Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria

Area of Science:

• Environmental Microbiology
• Wastewater Treatment Engineering
• Biogeochemical Cycles

Background:

• Autotrophic nitrogen removal relies on nitritation (ammonium to nitrite conversion).
• Traditional nitritation processes face challenges with efficiency and microbial community stability.
• Low-strength wastewater treatment requires robust and effective nitrogen removal strategies.

Purpose of the Study:

• To demonstrate a stable and robust nitritation process for low-strength wastewater.
• To investigate the cultivation of acid-tolerant ammonia-oxidizing bacteria (AOB) under acidic conditions.
• To identify the dominant AOB responsible for nitritation and understand their role in the process.

Main Methods:

• Cultivation of AOB in a laboratory nitrifying bioreactor using a mixture of real sewage and anaerobic digestion liquor.
• Operation of the bioreactor at a low pH range of 4.5-5.0.
• 16S rRNA gene sequencing to analyze the microbial community composition.
• Monitoring of nitrite and nitrate concentrations to determine the nitrite accumulation ratio.
• Measurement of ammonium removal rates and greenhouse gas emissions (nitric oxide and nitrous oxide).

Main Results:

• Stable nitrite accumulation (95 ± 5% NO2-/(NO2- + NO3-)) was maintained for over 300 days.
• A high volumetric ammonium removal rate (188 ± 14 mg N L-1 d-1) was achieved.
• "Candidatus Nitrosoglobus" dominated the nitrifying community, replacing typical activated sludge microorganisms.
• This dominant AOB species demonstrated tolerance to low pH and free nitrous acid (FNA), inhibiting nitrite-oxidizing bacteria (NOB).
• Acidic nitritation resulted in higher nitric oxide (NO) and lower nitrous oxide (N2O) emissions compared to previous systems.

Conclusions:

• Acid-tolerant "Candidatus Nitrosoglobus" can be effectively cultivated for robust nitritation in low-strength wastewater.
• The acidic conditions and FNA generated in situ effectively inhibit NOB, leading to nitrite accumulation.
• This novel acidic nitritation process offers a promising approach for efficient nitrogen removal with potentially reduced greenhouse gas emissions.