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• FBR is a viable option for end-of-pipe removal of nitrate-N. • Among the five carbon sources tested, acetate sustained the highest N removal rate. • A volume of 33 m3 is needed to remove 39 kg N/d in an acetate-fed reactor. • Using ethanol as a carbon source can cause sulfide production. • Using fish organic waste in N end-of-pipe removal will save operational costs. Stringent environmental legislation in Europe, especially in the Baltic Sea area, limits the discharge of nutrients to natural water bodies, limiting the aquaculture production in the region. Therefore, cost-efficient end-of-pipe treatment technologies to reduce nitrogen (N) discharge are required for the sustainable growth of marine land-based RAS. The following study examined the potential of fed batch reactors (FBR) in treating saline RAS effluents, aiming to define optimal operational conditions and evaluate the activated sludge denitrification capacity using external (acetate, propionate and ethanol) and internal carbon sources (RAS fish organic waste (FOW) and RAS fermented fish organic waste (FFOW)). The results show that between the evaluated operation cycle times (2, 4, and 6 h), the highest nitrate/nitrite removal rate was achieved at an operation cycle time of 2 h (corresponding to a hydraulic retention time of 2.5 h) when acetate was used as a carbon source. The specific denitrification rates were 98.7 ± 3.4 mg NO 3 −-N/(h g biomass) and 93.2 ± 13.6 mg NO x −-N/(h g biomass), with a resulting volumetric denitrification capacity of 1.20 kg NO 3 −-N/(m3 reactor d). The usage of external and internal carbon sources at an operation cycle time of 4 h demonstrated that acetate had the highest nitrate removal rate (57.6 ± 6.6 mg N/(h g biomass)), followed by propionate (37.5 ± 6.3 mg NO 3 −-N/(h g biomass)), ethanol (25.5 ± 6.0 mg NO 3 −-N/(h g biomass)) and internal carbon sources (7.7 ± 1.6–14.1 ± 2.2 mg NO 3 −-N/(h g biomass)). No TAN (Total Ammonia Nitrogen) or PO 4 3- accumulation was observed in the effluent when using the external carbon sources, while 0.9 ± 0.5 mg TAN/L and 3.9 ± 1.5 mg PO 4 3--P/L was found in the effluent when using the FOW, and 8.1±0.7 mg TAN/L and 7.3 ± 0.9 mg PO 4 3--P/L when using FFOW. Average sulfide concentrations varied between 0.002 and 0.008 mg S2-/L when using the acetate, propionate and FOW, while using ethanol resulted in the accumulation of sulfide (0.26 ± 0.17 mg S2-/L). Altogether, it was demonstrated that FBR has a great potential for end-of-pipe denitrification in marine land-based RAS, with a reliable operation and a reduced reactor volume as compared to the other available technologies. Using acetate, the required reactor volume is less than half of what is needed for other evaluated carbon sources, due to the higher denitrification rate achieved. Additionally, combined use of both internal and external carbon sources would further reduce the operational carbon cost. [ABSTRACT FROM AUTHOR]