Reducing phosphorus discharge from high-density, flow-through aquaculture systems

Brit Johnson

doi:10.7273/000004294

Phosphorus (P) discharged from flow-through (FT) aquaculture operations into receiving waters has caused environmental concerns because of the role of P in the eutrophication process. The physical characteristics and effluents of rainbow trout (Oncorhynchus mykiss) FT farms in Idaho were characterized to identify and quantify factors affecting P discharge, with a focus on transport and removal of fecal particles. Results showed mean raceway water velocities of 0.05 m/s which were well below the 0.1-0.4 m/s range recommended for prevention of waste solids settlement. The mean solids settling velocities were 0.16 cm/s and 2.31 cm/s for particles smaller and larger than 814 µm, respectively. The mean solid and dissolved P concentrations in the effluent were 0.04 mg/L and 0.06 mg/L, respectively. The mean concentrations of total suspended solids greater than 10, 53, and 105 µm were 1.93 mg/L, 1.34 mg/L, and 1.01 mg/L, respectively. Laser diffraction particle sizing showed that the median effluent particle size was 250.1 µm. The results indicated that solids transport and solids removal directly affected P discharge. The effects of these factors were estimated. Excess P discharge due to inadequate solids transport was estimated at 2-10%. While, the effect of theremoval of effluent particles over 100 µm would result in reductions of solid phase P discharge ~ 60% (20% total P reduction). To address particle transport and removal factors baffle systems and high-rate filtration were designed and tested. Hinged and moving baffle systems were used to enhance particle transport that resulted in average under baffle velocities of 0.28 m/s (hinged) and 0.40 m/s (moving). Consequently average particle transport efficiencies increased from 15% (control) to 55% (hinged) and 75% (moving). A baffle design procedure for a range of facility and waste characteristics was also developed. To improve solids removal, two high-rate filtration media, reticulated foam and Fuzzy® media (Schreiber Technologies, Trussville, Alabama, USA) were characterized during bench top tests with simulated effluent. The reticulated foams were capable of handling higher hydraulic loading rates (>2,037 Lpm/m2 [50 gpm/ft2 ]) at lower head losses than the Fuzzy® media. Thus, even though the Fuzzy® media at ~15% compression captured the smallest particles, the reticulated foam was chosen as the most suitable medium for future trials. The 30 ppi (pores per inch) reticulated foam media was tested on FT effluent from a commercial farm. Solids capture results showed that the 30 ppi reticulated foam medium led to a 30% reduction in both suspended solids and solid phase P discharge, which corresponded to an 11% total P reduction. Both the baffle designs and the characterized high rate filtration media were effective in addressing solids transport and removal factors, respectively..

Reducing phosphorus discharge from high-density, flow-through aquaculture systems

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