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Fig. 7.1 Diagram of the first system by Naegel (1977) growing Tilapia and common carp in combination with lettuce and tomatoes in a closed recirculation system
The combination of fish and plant cultivation in coupled aquaponics dates back to the first design by Naegel (1977) in Germany, using a 2000 L hobby scale system (Fig. 7.1) located in a controlled environment greenhouse. This system was developed in order to verify the use of nutrients from fish waste water under fully controlled water recirculating conditions intended for plant production including a dual sludge system (aerobic/anaerobic wastewater treatment). Naegel based his concept on the open pond aquaponic system of the South Carolina Agricultural Experiment Station, in the USA, where excess nutrients from the fishponds, stocked with channel catfish (Ictalurus punctatus), were eliminated by the hydroponic production of water chestnuts (Eleocharis dulcis) (Loyacano and Grosvenor 1973). By including nitrification and denitrification tanks to increase the nitrate concentration inside his system, Naegel (1977) attempted a complete oxidation of all nitrogenous compounds, reaching nitrate concentrations of 1200 mg/L, and demonstrating the effectiveness of the nitrification step. Although the system was stocked at a low density (20 kg/msup3/sup each) using tilapia (Tilapia mossambica) and carp (Cyprinus carpio), the tomatoes (Lycopersicon esculentum) and iceberg lettuce (Lactuca scariola) grew well and produced harvestable yield. These first research results led to the concept of coupled aquaponic systems, in which the plants eliminate the waste produced by the fish, creating adequate growth, demonstrating highly efficient water use in both units. The principle of coupled aquaponics was first described by Huy Tran at the World Aquaculture Conference in 2015 (Tran 2015).
Coupled aquaponic systems do not necessarily use mechanical particulate filtering in the classical sense and keep consistent nutrient flow between the aquaculture and hydroponic units. The main challenge is how to manage the faecal load in the coupled aquaponic system where the plants absorb the nutrients and particulate waste can be removed from the system by filter presses or geotextiles.
The development of modern agriculture, human population growth and shrinking resources worldwide, has promoted the development of coupled aquaponic systems. Since fish farming is considerably more efficient in protein production and water use compared with other farmed animals and since closed systems are largely siteindependent, coupled aquaponic systems have been able to develop worldwide (Graber and Junge 2009), under arid conditions (Kotzen and Appelbaum 2010; Appelbaum and Kotzen 2016) and even in urban settings (König et al. 2016). Most described systems belong to domestic, small-scale and semi-commercial installations (Palm et al. 2018) that are driven by hobby aquarists, enthusiasts or smaller start-up companies. New research results, summarized in this chapter, demonstrate both the potentials and constraints regarding the continued development of these systems into commercial aquaponics, being capable of making a significant contribution to future food production.