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

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.Kalojen kasvatus kiertovesijärjestelmässä tuottaa jätevettä, jonka ravinnepitoisuus on niin suuri, että jätevettä voidaan hyödyntää leväbiomassan tuottamiseen ja samalla vähentää ravinnepäästöjä ympäristöön. Tässä työssä tutkittiin luontaisesti Suomessa esiintyvien mikrolevien kasvua ja kykyä poistaa ravinteita kiertovesikasvatuksen jätevedestä sekä kasvatettujen mikrolevien poistamista vedestä ravintonsa suodattavien vesikirppujen (Daphnia magna) avulla. Testatuista leväryhmistä viherleviin kuuluvat levät kasvoivat ja vähensivät tehokkaasti jäteveden liuenneita ravinteita, nitraattia ja fosfaattia (NO3-N ja PO4-P), kun taas muiden leväryhmien edustajat kasvoivat huonosti eivätkä vähentäneet ravinteita oleellisesti. LED-valojen spektri ei vaikuttanut kolmen tutkitun viherlevän kasvuun ja jäteveden ravinteiden poistokykyyn, mutta neljännellä tutkimuslajilla (Haematoccocus pluvialis) ravinteiden käyttö tehostui tietyllä LED-valon spektrillä. Jäteveden suodatus leväkasvatusta varten ei näytä olevan tarpeen, koska biologiset epäpuhtaudet eivät merkitsevästi vähentäneet mikrolevien kasvua, ravinteiden poistokykyä eivätkä myöskään aminohappo- tai rasvahappokoostumusta, poikkeuksena H. pluvialis. Kun jätevedessä kasvatettuja leviä syötettiin vesikirpuille, vesikirppujen paino 2–3-kertaistui neljässä vuorokaudessa. Vesikirput pystyivät poistamaan vedestä 80 % Monoraphidium griffithii - ja 70 % H. pluvialis -levistä, mutta vain 20 % Selenastrum sp. -levästä 48 h aikana. Jos ruokana oli Selenastrum sp. -levä, vesikirput vapauttivat veteen PO4-P:a. Tämän tutkimuksen perusteella mikrolevien kasvatus on mahdollista yhdistää vesikirppujen tuottamiseen jäteveden puhdistamiseksi myös pohjoismaisissa ilmasto-olosuhteissa, mutta tehokkuuden maksimoimiseksi on oleellista käyttää käyttötarkoitukseen sopivinta mikrolevää

Efficiency of Daphnia magna in removal of green microalgae cultivated in Nordic recirculating aquaculture system wastewater

The increase of global aquaculture production has boosted the development of recirculating aquaculture systems not only because they reduce water use but also provide opportunities for waste management and the use of released nutrients. The dissolved nutrients can be efficiently removed from recirculating aquaculture system wastewater by microalgae, and microalgae can be harvested from the wastewater with low costs by zooplankton such as Daphnia. The purpose of this study was to evaluate the feasibility of using microalgae and Daphnia for bioremediation of recirculating aquaculture system wastewater in Nordic conditions. We evaluated the growth and filtration efficiency of the waterflea Daphnia magna on four green microalgae species cultured at 17 ± 0.3 °C in recirculating aquaculture system wastewater as compared to microalgae medium. There was no difference in the growth of Daphnia fed with Monoraphidium griffithii and Selenastrum sp. cultured in either media, while with Chlamydomonas reinhardtii and Haematococcus pluvialis growth of Daphnia was slower when cultured in wastewater than in algae medium. Higher filtration efficiency was achieved with the Daphnia density of 100 individuals L−1, than with higher tested densities (200 and 300 ind. L−1). After 48 h, Daphnia had removed 80% of M. griffithii, 70% of H. pluvialis, but only 20% of Selenastrum sp. from the wastewater. Phosphate was re-released to water after 72 h feeding on Selenastrum sp., but not when feeding on M. griffithii. Taken together, of the four tested green microalgae, M. griffithii was found to be the most suitable microalga species for the microalgae-Daphnia bioremediation system. The results support the potential of using microalgae and Daphnia for bioremediation and for producing biomass in Nordic recirculating aquaculture system wastewater.peerReviewe

Screening of microalgae and LED grow light spectra for effective removal of dissolved nutrients from cold-water recirculating aquaculture system (RAS) wastewater

Popularity of recirculating aquaculture systems (RAS) is increasing. Because of the high water recirculation rate, dissolved nutrients originating from fish feed are concentrated enough in RAS wastewater (WW) to enable growth of primary producers, e.g. microalgae. This study evaluated nitrate-nitrogen (NO3-N) and phosphate-phosphorus (PO4-P) removal efficiency of ten temperate zone freshwater microalgae species during their exponential growth phase in unfiltered RAS WW at 17 ± 0.5 °C. Growth and nutrient uptake efficiency of six green and four non-green microalgae strains were compared between WW and reference growth medium in batch monocultures. The effect of three different LED grow light spectra on growth and nutrient uptake efficiency were compared for four green microalgae in WW. The specific growth rate (SGR, 0.5-0.8 d−1) and removal of NO3-N (N%, 57–96%) and PO4-P (P%, 78–94%) of green microalgae in WW in 4 days were comparable to the results obtained in the reference medium (SGR, 0.6-0.9 d−1, N%, 59–99%; P%, 86–99%). In contrast, non-green microalgae had negligible growth (SGR, from -0.1 to 0.2 d−1) and poor nutrient removal (N%, 1–29%; P%, 0–34%) in both growth media after 9 days. The three LED spectra did not differ on their effect on growth and nutrient removal of three green microalgae in WW after 4 days, while the fourth tested species, Haematoccocus pluvialis, had its highest nutrient removal after 8 days under a specific LED spectrum. Current results show that RAS WW supports well green microalgae growth in batch cultures in temperatures common in Nordic RAS and that continuous spectrum LED grow lights can induce high removal of dissolved nutrients. Our findings lend support to the concept of using temperate zone microalgae for nutrient removal and recycling from RAS WW.peerReviewe

Original data for article: Filtration of Nordic recirculating aquaculture system wastewater: effects on microalgal growth, nutrient removal and nutritional value

The data processed and analyzed in this study is divided if five files: Growth_and_Nutrient_removal.txt, FA_concentration.txt, FA_proportion.txt, AA_concentration.txt and AA_proportion.txt. For detailed descriptions of the motives, methods and results of the study, see the article

Filtration of Nordic recirculating aquaculture system wastewater : Effects on microalgal growth, nutrient removal, and nutritional value

Microalgal bioremediation of recirculating aquaculture system (RAS) wastewater represents an alternative for wastewater treatment with the potential to generate valuable biomass. This study evaluated the effects of removing biological contamination and suspended solids from Nordic area RAS wastewater through filtration with 0.45 μm filters on the performance and nutritional value of microalgae. All three tested green microalgae (Haematococcus pluvialis, Monoraphidium griffithii, and Selenastrum sp.) were able to grow in raw (unfiltered) and filtered RAS wastewater. Cultivation in raw RAS wastewater decreased the ω-3 and ω-6 fatty acid content of H. pluvialis as compared to filtered wastewater, while no differences in cell density, nutrient removal, or fatty acid and amino acid contribution were seen for any microalgae between the treatments. Filtration of wastewater significantly reduced the content of actinobacterial fatty acid biomarkers in microalgal cultures compared to raw wastewater. The difference in actinobacterial fatty acid content between raw and filtered wastewater was species-specific. Our results suggest that with careful selection of microalgal species, RAS wastewater can be used for the production of high-quality microalgal biomass for further applications, such as aquaculture feeds, with no need to remove indigenous biological contaminants and suspended solids.peerReviewe