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8.3 Distillation/Desalination Loop

5 months ago

5 min read

In decoupled aquaponics systems, there is a one-way flow from the RAS to the hydroponics unit. In practice, plants take up water supplied by RAS, which in turn is topped up with fresh (i.e. tap or rain) water. The necessary outflow from the RAS unit is equal to the difference between the water leaving the HP system via plants (and via the distillation unit) and the water entering the hydroponics unit from the mineralization reactor, if the system includes a reactor (Fig. 8.4). A simplified summary is that the long-term water flux requirement from RAS to HP is equal to the crop water consumption by evapotranspiration and plant water storage in the plant biomass.

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Fig. 8.4 Scheme of water fluxes and different concentrations of nutrients in a decoupled aquaponics system, where Q, flow volume in L; ρ, nutrient concentration in mg/L; RAS, recirculating aquaculture system; MIN, mineralization reactor; DIS, distillation unit; and X, unknown/flexible flow parameter

However, in terms of mass balances, the amount of nutrients leaving the hydroponics system via the plants needs to be replaced to assure a constant equilibrium. This poses a dilemma, as the maximum tolerable nutrient concentration in RAS is much lower than what is necessary in HP. The high nutrient flows (ρsubRAS/sub $\times$ QsubRAS/sub) for HP can thus not be accomplished by the low RAS nutrient concentrations. Instead, without a distillation/desalination loop, the nutrient concentration would increase in the RAS while decreasing in the hydroponics system. A possible remedy is to discharge RAS water (and thus also nutrients) to decrease the nutrient concentration there and add fertilizer to the hydroponics nutrient solution. In terms of environmental and economic impact, this solution is less satisfying and does not serve the aim of a closed loop combined production.

The implementation of a distillation unit as shown in Fig. 8.3 represents a potential solution for this dilemma. Such distillation technologies (e.g. thermal membrane distillation) have the potential to separate dissolved salts and nutrients from water (Shahzad et al. 2017; Subramani and Jacangelo 2015). In the context of multi-loop aquaponics systems, and as an alternative to additional fertilization and water bleed-off with corresponding extra costs, this technology could not only provide fresh water to the system but also achieve desired nutrient concentrations for the respective subsystems (Goddek and Keesman 2018).

For the implementation (i.e. sizing) of such a distillation unit, simple mass balance equations can be used. The remaining system, however, must be sized beforehand (either via rules of thumb or via mass balance equations; see Sect. 8.5), because the nutrients that enter the system should be in equilibrium with the bioavailable nutrients taken up by the crop (Note: the sweet spot of decoupled systems is its flexibility. Consequently, one can also oversize the hydroponics part of the system although that will necessitate the use of more fertilizer). The easiest way to estimate nutrient uptake is to use the assumption that nutrients are taken up/absorbed much the same as dissolved ions in irrigation water (i.e. no elementspecific chemical, biological or physical resistances). Consequently, to maintain equilibrium, all nutrients taken up by the crop as contained in the nutrient solution need to be added back to the hydroponics system (Eq. 8.4).

$\phi{RAS}+\phi{MIN}-\phi_{HP}=0$ (8.4)

where ϕsubRAS/sub is the nutrient flow from the RAS system to the hydroponics system, ϕsubMIN/sub is the nutrient flow from the mineralization unit to the hydroponics system and ϕsubHP/sub is the nutrient plant uptake. For this equation, it is assumed that the distillation system has an efficiency of close to 100%. Thus, QsubDIS/sub goes back to the hydroponics subsystem.

Consequently:

$(\rho{HP}\times Q{HP})=(\rho{RAS}\times Q{RAS})+(\rho{MIN}\times Q{MIN})$ (8.5)

where Q is the flow volume in L, and ρis the nutrient concentration in mg/L.

As stated above, the flow from RAS to the hydroponics unit is the difference of the sum of the water flows leaving the hydroponics system (i.e. QsubHP/sub + QsubX/sub) and the inflow from the bioreactor (QsubMIN/sub), i.e. QsubRAS/sub = QsubHP/sub + QsubX/sub - QsubMIN/sub, which leads us to the following equation:

$(\rho{HP}\times Q{HP})=(\rho {RAS}\times Q{HP})+(\rho {RAS}\times Q{X})-(\rho{RAS} \times Q{MIN})+(\rho{MIN}\times Q{MIN})$ (8.6)

The targeted variable is the distillation flow (QsubX/sub) that is required to maintain the nutrient concentration equilibrium in the hydroponics system. For this, Eq. 8.6 is solved for QsubX/sub in the following steps:

$(\rho{RAS}\times Q{X}) = (\rho{HP}\times Q{MIN})-(\rho{RAS}\times Q{HP})+(\rho{RAS}\times Q{MIN})$ (8.7)

$Q{X}=\frac{\rho{HP}\times Q{HP}}{\rho{RAS}}-\frac{\rho{MIN}\times Q{MIN}}{\rho{RAS}}-Q{HP}+Q_{MIN}$ (8.8)

Note that the distillation flow QsubX/sub is highly dynamic and depends on the evapotranspiration rate of the plants, which is climate-dependent. The dynamic outcome, however, can be used for sizing the distillation unit. To calculate the required inflow into the distillation unit, the following formula can be used:

$Q{DIS}=Q{X}\times \frac{100}{\eta_{DIS}}$ (8.9)

where Q is the flow volume in L and η the demineralization efficiency of the used device (in %).

Distillation technology can hence drastically reduce the water and environmental (i.e. fertilizer usage) footprint of multi-loop aquaponics systems. However, aquaponics systems become even more complex when considering their implementation. Even though this additional loop might not make any sense for small-scale systems, it has the potential to take larger commercial systems to a new level. Yet, one has to consider that thermal distillation technology requires high amounts of thermal energy and might not be economically reasonable everywhere. Regions with high global solar radiation levels or geothermal energy sources might be the most suitable for this technology. The economical sustainability of such systems is consequently also location dependent.

Another point to bear in mind is the high temperature of distilled water and brine from the distillation unit. Depending on the environmental conditions and the fish species used, the hot distillation water could be used to heat up the RAS water; the brine, however, needs to cool down before re-entering the HP subsystem.