ASIM User Forum - [Initial conditions of ASM2d fractions]

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From: Laila Bahgat (
Date: 04/14/2003

Actually my question is not directly related to ASIM
software specifically.
However I'll very much appreciate any information about the
following point regarding your experience with ASIM 4.0:

I've noticed while simulating model ASM2d using MATLAB, that
the initial conditions, number of reactors used and their
volumes can lead to simulation failure in some cases. In
such cases the simulation fails as the state variable Xpao
reaches zero value whereas three  equations of the set of
ODEs that make up the model contain the term (Xpp/Xpao) or
(Xpha/Xpao). Thus the simulation fails due to divide by zero
error while it may complete in some other cases using the
same initial conditions but a fewer number of reactors or
where the volume of the aerobic reactors is decreased.

I would like to know whether what I am experiencing is the
result of some error and if not how ASIM handles this issue
and what is the best set of typical values to use as initial

Thank you very much for your help.

Reply #1
From: Rolf Fankhauser (
Date: 04/15/2003
We solved this problem in ASIM as follows:
the process is of the form:

rho = (X1/X2) / (Kx + (X1/X2)), in case of X2=0 we set

because you can write rho in the form:

rho = 1 / (Kx*X2/X1 + 1) and for X2=0 => rho=1

I hope this answers your question.

Kind regards,


Reply #2
From: laila bahgat (
Date: 04/15/2003
Many thanks for the quick response.

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