Solids management and removal for intensive land-based aquaculture production systems

This review aims to identify and examine realistic aquaculture waste solids management strategies. The main reason for treating solids to be discharged from flow-through systems is to reduce potential negative impacts on the surrounding aquatic environment. In reuse and recycle systems, solids management will be required to maintain culture water quality. In such cases, solids management will often be designed to be combined with other unit processes. Solids concentrations in the untreated effluent from flow-though farms are low at around 5-50 mg l(-1), and do not appear to have altered greatly within the last 20 years. These solids can commonly carry 7-32% of the total nitrogen and 30-84% of the total phosphorus in the wastewater. Feed quality and feeding management can be manipulated to reduce the quantity of waste solids produced. Decreases in the specific water consumption within a farm, increase the concentration of solids in the effluent, which results in an increase in particle separation efficiency. Particles should be separated from the culture stock and the primary effluent flow quickly and efficiently (i.e. at high solids concentrations). This can be achieved by within-tank separation systems, often comprising a separate low-flow particle outlet. Rotating microscreens are commonly used at land-based intensive fish-farms in Europe. Screen mesh pore sizes of 60-200 mu m are common. There is little advantage in using pore sizes smaller than 60 mu m. Low concentration aquaculture solids usually settle discretely in sedimentation tanks, i.e. with no agglomeration of particles that would increase their settling velocity. Such basins are prone to many technical problems, and in most situations are unlikely to be suitable for the treatment of solids in the primary wastewater from aquaculture facilities. Sedimentation basins may be appropriate for secondary de-watering or thickening. Overflow rates of 1.0-2.7 m(3) m(-2) h(-1) have been reported. Bead filters and Rotation columns have been shown to perform well for solids removal in limited flow-rate recycle systems. The sludge produced by separation technology can be thickened and stabilised by the addition of lime, to kill pathogenic diseases and restrict putrefaction. The resulting sludge has been spread on agricultural land. (C) 2000 Elsevier Science B.V. All rights reserved.