Resource Recovery Through Halophyte Production in Marine Aquaponics: An Evaluation of the Nutrient Cycling and the Environmental Sustainability of Aquaponics

• Main Author: Boxman, Suzanne
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: Digital Commons @ University of South Florida 2015
• Subjects: Aquaculture; Integrated Multi-Tropic; Life Cycle Assessment; Nutrient Reuse; Denitrification; Environmental Engineering; Red drum; Sciaenops ocellatus
• Online Access: https://digitalcommons.usf.edu/etd/5915; https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=7111&context=etd

Aquaculture, the farming of aquatic animals and plants, is an important component of global food production, which supplies a nutritious protein source for millions of people. Interest in improving the sustainability of aquaculture has led to the development of aquaponics in which fish production is combined with plant production to create zero-discharge systems. A need for more fundamental science and engineering research on marine aquaculture and growing interest in production of halophytes motivated this novel research on marine aquaponics. One objective was to evaluate the growth and nutrient removal capacity of halophytes in marine aquaponics. Bench-scale studies were conducted to determine the best methodology to grow the halophytes sea purslane (Sesuvium portulacastrum) and saltwort (Batis maritima). The results indicated these species were important for nitrogen removal and function well under varying conditions of flow rate, species, or plant density. A prototype commercial-scale marine aquaponic system was evaluated through regular collection of water quality and plant growth data over a 9 month period. The system had a total volume of 50 m3 and contained: a swirl separator, uplfow media filter, a moving bed bioreactor, 61.4 m2 of hydroponic growing area, and a sand filter. Water quality parameters measured included: total ammonia nitrogen (TAN), nitrite (NO2-), nitrate (NO3-), total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), total suspended solids (TSS), and volatile suspended solids (VSS). TAN and nitrite concentrations in the fish tank effluent ranged from 0.04 to 2.42 mg/L TAN and 0.07 to 14.7 mg/L NO2--N, respectively. Nitrate concentrations increased to a maximum of 120 ± 5.7 mg/L NO3--N during the first 119 days of operation. To provide greater control over nitrate concentrations, the sand filter was converted into a downflow submerged packed bed biofilter. This reduced concentrations to a mean of 27.5 ± 13.7 mg/L NO3--N during the last 3 months. Dried plant samples were analyzed for nitrogen and phosphorus content. Nutrient uptake by plants ranged from 0.06 to 0.87 g N/m2/d and 0.01 to 0.14 g P/m2/d. It was estimated 0.55 kg/m2 of plant biomass could be harvested every 28 days. Red drum (Sciaenops ocellatus) were initially stocked at an average weight of 0.047 kg and grew to a harvestable size of 0.91 kg in approximately 12 months. A mass balance indicated that plants contributed to less than 10% of nitrogen and phosphorus removal and passive denitrification was the dominant nitrogen removal process. The second objective was to evaluate the environmental impact of aquaponics through life cycle assessment (LCA). LCAs were completed on freshwater aquaponic systems at commercial- and residential-scales. The system expansion method was used address co-production of 1 ton live-weight fish, recovered solids, plants, and water treatment. The results indicated that aquaponics contributed to significant water savings; however, aquaponics is subject to trade-offs from high energy use and the addition of industrial fish feeds. The methodology developed for freshwater aquaponics was applied to the prototype commercial-scale marine aquaponic system and was compared with two alternative scenarios of maximized plant production and a denitrification reactor with no plant production. The results indicated that a system with a denitrification reactor had the lowest environmental impact. Alternatively in the system with maximized plant production, the use of renewable energy sources would reduce the environmental impact and would contribute to greater water savings, while realizing the economic benefits of dual products. This is the first study to complete an in-depth evaluation of a commercial-scale marine aquaponic system and to evaluate aquaponics using LCA while accounting for the potential environmental offsets of multiple co-products.