ANAEROBIC, AEROBIC AND MIXOTROPHIC BIOELECTROCHEMICAL DENITRIFICATION

RODRÍGUEZ SIMÓN, CARLOS NORBERTO; BUSALMEN, JUAN PABLO; PEDETTA, ANDREA; BONANNI, PABLO SEBASTIAN

Congreso; SAMIGE 2022; 2023

Bioelectrochemical denitrification allows surpassing an usual limitation of traditional denitrification processes which is the availability of electrons and carbon sources for denitrifying bacteria. Being thought of as an anaerobic process, it is mostly studied in the lack of oxygen. In denitrifying systems where oxygen is present it is proposed that denitrification is feasible due to the existence of anaerobic microenvironments that prevent denitrifying bacteria being inhibited by contact with oxygen. But not all denitrifiers are inhibited by oxygen and, notably, all of them can use it as an electron acceptor. Oxygen does not have an inhibitory effect on the enzymes responsible for nitrate reduction, but on nitrate transporters at the internal membrane. In some denitrifiers nitrate reductases are located at the cytoplasm (NarGHI) and, as a consequence, nitrate reduction is then inhibited by the presence of oxygen. In other bacteria nitrate reductases are located in the periplasm (NapAB) and thus, the inhibition of internal membrane transporters by oxygen does not impede denitrification to occur. Finally, some bacteria have both types of nitrate reductases and are not only tolerant to oxygen, but can also use it as an electron acceptor in a process called aerobic denitrification. In this process bacteria use oxygen and nitrate simultaneously as electron acceptors, in a mixotrophic way of growth. The possibility for denitrifiers to develop in the presence of oxygen expands the experimental and technological setups where bioelectrochemical denitrification can be applied but, still, aerobic denitrification is rarely considered. In this work the efficiency on nitrate and ammonia removal of aerobic and anaerobic denitrification in autotrophic conditions with an electrode as sole electron source were compared. Higher removal rates were obtained under aerobic conditions (19.2 gN/m3.day vs 10.8 gN/m3.day under anaerobic conditions) in agreement with also higher cathodic current densities (0.14 A/m2 vs. 0.03 A/m2 for anaerobic reactors) and higher respiration rates measured under the same aerobic condition. Also, as the presence of oxygen allowed nitrification (bacterial ammonia oxidation), much higher ammonia removal was obtained in the presence of oxygen (8.4 gN/m3day and 0.5 gN/m3day, respectively). A metagenomic analysis of the populations showed a distinctive Nar/Nap profile between both conditions. Electrochemical analysis of these populations revealed that the electrochemical potentials at which denitrification is performed is produced at higher potentials in the presence of oxygen, with sharp current increase at potentials of -450mV vs Ag/AgCl and -250mV vs Ag/AgCl for anaerobic and aerobic conditions respectively.