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Coupled aquaponics depends on the nutrients that are provided from the fish units, either a commercial intensive RAS or tanks stocked under extensive conditions in smaller operations. The fish density in the latter is often about 15—20 kg/msup3/sup (tilapia, carp), but extensive African catfish production can be higher up to 50 kg/msup3/sup. Such different stocking densities have a significant influence on nutrient fluxes and nutrient availability for the plants, the requirement of water quality control and adjustment as well as appropriate management practices.
The process water quality with respect to nutrient concentrations is primarily dependent on the composition of the feed and the respective turnover rates of the fish. The difference between feed input and feed nutrients, assimilating inside the fish or lost through maintenance of the system, equals the maximum potential of plant available nutrients from aquaculture. As noted above, the nutrient concentrations should be adjusted to levels, which allow the plants to grow effectively. However, not all fish species are able to withstand such conditions. Consequently, resilient fish species such as the African catfish, Tilapia or carp are preferred aquaponic candidates. At the University of Rostock, whole catfish and its standard diet as output and input values were analysed to identify the turnover rates of the macronutrients N, P, K, Ca, Mg and S and the micronutrients Fe, Mn, Mo, Cu, Zn and Se. With the exception of P, more than 50% of the feed nutrients given to the fish are not retained in its body and can be considered potentially available as plant nutrients (Strauch et al. 2018; Fig. 7.12). However, these nutrients are not equally distributed inside the process water and the sediments. Especially macronutrients (N, P, K) accumulate in the process water as well as inside the solid fraction whilst the micronutrients, such as iron, disappear in the solid fraction separated by the clarifier. Figure 7.13 shows the nutrient output per clarifier cleaning after 6 days of sludge collection in an intensive African catfish RAS. The proportions of plant essential nutrients that are bound in the solids relative to the respective amounts that appear dissolved are significant: N = 48%, P = 61%, K = 10%, Ca = 48%, Mg = 16%, S = 11%, Fe = 99%, Mn = 86%, Mo = 100%, Zn = 48% and Cu = 55%.
Fig. 7.12 Unused nutrients in African catfish aquaculture that are potentially available for aquaponic plant production (original data)
Fig. 7.13 Distribution of macro- and micronutrients inside the process water and the solids. (Data from Strauch et al. (2018))
One key management factor is the availability of oxygen inside the system, which is crucial to keep the concentration of plant available nitrate in the process water high. Conventional clarifiers that are applied in many RAS remove carbon-rich solid wastes from the recirculation but will leave them in contact with the process water until the next cleaning interval of the sedimentation tank. During this time, the carbon-rich organic matter is utilized as a source of energy by denitrifying bacteria, accounting for significant losses of nitrate. It outgasses as nitrogen into the atmosphere and is lost. Under intensive production conditions, large quantities of organic sludge will accumulate inside the sedimentation tanks, with consequences for maintenance, replacement with freshwater and subsequently for the nutrient composition inside the process water. Figure 7.14 illustrates the nutrient concentrations in the holding tanks of African catfish RAS under three different stocking densities (extensive: 35 fish / tank, semi-intensive: 70 fish / tank, intensive: 140 fish/ tank). The higher the stocking density and the lower the resulting oxygen content inside the system, the lower is the plant available nitrate per kg feed inside the system.
Fig. 7.14 N-budget per kg feed and oxygen level in African catfish aquaculture under three different stocking densities (original data)
In general, with increasing fish intensity, the availability of oxygen inside the system decreases because of the consumption of the oxygen by the fish and aerobic sludge digestion inside the clarifier and the hydroponic subsystems. Oxygen levels can be maintained at higher levels, but this requires additional investment for oxygen monitoring and control. This issue is of tremendous importance for coupled aquaponics, right from the beginning of the planning phase of the systems because the different scenarios are decisive for the planned fish production, the resulting quality of the process water for the plant production units, and consequently for economic sustainability. Four principals of coupled aquaponic production systems with management consequences in terms of system design, maintenance procedures and nutrient availability for plant growth, with transitions between them, can be defined as follows:
Extensive production, oxygen resilient fish (e.g. tilapia, carp), no oxygen control, Osub2/sub above 6 mg/L, little water use with high nutrient concentrations, small investment, low BOD, high nitrate per kg feed.
Intensive production, oxygen resilient fish (e.g. African catfish), no oxygen control, Osub2/sub below 6 mg/L, high water use, medium investment, high BOD, low nitrate per kg feed, high nutrient concentrations.
Extensive production, oxygen demanding fish (e.g. Trout), oxygen control, Osub2/sub above 6—8 mg/L, high water use, medium investment, low BOD, high nitrate per kg feed, low nutrient concentrations.
Intensive production, oxygen demanding fish (e.g. Trout, pikeperch), oxygen control, Osub2/sub above 6—8 mg/L, high water use, high investment, low BOD, medium nitrate per kg feed.
In addition to the stocking density and the average amount of oxygen inside the system, the plant production regime, i.e. batch or staggered cultivation, has consequences for the plant available nutrients inside the process water (Palm et al. 2019). This is the case especially with fast growing fish, where the feed increase during the production cycle can be so rapid that there needs to be a higher water exchange rate and thus nutrient dilution can increase, with consequences for the nutrient composition and management.
The same oxic or anoxic processes that occur in the RAS as a part of the coupled aquaponic system also occur inside the hydroponic subsystems. Therefore, oxygen availability and possibly aeration of the plant water can be crucial in order to optimize the water quality for good plant growth. The oxygen allows the heterotrophic bacteria to convert organic bound nutrients to the dissolved phase (i.e. protein nitrogen into ammonia) and the nitrifying bacteria to convert the ammonia into nitrate. The availability of oxygen in the water also reduces anoxic microbial metabolism (i.e. nitrate- and/or sulphate-reducing bacteria, Comeau 2008), processes which can have tremendous effects on the reduction of nutrient concentrations. The aeration of the roots also has the advantage that water and nutrients are transported to the root surface, and that particles that settle on the root surface are removed (Somerville et al. 2014).