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The coupled aquaponics principle combines three classes of organisms: (1) aquatic organisms, (2) bacteria and (3) plants that benefit from each other in a closed recirculated water body. The water serves as a medium of nutrient transport, mainly from dissolved fish waste, which is converted into nutrients for plant growth by bacteria. These bacteria (e.g. Nitrosomonas spec., Nitrobacter spec.) oxidize ammonium to nitrite and finally to nitrate. Therefore, it is necessary for the bacteria to receive substantial amounts of ammonium and nitrite to stabilize colony growth and the quantity of nitrate production. Consequently, in a coupled aquaponic system, volumes are critically important, i) the aquaculture unit following the principles of recirculating aquaculture systems (RAS), ii) the bacterial growth substrate and iii) the space for the plant units and the amount of plants to be cultivated. Together, they form the aquaponics unit (Fig. 7.2).
Fig. 7.2 Principle of coupled aquaponic system with fish, bacteria and plants in a fully closed water recirculation
The specific biological-chemical components of the process water have particular importance for coupled aquaponic systems. With food or uneaten feed particles, the organic fish waste and the bacteria inside the process water, an emulsion of nutrients combined with enzymes and digestive bacteria support the growth of fish and plants. There is evidence that compared to stand-alone systems such as aquaculture (fish) and hydroponics (plants), the growth of aquatic organisms and crops in a coupled aquaponics can be similar or even higher. Rakocy (1989) described a slightly higher yield of tilapia (Tilapia nilotica, 46.8 kg) in coupled aquaponics in contrast to standalone fish culture (41.6 kg) and slight increases in Summer Bibb lettuce yield (385.1 kg) compared to vegetable hydroponic production (380.1 kg). Knaus et al. (2018b) recorded that aquaponics increased biomass growth of O. basilicum, apparently due to increased leaf generation of the plants (3550 leaves in aquaponics) compared to conventional hydroponics (2393 leaves). Delaide et al. (2016) demonstrated that aquaponic and hydroponic treatments of lettuce exhibited similar plant growth, whereas the shoot weight of the complemented aquaponic solution with nutrients performed best. Similar observations have been made by Goddek and Vermeulen (2018). Lehmonen and Sireeni (2017) observed an increased root weight, leaf area and leaf colour in Batavia salad (Lactuca sativa var. capitata) and iceberg lettuce (L. sativa) with aquaponics process water from C. gariepinus combined with additional fertilizer. Certain plants such as lettuce (Lactuca sativa), cucumbers (Cucumis sativus) or tomatoes (Solanum lycopersicum) can consume nutrients faster, and as a result flower earlier in aquaponics compared with hydroponics (Savidov 2005). Also, Saha et al. (2016) reported a higher plant biomass yield in O. basilicum in combination with crayfish Procambarus spp. and a low start-up fertilization of the aquaponic system.
The basic system design of coupled aquaponics consists of one or more fish tanks, a sedimentation unit or clarifier, substrates for the growth of bacteria or suitable biofilters and a hydroponic unit for plant growth (Fig. 7.3). These units are connected by pipes to form a closed water cycle. Often, after the mechanical filtration and the biofilter, a pump sump is used (one pump or one loop system) which, as the deepest point of the system, pumps the water back to the fish tanks from where it flows by gravity to the hydroponic unit.
Fig. 7.3 Basic technical system design of a coupled aquaponic system with fish tank, sedimenter, biofilter, hydroponic unit and a sump where the water is pumped or airlifted back to the fish tanks and flows by gravity along the components
Coupled aquaponic systems are used in different scales. The closed-loop principle can be used in domestic systems (mini/hobby/backyard-coupled), demonstration units (e.g. living walls coupled), commercial aquaponics and aquaponics farming (with soil) ranging from small/semi-commercial to large-scale systems (Palm et al. 2018). A recent development in aquaponics has included partial fertilization, which is dependent on the tolerance of the fish species. This, however, can result in a shortterm nutrient peak in the system but can be compensated through the nutrient retention by the plants. In coupled aquaponics, an optimal ratio of the production area (or fish volumes) of the aquaculture unit with the resulting feed demand as well as an adequate amount of plants to be cultured in the hydroponic unit (plant production area) must be achieved. (For discussions on the role of evapotranspiration and solar radiation within the systems, see Chaps. 8 and 11). For gravel aquaponics, Rakocy (2012) as a first attempt suggested 'component ratio principles', with a fish-rearing volume of 1 msup3/sup of fish tank volume to 2 msup3/sup hydroponic media of 3 to 6 cm pea gravel as a rule of thumb. Ultimately, the amount of fish determines the yield of crops in coupled aquaponics. Additionally, the technical conditions of the fish-rearing unit must be adapted according to the needs of the cultivated aquatic species.