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[Display omitted] • Upgrading nitrogen removal performances in wastewater treatment plants by BES. • Enhancing the polishing action of secondary treatments to avoid tertiary treatments. • Nitrate and ammonium removal without nitrous oxide accumulation. • Disinfection of wastewater via electrochemical anodic oxidation. Economic options to retrofit wastewater treatment plants (WWTPs) without tertiary treatments need to be explored. In this regard, bioelectrochemical systems (BES) can be hybridized with existing technologies, upgrading the removal performance of original techniques while avoiding replacement costs. Yet, few demonstrations of merged systems have been given. For the first time, in this work it was built a lab-scale model of a BES merged with a secondary settler, namely e-settler, to enhance the polishing performance of already existing WWTPs. In particular, to concomitantly increase nitrogen removal and perform wastewater (WW) disinfection, avoiding further tertiary treatments. In the e-settlers, nitrogen removal was increased through bioelectrochemical stimulation. Concomitant ammonium and nitrate removal without nitrite accumulation and a negligible amount of nitrous oxide emissions were observed. Ti-MMO as anode material showed a high disinfectant action. In conclusion, it was demonstrated how a simple bioelectrochemical set-up can upgrade existing WWTPs. The following step requires the study at a larger scale, identifying optimal operational and structural parameters for the in-situ application. The main limitations of the e-settlers were discussed, linking them to possible solutions that need to be deepened in a lab-scale model of conventional secondary treatments (activated sludge followed by secondary settler). [ABSTRACT FROM AUTHOR]

[Display omitted] • Continuous aeration in membrane-aerated biofilms resulted in full nitrification. • Intermittent aeration suppressed nitrite oxidation and supported anammox process. • Biofilm pH presented periodic upshifts with aeration switches. • Free ammonia speciation likely caused the inhibition of nitrite-oxidizing bacteria. • Heterotrophs established nitrous oxide-reduction zones under intermittent aeration. Membrane-aerated biofilm reactors (MABR) are being applied for autotrophic nitrogen removal, yet control of nitrogen turnover remains challenging in MABR counter-diffusion biofilms. In this study, we regulated microbial activities in two lab-scale MABRs by providing continuous versus intermittent aeration. Nitrogen consumption by different functional microbial groups was estimated from bulk measurements via a mass balance approach. Nitrite-oxidizing bacteria (NOB) proliferated under continuous aeration while they were significantly suppressed under intermittent aeration, and NOB suppression activated anaerobic ammonium oxidation. Nitritation performance in the MABR was studied through long-term bulk measurements and in situ biofilm microprofiles of dissolved oxygen (DO) and pH. During intermittent aeration pH effects rather than DO effects determined nitritation success, especially ammonia speciation, which serves as substrate and inhibitor in nitrification processes. Biofilm transition phases were monitored upon aeration switches. Canonical correspondence analysis suggested that the relative transition after anoxia and aeration intermittency were less decisive for biofilm performance than the relative aeration duration. Heterotrophic bacteria displayed minor denitrification rates with aeration control, but contributed to mitigation of nitrous oxide (N 2 O) emissions. N 2 O production hotspots were identified at the top of the anoxic biofilm zone under continuous aeration. Instead, under intermittent aeration an anoxic N 2 O reduction zone was established. Our observations support intermittent aeration control of MABRs as a simple strategy for energy-efficient nitrogen removal with low N 2 O emission.. [ABSTRACT FROM AUTHOR]