Brittleness/high dependency of `high_rate_wastewater_treatment_system` on feed

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