Hello there, During an experiment I had for my MSc Thesis for ammonia extraction, I used WWTP sludge and biogas digestate with an evaporation/distillation unit. I tried different combinations of sample vol. and evaporating flask vol. and then I added vacuum pressure. At first pH was dropping according to literature with the exceptions of some cases where pH was increasing at the end. As soon as I turned on the pump and applied vacuum, the pH just kept rising! So during evaporation, we get from the sludge, NH3 and CO2 in the vapours. As I get it, CO2, turning into carbonic acid, gives you H+, making the solution more acidic (but as you drop the pressure, the PH goes up). On the other hand you got the volatile ammonia molecules from the sludge. My VERY personal (and very civil engineering) theory is that when you create a sudden vacuum in the ammonia equilibrium in the sludge/digestate mixture by sucking the ammonia molecules, you drive the equation into ammonia production. The fact that almost all the cases with pressure had a lower NH4-N concentration than the one with atm. pressure also makes me think of my theory as plasuible. Any ideas of what might be going on?