Waste treatment capacity of raft hydroponic lettuce production in an integrated fish culture system and the contribution of lettuce to treatment capacity

Abstract

Two experiments were conducted to determine: 1.) the waste treatment capacity of raft hydroponic lettuce production in an integrated fish culture system and 2.) the contribution of lettuce plants, Lactuca saliva, cv. Paris Island, to the documented treatment capacity. Six identical integrated systems were used, each consisting of a 12.8 m3 rearing tank, a 1.9 m3 clarifier, and two, 2.1 M3 hydroponic tanks (6.03 m L x 1. 19 m W x 0.28 m D). In the first experiment male Oreochromis niloticus were stocked at two densities 5.6 kg/M3 (132 fish) and 9.4 kg/M3 (220 fish); each replicated three times. Feed was supplied at 0.93% of mean body weight in both treatments and increased weekly at approximately 1 g/fish/day. Ammonium, nitrite, nitrate, pH, lettuce biomass, temperature, fish weight, BOD, and COD were monitored. A maximum sustainable feeding rate (highest feed input with total ammonia nitrogen <5 mg/litre and nitrite <2 mg/litre) of 195 g/M2 of hydroponic growing area/day was reached in week nine of the experiment when N02-N concentrations increased above 2 mg/litre in the high feed system (220 fish). In both treatments, fish biomass increased 100% during the 13-week experiment. Mean lettuce production (347 g/m2/day in low feed and 291 g/m2/day in high feed) was below potential production rates due to solids accumulation on roots. In the second experiment, all six systems were stocked with male Oreochromis niloticus at a density of 14.5 kg m3 (170 fish). Three systems were planted with Paris Island lettuce and three received no lettuce. Feeding and lettuce protocols were the same as in the first experiment. Ammonium, nitrite, nitrate, pH, lettuce biomass production, temperature, fish weight, BOD and COD were monitored. A maximum sustainable feeding rate of 186 g/M2 of hydroponic growing area/day was reached in week eleven in both treatments when N02-N concentrations increased above 2 mg/litre. Systems planted with lettuce removed about 25% more of both N03-N and total dissolved solids than did the unplanted systems. Planted systems were also able to dampen NH4+ spikes more effectively than unplanted systems, presumably through direct plant uptake.