Comparisson of Coupled and decoupled aquaponics - Implications for future system design
Introduction
In classical aquaponic systems the aquaculture unit and the hydroponic unit are arranged in a single closed loop where process water is directed from the aquaculture to the hydroponic unit and back (Rakocy et al. 2004). Inevitably, such systems (coupled aquaponic systems, 1-circle/loop systems) provide the same water quality for both cultured species, fish and plants, which necessarily represents a compromise with regard to the optimal species-specific rearing conditions. Therefore, current efforts aim at decoupled systems arranged in separate loops. Process water is recirculated within the respective unit, thereby allowing a better control with regard to species-specific requirements (Kloas et al. 2015).
Material & methods
Experiments were conducted at the aquaponic research facility (Fig.1) of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB, Berlin, Germany). Briefly, RAS with a volume of 16.5 m3 each were stocked with tilapia (Oreochromis niloticus). Hydroponic units (NFT) with a volume of 200 L each were stocked with 15 tomato plants (Solanum lycopersicum) each.
Results
Biomass of fish showed no differences between coupled and decoupled systems at the end of the experiment. Tomato plants revealed best growth in decoupled aquaponic system (123.5 kg) compared to coupled system (90.0 kg). The pH in decoupled hydroponics was 6.4 (±0.7) compared to 7.0 (±0.4) in the coupled system over the experimental period.
Discussion
The results of the study demonstrated that decoupled systems provide an excellent alternative to classical coupled aquaponic systems especially in terms of the improvement of plant production. The regulation of the pH is thereby one of the most critical factors to consider. Optimal availability of essential nutrients should be guaranteed at a pH of ≤ 6.5 for hydroponics (Hochmuth 2001). In contrast, optimal pH in RAS should be maintained at ≥ 7.0 to ensure optimal conditions for microbial nitrification in the biofilter and thereby safeguard fish welfare (Chen et al. 2006). A decoupling of the hydroponic unit allows short-term modulation of the water chemistry within the unit and is thus easier to control with respect to optimal conditions (e.g. pH). As expected the pH in decoupled hydroponic unit was considerably lower, improving nutrient availability for the plants. Additionally, in decoupled aquaponics the application of fertilizer can be carried out in a much smaller water volume since fertilizer is only supplemented to the nutrient media reservoir (V= 200L, Fig.1) compared to the whole system (V=16.5 m3), as performed in coupled aquaponics.
With respect to future system design for commercial applications the decoupled approach is recommended due to the improved control. For smaller systems (e.g. for schools, showcases or backyards) both approaches are feasible but increased care and workload needs to be considered in coupled systems.
References
Chen, S.; Ling, J.; Blancheton, J.-P., Nitrification kinetics of biofilm as affected by water quality factors. Aquacultural Engineering 2006, 34 (3), 179-197.
Hochmuth, G.J.: Fertilizer management for greenhouse vegetables, Florida greenhouse vegetable production handbook, vol 3, Horticultural Sciences Department, Florida Cooperative Extension Service, University of Florida 2001, webpage: http://edis.ifas.ufl.edu/CV265
Kloas W, Groß R, Baganz D, Graupner J, Monsees H, Schmidt U, Staaks G, Suhl J, Wittstock B, Wuertz S, Zikova A, Rennert B (2015) A new concept for aquaponic systems to improve sustainability, increase productivity, and reduce environmental impacts. Aquaculture Environment Interactions, 7 (2) 179-192
Rakocy, J., Shultz, R.C., Bailey, D.S. and Thoman, E.S. (2004). Aquaponic Production of Tilapia and Basil: Comparing a Batch and Staggered Cropping System. Acta Hortic. 648, 63-69
