Brittleness/high dependency of high_rate_wastewater_treatment_system on feed #240

Hi BIOSTEAM team,
I'd like to use the high_rate_wastewater_treatment_system for wastewater treatment.
However, the system fails to converge in, for me, unintuitive ways.

Below is a contained example that fails to converge:

import biosteam as bst
import thermosteam as tmo
chemicals = bst.Chemicals(['CO2', 'AceticAcid', 'FormicAcid', 'H2O', 'O2', 'Methanol', 'Methane', 'N2'])
bst.settings.set_thermo(chemicals)
feed = bst.Stream(
	CO2=0.0468,
	AceticAcid=0.624,
	FormicAcid=0.00169,
	H2O=1.03e5,
	O2=0.0323,
	Methanol=4.39,
	T=273.15+37,
)
wwt_sys = bst.create_high_rate_wastewater_treatment_system(ins=feed)
wwt_sys.simulate()

Troubleshooting to figure out the feed components that caused failure to converge resulted in the following:

1. No convergence when removing O2, CO2 or both O2 and CO2
2. Convergence when removing Methanol from feed, but the solution has negative O2 flows in the outs of R603 and S601
3. Convergence when increasing O2 by 3 to 3.0323. In this case no negative O2 flows are observed
4. No convergence when changing water inflow by orders of magnitude

Am I misunderstanding something fundamental here?
Thanks for any input!

Best,
M