Water treatment of recirculating aquaculture system (RAS) effluent water through microalgal biofilms

This research studied the growth of microalgae (Chlorella vulgaris and Phaeodactylum tricornutum) on a biofilm reactor using effluent water from salmon production in a recirculating aquaculture system (RAS). RAS effluent water contains considerable amounts of nitrate and small amounts of phosphate, that stem from dissolved excess feed and fish faeces. In microalgae growth experiments, we tested a twin-layer biofilm reactor, which has one layer for substrate distribution (RAS effluent water-based medium) and adhered onto that, a carrier layer for biofilm cultivation. First, we tested five different carrier materials (newsprint, filter paper, polypropylene, viscose/polyester mix, viscose) to assess the microalgae's attachment ability of the material, where the viscose fibre material proved to be the most suitable. The biofilm reactor design had to be improved for saltwater suitability, as water evaporation caused changes in salinity and nutrient concentrations and ultimately led to the formation of salt crusts on the biofilm and clogging of the irrigation system. A dilution of the medium with osmosis water compensated the evaporation rate and a technical improvement of the irrigation system established stable cultivation conditions. The biofilm reactor was then tested for all three water types (Chlorella for freshwater, Phaeodactylum for brackish water and saltwater) that are discharged during a RAS production cycle for salmon. Microalgae paste was used for inoculation of the biofilm carrier material and after a short maturation phase the biofilm reactor was harvested every three days. This study demonstrated that a complete uptake of nitrate and phosphate from RAS effluent water through microalgae cultivation is possible, and the biofilm reactor is able to handle changes in nutrient concentrations and salinity. Biomass productivity for Phaeodactylum cultivated on brackish RAS medium was highest (15.28 g m⁻² d⁻¹), compared to saltwater RAS medium (4.35 g m⁻² d⁻¹) and Chlorella on freshwater RAS medium (4.25 g m⁻² d⁻¹).