[Delphinus]

I'm not sure I follow?

To me, the alkalinity of the effluent is directly proportional to the pH you're running your reactor at. If the alkalinity is too low then the pH is too high, so increase the CO2 to decrease the pH and thus increase the alkalinity.

So if you're running 70 ml/min and 60bpm, and your effluent pH is (say) 6.7, and the levels are not being maintained, I'd increase to 100ml/min and increase the bubble rate to whatever it is needed to make the effluent stay at 6.7. Retest after a week or so and see where it goes. Maybe at the higher flowrate you need to make your pH setpoint lower like 6.6 (I don't think you need to though, I think 6.7 is an ideal target and the alkalinity should be the same no matter what the flowrate is, so long as the pH is still the same).

As for the bubble of CO2 at the top .. Do you not have a recirculation line on your reactor? If not you should see about getting one put in. It's just a 1/4" line from the top of the chamber to the pump intake (usually just T-d into the CO2 input line). It gives the CO2 that builds up at the top, a place to go (gets re-used basically). If you do have a recirc line, see if it's blocked or see if you need to shim the reactor so that the recirc line is at the high point (gas wants to go up, so make the gas go towards the recirc input).