Single-step nitrification models erroneously describe batch ammonia oxidation profiles when nitrite oxidation becomes...

Area of Science:

• Environmental Microbiology
• Biochemical Engineering
• Water Quality Management

Background:

• Nitrification is a crucial two-step biological process converting ammonium-nitrogen (NH(4)(+)-N) to nitrite-nitrogen (NO(2)(-)-N) and then to nitrate-nitrogen (NO(3)(-)-N).
• Accurate kinetic modeling of nitrification is essential for optimizing wastewater treatment and understanding nitrogen cycling in ecosystems.
• Previous models often simplify nitrification to a single composite reaction, potentially overlooking the distinct kinetics of each step.

Purpose of the Study:

• To investigate the adequacy of modeling the complete nitrification process (NH(4)(+)-N to NO(3)(-)-N) as a single biochemical reaction.
• To determine the kinetic parameters for the individual steps: NH(4)(+)-N to NO(2)(-)-N oxidation and NO(2)(-)-N to NO(3)(-)-N oxidation.
• To assess the impact of rate-limiting steps on the accuracy of composite nitrification models.

Main Methods:

• Utilized selective inhibitors (allylthiourea and sodium azide) to uncouple the two stages of nitrification in a mixed nitrifying consortium.
• Employed a rapid extant respirometric technique to determine kinetic parameters (q(max) and K(S)) for each oxidation step.
• Derived stoichiometric coefficients relating nitrogen removal, oxygen uptake, and biomass synthesis using an electron balance equation.

Main Results:

• Ammonium-nitrogen to nitrite-nitrogen oxidation kinetics were independent of nitrite-nitrogen concentrations up to 100 mg/L.
• Nitrite-nitrogen to nitrate-nitrogen oxidation was noncompetitively inhibited by ammonium-nitrogen but unaffected by nitrate-nitrogen up to 250 mg/L.
• Single-step modeling adequately represented nitrification when NH(4)(+)-N to NO(2)(-)-N oxidation was solely rate-limiting; however, it yielded erroneous parameters when both steps were rate-limiting.

Conclusions:

• Modeling nitrification as a single composite reaction can lead to inaccurate kinetic parameter estimations when both oxidation steps are simultaneously rate-limiting.
• Independent quantification of NH(4)(+)-N to NO(2)(-)-N and NO(2)(-)-N to NO(3)(-)-N oxidation is necessary for precise kinetic analysis in such scenarios.
• This study highlights the importance of considering the distinct kinetics of sequential reactions in biological processes for accurate modeling and process optimization.