Reducing the Environmental Impact of High-density Fish Production: an Integrated Approach to Solids Treatment for Recirculating Aquaculture Systems Using Expandable Granular Biofilters

Over the past decade, the environmental impact of wastes discharged from high-density flow-through fish-production systems has been viewed with increasing concern. Recirculating aquaculture systems are gaining wider acceptance because of their ability to reduce waste discharges, improve quality control, and reduce costs. The crucial processes that must be addressed in treating recirculating water are solids capture, biofiltration, aeration, degasification, and ion balance. One of the greatest potentials for cost reduction rests with the development of designs that integrate two or more of these processes. Expandable granular biofilters (EGBs) integrate solids capture and biofiltration in a single unit process, potentially reducing the cost of treatment. Backwash frequency, a major operational parameter, influences the rate of nitrification and the volume of sludge produced. Computer and mass balance models allow the relationship between solids retention time, sludge production, and nitrification rates to be examined. While infrequent backflushing decreases water loss and sludge production, nitrification rates decline for extended solids retention times. Declining nitrification rates reflect ammonification of the accreting solids mass and a decrease in the availability of oxygen due to heterotrophic competition and increased mass-transfer resistance. Nitrification appears to be optimized with solids retention times in the range of 2-3 days, for filters utilized as the primary solids capture device. Studies indicate that biofiltration can be optimized in the presence of solids capture, by manipulating the backwash regime. This paper focuses on the primary in-filter stabilization and the post-discharge, predisposal digestion of aquacultural solids. However, because in-filter stabilization of solids is accomplished concomitantly with nitrification in EGBs, the interplay of these two processes are considered. 1 For citation, use: Golz, W. J., S. Chen, and R.F. Malone. 1995. Reducing the Environmental Impact of High Density Fish Production: An Integrated Approach to Solids Treatment for Recirculating Aquaculture Systems Using Expandable Granular Biofilters. In Water Effluent and Quality, with Special Emphasis on Finfish and Shrimp Aquaculture: Proceedings of the Twenty-Fourth U.S.-Japan Aquaculture Panel Symposium, 157-164. 2 Author to whom all correspondence should be addressed: [email protected]. W. J. Golz, et al. / Reducing the Environmental Impact of High-Density Fish Production 2