Economic Evaluation of Hydroponics and Other Treatment Options for Phosphorus Removal in Aquaculture Effluent

HORTSCIENCE, VOL. 35(6), OCTOBER 2000 Consumer demand for fish has been increasing despite declining ocean fish catches. Aquaculture, the cultivation of freshwater and marine plants and animals, is one of the fastest growing segments of U.S. agriculture. In the period from 1987 to 1992, sales of farm-raised trout increased by almost 20% to over $80 million in the United States (Terlizzi et al., 1995). Other sectors of the industry are growing even faster, with an overall increase in sales of almost 52% (to $504 million) during this time period. Three different methods have been used for aquacultural production: pond culture, flowthrough systems, and recirculating systems. Pond systems, the most widely practiced form of aquaculture in the United States, have been used for the production of catfish (Ictaluris sp.) and many other species. Flow-through systems involve the continual flow of water through a tank or raceway. Often, these systems have been used in conjunction with a high-yielding spring for trout (Oncorhynchus sp.) production. Recirculating systems are semi-closed systems, in which water flowing through a series of tanks or raceways is captured, treated, and reused. Recirculating systems use the least amount of water, which is an advantage in areas with either limited water resources or stringent discharge standards. A high degree of management expertise is needed to manage oxygen levels and water quality in these systems. With the increase in production of fish also comes an increase in discharge of nutrient pollutants. Wastewater from aquaculture can pollute streams by adding excess nitrogen, phosphorus, and organic matter. Removal of an aquaculture effluent. Thin-film technology is a hydroponic crop production system in which plants grow in water that flows continuously as a thin-film over their roots. Water flow across the roots decreases the stagnant boundary layer surrounding each root, thus enhancing the mass transfer of nutrients to the root surface and permitting crops to maintain high productivity at steady-state P levels above 0.3 mg·L (Chen et al., 1997). In our initial study, lettuce plants were grown in long (21.9 m, 126 plants) troughs on rainbow trout effluent flowing from one end of the trough to the other. This system removed P from an inlet concentration of ≈0.7 mg·L to an outlet concentration of a few μg·L. However, as solution P concentrations dropped below ≈0.3 mg·L, tissue P concentrations decreased. Even so, growth was sustained until the P concentration within the plant dropped below the critical deficiency level (0.35% to 0.4% P on a dry weight basis for lettuce). At that point, P deficiency symptoms appeared, growth rate decreased, and the plants became unmarketable. Thus, conventional hydroponic technology (where all plants in the trough are the same age) could only remove ≈50% of the P while producing a marketable product. Although lettuce can remove P to <0.3 mg·L, a reduction in growth coincides with a further reduction in solution P concentrations. As a result, the conveyor production strategy was developed to sustain plant productivity and health while removing dissolved P levels to <0.01 mg·L. Conveyor production system. Fundamental concepts of plant nutrition were utilized to develop the conveyor production system, which produced healthy lettuce and basil without an apparent reduction in growth, while simultaneously removing P to very low levels (μg·L). [For a mechanistic understanding of plant nutrient uptake, see Adler et al. (1996d, 1996e)]. Plants have the capacity to absorb and store nutrients in excess of their immediate needs, a process called luxury consumption (Marschner, 1995). The conveyor crop production strategy enables plants to store P early in their growth cycle. This stored reserthese nutrients from wastewater is an important operation because these compounds play a critical role in eutrophication. Emphasis has been placed on phosphorus removal for two reasons: 1) phosphorus is often the most critical nutrient in eutrophication of freshwater; and 2) nitrogen removal processes are less efficient and more expensive (Ramalho, 1983). All states in the Northeastern United States have regulations regarding the discharge of aquacultural effluents (Ewart et al., 1995). Therefore, treatment of fishery effluents needs to be considered when planning aquacultural production systems. Aquacultural effluents are difficult to treat because they contain large volume flows carrying relatively dilute nutrients (e.g., <1 mg·L of P) (Heinen et al., 1996). However, treating the nutrients in aquacultural effluents may be important because, depending upon the receiving water, the total nutrient mass loading can contribute significantly to environmental degradation. The Freshwater Institute maintains a highdensity recirculating system near Shepherdstown, W. Va., which has the capacity to produce ≈22.7 t of rainbow trout (Oncorhynchus mykiss Walbaum) annually. Daily production of trout effluent is ≈109 m. Adler et al. (1996a, 2000) have considered several nontraditional techniques for treatment of the effluent from this system, including hydroponic crop production in greenhouses. Hydroponic production of horticultural crops such as basil (Ocimum basilicum L.) and lettuce (Lactuca sativa L.) may be a way to treat the wastewater and also produce a profit for growers. Predominant thinking regarding the use of food crops to clean aquaculture effluents has been that plants cannot remove nutrients in water to low levels without a reduction in productivity and quality. Because greenhouse space is expensive, maintaining maximum productivity is critical to sustaining a profitable operation. Thin-film technology production system. Conventional hydroponic production of lettuce and basil using thin-film technology, also known as Nutrient Film Technique (NFT), was investigated as a method to remove P from Economic Evaluation of Hydroponics and Other Treatment Options for Phosphorus Removal in Aquaculture Effluent