Reducing phosphorus discharge from flow-through aquaculture I: facility and effluent characterization

Brit True; William Johnson; Shulin Chen

doi:10.1016/j.aquaeng.2004.08.001

Phosphorus (P) discharged from flow-through (FT) fish culture operations has caused environmental concerns linked to the role of P in the eutrophication process. Physical dimensions and flow rates of five rainbow trout (Oncorhynchus mykiss) FT farms in Southern, Idaho, USA, and the effluent of two of those farms were characterized to identify and quantify factors affecting P discharge. The results indicated that the mean water current velocities in raceways were of an order of magnitude below the 0.1–0.6m/s range recommended to prevent solids settlement [Boersen, G., Westers, H., 1986. Waste solids control in hatchery raceways. Prog. Fish Cult.; Alley, E.R., 2000. Water Quality Control Handbook. McGraw-Hill, New York] and thus lead to waste settlement in the rearing areas of raceways. The average measured solids settling velocities were 0.16 and 2.31cm/s for particles smaller and larger than 814μm, respectively. The mean particulate P discharge was 0.04mg/L, composing 3.4% by mass of the discharged suspended solids and representing 40% of total P discharge. The mean dissolved P discharge was 0.06mg/L representing 60% of the total P discharge. No significant statistical difference was found in P contents associated with different particle sizes (P > 0.11). The average concentrations of suspended solids that were greater than 10, 53, and 105μm were 1.93, 1.34, and 1.01mg/L, respectively; suggesting that 69% of discharged solids mass was greater than 53μm and 52% was greater than 105μm. Laser diffraction particle size analysis revealed a volumetrically weighted median effluent particle size of 250.1μm; with 89% of the discharged particles larger than 53μm and 76% greater than 105μm. The results suggested that the removal of particles over 100μm from the effluent would result in approximate total P discharge reductions of 20%. Overall, the results confirmed that transport and removal are important factors affecting P discharge. The information obtained from this work can be used to design processes and systems to reduce P discharge from these operations.

Reducing phosphorus discharge from flow-through aquaculture I: facility and effluent characterization

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