Roshanak Halvaeifard, Anammox recovers the alkalinity that was lost by oxidising the nitrogen. If we take the nitrogen to be ammonia the reactions are:

Nitritation: NH3 + 1.5 O2 -> NO2- + 2H2O + H+

Anammox: NH3 + NO2- -> N2 + H2O+OH-

Sum: 2NH3 + 1.5 O2 -> 3H2O

So, you can say anammox produce alkalinity and partial nitrification-anammox doesn't change alkalinity.

Usually wastewater is closer to neutral pH so we will find reduced nitrogen as ammonium, in which case the process appears to consume alkalinity and reduce pH:

Nitritation: NH4+ + 1.5 O2 -> NO2- + 2H2O + 2H+

Anammox: NH4+ + NO2- -> N2 + 2H2O

Sum 2NH4+ + 1.5 O2 -> 3H2O + N2 + 2H+

Henrik Rasmus Andersen : Thanks for your nice answer, then can we say that the amount of produced alkalinity in partial nitritation-ANAMMOX depends on ANAMMOX/nitritation ratio? I mean, by proceeding of ANAMMOX over nitritation, amount of alkalinity is increased in the system..

Roshanak Halvaeifard , The ratio of nitritation to the anammox reaction influence the alkalinity change, but I cannot see one can have a positive effect on the alkalinity, as one cannot have more ammonia oxidized by anammox than there are nitrite which formation removes an equivalent of alkalinity.

Nitritation: NH3 + 1.5 O2 -> NO2- + 2H2O + H+

Anammox: NH3 + NO2- -> N2 + H2O+OH-

Henrik Rasmus Andersen Thank you much for the clarification and your nice answers.

each mg/L ammonia consumed by anammox generates 1.7 mg/L alkalinity