High-rate nitrification at low pH in suspended- and attached-biomass reactors

Area of Science:

• Environmental Microbiology
• Biogeochemical Cycles
• Wastewater Treatment

Background:

• Nitrification is a key process in the nitrogen cycle, typically occurring at neutral pH.
• Low pH environments were previously considered inhibitory to efficient nitrification.
• Chemolithotrophic bacteria are known to perform nitrification.

Purpose of the Study:

• To investigate high-rate nitrification at low pH using biofilm and suspended-biomass reactors.
• To identify the bacterial genera responsible for nitrification under acidic conditions.
• To assess the efficiency of ammonium uptake and biomass growth at low pH.

Main Methods:

• Cultivation of nitrifying bacteria in biofilm and suspended-biomass reactors at low pH.
• Measurement of nitrification rates and specific nitrification rates.
• Isotope labeling (15N) to track ammonium assimilation and biomass growth.
• Fluorescence in situ hybridization (FISH) for bacterial identification.

Main Results:

• Achieved high nitrification rates (up to 5.6 g N/L/day) in a biofilm reactor at pH 4.3.
• Observed significant biomass growth (yield of 0.1 g biomass/g N oxidized) in a suspended-biomass reactor at pH 3.8.
• Identified Nitrosomonas bacteria as the dominant nitrifiers, indicating widespread low-pH nitrification capability.
• Demonstrated efficient ammonium uptake and acid tolerance in these bacteria.

Conclusions:

• Autotrophic nitrifying bacteria, specifically Nitrosomonas, can achieve high nitrification rates at low pH.
• These bacteria possess efficient ammonium uptake systems and mechanisms to cope with acidic environments.
• Low-pH nitrification is more widespread than previously understood, with implications for nitrogen cycling and wastewater treatment.