I think you must provide more data in order to understand what might be the situation. Please describe the treatment process and some of the key working parameters (influent composition, retention time, biomass concentration, recirculations etc.).

thank you Georgios Samiotis for kindly response

i have done this test without mass inoculation, and just i areated.

Often an anaerobic effluent with high ammonia/ ammonium also contain a significant concentration of CO2. If the initial pH not too high it will be natural that CO2 degasses with aeration. This causes increased pH.

thank you dear Prof. Andersen

I have been studying the effect of aeration on ammonia at pure condition (synthetic wastewater using NH4Cl).

during aeration the i saw the inceasing in pH From 7.7 to 8.25

i think this fact occurred by oxidation of the ammonium to ammonia and CO2 degasification.

The equilibrium in aqueous medium is:

NH4 (+) (aq) + H2O (l) NH3 (g) + OH (-) (aq).

By adding air be favoured the removal of ammonia and the equilibrium shifts to the right increase the pH value.

This would agree with your observations.

Oscar, I disagree with your reaction. It is not balanced. Try to count the hydrogen atoms in your reaction. I think the reaction that occur then ammonia degassing decreases pH is according to the reaction: NH4+ -> NH3(g) + H+ In this case apparently this reaction isn’t significant and I suspect there is a supersaturation of CO2 in the solution. Likely the artificial media contain a carbonate salt and an acidic chemical. The reaction which makes the pH go up is: HCO3- -> CO2(g) + OH-

Sorry, I've wrong.

NH4(+) is hydrolized to NH3(g) and increase acid, pH value decreased.

If there are carbonated species in synthetic wastewater:

CO3(2-) + NH4(+) --> HCO3(-) --> CO2(g) + OH(-). Ok. Air removal CO2(g) and pH is increased.

You're right, I'm agree with you.

Thank you all for good responses.

now, what do you think about redox (oxidation reduction potential) during aeration? regard to this equation ? increasing or decreasing?

It depends on the equiibrium with oxygen in air. If the solution take oxygen from the atmosphere the redox level increases.

If you already have oxygen equilibrium I expect that your redox decreases due to the effect of increasing pH. Ammonia and CO2 doesn't really react with the redox electrode.

thank it is good response, but i dont understand how the pH effect on redox!

during aeration with increasing the pH, the redox decreased but i don't found a reasonable response.

Hooshyar,

As a first approximation, I think the answer can get using the Nernst equation. It should be recognized how hydrogen ions act (reactants or products). If the hydrogen concentration decreases (rising pH values) and they are on the side of the reactants (they would be at denominator in the logarithm term of the Nernst equation), the term logarithmic increases and the potential is made more negative (reducing). The opposite would happen if the hydrogen ion is a product.

Thank you

I agree with Oscar, Consider the reaction of oxygen with the electrode: O2 + 4H+ + 4e- -> 2H2O.

Ok.

Henrik.

The hydrogen activity (approx. hydrogen ion concentration) with redox potential:

E = E(0) - RT/nF * ln [1/(p(O2)*[H+]^4)], p(O2) is the partial pressure of Oxygen (this pressure can be expressed as O2 solubility by Henry Law, assume that fugacity of the gas is equal to the pressure., ideal gas). When pH is increased, redox potential is more negative because hydrogen-ion concentration is low. In this case we assume that water activity does not change since it is the solvent, and can be included into E (0) (reference.states).

thanks again for dear Oscar and Henrik for excellent resonses

Hello please how will you prevent this increase in ph without adding acid?

If CO2 supersaturated water is aerated it cannot be avoided that the pH will increase. Acid (CO2, H2CO3) is removed from the solution.

To keep the pH you should avoid the gas exchange. Don't aerate.

Passive treatment systems for mine drainage use no energy other than gravity, but they require greater area than active treatment systems. Researchers are considering “hybrid” systems that have passive and active components for increased efficiency, especially where space limitations render passive-only technology ineffective. Flow-through reactor field experiments were conducted at two large net-alkaline anthracite mine discharges in central Pennsylvania. Assuming an Fe removal rate of 20 g m−2 day−1 and Fe loading from field data, 3.6 × 103 and 3.0 × 104 m2 oxidation ponds would be required for the passive treatment of Site 21 and Packer 5 discharges, respectively. However, only a small area is available at each site. This paper demonstrates aeration to drive off CO2, increase pH, and increase Fe(II) oxidation rates, enabling treatment within a small area compared to passive treatment methods, and introduces a geochemical model to accurately predict these rates as well as semi-passive treatment system sizing parameters. Both net-alkaline discharges were suboxic with a pH of ≈5.7, Fe(II) concentration of ≈16 mg L−1, and low Mn and Al concentrations. Flow rates were ≈4000 L min−1 at Site 21 and 15,000 L min−1 at Packer 5. Three-h aeration experiments with flow rates scaled to a 14-L reactor resulted in pH increases from 5.7 to greater than 7, temperature increases from 12 to 22 °C, dissolved O2 increases to saturation with respect to the atmosphere, and Fe(II) concentration decreases from 16 to

Thank you Henri.  However I need 2 aerated to increase oxygen supply. 

It is possible to add oxygen to wastewater without aerating. The method depend on the purpose and if you are working in the laboratory, pilot scale or a full scale permanent installation.

Can you explain your experiment and why you desire to add oxygen without changing pH?

Thanks for your reply Henrik. I am working in the laboratory and I am aerating wastewater to nitrify it with activated sludge.

And that is the reason you cannot allow the pH to fluctuate? As you nitrify alkalinity is consumed so pH would change anyway in the biofilter.

Thanks for your reply Henrik. Yes as I nitrify ph would change as nitrification consumes alkalinity not producing it. My issue is that the ph has been increasing instead of decreasing. And nitrate level  remains same instead of increasing with time. We are thinking maybe evaporation stimulated also by aeration is creating saturation of the water but not very sure. I am thinking if I could control increase in ph I would probably be able to get more nitrate with time. Any suggestions please Thank you very much.

As I suggested in my answer to the original question: I believe the reason pH increase initially is that the water was supersaturated with CO2 compared to the atmosphere. Therefore CO2 is removed from solution with the initiation of aeration increasing the pH. If this is correct the alkalinity will not change while the pH increase.

I cannot see why you think this increase in pH should interfere with your nitrification experiment. As you nitrify alkalinity is consumed driving pH down and it is more common to have problems with nitrification as alkalinity is exhausted and pH drops below the tolerable level for the nitrifying bacteria.

Even I cannot understand your reason to avoid stripping CO2 from your wastewater while adding oxygen I can suggest you a method I have used in a laboratory setup that might achieve that you desire.

I suggest that you get a fine bubble aeration stone from an aquaria shop and attach it to a pure oxygen pressure bubble. Adjust your reduced pressure on the bottle to about 1½ bar and using a needle valve add a very low flow of oxygen into your wastewater in a glass bottle. Adjust the flow so the oxygen bubbles are so few and small that they dissolve in the water and do not reach the surface.

https://maswanord.dk/luftsten-bla.html?gclid=CjwKEAjwlKLHBRDztKr6wMnRthMSJAALcT-sb_kY6nbI04XDYUFsDrAUUZ0UtsoX40DscMJ5bO0VMxoCpZjw_wcB

Thank you very much Henrik. What worried me was instead of ph decrease I got ph increase. I reduced the flow of bubbles and closed up the vessel and ph stabilied a bit.  Thanks for the suggestion above. 

Infact this increase did not come down with time. It increased with time. We kept the reactor for days and experience gradual increase in ph hence my question

Hello Henrik thanks alot for your help above. Pls would you have an idea on why my nitrifingers system will be showing increase in cod even as bod decreases. Cod should decrease I think. Thanks 

One possibility could be that your system produces nitrite rather than nitrate. Nitrite is measured in the COD analysis (unlike nitrate and ammonia), and if your biomass lack nitrite oxidisers you will not measure it in the BOD analysis. BOD goes down as heterotrophs oxidise organic matter.

Another possibility is that nitrifiers are producing new cells (biomass) that isn't biodegradable in the BOD test. Nitrifying bacteria could easily survive in the BOD test. Nitrifiers are autotrophs so they produce biomass without consuming organic matter (from hydrogencarbonate).

Dear

Air stripping was obviously enhanced with the decrease in pH.

You may find useful to check the following related RG discussions: https://www.researchgate.net/post/Why_I_am_getting_a_negative_r_values_between_pH_and_Dissolved_Oxygen https://www.researchgate.net/post/What_is_the_relation_between_pH_and_dissolved_oxygen_in_a_bioreactor_cell_culture