Introduction
Despite intensive research and operational innovation for over the past 30 years, “aquaponic” systems that combines fish culture and hydroponic plant production are still considered "experimental" by most authors. The effluent from a recirculating aquaculture system (RAS) includes ammonia, nitrate, nitrite, phosphorus, potassium, and other secondary micronutrients that are necessary to sustain hydroponic plants production (Rakocy et al. 2006; Kloas et al., 2015). There are basically two types of hydroponic systems. A “coupled” aquaponics system, uses the hydroponic biomass (specifically the submerged roots) as a large biofilter that extracts the nutrients from the wastewater, purifying it and returning it to the fish tanks for sustaining this secondary biomass. To be effective, coupled aquaponics systems must be sized exactly to balance the nutrient production from the fish culture with the potential for nutrient uptake by the plants. This balance can be precarious to achieve and maintain. For example, the nitrification process necessary for water quality maintenance is efficient at a pH of 7.0 or higher but practically ceases at less than 6.0. The optimum for nutrient solubility is 6.5. If pH is too high, nutrients precipitate out of solution and plants display nutrient deficiencies. There is minimal acceptable overlap of even this simple water quality parameter. In an “uncoupled” aquaponics system, the fish production and hydroponics systems are separate, and are designed independently. The recirculating aquaculture operation treats the water using biofiltration and solids removal technologies, resulting in elevated nitrate levels and the accumulation of solid wastes in the waste stream. These effluents can be processed in different ways to the benefit of a hydroponics operation. However, the wastewater leaving the fish culture operation is not returned to the fish tanks. There are various designs of each type of aquaponics system, some successful but often not, due to inadequate understanding and respect for either of the two basic activities: fish farming or hydroponic farming. This article describes the design and efficiency of a decoupled aquaponic system consisting of a closed recirculating aquaculture system, waste stabilization tanks, a constructed wetland, and a hydroponic greenhouse.
Materials and Methods
The recirculating fish production system design includes concrete tanks, with a total volume of 120 m3 of water. Hybrid striped bass were raised at the maximum stocking density of 45 kg/m3. Each e tank was integrated with a water recirculation unit (Jug-Dujaković et al., 2010). Solids were removed from the aquaculture system by a micro-screen filter. The screen was backwashed at periodic intervals to discharge solids to a swirl separator, discharging between 5% and 8% of the fish culture system volume daily. The swirl-separator was used to separate settleable solids. Cleared supernatant was transferred directly to the hydroponic greenhouse. The concentrated solids were transferred to aerobic stabilization tanks where heterotrophic bacteria aerobically degraded and mineralized the solids for 3-4 weeks. These were then applied to the surface of a constructed wetland. Constructed wetlands are fabricated inside a concrete basin, with layers of coarse and fine gravel, topped with a reed bed of Phragmites. The periodic discharge from aerobic stabilization tanks was filtered through the constructed wetlands and the clear nutrient-rich water was transferred to the hydroponic greenhouse, the elevated nutrient levels available to a variety of hydroponically cultured vegetables. Maximal planting density was 30 plants per m². During the hydroponic growth cycle, the plants purified the water by utilizing the dissolved nutrients for their growth. The greenhouse area was scaled to efficiently extract the nutrients from water discharged from the recirculating aquaculture system. The clear treated water could then be sterilized with ozone and reused within the fish culture facility or discharged. After years of accumulation, the solids on the surface of the reed beds are removed and used as fertilizer.
Results and Discussion
The aquaculture process continuously generates both solid and liquid organic waste. Liquid waste includes nitrates a by-product of the biofiltration process) and other dissolved nutrients within the culture water. Approximately 25% of the feed given to fish is excreted as solid waste. If not stabilized the organic waste has a characteristic unpleasant smell, and since it is not mineralized and dissolved in water, it is mostly unusable for growing plants. Also, if solids are not properly removed, they will adhere to plant roots, decrease oxygen levels as they decompose and have an adverse effect on nitrifying bacteria (Jug-Dujaković et al., 2010). A substantial quantity and variety of organic vegetables can be cultured using the dissolved waste from a RAS, creating a secondary profit center. Those plant varieties studied and confirmed as appropriate for this process include various types of lettuce, water crest, tomatoes, cucumbers and herbs. The measured average levels of macronutrients in the waste stream from the mechanical drum filters and from the hydroponic greenhouse effluent demonstrates reduction of N-NO3 from 165 to 8.4, N-NO2 from 0.84 to 0.56, and P-PO4 from 8.2 to 0.7 ppm. Waste mineralization and water quality of the system was maintained at a feeding rate of 90 g of striped bass feed/m²/day for mixed vegetable production, which is in agreement with data from Rakocy et al. (2006). Gloger et al. (1995) reported that the raft hydroponic tanks removed an average of 0.56 g of total ammonia-nitrogen, 0.62 g of nitrite-nitrogen, 30.29 g of chemical oxygen demand, 0.83 g of total nitrogen and 0.17 g of total phosphorous per m2 of plant growing area per day using romaine lettuce.
In conclusion, a decoupled aquaponic system designed with appropriate pretreatment of wastewater before sending it to the hydroponic greenhouse was successful in producing fish and plants creating two profit centers, while effectively reducing aquaculture wastewater.
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References
Gloger, K.C., Rakocy, J.E., Cotner, J.B., Bailey, D.S., Cole, W.M., Shultz, K.A. 1995. Waste treatment capacity of raft hydroponics in a closed recirculating fish culture system. World Aquaculture Society, Book of Abstracts: 126-127.
Jug-Dujaković, J., Gavrilović, A., Skaramuca, B., Van Gorder, S., 2010. Design criteria and performance of an intensive closed recirculating aquaculture system. Proceedings of the European Equaculture Conference. Porto, Portugal, October, 5th-8th 2010.
Kloas, W., Groß, R., Baganz, D., Graupner, J., Monsees, H., Schmidt, U., Staaks, G., Suh, J., Tschirner, M., Wittstock, B., Wuertz, S., Zikova, A., Rennert, B., 2015. A new concept for aquaponic systems to improve sustainability, increase productivity, and reduce environmental impact. Aquaculture Environment Interactions. 7: 179–192.
Rakocy, J.E., Masser, M.P., Losordo, T.M., 2006. Recirculating aquaculture tank production systems: aquaponics—integrating fish and plant culture. Publication no. 454. Southern Regional Aquaculture Center, Stoneville, MS.