دانلود رایگان مقاله مقایسه هیدرولیک و راندمان حذف ذرات در نهر سلول مخلوط

ABSTRACT

We compared the hydrodynamics of replicate experimental mixed cell and replicate standard Burrows pond rearing systems at the Dworshak National Fish Hatchery, ID, in an effort to identify methods for improved solids removal. We measured and compared the hydraulic residence time, particle removal efficiency, and measures of velocity using several tools. Computational fluid dynamics was used first to characterize hydraulics in the proposed retrofit that included removal of the traditional Burrows pond dividing wall and establishment of four counter rotating cells with appropriate drains and inlet water jets. Hydraulic residence time was subsequently established in the four full scale test tanks using measures of conductivity of a salt tracer introduced into the systems both with and without fish present. Vertical and horizontal velocities were also measured with acoustic Doppler velocimetry in transects across each of the rearing systems. Finally, we introduced ABS sinking beads that simulated fish solids then followed the kinetics of their removal via the drains to establish relative purge rates. The mixed cell raceway provided higher mean velocities and a more uniform velocity distribution than did the Burrows pond. Vectors revealed well-defined, counter-rotating cells in the mixed cell raceway, and were likely contributing factors in achieving a relatively high particle removal efficiency-88.6% versus 8.0% during the test period. We speculate retrofits of rearing ponds to mixed cell systems will improve both the rearing environments for the fish and solids removal, improving the efficiency and bio-security of fish culture. We recommend further testing in hatchery production trials to evaluate fish physiology and growth.

4. Discussion

Our study provides the first assessment of a large MCR in a side-by-side field comparison with BP with and without fish. In addition, we evaluated the hydraulic characteristics with two sizes of fish. The modification of two BP into two MCR systems was undertaken as a research effort by hatchery managers at the Dworshak National Fish Hatchery, Idaho, to determine if personnel costs for pond cleaning could be reduced, fish health and performance could be improved and the overall fish rearing environment could be improved by improvements in water quality. The Dworshak National Fish Hatchery, Idaho, rears about 2 million Steelhead Trout Oncorhynchus mykiss annually that are released as smolts as part of a U.S. Army Corps of Engineers dam mitigation program. Fish are produced in 84 replicate Burrow’s type ponds measuring 22.86 by 5.18 m. Poor hydraulic conditions allow for inactive regions that, in turn, allow for the accumulation of waste solids. Modifications of the suite of BP were proposed as the least cost. In addition, the MCR system was proposed to improve bio-security measures to reduce transporting invasive species in the feces of fish (Bruce and Moffitt, 2010). Additional modifications of the design could have been made to separate the solid waste particles into a separate stream, but were not part of this study. Our analysis of the fluid responses in the two systems using a salt tracer showed both systems had measured mean water particle residence times that were lower than theoretical estimates. The variances around these measures were high. In the MCR, the fish effect could be observed to increase the variance, but fish reduced the variance in the BP (Table 2). The depletion of tracer was nearly 10 min faster in the MCR over the BP when no fish or small fish were present. Studies by Rasmussen et al. (2005) reported increased mean residence time ratios when fish were present and at two flow rates. Our studies found similar or lower mean hydraulic residence times occurred with fish in the rearing systems. Mixing rate in our studies increased with fish in the MCR, but decreased somewhat with larger fish in the BP. Rasmussen et al. (2005) reported decreases in mixing with fish at both flow rates they tested. Clearly the size and density of fish in the system can affect the hydraulic characteristics, and should be considered in system design and operation. Our measures of tracer were all at the outflow, but Rasmussen et al. (2005) measured both outlet-based and in-tank dispersion.