosed bloated heterotrophic community.

Hard to tease these things out.

Consumption of ammonia by any organism (except one producing nitrate as the product) won't impact alkalinity, whether it is an autotroph or heterotroph. How does it matter what organism converts ammonia into organic molecules of tissue?

 

[Dan_P]

QUOTE="Randy Holmes-Farley, post: 5607118, member: 45227"]Consumption of ammonia by any organism (except one producing nitrate as the product) won't impact alkalinity, whether it is an autotroph or heterotroph. How does it matter what organism converts ammonia into organic molecules of tissue?[/QUOTE]

Here is the reference and the authors’ equations. What do you think? Have I gone too far on the proverbial limb?

Engineering analysis of the stoichiometry of photoautotrophic,autotrophic, and heterotrophic removal of ammonia–nitrogen in aquaculture systems

https://www.researchgate.net/public...al_of_ammonia-nitrogen_in_aquaculture_systems


Heterotroph

Autotroph

 

[Randy Holmes-Farley]

The “autotroph” equation you show is actually converting ammonia to nitrate ( which is not the normal process), not consuming all the N. What us shown is closer to the nitrogen cycle.

 

[Randy Holmes-Farley]

When that nitrate is used by something else, the alk will be returned.

 

[Dan_P]

The “autotroph” equation you show is actually converting ammonia to nitrate ( which is not the normal process), not consuming all the N. What us shown is closer to the nitrogen cycle.

The equation is a combination of respiration and biomass production reactions. It seems we tend to only see the respiration reaction.

In bacteria the proportion of energy spent on biomass production depends on “where” the bacteria is on its growth curve. A “good enough” approximation might be to reference the bigger respiration reaction.

 

[Dan_P]

When that nitrate is used by something else, the alk will be returned.

Yes, and that is what I need to look at more closely for my alkalinity consumption mystery. First, I need to balance my check book

 

[Dan_P]

When that nitrate is used by something else, the alk will be returned.

Randy, I appreciate the discussion. Just to close it out for now, here is the notion I am entertaining.

How I started out thinking about the aquarium bio filter

Biomass -> NH3 -> NO3 -> N2 Alkalinity consumption 0 meq/L/day

With the discovery of alkalinity consumption in my fish only system, the above viewpoint must be incomplete. Could the ammonia to be shunted off to other consumers that also consume alkalinity? Shunting nitrate to other users doesn’t help because a nitrate reduction step by these users generates alkalinity. Does this approach explain alkalinity consumption variability?

Variability in consumption might actually be normal, reflecting a competition for ammonia by users that also have an alkalinity consumption rate that differs from autotrophs. But why would adding acetate stabilize AND reduce alkalinity consumption?

Stabilizing the consumption rate with carbon dosing might just equate to one ammonia user becoming the dominant ammonia consumer. What about the reduction in alkalinity use?

The new dominant user would have to consume nitrate not ammonia. Photoautotrophic and aerobic heterotrophic consumption of ammonia involves the consumption of alkalinity. Moreover, photoautotrophic consumption of nitrate which generates alkalinity would not be stimulated by acetate (?). However, anoxic heterotrophic consumption of nitrate would decrease overall alkalinity consumption and be stimulated by acetate dosing. Is this the answer?

I stopped dosing my aquarium (abruptly) and will continue observing nitrate and alkalinity levels. In a month or two there might be enough data to shed light on the alkalinity consumption question. I am not sure how much my small scale experiments will help but they will continue as well.