Author Topic: theta_K_X in ASM2dModTemp vs ASM1Temp  (Read 255 times)

arspr

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theta_K_X in ASM2dModTemp vs ASM1Temp
« on: March 10, 2020, 09:51:16 am »
Hi, I've just noticed this very strange default parametrization in ASUs:

  • ASM1Temp. theta_K_X = 1.116, (greater than 1), therefore K_X increases with temperature.
  • ASM2dModTemp. theta_K_X = 0.896, (smaller than 1), therefore K_X decreases with temperature.

Is this correct? It seems pretty weird, at least at first glance.

(I know, ASM1 and ASM2d have different default parameterizations, but more or less their core reactions and parameters are quite equivalent. A parameter evolving in complete opposite directions is, at least, strange).

FaPo

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Re: theta_K_X in ASM2dModTemp vs ASM1Temp
« Reply #1 on: March 10, 2020, 03:06:37 pm »
The parameterization for theta_K_X is somehow based on originally published K_X values at different temperatures, namely:
  • ASM1: K_X = 0.03 gCOD/gCOD at 20°C, K_X = 0.01 gCOD/gCOD at 10°C, hence theta_K_X > 1
  • ASM2: K_X = 0.10 gCOD/gCOD at 20°C, K_X = 0.30 gCOD/gCOD at 10°C, hence theta_K_X < 1

In the case of ASM2, theta_K_X < 1 indicates a decreasing half-saturation coefficient, thus increasing affinity (and overall hydrolysis rate) with increasing temperature.

The difficulty in defining unique trends for temperature dependence of half-saturation coefficients was already acknowledged by authors of ASM1, who wrote "Because half-saturation coefficients are not rate coefficients, but are parameters which influence the shape of a μ-S ... curve it is more difficult to generalize about the effects of temperature on them" (Henze et al., 1987).
Therefore, you may also wish to assume negligible temperature effect on K_X and set theta_K_X = 1.

arspr

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Re: theta_K_X in ASM2dModTemp vs ASM1Temp
« Reply #2 on: March 10, 2020, 06:56:06 pm »
OK, I see. Many thanks for your answer