1 Evaluation and Improvements of Solids Removal Systems for Aquaculture Raul H. Piedrahita Department of Biological and Agricultural Engineering University of California, Davis Walter H. Zachritz, II Southwest Technology Development Institute New Mexico State University, Las Cruces Kevin Fitzsimmons Environmental Research Laboratory University of Arizona, Tucson Charles Brockway Department of Civil and Agricultural Engineering University of Idaho, Kimberly Introduction The effective management of solids in aquaculture waters is one of the major obstacles to the continued development of the aquaculture industry. Solids derived from aquaculture operations are composed of unconsumed feed particles and feces. Solids removal from aquaculture waters is an integral component of most aquaculture systems, whether they are flow through or recirculating. Solids in flow through system effluents may lead to possible environmental degradation due to the organic and nutrient content in the solids. In recirculating systems, the presence of solids in the water makes water treatment and water quality maintenance more difficult, increasing water treatment costs, and increasing the health risks to the crop due to water quality degradation. By removing solids from the water, substantial proportions of the organic and nutrient (nitrogen and phosphorus) loads are prevented from contributing to the degradation of water quality. Degradation of water quality by solids may come from the consumption of oxygen to oxidize organics present and from the release of inorganic nutrients. Regulatory agencies have recognized this, and use solids loads as one of the main criteria when evaluating an aquaculture operation. Settling basins have been the most common solids removal method used by the trout industry in the Western States. These basins can be effective in reducing suspended solids concentrations to very low levels, but they must be managed carefully to achieve high nutrient removal rates as well. Whereas settling basins appear to be the system of choice for the trout industry in some areas, they may not be practical or effective for other types of aquaculture operations. In those cases, other types of solids removal systems such as various configurations of microscreen filters, granular media filters, or constructed wetlands may be more effective components of a water treatment system. However, selection of the "best" treatment system for a particular aquaculture operation is difficult given the variety of processes available, and the lack of a uniform methodology for evaluation of water treatment effectiveness, and of an accounting of economic and practical considerations.