Use of Recirculating Aquaculture Systems (RAS) for Atlantic salmon production is rapidly expanding both globally and domestically in the U.S. Domestic production, currently under 5 ,000 MT a year, is projected to reach 350,000 MT a year within 10 years. However , treating solid wastes from RAS systems is a physical bottleneck in the rapidly expanding U.S. and global RAS Atlantic salmon industry. 1,000 MT of salmon produced in RAS generate 16.7-20 MT of sludge daily, which translates to 6,100-7,300 MT sludge per year.
P erformance, biotic/abiotic operational parameters, and efficiency of a pilot-scale Atlantic salmon marine solid waste digester was tested under intensive industry-like conditions in a high density (65kg/m3 biomass ) pilot-scale 0.5 MT salmon saltwater RAS operation. The system attained a stable output of 500-600 L/day biogas (70% methane) and 83% reduction in COD with a HRT of 14 days in a combined hydrolysis/methanogenic 1, 000L modified UASB bioreactor. Energy recovery of 1.56 kWh from biogas production achieved 11.7% of the daily energy requirements for the system.
Water quality analyses of the influent and effluent of the anaerobic digester showed that not only are COD and TS reduced in the effluent, but phosphate is also reduced. In addition, alkalinity and pH are increased , which can offset reduction of alkalinity in the nitrification biofilter and prevent phosphate accumulation in the RAS.
Results demonstrate that this technology eliminates a major process bottleneck in the rapidly expanding U.S. and global RAS Atlantic salmon industry by digesting sludge in saline water. It reduces an environmental liability (RAS solid waste) and transforms the majority of the waste into an economic asset, biogas, which can partially offset ener gy cost of the RAS operation.