WATER AND NUTRIENT MANAGEMENT AND SAVINGS IN CLOSED-LOOP AQUAPONIC TILAPIA/VEGETABLE PRODUCTION

Integrated co-cultivation systems, such as aquaponics, have the potential to revolutionize aquatic food and protein production on a large scale and reinvigorate the aquaculture industry. When recirculating aquaculture systems (RAS) and controlled environment agriculture (CEA) are integrated together to form aquaponic systems, water and nutrient use efficiencies improve, costs per system decrease, environmental contamination decreases, and profitability can improve. The conservation of water and nutrient resources through re-use and re-tasking of initial fish feed is the central keystone for a sustainable domestic aquaculture business sector.

Auburn University School of Fisheries, Aquaculture, and Aquatic Sciences operates a closed-loop, commercial-scale aquaponics system producing tilapia and a variety of vegetables. The incoming water for the 50,000-gallon tilapia system is flowed by gravity from an 11-acre reservoir. Nutrients, mainly nitrogen eventually converted to nitrate, in the form of commercial fish feed are fed daily to 5,000 to 6,000 Nile tilapia, representing the first use of water and nutrients. Nutrient rich water is used a second time to water 2,880 meters squared of vegetables in a 96' x 30' commercial greenhouse. Excess water and nutrients are caught and used a third time to fertilize outdoor raised beds.

Water volumes and nitrate levels were measured prior to usage at each of the three usage points, 1) entering fish tank, 2) prior to watering plant greenhouse, 3) prior to watering raised beds. Water and fish feed were assigned an initial cost. All re-used and re-tasked water and nitrates were subtracted from the initial cost of both water and nutrients to ascribe savings to the overall system. Water use efficiencies were used to compare across production methods for both tilapia and vegetables separately The following formula was used to calculate a water use efficiencies for plants and fish: Total water volume delivered (gal/m²) divided by marketable fruit or fish yield (number/m²), and expressed as gal/fruit. A water use efficiency for the combined system was  calculated using the following formula: Total water volume delivered for the initial fish culture (gal/m²) divided by the combined total marketable yield of fruit and fish (kilogram/m²), and expressed as gal/kg.

Nutrient use efficiencies were estimated with the following formula: Total weight of nitrogen used (g/m²) divided b the marketable fruit yield (number/m²), and expressed as g/fruit. Nutrient use efficiencies were calculated separately for fish and plants and combined. Results were compared across production methods for both tilapia and vegetables

The results show savings of both water and nutrients due to increased water and nutrient us efficiencies.