Clean technology in aquaculture — a production without waste products?

Abstract

A theoretical model linking the production of salmon, mussels and seaweed in floating, enclosed units is based on field data, laboratory tests and literature data. First step is the production of 300 metric tons of salmon in 12 production units of 500 m³ each and a total water flow of 60 m³ min⁻¹. Using a standard high energy dry feed with a feed conversion factor of 1, a production of 300 metric tons of salmon is calculated to produce 15 metric tons of nitrogen (N) and 2.4 metric tons of phosphate (P), of which 13 metric tons of N and 0.7 metric tons of P are dissolved. Outlet water is transferred from the salmon units to 12 closed units (500 m³ each) with blue mussels. A standing stock of 112.5 metric tons of mussels (WW) is necessary to filtrate 60 m³ min⁻¹. If all particles are filtrated through the mussels, 25% of the nitrogen is accumulated as increased biomass. 25–30% is released as faeces and 45–50% as dissolved matter. Particles are removed by a sedimentation trap. Outlet water from the 12 mussel filter units, containing 13.9 metric tons of dissolved N (0.9 metric tons from the mussels), is transferred to closed units (1000 m³) with seaweeds. Concentration of N is calculated to 0.44 g N m⁻³ and hydraulic retention time in the seaweed production unit is 4 hours. With a DW of 20% (4% N of DW) and an estimated growth rate of 10% d⁻¹ during the production period, a standing stock of 45 metric tons (FW) of seaweed is theoretically needed to bind up all dissolved N from the salmon and mussel production.