What is eating up my alkalinity?

[Randy Holmes-Farley Verified Community Expert]

The only significant bacterial process I know of that likely consumes alkalinity in a reef tank (aside from a sulfur denitrator) is conversion of ammonia to nitrate.

It consumes 2.3 dKH for each 50 ppm of nitrate added. If nitrate is steady at any level, there is no ongoing alk depletion.

 

[blub_blub]

Thanks everyone for your input!! It seems I need a fuller picture, ordered a better phosphate test and a magnesium test, they should arrive in the next few days!

 

[Randy Holmes-Farley Verified Community Expert]

I personally do not think home magnesium tests are worth the cost and bother due to frequent inaccuracy.

 

[JonasRoman]

The only significant bacterial process I know of that likely consumes alkalinity in a reef tank (aside from a sulfur denitrator) is conversion of ammonia to nitrate.

It consumes 2.3 dKH for each 50 ppm of nitrate added. If nitrate is steady at any level, there is no ongoing alk depletion.

I disagree in one point here: if the nitrification is equal to denitrification (Nitrate is not changing) there is actually a nettoconsumption of alkalinity. It produces less alkalinity to reduce one unit NO3 than it consumes alk to create equal NO3 from nitrification, so its actually a depletion of alka even when No3 stays steady. The denitrification reset only approx 50% of the alka that nitrification consumes.

 

[Randy Holmes-Farley Verified Community Expert]

I disagree in one point here: if the nitrification is equal to denitrification (Nitrate is not changing) there is actually a nettoconsumption of alkalinity. It produces less alkalinity to reduce one unit NO3 than it consumes alk to create equal NO3 from nitrification, so its actually a depletion of alka even when No3 stays steady. The denitrification reset only approx 50% of the alka that nitrification consumes.

What is the basis for thinking that? The equations that I have show it matches exactly.

In short, organic N to ammonia is net no alk change in either direction.

Ammonia to nitrate and back to ammonia is also no net change.

So I’m not seeing where alk is lost or gained.

 

[JonasRoman]

What is the basis for thinking that? The equations that I have show it matches exactly.

In short, organic N to ammonia is net no alk change in either direction.

Ammonia to nitrate and back to ammonia is also no net change.

So I’m not seeing where alk is lost or gained.

I do not mean nitrate back to ammonium, I mean nitrate reduction to N2, thus the denitrication, where nitrate act as electron acceptor. Isnt that the main reason why nitrate is reduced?
So I mean nitrification versus denitrification . In that 2 reactions if they are equal (nitrate is not changing), the nitrate reduction to N2 is not giving back same amount of alkalinity as nitrification gives (ammonium to nitrate).
If we say one ammonium is oxidized to NO3 through the nitriciation that creates two H ions.
If now this Nitrate ione is reduced to N2 via denitrification, that consumes only 1 H ion. So its a nettoincrease by 1 H ion for each ammonium that is going from ammonium to N2, through nitrification-denitrifiction.

 

[ReneReef]

I do not mean nitrate back to ammonium, I mean nitrate reduction to N2, thus the denitrication, where nitrate act as electron acceptor. Isnt that the main reason why nitrate is reduced?
So I mean nitrification versus denitrification . In that 2 reactions if they are equal (nitrate is not changing), the nitrate reduction to N2 is not giving back same amount of alkalinity as nitrification gives (ammonium to nitrate).
If we say one ammonium is oxidized to NO3 through the nitriciation that creates two H ions.
If now this Nitrate ione is reduced to N2 via denitrification, that consumes only 1 H ion. So its a nettoincrease by 1 H ion for each ammonium that is going from ammonium to N2, through nitrification-denitrifiction.

Denitrification consumes 2 H+.
Randy is correct. Please check your sources.

 

[Randy Holmes-Farley Verified Community Expert]

Some balanced equations:

Nitrification
NH3 + 2O2 --> NO3- + H+ + H2O
(Depletes 1 unit of alk for each nitrate produced)

Typical denitrification
4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2
(Adds one unit of alk for each nitrate consumed)

 

[JonasRoman]

I do not mean nitrate back to ammonium, I mean nitrate reduction to N2, thus the denitrication, where nitrate act as electron acceptor. Isnt that the main reason why nitrate is reduced?
So I mean nitrification versus denitrification . In that 2 reactions if they are equal (nitrate is not changing), the nitrate reduction to N2 is not giving back same amount of alkalinity as nitrification gives (ammonium to nitrate).

Some balanced equations:

Nitrification
NH3 + 2O2 --> NO3- + H+ + H2O
(Depletes 1 unit of alk for each nitrate produced)

Typical denitrification
4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2
(Adds one unit of alk for each nitrate consumed)

Hmm...really?
The diff between our examples is that you use NH3. Shouldnt you write NH4 from start?. I mean: yes, the bacteria uses NH3, but as the equlibrium is far "to the left", to the NH4 side, it means that immediately when a bacteria have used one NH3 for the nitrogen cycle, one NH4 is converted to : NH3+H+. Thus giving a netto addition of one H, if we using your formula, where there is no H netto addition as you use NH3 and not take in count that at same time a NH4 is converted to NH3+H. Thus an addition of one H. In "my" formula that is included as I use NH4, which seems to be a standard when calculate these reactions on waste water?

Jonas

 

[EnterName]

Hmm...really?
The diff between our examples is that you use NH3. Shouldnt you write NH4 from start?. I mean: yes, the bacteria uses NH3, but as the equlibrium is far "to the left", to the NH4 side, it means that immediately when a bacteria have used one NH3 for the nitrogen cycle, one NH4 is converted to : NH3+H+. Thus giving a netto addition of one H, if we using your formula, where there is no H netto addition as you use NH3 and not take in count that at same time a NH4 is converted to NH3+H. Thus an addition of one H. In "my" formula that is included as I use NH4, which seems to be a standard when calculate these reactions on waste water?

Jonas

@Lasse and I had the same question back in June. Here is what I went through after Lasse showed me the nitrification formula in waste water treatment

NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O​

This is more or less what I found:

The extra H in NH₄ has to come from somewhere. If an organism produces NH₃ a majority of it will be converted to NH₄ by stealing an H⁺ from somewhere. Probably like

NH₃ + H₂O → NH₄⁺ + OH⁻​

This is alkalinity positive (1 eq) and you can then use the waste water treatment equation, but insert the additional OH⁻:

NH₄⁺ + 2O₂ + OH⁻ → NO₃⁻ + 2H⁺ + H₂O + OH⁻​

Combined with the +1 from the step above, this would only be -1 alkalinity equivalent per mole of N instead of -2. We can further simplify by reacting one of the H⁺ with OH⁻. This gives us:

NH₄⁺ + 2O₂ + OH⁻ → NO₃⁻ + H⁺ + 2H₂O​

If we scramble everything together step by step we get:

NH₃ + H₂O + 2O₂ → NH₄⁺ + OH⁻ + 2O₂
(Ammonia is converted to ammonium according to the NH₃/NH₄ equilibrium which produces an OH⁻ ion)

NH₄⁺ + OH⁻ + 2O₂ → NO₃⁻ + OH⁻ + 2H⁺ + H₂O
(ammonium is then oxidized to nitrate, but we keep track of the additional OH⁻ ion)
​

or in short

NH₃ + H₂O + 2O₂ → NO₃⁻ + H⁺ + 2H₂O
​

or even shorter

NH₃ + 2O₂ → NO₃⁻ + H⁺ + H₂O
​

which is Randy's equation.

 

[Randy Holmes-Farley Verified Community Expert]

Hmm...really?
The diff between our examples is that you use NH3. Shouldnt you write NH4 from start?. I mean: yes, the bacteria uses NH3, but as the equlibrium is far "to the left", to the NH4 side, it means that immediately when a bacteria have used one NH3 for the nitrogen cycle, one NH4 is converted to : NH3+H+. Thus giving a netto addition of one H, if we using your formula, where there is no H netto addition as you use NH3 and not take in count that at same time a NH4 is converted to NH3+H. Thus an addition of one H. In "my" formula that is included as I use NH4, which seems to be a standard when calculate these reactions on waste water?

Jonas

No. Unprotonated ammonia is what is produced from metabolism of organics. It will get protonated when it enters the water, but that process adds that same amount of alkalinity that would be lost deprotonating it. So it doesn’t really matter which you think of it coming from as long as you track it all the way back to organic foods.

Hence the trip from organic food to N2 has no effect on alk, whether you take into account the protonation and deprotonation steps (which cancel out) or not.

 

[JonasRoman]

@Lasse and I had the same question back in June. Here is what I went through after Lasse showed me the nitrification formula in waste water treatment

NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O​

This is more or less what I found:

The extra H in NH₄ has to come from somewhere. If an organism produces NH₃ a majority of it will be converted to NH₄ by stealing an H⁺ from somewhere. Probably like

NH₃ + H₂O → NH₄⁺ + OH⁻​

This is alkalinity positive (1 eq) and you can then use the waste water treatment equation, but insert the additional OH⁻:

NH₄⁺ + 2O₂ + OH⁻ → NO₃⁻ + 2H⁺ + H₂O + OH⁻​

Combined with the +1 from the step above, this would only be -1 alkalinity equivalent per mole of N instead of -2. We can further simplify by reacting one of the H⁺ with OH⁻. This gives us:

NH₄⁺ + 2O₂ + OH⁻ → NO₃⁻ + H⁺ + 2H₂O​

If we scramble everything together step by step we get:

NH₃ + H₂O + 2O₂ → NH₄⁺ + OH⁻ + 2O₂
(Ammonia is converted to ammonium according to the NH₃/NH₄ equilibrium which produces an OH⁻ ion)

NH₄⁺ + OH⁻ + 2O₂ → NO₃⁻ + OH⁻ + 2H⁺ + H₂O
(ammonium is then oxidized to nitrate, but we keep track of the additional OH⁻ ion)
​

or in short

NH₃ + H₂O + 2O₂ → NO₃⁻ + H⁺ + 2H₂O
​

or even shorter

NH₃ + 2O₂ → NO₃⁻ + H⁺ + H₂O
​

which is Randy's equation.

Thats not the "issue " , that's correct as you write. The nitrification from NH3 gives an nettoaddition of one H. 2 H if you go from NH4, but one H if we "use" NH3.
But then, when one NO3 is going to be reduced to N2 that's only consumes 0.5 H (or gives 0.5 OH back) which gives you totally a nettoaddition of 0.5 H from NH3 to N2.

 

[EnterName]

Thats not the "issue " , that's correct as you write. The nitrification from NH3 gives an nettoaddition of one H. 2 H if you go from NH4, but one H if we "use" NH3.
But then, when one NO3 is going to be reduced to N2 that's only consumes 0.5 H (or gives 0.5 OH back) which gives you totally a nettoaddition of 0.5 H from NH3 to N2.

Ah okay, sorry, then I misunderstood the discussion.

I will try to make sense of it again, maybe I can find what's off.

 

[JonasRoman]

Ah okay, sorry, then I misunderstood the discussion.

I will try to make sense of it again, maybe I can find what's off.

Hmm wait. You are right. I calculated wrong. To denitrify one NO3 to 0.5N2 it takes one H. That one that was created if we go from NH3 to NO3. So yes +-0.

That bring back the question what really happens as NH3 is removed from water when NH3 is going into the nitrogen cycle. So when this NH3 is removed I can't see anything else that a NH4 is going to NH3+H
I mean, yes the nitrification and denitrification of NH3 to 0.5N2 is +- 0 for alk, but this will induce the release of an H ions from a NH4 ion in the water. So shouldn't we at the end of the day when that is taken in count get a netto addition of one H.

 

[Randy Holmes-Farley Verified Community Expert]

If you do not trace back to the way ammonia is added, you are missing a step that does impact alk.

 

[JonasRoman]

If you do not trace back to the way ammonia is added, you are missing a step that does impact alk.

I get what you mean, but is that relevant? At least I am not talking about a balance of alk from organics to N2 but from ammoniak to N2. The nitrogen cycle. What that do with alkalinity. As we are talking about when nitrate are not changing=nitrification equals denitrification. And that now balanced nitrogen cycle is actually producing H ions.
So, we may talk about different things then and seems to have same stochionetric formulas.

Good discussion

 

[Randy Holmes-Farley Verified Community Expert]

I get what you mean, but is that relevant? At least I am not talking about a balance of alk from organics to N2 but from ammoniak to N2. The nitrogen cycle. What that do with alkalinity. As we are talking about when nitrate are not changing=nitrification equals denitrification. And that now balanced nitrogen cycle is actually producing H ions.
So, we may talk about different things then and seems to have same stochionetric formulas.

Good discussion

It is relevant if one is discussing where alkalinity is going. If nitrate is stable at any level, the above discussion shows there is no significant ongoing alk addition or depletion from processes relating to the nitrogen cycle (which is the subject of this thread).

I certainly agree that if one is adding ammonia or starts with high ammonia, then no, the organic source is unimportant.