Sometimes closed circuits behave in an aparent ilogical way because the inlet pressure of the pump is afected by its out output pressure.
In this reactor (As oposed to downflow reactors) the inlet pressure in the pump basically equals its output pressure less the pressure drop across the media, so when the inlet valve and CO2 are closed the pressure at the ilet is lower by the pressure drop across the media. In this conditions if line 2 (connected to the top of the chamber) is open it is easier for the pump to pull air in then to pull the water up trough the media.
This effect is good to pull in the CO2 which then is compressed into the bottom of the media (The higher pressure side of the chamber) by the pump's discharge, This compression increases the amount and speed of the CO2 being dissolved in the water.
To prevent air suction from the line 2 the pressure created by the inlet water has to be higher than the pressure drop from the media upflow minus the pressure increase created by the CO2 flow. In other wordsn and I just realized, using the inlet valve to control the effluent will not work unless the line 2 is closed or line 1 is closed and line 2 is used to handle all the effluent.
From These here are alternatives you can test:
a) Use Line 1 for effluent, leave line 2 slightly open, open inlet valve slightly more than the effluent valve 1 but still effluent valve 1 shall control the effluent. This will pressurize the reactor up to the return pumps discharge. to prevent to high of a pressure turn vlave one fully open, slowly open inlet valve until effluent is about 10 % higher than what you look for, then start slowly close valve one to restrict the effluent beck from the 10% high to the target. You may need to adjust valve on a couple of times as the effluent will increase back up ad the pressure inside the reactor increases.
b) Use line 2 for effluent and leave it fully open, close line 1, use inlet valve to control effluent flow. This option will work but looses the benefit of increased reactor pressure.
c) Use line 1 for effluent but fully open, keep line 2 closed and use inlet valve to control the effluent amount. This will work but may create CO2 accumulation over time and potentially the recirc pump may loose priming and run dry so it is not one I would prefer
d) Switch the feed to a powerhead or aqualift, remove the inlet valve, use valve one for effluent control and leave line 2 slightly open to vent any CO2 accumulation. The powerhead pressure will be enough for good operation but not high enough to create any leaks.
So from those above atlhough all would work the one that rewuires less attention and trimming is (d)
