Design and management of conventional fluidized-sand biofilters

Authors: SUMMERFELT, STEVE - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2005
Publication Date: 5/15/2006
Citation: Summerfelt, S. 2006. Design and management of conventional fluidized-sand biofilters. Aquacultural Engineering. 34(3):275-302.

Interpretive Summary: Fluidized-sand beds are an efficient, relatively compact, and cost-competitive technology for removing dissolved wastes from recirculating aquaculture systems, especially in relatively cool or coldwater applications that require maintaining consistently low levels of ammonia and nitrite. This paper describes several types of flow injection mechanisms used in commercial fluidized-sand biofilters and provides criteria for design of flow distribution mechanisms at the bottom of the fluidized bed. This paper also summarizes the most critical aspects of sand selection, as well as methods for calculating or experimentally measuring fluidization velocities and pressure drop for a given filter sand size distribution. Estimates of nitrification rate, ammonia removal efficiency, carbon dioxide production, and oxygen consumption across fluidized-sand biofilters are also provided for various conditions. Fluidized-sand biofilter operational and management practices are also described.

Technical Abstract: Fluidized sand biofilters (FSBs) are relatively compact, efficient, and cost-competitive biofilters, especially in recirculating systems that require maintaining consistently low levels of ammonia and nitrite. Filter sand is low cost (often $70-200/m3 of sand delivered) and has a high specific surface area (4,000 to 20,000 m2/m3), so the cost to supply surface area in FSBs is low (i.e., $0.05-0.004/m2). FSBs can be used to treat small or large flows, with single FSBs treating as much 190 L/s of water flow. FSBs are generally tall (2-6 m tall) and can be fabricated as circular or rectangular vessels from plastic, fiberglass, concrete, or enamel-coated steel tanks, and can sometimes be assembled by personnel on site. FSBs can be relatively easy to manage as they do not filter solids from the passing flow and the microbial biomass in the expanded bed can be readily harvested by siphoning the lightest (i.e., thickest and oldest), biofilm coated particles from the top of the bed. This paper describes several types of flow injection mechanisms used in commercial FSB and provides and management recommendations for FSBs.