Denitrification and dissimilatory nitrate reduction to ammonium in digested sludge

Area of Science:

• Environmental microbiology
• Biogeochemical cycles

Background:

• Denitrification is crucial for nitrogen removal in wastewater treatment.
• Understanding the metabolic capabilities of denitrifiers is essential for optimizing treatment processes.

Purpose of the Study:

• To investigate the denitrification and nitrate-to-ammonium reduction potential of digested sludge.
• To characterize the activity of denitrifying enzymes and the fate of nitrogen oxides.

Main Methods:

• Acetylene inhibition technique to assay denitrification.
• 13N tracer methods to quantify nitrate reduction to ammonium.
• Measurement of gaseous nitrogen oxides (NO, N2O, N2).

Main Results:

• Digested sludge readily produced NO, N2O, and N2 from nitrate/nitrite, indicating active denitrifying enzymes.
• Acetylene effectively inhibited N2O to N2 reduction at higher nitrate levels.
• Fresh sludge converted 60-70% of 13NO3- to 13NH4+, with the remainder likely lost to denitrification.
• Denitrification capacity was maintained in sludge and a long-term methanogenic culture.

Conclusions:

• Digested sludge possesses significant denitrification and nitrate-to-ammonium reduction capabilities.
• Denitrifying enzymes are active and capable of producing various nitrogen oxides.
• The persistence of denitrification capacity suggests the prevalence of denitrifiers with flexible anaerobic metabolism.