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AquaticEcoDynamics / libaed-water

Code for the AED water quality model
GNU General Public License v3.0
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Xon equal to 2 or 3? #28

Closed MichaelBarryBMT closed 2 years ago

MichaelBarryBMT commented 2 years ago

Hi Matt and Casper Me again. This is a small issue we talked about a while ago Matt - the consumption of O2 via nitrification of ammonium. The ratio of moles N:O2 is Xon and is set to 3 in the library. This pull request is about whether Xon could be 2, based on the reactions below? I guess I am keen to know what the underlying reactions are so that I can write them up in the TUFLOW WQM manual correctly - not an issue for me either way, but I just need the TUFLOW manual to be correct! Thanks MB image

matthipsey commented 2 years ago

Hey MB, Ive had a look. The O to N elemental stoichiometry is 6:2 in your diagram, making the the DO : NH4 stoichiometry in the model to be 3:2 (since DO is O2). Therefore the ratio should be 1.5 ...?

However, After reading the attached paper, It seems the effective amount is closer to 1.8 due to various inefficiencies in the dual reaction process. Based on their equation 18, I get the O2 : NH4 ratio to be = 1.851 / (1+0.0298) ~ 1.8. Granted this is wastewater which may have some differences, and maybe a good option is to make this a user modifiable option.

@danparaska - could you please confirm if this sounds reasonable for Xon? (Also, note this paper made give you the details for Xon1, Xon2 and Xon3 that are here?)

MH

MichaelBarryBMT commented 2 years ago

Thanks Matt I think the elemental stoichiometry in my diagram is 2 Ns : 8 Os, rather than 6 Os? 6 Os are definitely used in the first reaction, but then another 2 Os are used in the second. So I think it is N:O2 of 2:4 (i.e. 1:2), which is close to 1.8? I think offering this as a user definable variable might be useful in some situations, but my feel is that the majority of FV WQM consulting users will not know the difference (or where to start looking!!) so I am probably going to hardwire it to whatever you and Dan decide on is most reasonable. MB

matthipsey commented 2 years ago

Yes, sorry you are right about the 2nd step oxygen adding another. Though in the linked paper it is a bit less than one. So to summarise - according to your diagram which is the "conventional" view, the stoichiometry should be 2:1, and based on various publications it may be slightly lower in practice, down to 1.8.

For now I'll accept the commit to change to 2, but will leave open until we confirm and update our docs.

Then in a future update we will add Xon as an input variable to nml. Also, note that we are currently finalising a f(S) limitation on nitrification based on some further lit review.