Activated sludge processes nitrify if nitrite ions or nitrate ions are produced in the aeration tank. Despite the various mechanisms of ammonium ion loss and production in an aeration tank (Table 12.1), a decrease in ammonium ion concentration across an aeration tank does not indicate nitrification. Nitrification is demonstrated by the production of nitrite ions or nitrate ions. However, if testing for nitrite ions or nitrate ions in the mixed liquor e¿uent is not performed, nitrification may be suspected by the presence of biological, chemical, and physical indicators (Table 12.2).
Biological indicators of nitrification include (1) the growth of algae and duckweed in secondary clarifiers, (2) a significant increase in mixed liquor oxygen demand, and (3) a significant decrease in mixed liquor dissolved oxygen level. Algae and duckweed obtain their nitrogen nutrient from nitrate ions. Therefore their presence in secondary clarifiers is an indicator of nitrate production or nitrification in the aeration tank.
Duckweed is the smallest and the simplistic of the flowering plants (Figure 12.1). Duckweed reproduces rapidly and floats on the water surface. It is used to control alga growth on oxidation ponds and remove nitrate ions in e¿uent lagoons before the e¿uent is discharged to receiving water. There are three genera of duckweed that are found in activated sludge process. These genera are Lemna, Spi-rodella, and Wolffia.
When nitrification occurs in an aeration tank, large quantities of
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