Posted Fri, 11 Sep 2020 11:31:19 GMT by Rodríguez Sánchez, Alberto

Rather quite slowly, (when and where I find some spare time), I continue my training on ASM models (and WEST). Now I'm reading the obvious and classic "Methods for Wastewater Characterization in Activated Sludge Modeling" manual and Chapter 15 does include an important surprise.

It says that heterotrophic yield is lower under anoxic conditions (about 0.52 - 0.54) than the aerobic one (the common 0.67). And it explicitly says that ASM1 and ASM2d models don't include this feature which could lead to some deviations in the simulations.

As WEST implements slightly modified versions, (as example ASM2d[b]Mod[/b]Temp), and this change/option seems rather trivial to implement, I have some questions about it:
[li]The obvious one. Could it be implemented in future versions? In fact, why hasn't it been included yet?[/li]
[li]Or, maybe, is it a superseded observation? I mean. This book is "somehow old" and maybe other contradictory researches have been made afterwards, and that statement is not so clear o valid any more. [/li]

Thank you in advance.
Posted Mon, 14 Sep 2020 11:11:17 GMT by Fabio Polesel
Thank you very much for the very relevant questions, which I will try to address here.

From a thermodynamic standpoint, it is correct that (for the same carbonaceous substrate or wastewater source) the anoxic yield is lower than the aerobic one. This theoretical consideration has been confirmed experimentally for multiple wastewater sources (e.g., Muller et al., Water Res. 2003).

As to yield parameter values, process model implementations in WEST have been made to adhere as much as possible to their original formulation. This means that ASM1, ASM2d(Mod) consider the same value for the aerobic and anoxic heterotrophic yield, while ASM3-based models consider a lower value for the anoxic yield.

This shouldn’t prevent users (and us) from modifying ASM2d(Mod) and implementing a lower anoxic yield to increase the realism of this model. However, this change may affect the extent and the kinetics of other processes (e.g., hydrolysis), and should be therefore accompanied by a new parameterization for these processes.
Posted Tue, 15 Sep 2020 06:43:15 GMT by Rodríguez Sánchez, Alberto
Many thanks for your answer.

Now, I'm going to try to convince you to add something like a correction factor N_Y_H (or even directly a Y_NO_H parameter) in future versions of WEST at least in ASM2d[u][b]Mod[/b][/u]Temp. (I'm not so sure about the "pure" ASM1Temp).  ;)

[li]It would be fully reversible. N_Y_H = 1.0 would give the very same results as the "official" ASM2d model. Maybe, you could even use 1.0 as the default value.[/li]
[li]As I've highlighted, you've actually departed from "official" ASM2d in your Mod version. As discussed in the threads I post at the end, the WEST "Mod" implementation of ASM2d has slower decay rates than "official" ASM2d no matter what you do. And you are keeping the very same default values. Therefore, you are currently making "irreversible" changes over "official" ASM2d... So you don't actually fear modifying the "official" models if the current scientific and technical development gives you a good reason as it seems to be the case.[list]
[li]More over, in that Chapter 15 from the IWA manual I've mentioned in the opening post, that reduced decay rate in anoxic conditions is also suggested. With this change, your "Mod" version would address both effects.[/li]
[li](And of course, I assume everyone who use these models know everything is interlocked. If you change something wherever you want in the model, everything else is going to be modified, more or less, but modified any way. So, yes, a hypothetical N_Y_H < 1.0 is going to have consequences. But analyzing them could in fact be an interesting exercise).[/li]

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