Small Scale Carbon Dosing Demo

[Dan_P]

I will be repeating this experiment, but the initial demonstration results were interesting enough to share. Your ideas and comments will be useful in design of followup work.

The idea was to dose acetate on a very small system (100 mL) to observe what would happen to ammonia and nitrate levels. The plot and footnotes below summarize the findings. The dosing levels correspond to weeks 5 and 10. I used ppm of carbon instead of mL per gallon because I was interested in the stoichiometry.

As I suspected ammonia is consumed before nitrate and when the system is starved of PO4, nitrate is not consumed. The latter observation is probably not a surprise, but the implication of the preference for ammonia is potentially noteworthy.

For systems that have sufficient phosphate and are producing a high enough level of ammonia every day, dosing will not reduce nitrates until the bacterial population is large enough to become ammonia nitrogen limited and only then will nitrates decrease. I haven't proved this but it's a new idea to consider when diagnosing a stalled nitrate reduction during carbon dosing.

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[ramseynb]

That's really interesting! I'm curious about what a similar test with a tank that's nitrate starved but has phosphates would look like. I have low nitrate in my tank with some phosphates (most likely leached from rock). I know there's many products to absorb phosphates, but I'm wondering how much nitrate dosing you would need to do to lower the phosphates. I know the redfield ratio is 16:1 but I'm not sure how applicable that is to our tanks.

 

[Dan_P]

That's really interesting! I'm curious about what a similar test with a tank that's nitrate starved but has phosphates would look like. I have low nitrate in my tank with some phosphates (most likely leached from rock). I know there's many products to absorb phosphates, but I'm wondering how much nitrate dosing you would need to do to lower the phosphates. I know the redfield ratio is 16:1 but I'm not sure how applicable that is to our tanks.

I find the Redfield ratio useful for thinking about your question. If I remember correctly N of 16:1 is equivalent to NO3O4 of 10:1. For my small scale experiment, 7 ppm of NO3 was removed but phosphate concentration did not change from the added 0.1 ppm. Why was 0.7 ppm PO4 not required? Investigating this would help in understanding what the ratio might have to be. If I had thought of it, I could have kept dosing carbon AND started dosing nitrate until the PO4 was gone. Next time! Thanks.

 

[Kevin Guthrie]

Was this in an established system or a lab setting? Another possible cause of the drop would be absence of and then unintended introduction of one of the key bacteria such as nitrobacter.

If you repeat the experiment, can you also measure and graph the nitrite and maybe even the phosphate levels?

 

[Dan_P]

Was this in an established system or a lab setting? Another possible cause of the drop would be absence of and then unintended introduction of one of the key bacteria such as nitrobacter.

If you repeat the experiment, can you also measure and graph the nitrite and maybe even the phosphate levels?

Good thoughts! Thanks.

The test was done on aquarium water that had been aerated for several weeks, observing the rise in ammonia and nitrate over time, presumably with the death of phytoplankton and consumption of DOC. Nitrite was not detected. It was this sample that was dosed with acetate "œto see what would happen", whether a carbon source would reverse ammonia and nitrate accumulation.

I was surprised by the nitrate production. It was repeatable and time profiles were similar from experiment to experiment. I had assumed ammonia oxidation needed a substrate and that it took weeks to establish the bacterial population to do this. Changing the bottle every day did not impact nitrate production, though recently I remembered that I did not change the airstone every day. Maybe that was my biofilter surface, though nitrate production started within days.

The experimental setup would not have prevented introduction of bacteria.

I will definitely track phosphate in the next go round. Checking for nitrite would be a simple add on.

 

[ramseynb]

I suspect the ratio will not be the same in our tank, but it would be interesting to see how close it is! I'm definitely following!

 

[OzIA]

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