The use of biological traps for water treatment in recirculating aquaculture systems

Abstract

Wastewater (WW) of recirculating aquaculture systems (RAS) has a high concentration of dissolved nutrients, which enable bioremediation of RAS WW with microalgae. Biological harvesting by filter-feeding organisms offers an alternative for the expensive mechanical and chemical harvesting of microalgae with opportunities for further utilization of the produced biomass. This thesis evaluated if the combination of microalgae and waterflea (Daphnia magna) cultivation in Nordic RAS WW (ca. 17 °C) can be used to trap the dissolved nutrients. Green microalgae had comparable growth and removal of nitrate-nitrogen (NO3-N) and phosphate-phosphorous (PO4-P) in unfiltered RAS WW to those in the reference algal medium while non-green microalgae had insignificant growth and nutrient removal capacity. Growth and nutrient removal of three green microalgae in unfiltered WW did not differ between the three tested LED spectra, while the fourth tested species, Haematoccocus pluvialis, showed higher nutrient removal under a specific LED spectrum. Filtration of WW from RAS for growing microalgae is not needed as biological contaminants within WW did not significantly decrease the microalgal growth, nutrient removal, and amino acid and fatty acid composition, with the exception of H. pluvialis. When green microalgae were cultivated in WW and fed to D. magna, Daphnia’s weight increased 2–3 times in 4 days. D. magna removed 80 % of Monoraphidium griffithii, 70 % of H. pluvialis, and 20 % of Selenastrum sp. from WW in 48 h. Only when Selenastrum sp. was used as a diet, D. magna re-released PO4-P into solution. In conclusion, the efficiency of microalgae–Daphnia bioremediation system in Nordic RAS WW can be improved with a careful selection of microalgal species, supporting the concept of circular economy and sustainable WW management.