PERFORMANCE OF MARINE ANAMMOX UNDER RAS CONDITIONS: EFFECTS OF OXYGEN AND NITROGEN

Introduction

As aquaculture production intensifies, concerns regarding nutrient discharges to the environment are increasing. Recirculating aquaculture systems (RAS) enable high water reuse while maintaining controlled farming environments (Van Rijn 2013). In RAS aerobic biofilters, nitrifying bacteria oxidize the ammonium (NH4⁺) excreted by the fish into nitrate (NO3^-) via nitrite (NO2^-). NO3^- is then typically removed through unsustainable water exchanges or converted to nitrogen gas (N2) via complex denitrification (Chen 2002, Preena et al. 2021).

The anammox (anaerobic ammonia oxidation) process has emerged as a cost-effective and environment-friendly nitrogen removal alternative, where NH4⁺ and NO2^- are directly converted into N2 (Kartal et al. 2012). We previously demonstrated that the marine anammox bacteria Candidatus Scalindua effectively removed NH4⁺ and NO2^- from synthetic RAS wastewater under controlled laboratory conditions (Micolucci et al. 2023).

Material and methods

To bridge the gap between controlled laboratory conditions and real-world applications, we further evaluated the performance of Ca. Scalindua under RAS-relevant conditions, characterized by low levels of NH4⁺ and NO2^-(<1 mg.L^-1), elevated NO3^- (up to 400-1000 mg.L^-1) and the presence of oxygen (O2), necessary for the fish and the nitrifying bacteria. Upflow reactors were operated under these RAS conditions and monitored over time. Nitrogen removal performance was assessed from influent and effluent concentrations, and microbial community dynamics were analysed using 16S rRNA gene sequencing and FISH.

Results and discussion

High nitrogen removal efficiencies (>70%) were maintained at low substrate concentrations typical of RAS by increasing flow rates to sustain nitrogen loading and maintaining trace elements supplementation (preliminary results). Elevated NO3^- concentrations encountered (up to 1000 mg.L^-1) did not affect nitrogen removal but reduced the relative abundance of Ca. Scalindua in the granule (Roques et al. 2024, Roques et al. 2026). Furthermore, nitrogen removal efficiencies remained high and stable when the biomass was gradually exposed to increasing O2 concentrations up to 4.5 ppm, the highest level tested so far (Figure 1). These results suggest the potential applicability of anammox under real RAS conditions in marine environments, despite its characterization as a strictly anaerobic process, suggesting a higher tolerance to oxygen under RAS conditions. Further validation at pilot scale is required to confirm long-term stability and process integration.

Figure 1. Performance of the marine anammox Ca. Scalindua under increasing O2 concentrations.

Acknowledgments

The studies were conducted within the frame of the MIRAI project and was supported by FORMAS (2020-00867), Kungl. Skogs- och Lantbruksakademien (GFS2024-0148), STINT (MG2019-8483), JSPS KAKENHI (JP23KJ1642, JP24KK0197), JSPS Bilateral Program (JPJSBP120259928) and FY2025 JSPS Invitational Fellowship for Research in Japan (S25124).

References

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