The accumulation of nitrogenous compounds in recirculating aquaculture systems (RAS) represents a key challenge for the sustainability of the sector. The Anammox process (anaerobic ammonium oxidation) emerges as an efficient and low-energy alternative for nitrogen removal without the need for external organic carbon. However, its application under low salinity conditions, typical of freshwater or brackish water cultures, still presents microbiological limitations that remain poorly explored. This study preliminarily evaluated the performance of an Anammox consortium operated in a sequencing batch reactor (SBR) under freshwater and low salinity conditions (5 practical salinity units, PSU). Initial concentrations of 38 mg/L of ammonium (NH₄⁺-N) and 45 mg/L of nitrite (NO₂⁻-N) were used, with 6-hour operation cycles at 30 °C. Ammonium and nitrite consumption were evaluated, as well as the quantification and condition of the biomass through measurements of total suspended solids (TSS) and volatile suspended solids (VSS) .
The results show ammonium and nitrite conversion efficiencies of 75% and 95.96%, respectively, under freshwater conditions. When salinity was increased to 5 PSU, these efficiencies decreased to 41% and 49.68%. Simultaneously, a reduction in total suspended solids concentration (from 9.28 to 3.34 g/L) and volatile suspended solids (from 6.26 to 2.38 g/L) was observed, suggesting a possible impact on the active biomass or its retention within the system. These results indicate partial inhibition of the Anammox metabolism in brackish environments, possibly due to osmotic effects or alterations in the microbial consortium structure. However, with extended operation time and appropriate adaptation strategies, the process may stabilize and progressively improve its removal efficiency.