•5 min read
Whilst aquaponics is seen to be one of the key food production technologies which 'could change our lives' (van Woensel et al. 2015), in terms of sustainable and efficient food production, aquaponics can be streamlined and become even more efficient. One of the key problems in conventional aquaponics systems is that the nutrients in the effluent produced by fish are different than the optimal nutrient solution for plants. Decoupled aquaponics systems (DAPS), which use water from the fish but do not return the water to the fish after the plants, can improve on traditional designs by introducing mineralization components and sludge bioreactors containing microbes that convert organic matter into bioavailable forms of key minerals, especially phosphorus, magnesium, iron, manganese and sulphur that are deficient in typical fish effluent. Contrary to mineralization components in one-loop systems, the bioreactor effluent in DAPS is only fed to the plant component instead of being diluted in the whole system. Thus, decoupled systems that utilize sludge digesters make it possible to optimize the recycling of organic wastes from fish as nutrients for plant growth (Goddek 2017; Goddek et al. 2018). The wastes in such systems mainly comprise fish sludge (i.e. faeces and uneaten feed that is not in solution) and thus cannot be delivered directly in a hydroponics system. Bioreactors (see Chap. 10) are therefore an important component that can turn otherwise unusable sludge into hydroponic fertilizers or reuse organic wastes such as stems and roots from the plant production component into biogas for heat and electricity generation or DAPS designs that also provide independently controlled water cycling for each unit, thus allowing separation of the systems (RAS, hydroponic and digesters) as required for the control of nutrient flows. Water moves between components in an energy and nutrient conserving loop, so that nutrient loads and flows in each subsystem can be monitored and regulated to better match downstream requirements. For instance, phosphorous (P) is an essential but exhaustible fossil resource that is mined for fertilizer, but world supplies are currently being depleted at an alarming rate. Using digesters in decoupled aquaponics systems allows microbes to convert the phosphorus in fish waste into orthophosphates that can be utilized by plants, with high recovery rates (Goddek et al. 2016, 2018).
Although decoupled systems are very effective at reclaiming nutrients, with nearzero nutrient loss, the scale of production in each of the units is important given that nutrient flows from one part of the system need to be matched with the downstream production potential of other components. Modelling software and Supervisory Control and Data Acquisition (SCADAS) data acquisition systems therefore become important to analyse and report the flow, dimensions, mass balances and tolerances of each unit, making it possible to predict physical and economic parameters (e.g. nutrient loads, optimal fish-plant pairings, flow rates and costs to maintain specific environmental parameters). In Chap. 11, we will look in more detail at systems theory as applied to aquaponics systems and demonstrate how modelling can resolve some of the issues of scale, whilst innovative technological solutions can increase efficiency and hence profitability of such systems. Scaling is important not only to predict the economic viability but also to predict production outputs based on available nutrient ratios.
Another important issue, which requires further development, is the use and reuse of energy. Aquaponics systems are energy and infrastructure intensive. Depending on received solar radiation, the use of solar PV, solar thermal heat sources and (solar) desalination may still not be economically feasible but could all be potentially integrated into aquaponics systems. In [Chap. 1, we present information about innovative technical and operational possibilities that have the capacity to overcome the inherent limitations of such systems, including exciting new opportunities for implementing aquaponics systems in arid areas.
In Chap. 2, we also discuss in more detail the range of environmental challenges that aquaponics can help address. Pathogen control, for instance, is very important, and contained RAS systems have a number of environmental advantages for fish production, and one of the advantages of decoupled aquaponics systems is the ability to circulate water between the components and to utilize independent controls wherein it is easier to detect, isolate and decontaminate individual units when there are pathogen threats. Probiotics that are beneficial in fish culture also appear beneficial for plant production and can increase production efficiency when circulated within a closed system (Sirakov et al. 2016). Such challenges are further explored in Chap. 5, where we discuss in more detail how innovation in aquaponics can result in (a) increased space utilization efficiency (less cost and materials, maximizing land use); (b) reduced input resources, e.g. fishmeal, and reduced negative outputs, e.g. waste discharge; and (c) reduced use of antibiotics and pesticides in self-contained systems.
There are still several aquaponic topic areas that require more research in order to exploit the full potential of these systems. From a scientific perspective, topics such as nitrogen cycling (Chap. 9), aerobic and anaerobic remineralization (Chap. 10), water and nutrient efficiency (Chap. 8), optimized aquaponic fish diets (Chap. 13) and plant pathogens and control strategies (Chap. 14) are all high priorities.
In summary, the following scientific and technological challenges need to be addressed:
Nutrients: As we have discussed, systems utilizing sludge digesters make it possible to optimize the recycling of organic waste from fish into nutrients for plant growth, such designs allow for optimized reclamation and recycling of nutrients to create a near-zero nutrient loss from the system.
Water: The reuse of nutrient-depleted water from greenhouses can also be optimized for reuse back in the fish component utilizing condensers.
Energy: Solar-powered designs also improve energy savings, particularly if preheated water from solar heaters in the greenhouses can be recirculated back to fish tanks for reuse.
The ability to recycle water, nutrients and energy makes aquaponics a potentially unique solution to a number of environmental issues facing conventional agriculture. This is discussed in Chap. 2.