Bioelectrochemical removal of ammonium in single-chamber systems using two different side-stream anammox sludges

Anaerobic ammonium oxidation (anammox) is a specialised biological process used for nitrogen removal from wastewater, where ammonium is oxidised with nitrite as the electron acceptor. However, this process is constrained by the slow growth rate of anammox microorganisms and the requirement for a continuous and stable supply of nitrite, which limits both the process efficiency and operational stability. This study evaluated the feasibility of replacing nitrite with a poised electrode as the electron acceptor in bioelectrochemical systems. Two distinct sludge sources were examined, originating from separate side-stream anammox processes: a Deammonification (DeAmmon®) MBBR process and an Integrated Fixed-Film Activated Sludge (IFAS-ANITA™Mox) process. Reactor operation was conducted in three sequential stages: (i) biological operation with nitrite supply, (ii) electrode installation with an applied anodic potential of 600 mV accompanied by a gradual reduction in nitrite supply, and (iii) operation at 600 mV without nitrite addition. Following complete nitrite removal, the DeAmmon-seeded bioelectrochemical reactor achieved an average NH₄ + -N removal efficiency of 87.6% (maximum 96.4%) at 20 mg/L influent NH₄ + -N and 94.7% (maximum 99.8%) at 40 mg/L influent NH₄ + -N. In contrast, the IFAS-inoculated reactor achieved an average removal efficiency of 76.7% (maximum 86.6%) at an influent NH₄ + -N concentration of 40 mg/L. The microbial community analysis revealed notable differences in key taxa between the two reactors. The IFAS-inoculated system exhibited the highest abundance of the anammox bacterium Candidatus Brocadia . In contrast, the DeAmmon-inoculated reactor had a more diverse nitrogen-transforming community, including nitrifiers such as Nitrosomonadaceae and denitrifiers such as Comamonadaceae .