Scaling aquaponic systems: Balancing plant uptake with fish output
Introduction
Aquaponics, the integrated culture of fish and plants, can reduce the environmental impacts of aquaculture (Adler et al., 2003, Tyson et al., 2011). In aquaculture, as with any agricultural enterprise where animals are concentrated, wastes accumulate and will degrade the production system and the landscape unless handled appropriately. Aquacultural wastes are challenging as they are either suspended or dissolved in water. Physical separation methods such as sedimentation or filtration are effective at suspended solids removal. However, limiting dissolved nutrient release is more difficult due to the large volumes of dilute liquid waste. Aquaponics is one of the few techniques available that can remove the low concentrations of dissolved N and P generated via aquaculture.
In an aquaponic system, effluent containing nutrients generated through fish rearing is passed through the rooting zone of plants where it provides a nutrient source. The plants use the nutrients to support growth and, depending on the plant chosen, can represent an additional commodity for the fish farmer. Nutrient uptake and sequestration in plant biomass removes nutrients from the effluent thereby improving water quality.
To be most effective the aquaponics system must be sized correctly with the optimum balance between nutrient production from fish culture and nutrient uptake by the plant component. Insufficient plant growing area will result in an accumulation of nutrients in recirculating systems or the excessive release of nutrients in flow through systems. Too large a plant growing area may improve water quality but will also lead to slower plant growth rates and reduced production of plant crops.
Waste generation by fish is directly related to the quantity and quality of feed applied. Best management practices, including the use of high quality diets designed to be highly digestible and frequent removal of solid wastes minimize the generation of dissolved nutrients (Fornshell and Hinshaw, 2008). Despite this, it is estimated that between 30 and 65% of feed N and up to 40% of feed P is excreted (Schneider et al., 2005). Factors regulating plant nutrient uptake include light intensity, root zone temperature, air temperature, nutrient availability, growth stage and growth rate. Empirical data or simulation models (Pagand et al., 2000, Papatryphon et al., 2005) can estimate nutrient release from the fish growing component, however it is more difficult to estimate removal rates for the wide range of potential plant crops produced under a variety of growing conditions.
Rakocy and colleagues in the U.S. Virgin Islands have devoted considerable efforts to the determination of the appropriate ratio of fish feed to plant growing area in a recirculating fish culture system growing tilapia (Oreochromis spp.) and a variety of vegetable crops. They recommended a ratio of 60–100 g feed/m2 of plant growing area (Rakocy et al., 2006). Al-Hafedh et al. (2008) cultured tilapia (Oreochromis nilotica) in a recirculating system in Saudi Arabia and determined that the optimum ratio for their system was 56 g feed/m2. Endut et al. (2010) cultured African catfish (Clarias gariepinus) in conjunction with water spinach (Ipomoea aquatica) in a recirculating system in Malaysia and established that the optimum ratio for their system was 15–42 g fish feed/m2.
It is not clear how transferrable these studies are to different plant production systems (gravel beds, nutrient film technique etc.), systems growing different fish species (percids, centrarchids, salmonids, etc.) or to other fish culturing systems such as flow-through raceways due to differences in water temperature and quantity of nutrients generated in fish waste. Additionally, there is no information as to whether the feed to plant growing area ratio might vary seasonally as light and temperature regimes change. Current methods used to obtain this data require experiments that take months to complete so growers whose systems are dissimilar to those above are forced to use their best estimate as to the optimum ratio for their system. Furthermore, growers who are just beginning to integrate aquaponics into their fish growing operation have little data they can use to predict the optimum plant growing space needed.
We describe a new method where the plant component was temporarily isolated from the fish rearing operation so that nutrient removal could be evaluated independently. The data generated can then be used estimate the required retention time to achieve water quality goals. Two experiments were conducted to explore the capabilities of the method. The first examined the removal capabilities of two cultivars (lettuce (Latuca sativa ‘Red Sails’) and nasturtium (Tropaeolum majus ‘Whirlibird Mix’)) and the second examined the nutrient removal ability of lettuce over the entire cropping period.
Section snippets
Site description
The West Virginia University Aquaculture Facility consists of a fish rearing building and an adjacent plastic covered greenhouse where plants were grown. Trout (Oncorhynchus mykiss) were reared in linear raceways which consisted of 8 raceway sections. Each section consisted of a rearing zone (7.3 × 0.9 m) followed by a quiescent zone (1.8 × 0.9 m). The raceways were positioned such that there were 2 series of 4 raceways on a side and water flowed serially from one raceway section into the next. The
Effect of crop on nutrient removal
In the first experiment removal rates of TAN, nitrate and phosphate by two crops, lettuce and nasturtium, were assessed. TAN concentrations were substantially reduced by both crops (Fig. 1). Lettuce reduced the TAN concentration by 81% to 0.11 mg/L while nasturtium reduced the concentration by 89% to 0.06 mg/L. Although the experiment lasted for 4 h, TAN was removed over 3 h with no additional removal occurring in the last hour. In addition to removing TAN, nasturtium also removed nitrate from the
Acknowledgement
This research was supported by a US Department of Agriculture Special Grant (2010-343386-21745).
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