Unveiling microbial electricity driven anoxic ammonium removal

Direct ammonium oxidation to nitrogen gas (Dirammox) in Alcaligenes strain HO-1: The electrode role

“…Electricity-driven ammonium (NH 4 + ) oxidation to dinitrogen gas (N 2 ) has been recently proved using ANAMMOX bacteria [ 1 ]. A similar process has been observed in the presence of feammox [ 2 , 3 ] or nitrifying bacteria, even though the mechanism is still mostly unknown in this latter case [ [4] , [5] , [6] , [7] , [8] , [9] ]. However, the electrical stimulation of heterotrophic nitrifying microorganisms has not been evaluated yet [ 10 , 11 ].…”

“…Besides, Alcaligenes did not have an apparent increase of ammonium removal activity in the presence of an electrode, and electrons were discharged on the electrode during the NH 4 + removal process. The results suggested that hydroxylamine, an intermediate of ammonium conversion into dinitrogen gas [ 14 ], could be involved in the electrochemical signal, a process suggested previously with nitrifying-enriched communities [ 5 , 8 ]. If NH 2 OH oxidation to N 2 is considered a potential oxidation reaction, current density would then represent about 3.8% of the NH 4 + removed in the presence of aeration and 16.6% in the absence of aeration.…”

“…Hence, a comparatively higher generation and oxidation of electroactive intermediates were observed. Taking into account that ammonium removal in Alcaligenes occurs via hydroxylamine [ 14 ] and the electroactivity of NH 2 OH [ 5 , 8 ], it was considered that the transient accumulation of hydroxylamine and its consequent oxidation could be responsible for current density generation. Considering the NH 2 OH oxidation to N 2 ( n = 1e − ), the current density observed represents 3.8 ± 0.3% of the ammonium removed in the presence of aeration and 10.9% and 22.3% in the first and second batch in the absence of aeration, hence representing significant support to the overall ammonium oxidation process.…”

“…Microbial electrochemical technologies (METs) are potential alternatives to suppress aeration in wastewater treatment plants due to the ability of some microorganisms to use an electrode as an electron acceptor instead of oxygen [ 1 , 8 , 18 ]. Considering the pivoting role of hydroxylamine in electricity-driven nitrification [ 5 , 8 ], one might hypothesise those heterotrophic nitrifying microorganisms such as Alcaligenes , could also be able to use an electrode as the terminal electron acceptor. Using an electrode instead of oxygen could suppress external aeration while being a barrier for competing for non-electroactive microorganisms.…”

Enhancement of Ammonium Oxidation at Microoxic Bioanodes

“…the formation of N 2 O was prevented. Complete removal of NH 4 + to N 2 was observed at bioanodes with mixed cultures and pure anammox cultures (referred to as electro-anammox), respectively. − With these insights, bioelectrochemical systems are a promising selective and energy efficient alternative to conventional N treatment technologies. The reported NH 4 + oxidation rates in these systems were low compared to existing NH 4 + removal technologies, i.e.…”