Role of Ammonia Oxidation in Organic Micropollutant Transformation during Wastewater Treatment: Insights from Molecular, Cellular, and Community Level Observations

Abstract

Organic micropollutants (OMPs) are a threat to aquatic environments, and wastewater treatment plants may act as a source or a barrier of OMPs entering the environment. Understanding the fate of OMPs in wastewater treatment processes is needed to establish efficient OMP removal strategies.Enhanced OMP biotransformation has been documented during biological nitrogen removal and has been attributed to the cometabolic activity of ammonia oxidizing bacteria (AOB) and, specifically, to the ammonia monooxygenase (AMO) enzyme. Yet, the exact mechanisms of OMP biotransformation are often unknown. This review aims to fundamentally and quantitatively evaluate the role of ammonia oxidation in OMP biotransformation during wastewater treatment processes. OMPs can be transformed by AOB via direct and indirect enzymatic reactions: AMO directly transforms OMPs primarily via hydroxylation, while biologically produced reactive nitrogen species (hydroxylamine (NH2OH), nitrite (NO2 − ) and nitric oxide (NO)) can chemically transform OMPs through nitration, hydroxylation and deamination and can contribute significantly to the observed OMP transformations. OMPs containing alkyl-, aliphatic hydroxyl-, ether-, and sulfide functional groups as well as substituted aromatic rings and aromatic primary amines can be biotransformed by AMO, while OMPs containing alkyl groups, phenols, secondary amines and aromatic primary amines can undergo abiotic transformations mediated by reactive nitrogen species.Higher OMP biotransformation efficiencies and rates are obtained in AOB-dominant microbial communities, especially in autotrophic reactors performing nitrification or nitritation, than in non-AOB-dominant microbial communities. The biotransformations of OMPs in wastewater treatment systems can often be linked to ammonium (NH4 + ) removal following two central lines of evidence:(i) similar transformation products (i.e. hydroxylated, nitrated and desaminated TP) are detected in wastewater treatment systems as in AOB pure cultures; (ii) consistency in OMP biotransformation (rbio, µmol/g VSS/d) to NH4 + removal (rNH4+, mol/g VSS/d) rate ratios (rbio/rNH4+) are observed for 3 individual OMPs across different systems with similar rNH4+ and AOB abundances. In this review, we conclude that AOB are the main driver of OMP biotransformation during wastewater treatment processes. The importance of biologically driven abiotic OMP transformation is quantitatively assessed and functional groups susceptible to transformations by AMO and reactive nitrogen species are systematically classified. This review will improve the prediction of OMP transformation and facilitate the design of efficient OMP removal strategies during wastewater treatment.

Comment on “Role of Ammonia Oxidation in Organic Micropollutant Transformation during Wastewater Treatment”: Overlooked Evidence to the Contrary

“…In their critical review, Su, Smets, and coauthors extensively summarized studies on the role played by ammonia-oxidizing bacteria (AOB) in organic micropollutant (OMP) transformation using three levels of evidence: molecular, cellular, and community. They also comprehensively covered the abiotic reactions with the N-species formed from nitrification.…”

“…In the following, we will present our main arguments and provide the overlooked evidence contradicting the authors’ conclusion. (In the following, all page numbers and references to graphical elements refer to Su et al 2020 …”

Response to “Comment on ‘Role of Ammonia Oxidation in Organic Micropollutant Transformation during Wastewater Treatment’: Overlooked Evidence to the Contrary”

“…We thank Profs. Fenner and Men for their critical comments on our manuscript that examined the role of ammonia oxidation in organic micropollutant (OMP) transformation during wastewater treatment . Their comments highlight the challenge in unraveling the contribution of different functional microorganisms in OMP transformation as well as their individual contributions at the community and system level.…”

“…The cited studies demonstrating efficient heterotrophic OMP biotransformation were conducted under sufficient COD (e.g., additional supply of acetate (600 mg COD/L) in Kennes-Veiga et al (2021)) and fully aerated conditions (>5–9 O 2 mg/L) but relatively low ammonium concentrations (<2–29 mg N/L), where the activities and cometabolic transformations of aerobic heterotrophs were favored over AOB. In addition to activated sludge reactors treating municipal wastewater, we also examined BNR reactors treating wastewater with high NH 4 + and no (e.g., lab-scale nitritation and partial nitritation-anammox systems) or low organic carbon contents (e.g., autotrophic nitrifying reactor), where AOB were highly enriched in the microbial community with relative abundances of 44 ± 28% and 29 ± 17%, respectively . In these autotrophic systems, AOB are dominant in microbial community and can drive the overall OMP biotransformation.…”

“…Fenner and Men for their critical comments 1 on our manuscript that examined the role of ammonia oxidation in organic micropollutant (OMP) transformation during wastewater treatment. 2 Their comments highlight the challenge in unraveling the contribution of different functional microorganisms in OMP transformation as well as their individual contributions at the community and system level. In our review paper, to reduce system complexity, we first examined the mechanisms and kinetics of OMP biotransformation by ammonia oxidizing bacteria (AOB) at the molecular and cellular level, from which we inferred their potential importance for OMP biotransformation in complex microbial communities.…”