Enhancement of Ammonium Oxidation at Microoxic Bioanodes

Abstract

Bioelectrochemical systems (BESs) are considered to be energy-efficient to convert ammonium, which is present in wastewater. The application of BESs as a technology to treat wastewater on an industrial scale is hindered by the slow removal rate and lack of understanding of the underlying ammonium conversion pathways. This study shows ammonium oxidation rates up to 228 ± 0.4 g-N m^-3 d^-1 under microoxic conditions (dissolved oxygen at 0.02-0.2 mg-O₂/L), which is a significant improvement compared to anoxic conditions (120 ± 21 g-N m^-3 d^-1). We found that this enhancement was related to the formation of hydroxylamine (NH₂OH), which is rate limiting in ammonium oxidation by ammonia-oxidizing microorganisms. NH₂OH was intermediate in both the absence and presence of oxygen. The dominant end-product of ammonium oxidation was dinitrogen gas, with about 75% conversion efficiency in the presence of a microoxic level of dissolved oxygen and 100% conversion efficiency in the absence of oxygen. This work elucidates the dominant pathways under microoxic and anoxic conditions which is a step toward the application of BESs for ammonium removal in wastewater treatment.

Keywords: Ammonium; ammonia-oxidizing microorganisms; bioanode; electro-anammox; oxygen.

Scheme 1. Schematic Diagram of the BES Reactor

Scheme 2. Timeline of the Experiments

The red blocks represent the periods with oxygen (+ O₂), and the blue blocks represent the periods without oxygen (− O₂). The breaks stand for the periods where reactors stabilized from switching between different conditions or disruption due to sampling. The data collected during these periods were not used in this study.

Figure 1

Change of NH₄⁺ concentrations in the anode compartment of duplicate bioelectrochemical reactors: (I) batch phase (+ O₂), (II) microoxic continuous phase (+ O₂), (III) anoxic continuous phase (− O₂), and (IV) microoxic continuous phase (+ O₂). An axis break is introduced to remove data points of the experiments in which the aim is not related to this section.

Figure 2

NH₄⁺ oxidation product distribution with and without oxygen: (I) batch phase (+ O₂), (II) microoxic continuous phase (+ O₂), (III) anoxic continuous phase (− O₂), and (IV) microoxic continuous phase (+ O₂). The distribution was represented by eight samples which were collected at the beginning and end of each phase. Each set of bars was from both R1 (left bar) and R2 (right bar).

Figure 3

Effect of ATU addition on NH₄⁺ oxidation in the anode compartment with oxygen (+ O₂) (a) (days 162 to 169) and without (− O₂) (b) (days 99 to 103).

Figure 4

Comparison of NH₄⁺ oxidation rates in the anode compartment for both with and without NH₂OH under microoxic conditions (+ O₂) (a) (days 153 to 158) and under anoxic conditions (− O₂) (b) (days 105 to 110).

Scheme 3. Proposed Pathways of NH₄⁺ Oxidation at a Bioanode

Pink and blue arrows indicate the reactions take place under microoxic and anoxic conditions. The green flash icons indicate the potential processes in which the electrode has been involved. Under microoxic conditions, they are (1) NH₄⁺ oxidation to NH₂OH by AOMs with oxygen as the electron acceptor; (2) bioelectrochemical NH₂OH oxidation to NO₂^–; (3) NO₂^– was removed, and the product was N₂, presumably by the traditional anammox; (4) NO₂^– was oxidized to NO₃^– via nitration by nitrite-oxidizing bacteria; (5) (bio) electrochemical NH₂OH oxidation to N₂O; under anoxic conditions, electro-anammox is likely the main activity; (6) NH₄⁺ oxidation to NH₂OH by anammox bacteria with the electrode as the electron acceptor; (7) NH₂OH was condensed with NH₄⁺ to produce N₂H₄; (8) oxidation of N₂H₄ to N₂.