Using microalgae for removing nutrients from RAS aquaculture effluents - a viable strategy? A case study.

Abstract: The increased global demand for fish has caused a tremendous increase in aquaculture production during the last decade, emphasising the importance of systems with a low climate footprint. One such system is a recirculating aquaculture system (RAS) which is land-based as opposed to traditional open systems. By implementing microalgae for nutrient removal in RAS, a more sustainable water use, and increased water reuse can be achieved. In this study the microalgae species (Chlorella vulgaris, Nitzschia pusilla, Staurastrum monticulosum and Scenedesmus sp.) and their removal of nutrients from aquaculture effluent from a land-based fish farm was examined. Simultaneously the microalgae’s ability to grow and produce biomass in the aquaculture effluent was studied. Treatments were carried out in 250ml flasks, and nutrient removal, cell density, chlorophyll florescence parameters, growth rates and biovolume was measured up to day 7 (end of experiment). The results show that C. vulgaris and Scenedesmus sp. were most efficient at removing ammonia, having removed 96% of ammonia after 7 days. C. vulgaris also had the highest removal rate of phosphorous at 88%. C. vulgaris and N. pusilla had the smallest biovolumes and yielded the highest biomass of 0.28g/L and 0.17g/L, respectively. The results indicate that using microalgae as bioremediators instead of biofilters for nutrient removal might be a possible strategy depending on the choice of species and design of the RAS – with the added possibility of producing biomass for valorisation.