Efficiency of Trout Raceway Quiescent Zones in Controlling Suspended Solids

Abstract

Best management practice for effluent control of trout hatchery discharges calls for separation of cleaning waste by collection in quiescent zones (QZs) at the end of each raceway. The efficiency of QZs in trapping solid waste over a range of fish densities, water velocities, and QZ areas, measured by overflow rate (OFR), were studied. Solids retention in the QZs was negligible at velocities of 2.0 and 4.4 cm/s (regardless of fish density) and at a fish density of 40 kg/m³ (regardless of velocity). Solids collection efficiency was affected more by OFR and fish density than by velocity. At OFRs of 0.5 and 1.0 cm/s, solids collection was significantly better at a fish density of 160 kg/m³ than at a density of 80 kg/m³. At a density of 80 kg/m³, solids retention was better at an OFR of 0.5 cm/s than at an OFR of 1.0 cm/s. Solids retention was not affected by OFR at a fish density of 160 kg/m³. Velocities in the range of 2.4–3.4 cm/s did not affect solids collection efficiency at either fish density. Thus, optimum conditions for solids collection in trout raceway QZs were 2.4–3.4-cm/s velocity and (1) a fish density of 160 kg/m³ at an OFR of 0.5 or 1.0 cm/s or (2) a density of 80 kg/m³ at an OFR of 0.5 cm/s. Under these conditions, up to 20% of the total solids load was collected in the raceway QZs. Although this value is lower than expected based on the literature, actual solids levels were less than 5 mg/L and solids concentrations in the effluents of downstream units of serial raceways were not additive.