There are several indicators of denitrification. Indicators of denitrification in the secondary clarifier include
• presence of numerous bubbles,
• rising solids that have numerous bubbles on their surface,
• increase in alkalinity, and possibly, pH across the clarifier, and
• reduction in redox potential across the clarifier.
Because the atmosphere contains approximately 80% molecular nitrogen, the capture and measurement of escaping molecular nitrogen from the surface of the clarifier to demonstrate denitrification is not practical. The captured gas may be contaminated by atmospheric molecular nitrogen.
However, capturing and measuring nitrous oxide can be used as an indicator of denitrification. Only a very small quantity of nitrous oxide is present in the atmosphere at low elevations, and the accumulation of nitrous oxide serves as an indicator of denitrification.
Proper mass balance for nitrogen (including the nitrogenous gases released during denitrification) is difficult to make in an activated sludge process. This difficulty is due to the numerous conversions of nitrogen that occur in the activated sludge process. These conversions include not only aerobic nitrification and anoxic denitrifica-tion but also assimilatory nitrate reduction, organotrophic nitrification, anaerobic ammonium ion oxidation, and aerobic denitrification (Figure 31.1).
Denitrification usually occurs in the secondary clarifier if the sludge is retained too long, and the sludge blanket becomes deoxy-genated. Denitrification in the secondary clarifier also can be associated with clarifier design. Flat-bottom clarifiers with a central sludge takeoff are very susceptible to denitrification. This is due to the accumulation of sludge at the perimeter of the clarifiers.
Denitrification can be controlled in the secondary clarifier by increasing the RAS rate or ensuring that the mixed liquor is well aerated before it enters the clarifier. Design problems contributing to denitrification may be overcome with appropriate baffling.
Denitrification also can be controlled by recycling the RAS to a special anoxic zone at the inlet of the aeration tank where the incoming wastewater itself is used as the carbon source or substrate. The recycled sludge and wastewater are kept in the anoxic zone for a spe-
Figure 31.1 Nitrogen conversions. In the environment or the activated sludge process, nitrogen can be converted from one form of nitrogenous compound to another form of nitrogenous compound. These conversions are due to biological, chemical, and physical events. Nitrate ions (NO3) are perhaps the most significant nitrogenous compounds. Nitrate ions can be used as a nutrient source for nitrogen by many organisms as they undergo assimilatory nitrate reduction to ammonium ions (NH¿). Nitrate ions also can be converted to molecular nitrogen (N2) as they undergo dissimilatory nitrate reduction. Nitrite ions can undergo as-similatory reduction and dissimilatory reduction as they are converted to ammonium ions and molecular nitrogen, respectively. Molecular nitrogen can be fixed or converted to ammonium ions by several nitrogen-fixing bacteria and numerous algae. Once fixed, nitrogen in the form of ammonium ions is easily assimilated into amino acids and proteins—forms of organic nitrogen. Assimilation of ammonium ions in an aeration tank results in an increase in the amount of MLVSS. At relatively high pH values ammonium ions are converted to ammonium (NH3) and escape from the water to the atmosphere as a gas. Under some anaerobic conditions, ammonium ions can be oxidized to nitrite ions and nitrate ions (anaerobic ammonia oxidation).
cific period of time. This period may be 30 minutes, which is generally accepted for significant denitrification to occur, or 60 to 120 minutes for the control of undesired, filamentous growth.
If nitrification is not required at an activated sludge process, the easiest control measure for denitrification is to prevent nitrification in the aeration tank. Nitrification does not occur in the aeration tank if nitrite ions or nitrate ions are not produced. Nitrification can be prevented by decreasing aeration intensity, decreasing aeration time, and increasing cBOD loading.
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