Variation of pH and its indication during biological nitrogen removal in SBRs

Deep understanding of pH variation during biological nitrogen removal is critical for ensuring the effective operation of nitrification/denitrification or nitritation/denitritation processes on a real-time basis. Two tank-type glass vessels, with a 6 L working volume for each, were used as sequencing batch reactors (SBRs) to investigate variation of pH in biological nitrogen removal process (nitritation, nitratation, denitratation and denitritation). Same amount of ammonium nitrogen (NH₄⁺-N) and nitrite nitrogen were used as initial nitrogen substances for oxidation operation mode, whereas nitrate (N₃^--N) and nitrite nitrogen (NO₂^--N) were used for reduction mode. Results showed that, pH decreased during nitritation and increased during denitritation. On pH profile, “ammonium valley” indicated the end of nitritation, instead of nitrification and no characteristic bending point appeared at the end of nitrification. There was no evident break point appeared on pH profile when denitratation finished. Moreover, the pH profile during denitritation is more convex than that during denitrification. Alkalinity consumption was carried out mainly in nitritation process and alkalinity was produced only in denitritation process. Same amount of total alkalinity will be produced during reduction of same amount of nitrate and nitrite, but the components of alkalinity are different.