Recirculating aquaculture systems (RAS) offer a solution to counter eutrophication risk through the treatment or recycling of nutrient-rich effluents. Applying the residual resource concept improves these effluents by adding value through recovery and recirculation of nutrients. The objective of this study was to determine the optimal solubilization oxidation-reduction (redox) conditions for increased solubilization of nutrients produced from marine RAS sludge. Thus, increasing its bioavailability for seaweed development and optimizing the utilization of sludge. The solubilisation of relevant nutrients as nitrogen (N), phosphorus (P), iron (Fe), and manganese (Mn) were examined under three redox potentials: aerobic, intermittent aeration and anaerobic. The results showed that anaerobic conditions led to the highest solubilization degree (0,55 ± 0,06 g sCOD/g TCOD) compared to aerobic (0,45 ± 0,07 g sCOD/g TCOD) and intermittent aeration (0,34 ± 0,09 g sCOD/g TCOD). The optimum fermented sludge supernatant (FSS) obtained was subsequently mixed with RAS overflow water (OW). This mix was evaluated against two conditions: FSS+OW with trace elements (TE) and OW alone, in a 10-day growth trial for assessment of both Ulva sp. specific growth rate and nutrient uptake efficiency. In this test, Ulva cultivated with FSS+OW presented the highest specific growth rates (SGR) (9,07 ± 1,49 %/day) than those treated with FSS+OW+TE (6,71 ± 1,95 %/day) and OW alone (4,95 ± 1,11 %/day). This indicates that the supernatant derived from anaerobically treated marine RAS sludge, when mixed with OW, can stimulate Ulva growth more effectively than the mix supplement with TE and OW alone. While previous research has demonstrated the feasibility of using RAS water for seaweed growth, the current study indicates the role of sludge in supplying nutrients to OW, increasing seaweed growth. The sludge utilisation could reduce the reliance on chemical trace element supplementation in seaweed farming, enhancing sustainability and at the same time reduce the costs of saline sludge treatment. These findings highlight the potential of valorising marine RAS effluents as a sustainable nutrient source in a circular bioeconomy for seaweed production.
Acknowledgement: This research was performed within “EASYTRAIN” network supported by the Marie Skłodowska‐Curie Action framework under the European Union’s Horizon 2020 research and innovation program [H2020‐MSCA‐ITN‐2020, grant agreement No 956129].