A REVIEW OF HYDRAULICS IN FLUIDIZED-BED BIOLOGICAL FILTERS S. T. Summerfelt, J. L. Cleasby ABSTRACT. Fluidized sand-beds are efficient and cost-effective methods for biological treatment within recirculating aquaculture systems, and have recently been the topic of considerable research. However, their hydraulic design has been largely undescribed in the aquaculture literature, and at this point, hydraulic criteria are typically obtained from preliminary laboratory fluidization studies on media samples obtained from distributors. This article summarizes and applies techniques developed in other disciplines for calculating fluidization hydraulics, based solely upon characteristics of the granular medium, for the purpose c>f designing biological fluidized-bed units for use in recycle aquaculture. These calculations require knowledge of five characteristics: (1) and (2) the medium's size gradation and effective diameter, estimated from sieve analysis provided by the supplier, (3) the medium's density, obtained from a property of materials table, and (4) and (5) the medium's sphericity and the bed porosity, both of which can be reasonably assumed, particularly with sands. Solutions to fiuidization calculations are presented as simplified curves useful for sensitivity analysis for optimizing unit design. This article also discusses the (1) characteristics which make a medium suitable for use in nitrifying fluidized beds in aquaculture, (2) design criteria for proper distribution of water at the bottom of the fluidized bed, and (3) special issues involving fluidization hydraulics and the application of fluidized beds to aquaculture. Keywords. Fluidize, Biofllter, Hydraulic, Expansion, Aquaculture, F luidized-sand biological filters are used to reduce dissolved constituents in wastewaters (Sutton and Mishra, 1991; Jewell, 1990; Cooper and Atkinson, 1981). Within recirculating aquaculture systems, fluidized-sand biological filters have been shown to be space and capital cost-efficient methods for providing nitrification (Burden, 1988; Losordo and Westerman, 1991). The performance of fluidized beds (both plastic pellet and sand) as nitrifying biofilters within recirculating aquaculture systems have been studied considerably, particularly during the last several years (Bullock et al., 1993; Watten, 1993; Weaver, 1991; Heinen and Hankins, 1991; Thomasson, 1991; Wimberly, 1990; Jewell and Cunmiings, 1989; Owsley et al., 1988; Burden, 1988; Burden and Malone, 1988; Paller and Lewis, 1988; Cooley, 1979). Currently, there are several large-scale commercial fish producers using fluidized-sand biological filters in their recirculating systems. The nitrification capacity of biological filters is largely dependent upon the total surface area available for biological growth and the efficiency of the area utilization. Greater anmionia removal capacity results from an increase in biofilm surface area. The efficiency of nitrification per unit surface area is dependent upon the accessibility of the Article has been reviewed and approved for publication by the Emerging Areas Div. of ASAE. This material is based upon work supported by the USDA- Agricultural Research Service under Agreement No. 59-1931 -8-111. The authors are Steven T. Summerfelt, Research Engineer, The Conservation Fund's Freshwater Institute, Shepherdstown, West Virginia, and John L. Cleasby, Anson Marston Distinguished Professor in Engineering, Dept. of Civil and Construction Engineering, Iowa State University, Ames. Corresponding author: Steven T. Summerfelt, Freshwater Institute, P. O. Box 1746, Shepherdstown, WV 25443; telephone: 304-876-1606; e-mail: <76622.2274@compuserve.com>. media surface to the substrate, the substrate concentration and loading, the mass transfer rate into and out of the biofilm, the growth phase of the biofilm (lag, log, stationary, and death phases), and by the competition with heterotrophic microbes for space and oxygen (Alleman and Preston, 1991; Manem and Rittman, 1992). Efficient use of the media surface area is provided in fluidized beds by the suspension and rolling of the media grains such that all portions are exposed to the solution. Mass transfer efficiency is increased at the biofilm surface on the particles within the fluidized bed because the high velocities and turbulence required for bed expansion decreases the thickness of the stagnant boundary layer surrounding the biofilm. Microorganism growth is a function of substrate concentration and loading (concentration times flow per unit surface area). Therefore, fluidized-sand biofilters can be managed to maintain the biofilm at or near log growth phase, which can be hypothesized the most efficient microbial phase for nutrient up-take. Biofilms can be managed by either introducing clean media (e.g., sand) while removing media supporting aged biofilms, or, by selecting a sand diameter that maintains a relatively ttiin steady-state biofilm, one with no net change in growtfi or decay. A thin steady-state biofilm can be obtained when (1) biofilm growth is balanced against continuous physical shearing due to hydraulic forces and physical particle-particle or particle-wall interactions (Chang et al., 1991) and (2) total biofilm mass is just equal to that which can be supported by the substrate loading (Rittman, 1982). In addition, the proportion of decaying organic matter within the biofilm is reduced by continuous shearing forces, which reduces the substrate available to heterotrophic organisms and reduces the competition placed on nitrifying bacteria. V0L.39(3):1161-1173 Transactions of the ASAE © 1996 American Society of Agricultural Engineers 0001-2351 / 96/3903-1161 1161