Desalination Unit

The purpose of the small-scale desalination plant is to concentrate the hydroponic nutrient solution and to direct demineralized water to the RAS part of the system. We can assume that 40% of the flow to the desalination unit is remineralized and can be reused within the RAS. A parameter optimization experiment allowed for determination of the optimal feed flow to keep the RAS nitrate levels just below 50ppm.

7.3. Results

The outcome of the sizing simulation with this model, and with α = 0.45, β = 0, η = 0,

suggests a cultivation area of 28m² (if 𝐹𝐹𝑖𝑖𝑖𝑖

𝑥𝑥 = 𝐴𝐴) for leafy greens (i.e. the average of the leafy

plants cultivation area requirement after the startup of the system). When remineralizing the fish sludge, Figure 7.2 shows that the system provides enough nutrients for twice the cultivation area.

Figure 7.2. Formula 1 is linked to the average feed input in a RAS system. Based on the sizing rule of thumb, approximately 28 m² can be cultivated in one-loop systems without remineralization, and 60 m² in one- or multi-loop aquaponic systems with remineralization units (for α = 0.45, β = 0, η = 0.90).

Based on these findings, the cultivation area has been set to 28m²; thus the remineralization loop was not needed to proof the necessity of an additional desalination loops for decoupled multi-loop aquaponic systems. This is because the remineralization loop as shown in Figure 7.1 provides the nutrients made available to the hydroponics loop unidirectionally, and correspondingly they do not increase the nutrient concentration in the RAS part of the system. Figure 7.3 and 7.4 show the nitrate concentrations of decoupled systems without and with an additional desalination unit, respectively.

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Figure 7.3. This graph shows what happens when the RAS and hydroponic systems are decoupled. The flow from RAS to hydroponics is now totally dependent on the crop specific evapotranspiration rate (Table 4). The starting position an optimal nitrate concentration for the lettuce. Even though Namibia has one of the highest average solar radiation levels in the world, the nitrate levels in the RAS are still suboptimal levels for the reared tilapia.

Figure 7.4. The parameter optimization experiment suggested a feed flow of 70 L h-1 _{to the} desalination unit. This figure shows that the RAS nitrate concentration is just below 60 ppm, while the hydroponic nitrate concentration is at least higher than approx. 500 ppm.

7.4. Discussion

The design development of multi-loop aquaponics systems has been advanced rapidly, primarily because of the potential of such systems to achieve higher yields while also reducing nutrient and water inputs. Unfortunately, optimal nutrient concentration levels cannot be achieved directly because of oversizing the hydroponic cultivation area to

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guarantee a sufficient water flow from the RAS to the hydroponics system in order to keep the RAS nutrient concentration at an optimal level. Consequently, nutrients for the hydroponic sub-system have to be added in form of artificial fertilizers. From an economical point of view, this means higher operational costs due to additional fertilization requirements.

Table 7.5 provides an overview of existing decoupled aquaponic systems in Europe. The schematic layouts of the respective systems and our proposed design can be found in Appendix A. In this appendix, it can be seen that only the commercial system of NerBreen contains an additional loop in order to both remineralize sludge as well as regulate the nutrient concentrations of the aquaculture and hydroponic subsystems. Their approach (A.2) is different to ours (A.1; Figure 7.1), since in their case the hydroponic and RAS nutrient concentrations are not directly regulated. Instead, the treated sludge is further concentrated to increase the nutrient concentration in the hydroponics, and increase water quality in the RAS. The research systems shown in Table 7.5 as well as A.3-A.5 decoupled their loops while discharging nutrient-rich sludge. It can be seen that without the implementation of on-site sludge treatment as well as the integration of nutrient concentrating technologies, high amounts of additional fertilization for the hydroponics and discharge of RAS-water are required.

Table 7.5. An overview of existing decoupled aquaponic systems in Europe.

Parameter Unit NerBreen Tilamur IGB Berlin Inagro

Country - Basque

Country, Spain Spain Germany Belgium

Purpose - Commercial Research Research Research

Fish Species - Nile Tilapia Nile Tilapia Nile Tilapia Pike perch Plant Species - Season

dependent: Tomato, pepper, fresh garlic, strawberry, herbs, lettuce

Tomato Tomato Tomato

Total RAS size m³ 300 45 16.5 160

Total HP size m² 3500 400 6.5 340

Total HP size m3 _{100 3 0.6 n/a}

HP type - 1/3 RAFT; 2/3 drip irrigation NFT/Rockwoo l (drip irrigation) NFT drip irrigation

Amount of Loops - 4 (RAS, HP,

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concentrator) removal to biogas)

RAS water discharge (incl. sludge)

% day-1 _{1 7 2.8 16}

HP water discharge % day-1 _{Zero Zero Zero Zero} 1 Remineralization

Technology - Vermiculture (Californian Red Worm)

Secondary

Clarifier - Biogas

HP fertilization % 40-60 60 38.1 (for

lettuce) n/a

1 _{discharge; water consumption depends on life stage of the tomato plants and on weather conditions as RAS}

and greenhouse were not specifically dimensioned to be coupled, but have been linked posteriorly, the RAS produces discharge water in excess to what the greenhouse may consume;

The outcome of our model (Figure 7.3) clearly shows that nutrient concentrations of the RAS and hydroponic systems would be far from optimal based on a simple decoupling of the systems. In particular, Figure 7.3 shows that the nitrate levels in the RAS are very high, and since the primary nutrient input (in form of fish feed) takes place in the RAS, it is impossible to achieve higher hydroponic nutrient concentrations while lowering the RAS water nutrient levels in proportion to the hydroponic counterpart. As multi-loop systems aim to reduce the discharge of water and nutrients, periodical bleed-off of aquaculture process water is not a desirable option. Additional nutrient supply via a possible remineralization loop as well as the addition of artificial fertilizer could raise the hydroponic nutrient content, however, it does not solve the problem of the high nitrate values in the RAS.

Apart from the suggested solution of implementing desalination technologies, a denitrification side loop has the potential to lower the RAS nitrate levels. Another option is to integrate the anaerobic sludge remineralization loop into the RAS sub-system, as it also promotes denitrification. Both options would reduce the total nitrogen availability in the system, which again leads to a higher fertilizer requirement. Also, having an independent

remineralization loop brings the advantage of providing the plants with NH4 (preferred form

of nitrogen) as well as ensuring optimal conditions for anaerobic bacteria.

Additional to proportioning the nutrient concentrations of each subsystem, such desalination units can also be used for their intended purpose: to desalinate sea or brackish water, thus increasing capacity of dry regions to produce food within a sustainable system (Shatat et al., 2013). However, it must be stated that the energy and cost required to install solar desalination technology currently cannot compete with fuel-based desalination methods yet (Ayoub and Malaeb, 2014).

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7.5. Conclusion

The present research quantitatively demonstrates that the implementation of desalination processes can contribute to the nitrate balances in multi-loop aquaponic systems to attain optimal growth conditions for both fish and plants, by concentrating the hydroponic nutrient solution while diluting the RAS process water. It builds on prior work by Goddek et al. (2016b) and Kloas et al. (2015), and shows how the integration of desalination technologies can improve the design and practical applications of multi-loop aquaponic systems.

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Chapter 8