Economic Evaluation of Hydroponics and Other Treatment Options for Phosphorus Removal in Aquaculture Effluent Paul R
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Economic Evaluation of Hydroponics and Other Treatment Options for Phosphorus Removal in Aquaculture Effluent Paul R. Adler1 U.S. Department of Agriculture–Agricultural Research Service, 45 Wiltshire Road, Kearneysville, WV 25430-9802 Jayson K. Harper Department of Agricultural Economics and Rural Sociology, The Pennsylvania State University, 214-A Armsby Building, University Park, PA 16802 Fumiomi Takeda U.S. Department of Agriculture–Agricultural Research Service, 45 Wiltshire Road, Kearneysville, WV 25430-9802 Edward M. Wade and Steven T. Summerfelt The Conservation Fund’s Freshwater Institute, P.O. Box 1889, Shepherdstown, WV 25443 Consumer demand for fish has been in- these nutrients from wastewater is an impor- an aquaculture effluent. Thin-film technology creasing despite declining ocean fish catches. tant operation because these compounds play is a hydroponic crop production system in Aquaculture, the cultivation of freshwater and a critical role in eutrophication. Emphasis has which plants grow in water that flows continu- marine plants and animals, is one of the fastest been placed on phosphorus removal for two ously as a thin-film over their roots. Water growing segments of U.S. agriculture. In the reasons: 1) phosphorus is often the most criti- flow across the roots decreases the stagnant period from 1987 to 1992, sales of farm-raised cal nutrient in eutrophication of freshwater; boundary layer surrounding each root, thus trout increased by almost 20% to over $80 and 2) nitrogen removal processes are less enhancing the mass transfer of nutrients to the million in the United States (Terlizzi et al., efficient and more expensive (Ramalho, 1983). root surface and permitting crops to maintain 1995). Other sectors of the industry are growing All states in the Northeastern United States high productivity at steady-state P levels above even faster, with an overall increase in sales of have regulations regarding the discharge of 0.3 mg·L–1 (Chen et al., 1997). almost 52% (to $504 million) during this time aquacultural effluents (Ewart et al., 1995). In our initial study, lettuce plants were period. Therefore, treatment of fishery effluents needs grown in long (21.9 m, 126 plants) troughs on Three different methods have been used to be considered when planning aquacultural rainbow trout effluent flowing from one end of for aquacultural production: pond culture, flow- production systems. Aquacultural effluents the trough to the other. This system removed P through systems, and recirculating systems. are difficult to treat because they contain large from an inlet concentration of ≈0.7 mg·L–1 to Pond systems, the most widely practiced form volume flows carrying relatively dilute nutri- an outlet concentration of a few µg·L–1. How- of aquaculture in the United States, have been ents (e.g., <1 mg·L–1 of P) (Heinen et al., ever, as solution P concentrations dropped used for the production of catfish (Ictaluris 1996). However, treating the nutrients in aquac- below ≈0.3 mg·L–1, tissue P concentrations sp.) and many other species. Flow-through ultural effluents may be important because, decreased. Even so, growth was sustained systems involve the continual flow of water depending upon the receiving water, the total until the P concentration within the plant through a tank or raceway. Often, these sys- nutrient mass loading can contribute signifi- dropped below the critical deficiency level tems have been used in conjunction with a cantly to environmental degradation. (0.35% to 0.4% P on a dry weight basis for high-yielding spring for trout (Oncorhynchus The Freshwater Institute maintains a high- lettuce). At that point, P deficiency symptoms sp.) production. Recirculating systems are density recirculating system near Shepherds- appeared, growth rate decreased, and the plants semi-closed systems, in which water flowing town, W. Va., which has the capacity to pro- became unmarketable. Thus, conventional through a series of tanks or raceways is cap- duce ≈22.7 t of rainbow trout (Oncorhynchus hydroponic technology (where all plants in the tured, treated, and reused. Recirculating mykiss Walbaum) annually. Daily production trough are the same age) could only remove systems use the least amount of water, which of trout effluent is ≈109 m3. Adler et al. (1996a, ≈50% of the P while producing a marketable is an advantage in areas with either limited 2000) have considered several nontraditional product. Although lettuce can remove P to water resources or stringent discharge stan- techniques for treatment of the effluent from <0.3 mg·L–1, a reduction in growth coincides dards. A high degree of management expertise this system, including hydroponic crop produc- with a further reduction in solution P concen- is needed to manage oxygen levels and water tion in greenhouses. Hydroponic production of trations. As a result, the conveyor production quality in these systems. horticultural crops such as basil (Ocimum strategy was developed to sustain plant pro- With the increase in production of fish also basilicum L.) and lettuce (Lactuca sativa L.) ductivity and health while removing dissolved comes an increase in discharge of nutrient may be a way to treat the wastewater and also P levels to <0.01 mg·L–1. pollutants. Wastewater from aquaculture can produce a profit for growers. Conveyor production system. Fundamen- pollute streams by adding excess nitrogen, Predominant thinking regarding the use of tal concepts of plant nutrition were utilized to phosphorus, and organic matter. Removal of food crops to clean aquaculture effluents has develop the conveyor production system, been that plants cannot remove nutrients in which produced healthy lettuce and basil with- water to low levels without a reduction in out an apparent reduction in growth, while Received for publication 15 Mar. 1999. Accepted productivity and quality. Because greenhouse simultaneously removing P to very low levels for publication 12 Sept. 1999. Use of trade names space is expensive, maintaining maximum (µg·L–1). [For a mechanistic understanding of does not imply endorsement of the products named productivity is critical to sustaining a profit- plant nutrient uptake, see Adler et al. (1996d, nor criticism of similar ones not named. The cost of able operation. 1996e)]. Plants have the capacity to absorb publishing this paper was defrayed in part by the Thin-film technology production system. and store nutrients in excess of their immedi- payment of page charges. Under postal regulations, this paper therefore must be hereby marked adver- Conventional hydroponic production of lettuce ate needs, a process called luxury consump- tisement solely to indicate this fact. and basil using thin-film technology, also tion (Marschner, 1995). The conveyor crop 1To whom reprint requests should be addressed. known as Nutrient Film Technique (NFT), production strategy enables plants to store P E-mail address: [email protected] was investigated as a method to remove P from early in their growth cycle. This stored reser- HORTSCIENCE, VOL. 35(6), OCTOBER 2000 993 FEATURE voir of P can be remobilized to meet current is to compare the cost of hydroponic produc- Six connected troughs, each 3.66 m long, plant needs and supplement the lower P influx tion alternatives with those of traditional formed the foundation for the conveyor pro- rate, which occurs as P drops below ≈0.3 wastewater treatment options (chemical duction scheme. The troughs, roughly 10.2 cm mg·L–1 in the effluent. Phosphorus re- precipitation, physical removal, and land × 3.66 m (Genova Products, Davison, Mich.), mobilization will maintain growth as long as application). were covered with 1.6-mm PVC, having 3.18- the tissue P concentration remains above the cm holes evenly spaced 17.5 cm apart, and critical deficiency level. At the front end of the MATERIALS AND METHODS planted with 21 seedlings. With this produc- thin-film troughs, where nutrient concentra- tion strategy, the rate of biomass production tions were highest, young plants absorbed and Rainbow trout effluent characteristics. The per unit area, hydraulic loading rate, and efflu- stored nutrients in excess of their immediate effluent was from the recirculating system for ent P concentrations were relatively constant. needs. Luxury consumption of nutrients dur- rainbow trout production at The Conservation Each of the six sections represented 4 d in the ing this early growth phase sustained the plants Fund’s Freshwater Institute, Shepherdstown, system, so both lettuce and basil were in the when they were moved towards the trough W. Va. The bulk effluent typically has a pH of system for 24 d, for total production cycles of outlet, where nutrient concentrations in solu- 7.2 and contains about 6 mg·L–1 total sus- 44 d. Every 4 d, plants were harvested at the tion were too low for absorption kinetics to pended solids and the following macronutri- outlet end of the system, the plants in the –1 meet their growth needs. Cellular nutrient ents (mg·L ): NO3-N (25), P (0.7), K (5), Ca remaining five sections were moved down one concentrations were sufficient to sustain (55), Mg (20), and S (9). In contrast, the spring position and 20-d lettuce or basil seedlings growth even after the concentrations of nutri- water that supplied the fish culture system were set into the system at the inlet end (Fig. –1 ents in the water became limiting. This con- typically contained (mg·L ): NO3 (3), P 1). This cycle was repeated five times to move veyor crop production scheme permitted the (<0.001), and K (3). In the effluent, nutrients a given set of plants completely through the removal of P to very low levels (µg·L–1) with- most limiting to the plants were Fe, Mn, Mo, system to harvest. The number of sections can out an apparent reduction in plant productivity and K, in decreasing order. A plant’s produc- be greater or less than six.