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Water and Nitrogen Budgets for Aquaponic Systems

Designing agricultural production systems for zero discharge of water to the environment protects groundwater quality and makes water permitting easier to obtain.  In Zero Agricultural Discharge Systems (ZADS) what goes in does not come out, except as a harvestable product.  One potential ZADS production arrangement combines hydroponic plant production and recirculating aquaculture systems into what is known as aquaponics.  The potential for plants to use the by-products of aquaculture and keep recirculating water clean have been documented (Adler, 1996; Adler et al., 2000; Lin et al., 2002).

Two major components of both hydroponic and aquaculture systems are water and nitrogen.  Most recirculating aquaculture systems replace 5 to 10% of system water daily to help prevent the buildup of toxic levels of ammonia and other fish by-products (Masser et al., 1999).  A single plant can use between 1 to 5 liters of water per day depending on its size, maturity, and the growing season or temperature.   If we assume an average of 3 liters of water use per plant per day, 100 plants could satisfy the water replacement requirements of a recirculating aquaculture tank containing 3000 (at 10%) or 6,000 liters (at 5% replacement).  In general, the higher the water replacement percentage, the higher the fish stocking density the system will permit.

The main nutrient in plant production – nitrogen – could be supplied by fish in an aquaponic system.  Sufficient nitrification to convert 75% of the ammonia to nitrate would be required since the recommended nitrate to ammonium ratio in hydroponics is 75:25 (Cockx and Simonne, 2003).   One hundred kilograms of fish could produce an average of 40 grams of ammonia per day (Tetzlaff and Heidinger, 1990).  Converting to elemental nitrogen (divide by 3.29) and allowing for nitrogen volatilization (25%), an average of 9 grams of N could be produced per day.  An average nitrogen requirement of vegetable plants is 100 kg/ha depending on plant size, type, and length of growing season.  If we assume an average plant density (Hickman, 1998) of one plant per 0.4 meter squared (25,000 plants per hectare), each plant would need 4 grams of elemental N during the growing season.  A fish production rate of 9 grams of N per day would support 270 cucumber plants over a 120 day production cycle.

No water is wasted in either system and up to 4% of the variable cost in greenhouse vegetable production could be saved.  Since certain other plant nutrients can fall below sufficiency standards in aquaponics without supplemental fertilization (McMurtry et al., 1990), nutrient application methods to make up this deficit by supplying specific elements without adversely impacting fish and nitrifying bacteria need further investigation.

Adler, P.R., J.K. Harper and F. Takeda.  2000.  Economic options of hydroponics and other treatment options for phosphorus removal in aquaculture effluent. HortScience, Vol.35(6):993-999.

Adler, P.R., F. Takeda, D.M. Glenn and S.T. Summerfelt.  1996.  Utilizing byproducts to enhance aquaculture sustainability.  World Aquaculture, June 1996, 27(2):24-26. 

Cockx, E. and E.H. Simonne.  2003.  Reduction of the impact of fertilization and irrigation on processes in the nitrogen cycle in vegetable fields with BMPs. UF/IFAS, Fla. Coop. Ext. Serv. HS948: 22pgs.

Hickman, G. W.  1998.  Commercial greenhouse vegetable handbook.  University of California Division of Agriculture and Natural Resources Publication 21575:24pgs.

Lin, Y.F., S.R. Jing, D.Y.Lee, and T.W. Wang.  2002.  Nutrient removal from aquaculture wastewater using a constructed wetlands system.  Aquaculture, 209:169-184.

Masser, M.P., J. Rakocy, and T.M. Losordo.  1999.  Recirculating aquaculture tank production systems: management of recirculating systems.  Southern Regional Aquaculture Center Publication No. 452:12 pgs.

McMurtry, M.R., P.V. Nelson, D.C. Sanders and L. Hodges.  1990.  Sand culture of vegetables using recirculated aquacultural effluents.  Applied Agricultural Research, vol.5, no.4:280-284. 

Tetzlaff, B.L., and R.C. Heidinger.   1990.  Basic principles of biofiltration and system deesign.  SIUC Fisheries and Illinois Aquaculture Center, Bulletin 9:17 pgs.

(Richard Tyson, Seminole Co. Coop. Ext. Serv.  - Vegetarian 04-09)