Abstract
There has been an increase in the use of Recirculating Aquaculture Systems (RAS) to produce Atlantic salmon smolts (Salmo salar). However, concerns have been raised regarding the environmental pollution associated with aquaculture wastewater. RAS water contains substantial amounts of inorganic nitrogen and phosphorous which can impact the local ecosystems. Nitrogen accumulation in RAS can also pose a potential threat to the fish and need to be treated before reaching a toxic level. Meanwhile, nitrogen and phosphorus deficiency pose a global threat, as binding nitrogen from the atmosphere is responsible for 1% of the energy consumption and 1.4% of the CO2 footprint on a global scale. Also, the “peak phosphorous” theory describes how the world will be depleted of phosphorous within the end of the century, posing a serious threat to the worlds food security. The present study aimed to address the challenge of nutrient management in RAS. By cultivating the microorganisms present in the RAS water as a mixed community biofilm in a bioreactor, this system can utilize nutrients in the RAS water, thus contributing to the water treatment process. Additionally, the biofilm product can be harvested and utilized as a potential feedstock or fertilizer ingredient due to the valuable biochemical composition associated with microalgal biomass. This study therefore aimed to develop a growth system for this mixed biofilm of microorganisms present in the RAS water from Hardingsmolt AS. Important influencing factors for biofilm development and cultivation was determined. Additionally, the biochemical composition of the biomass produced under different harvest frequencies, was determined, and compared to the biomass obtained in a pilot study on-site at Hardingsmolt AS. Photoautotrophic organisms were found to dominate the biofilm consortium, indicating light as an important factor for cultivation. Further, the biomass yield and productivity significantly increased with the addition of silica to the RAS water, keeping a high nutrient concentration, and harvesting of the biofilm every 12 days. The biochemical profiles of the biofilm product under the effect of different harvest frequencies (4 days, 8 days, 12 days) were characterized. The 12 day harvest frequency condition had significantly higher values for total protein (363 mg g-1 DW), nitrogen, and carbon contents and sum non-essential amino acids (208 mg g-1 DW). When the biofilm product obtained in the lab was compared to the one from the on-site pilot reactors, no significant differences was found. The valuable polyunsaturated fatty acids, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) was both present in all groups, where the 12 day harvest frequency had somewhat, but not significantly, higher contents of both DHA and EPA. High levels of palmitoleic acid were also found in all three conditions. The results obtained in the present study, demonstrated the successful cultivation of mixed community biofilm from RAS water (Hardingsmolt AS), which was recognized as a promising component in both RAS water treatment and as a potential feedstock ingredient.