Hydrodynamics of an integrated fish and periphyton recirculating aquaculture system

Marine aquaculture helps to mitigate a number of environmental problems, such as overfishing, loss of biodiversity, and eutrophication. Periphyton-based biofilters are a promising technology for marine aquaculture water treatment because periphyton repurposes nutrients as fish feed and produces dissolved oxygen (DO). Integration of periphyton biofilters into recirculating aquaculture systems (RAS) also preserves water and prevents pollutant discharges. In this study, we examined the effect of system hydrodynamics on two pilot-scale (2500 L) RAS with integrated periphyton biofilters, which were used to grow Ariopsis felis (hardhead catfish). Periphyton was harvested weekly from hanging nets. Conservative tracer tests conducted at varying fluid velocities indicated the presence of dead zones along tank edges. Growth of periphyton biomass was found to be primarily dependent on the nutrient mass loading rate. Improved mass transport and uptake of aqueous nutrients by periphyton occurred at higher fluid velocities. DO production by periphyton photosynthesis also increased with increasing fluid velocities. Mass balances on C, N, and DO were carried out to elucidate the nutrient transformation pathways and quantities. DO analysis revealed that periphyton provided 1.31 ± 0.20 mg DO/(L*m²*day) during daytime hours. This was nearly enough to support microbial and fish respiration without the use of a blower. Periphyton and filamentous algae, Oscillatoriaceae, removed 32 ± 4 % of the input nitrogen and 61 ± 3 % of input carbon from the feed that was not taken up by the cultured fish. The overall water quality goals for the catfish were either met or exceeded through application of periphyton biofilters in the RAS.