The Recirculating Aquaculture System (RAS) technology in North Carolina has potential economic benefits for producing market-stage fish for regional markets as well as supplying fingerlings to offshore cage culture operations in the U.S. However, the discharge of effluent from RAS, rich in nutrients, poses environmental risks. Integrated Multi-Trophic Aquaculture (IMTA) can mitigate these risks by balancing marine finfish culture with a salt-tolerant plant, Salicornia virginica, which can remove nutrients from the effluent and be used as a valuable crop for humans, livestock, fish, and as a seed oil source. The research aims to optimize production of S. virginica in a RAS-Geotube-Salicornia IMTA system at a pilot commercial scale and to evaluate nutrient and water mass balance relationships. The goal is to develop an uncomplicated, low-maintenance, and cost-effective system for RAS waste management with implications for the siting and the economics of operating intensive mariculture facilities in NC and other areas of the US.
An experiment was conducted to investigate the effects of water exchange rate on nutrient removal and plant yield of S. virginica grown in grow beds receiving effluent from a marine RAS (34-m3) producing black sea bass Centropristis striata. Effluent was clarified using geotextile fabric bag (Geotube) and filtered with an additional 25 um bag filter before application to the experimental system growing S. virginica. The experimental system consisted of twelve 200 L rectangular tanks (grow beds, 92 cm l x 92 cm w x 28 cm depth) operated at a volume of 150 L and a depth of 18 cm (deep-water culture system). Each grow bed contained twelve 50 mm grow cups containing expanded clay media and supported by a polystyrene foam base. S. virginica (grown from seed) were planted in each grow cup at a mean fresh weight of 117.8 ± 7.14 g. To study the effects of water exchange in the grow beds on plant growth and nutrient removal, two treatment exchange rates were compared; low exchange (100%/d, 150 L/d) and high exchange (300%/day, 450 L/d). Grow beds without plants were also maintained at a low exchange (100%/d, 150 L/d) as a control treatment. Four replicate grow beds were maintained per treatment. Influent and effluent water from the grow beds were collected every 21 days and analyzed for nitrogen and phosphorus concentration to evaluate nutrient removal efficiency. Plant biomass in each grow cup was taken at the beginning and the end of the experiment. Results on plant yields and nutrient concentrations and removal rates after 135 days of study under each treatment and recommendations for scale-up to a pilot commercial scale system will be presented.
The initial nutrient sample (d8) showed significant differences in nutrient removal efficiency. The nitrate removal efficiency was highest in the Low treatment (36.4 ± 1.66%) compared to the High treatment (10.6 ± 0.45%) and the Control (7.39 ± 1.39%). Phosphorus removal was higher in the Low treatment (12.74 ± 1.4%) than the Control (7.50 ± 0.91%) which was higher than the High treatment (1.80 ± 1.04%). The nitrate (initial = 19.3 mg/l) and phosphorus (initial = 5.83 mg/l) concentration in the Low treatment were reduced to 12.3 ± 0.32 mg/l and 5.06 ± 0.08 mg/l, High treatment to 17.3 ± 0.09 mg/l and 5.73 ± 0.06 mg/l, and the Control to 17.9 ± 0.37 mg/l and 5.39 ± 0.05 mg/l, respectively.