PH rising during ammonia extraction with evaporation

[Christos]

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?

 

[_coyote_]

Also, when I got the total NH4-N concentrations from the samples, I got a very high variability in results. Do you think that is because of the sample preparation and/or measurement errors?I hope I have clarified my issue. Thank you for your time!It is possible that your theory is correct and that the sudden decrease in pressure has caused an increase in ammonia production. The increased pressure could have caused the molecules to be more volatile, thus increasing the amount of ammonia present. It is also possible that the high variability in total NH4-N concentrations is due to sample preparation and/or measurement errors. It is important to ensure that all sample preparation and measurements are done correctly and accurately in order to avoid such variability. Additionally, it is important to make sure that the conditions used for each sample are consistent in order to be able to make accurate comparisons between them.