Numerical error while executing simulation experiment

I'm using WEST 2014. I received Numerical error display while using control center to simulate experiment. The display is " Numerical error: At t=0.8335869: Derivative value discovered that is either not a number (NaN) or not finite:.ASU_M(H20)". Pls help me to solve this.

The error message is actually quite clear: it suggests that a derivative is NAN which is typically the result of a division by zero.
If you are using a custom Category, you may have not initialized the M vector - which can be done in the base.msl file where M is defined (around line 96), or at the level of the project (the first option being obviously more effective).
Should this not help, you may need to carefully check all derived state variables and identify the culprit(s).

I used standard category and got error display. Not with custom category.

I meant: the "wwtp.base.msl" file.
It is part of the standard WEST library.
The declaration of the OBJ M should be at around line 96.

Vivek is using the standard model library, so nothing has been changed.

Moreover, he is using a fixed volume tank, so the mass of water is not changing during the simulation.

Further,I don't think the error is not related to initialization, as it occurs after some time during the simulation

Then I would definitely inspect all the derived state variables for one seems to be NAN.
If you cannot identify the problem, please send your case to mikebydhi

It may also be worthwhile to have a look at your integrator settings (use another algorithm and/or crank up the accuracy).