PRELIMINARY EVALUATION OF ORGANIC WASTE FROM TWO AQUACULTURE SYSTEMS AS A SOURCE OF INORGANIC NUTRIENTS FOR HYDROPONICS

Aquaponic (AP) and greenwater tank culture (GW) systems have been developed at the University of the Virgin Islands for the intensive production of fish. Aquaponics is the combined culture of fish and hydroponic plants in recirculating systems. Greenwater tank culture utilizes a mixed suspended growth process involving autotrophic and heterotrophic bacteria and phytoplankton for water treatment. Large quantities of organic waste are removed from these systems daily. This organic waste is a potential source of inorganic nutrients for hydroponics. Samples of AP and GW sludge were continuously aerated for 29 days to facilitate mineralization. EC increased from 4.6 to 6.0 mS/cm (3200 to 4200 ppm TDS) in AP sludge and from 1.8 to 3.4 mS/cm (1260 to 2380 ppm TDS) in GW sludge. Thirteen nutrients (Ca, Mg, K, NH₄, NO₃, PO₄, SO₄, Fe, Mn, Zn, Cu, B and Mo) were compared to a standard hydroponic lettuce formulation (Resh, 1995). Final concentrations of six AP nutrients and eight GW nutrients exceeded standard hydroponic values while concentrations of seven AP nutrients and five GW nutrients were less than standard hydroponic values. Na and Cl concentrations in AP and GW sludge exceeded the recommended values for hydroponics. Hydroponic nutrient formulations derived from aquaculture waste may require dilution, supplementation and source material (fish feed) without salt.