TABLE 121 Ammonium Ion Loss and Production in an Aeration Tank

Mechanism for Ammonium Ion Loss Ammonium Ammonium Ion or Production Ion Loss Production

Air-stripping of ammonia at high pH x

Deamination of amino acids and proteins x

Degradation of cationic polymers x

Nitrified to nitrite ions x

Nutrient source for nitrogen for bacteria x dissolved oxygen are consumed by nitrifying bacteria as they oxidize ammonium ions and nitrite ions. Therefore, when nitrification occurs, a significant biomass demand for dissolved oxygen occurs, and the dissolved oxygen level within the aeration tank decreases.

Chemical indicators of nitrification include (1) an increase in chlorine demand for disinfection of the secondary e¿uent or control of undesired filamentous organisms within the mixed liquor or return activated sludge (RAS), (2) a decrease in mixed liquor alkalinity/pH, and (3) an increase in secondary clarifier alkalinity/pH. If incomplete nitrification occurs and nitrite ions accumulate, the nitrite ions react quickly with the chlorine, resulting in poor coliform kill in the secondary e¿uent and poor filamentous organism control in the mixed liquor or RAS. The production of nitrite ions within the aeration tank results in destruction of alkalinity and a drop in pH. Denitrifi-cation in the secondary clarifier (as a result of nitrification in an aeration tank) returns alkalinity to the wastewater. The return of alkalinity results in an increase in alkalinity/pH.

TABLE 12.2

Indicators of Nitrification




Growth of algae in the clarifier

Growth of duckweed in the clarifier

Decreased mixed liquor dissolved oxygen level

Increased mixed liquor dissolved oxygen demand


Increased chlorine demand for control of filamentous organisms

Increased chlorine demand for final effluent disinfection

Decreased mixed liquor alkalinity/pH

Increased secondary clarifier alkalinity/pH


Denitrification: Molecular nitrogen in secondary clarifier

Denitrification: Sludge bulking in secondary clarifier

Figure 12.1 Duckweed. Duckweed is the smallest flowering plant. The plant produces a white flower. Duckweed floats on the water with it "root" system suspended beneath its leaves. The nitrogen nutrient for duckweed is the nitrate ion (NO3).

Physical indicators of nitrification include (1) sludge bulking or clumping in the secondary clarifier and (2) the presence of molecular nitrogen rising to the surface of the secondary clarifier. Sludge bulking or large clumps of dark solids rising to the surface of the secondary clarifier is the result of denitrification. The solids rise to the surface, because large numbers of insoluble molecular nitrogen are trapped in the solids. The molecular nitrogen is produced through the denitrification of nitrite ions or nitrate ions, which are produced through nitrification in the aeration tank.

If any of the biological, chemical, or physical indicators of nitrification occur in an activated sludge process that is not required to nitrify, then nitrification should be suspected. To determine the form of nitrification occurring, mixed liquor e¿uent testing for the concentrations of ammonium ions, nitrite ions, and nitrate ions should be performed.

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