There is lots of discussion going on on the internet and within academic worlds, whether decoupled aquaponics has a general advantage over conventional recirculating aquaponic systems. Figuring this out was our objective in the last years and led to the publication “Navigating towards Decoupled Aquaponic Systems: A System Dynamics Design Approach“. Following the KISS principle, I will briefly outline the main points of the publication and discuss them a bit in non-academic jargon (excluding the paper’s abstract).
The classical working principle of aquaponics is to provide nutrient-rich aquacultural water to a hydroponic plant culture unit, which in turn depurates the water that is returned to the aquaculture tanks. A known drawback is that a compromise away from optimal growing conditions for plants and fish must be achieved to produce both crops and fish in the same environmental conditions. The objective of this study was to develop a theoretical concept of a decoupled aquaponic system (DAPS), and predict water, nutrient (N and P), fish, sludge, and plant levels.
This has been approached by developing a dynamic aquaponic system model, using inputs from data found in literature covering the fields of aquaculture, hydroponics, and sludge treatment. The outputs from the model showed the dependency of aquacultural water quality on the hydroponic evapotranspiration rate. This result can be explained by the fact that DAPS is based on one-way flows. These one-way flows results in accumulations of remineralized nutrients in the hydroponic component ensuring optimal conditions for the plants. The study also suggests to size the cultivation area based on P availability in the hydroponic component as P is an exhaustible resource and has been identified one of the main limiting factors for plant growth.
Although many aquaponics systems are constructed and operated as a recirculating loop, commercial growers and researchers carry on expanding this initial aquaponics system design towards an independent control over each system unit (i.e. RAS, hydroponics, and nutrient recovery via sludge remineralization). Systems where fish, plants and, if applicable, remineralization are integrated as separate functional units comprising individual water cycles that can be controlled independently, are called decoupled aquaponic systems (DAPS). The difference between the concepts of one-loop and multi-loop (i.e. decoupled) aquaponic systems can be seen in Figures 1 and 2. In the context of recycling all nutrients that go into the system, decoupled aquaponics can be seen as a preferred option since they avoid additional discharge. Also they provide better growth conditions for both fish and plants.
Figure 3 shows a process flow drawing of a basic DAPS layout. Please note – this is only an example and can be adjusted modularly. The blue tags in the figure comprise the RAS component, the green tags the hydroponic component, and the red tags the remineralization components. The level of each component is illustrated numerically in the tags and refers to the vertical direction the flow needs to travel to. This means high numbers refer to high positioning and low numbers to low positioning.
Whereas RAS and hydroponics have been subjects of research for decades, the remineralization of fish sludge are still in their infancy. In the paper, we discussed the advantages and disadvantages of aerobic pre- and post-treatment of anaerobic digestion, however, we are currently looking into pure anaerobic digestion performances. We will keep you posted about our findings on this website.
Unfortunately, we have to disappoint everybody who got enthusiastic about building a decoupled aquaponics system in their backyard. Decoupled aquaponics systems require plenty of control equipment and only make sense, if one is willing to achieve high nutrient solutions in the hydroponic unit. Also, sizing the system is much more complex compared to sizing conventional one-loop systems. Determining the required evapotranspiration rate of the hydroponic plants that is required to avoid accumulation of nitrogen forms in the RAS additionally increases the complexity. Consequently, these kind of systems are most suitable for large scale commercial systems, especially because its ability to compete with commercial hydroponic systems.
The sweet spot of aquaponics for most people is its sustainable approach as well as the symbiotic effect the RAS water has on the plants and vice versa. From a commercial viewpoint, one cannot convince farmers with these arguments, even though they might be totally valid. In recent experiments, we could observe growth advantages of decoupled aquaponics systems. We observed an increased plant growth of 39% compared to a pure hydroponic control nutrient solution when supplementing the hydroponic component with additional fertilizer. Moreover, we were able to show that anaerobic digestates also increased plant growth. For now, it seems that both the RAS water and the digestates contain plant growth promoting rhizobacteria (PGPR) that could promote plant growth. We are currently planning more experiments on this issue and will also try to identify and isolate some of these PGPR.
Sensitive Fish Species
In the paper, we have explained why decoupled aquaponics is suitable for sensitive fish species. We found out that the use of artificial greenhouse lights leads to less fluctuation in RAS nutrient concentrations, as the plants’ evapotranspiration is more constant. To what degree artificial lighting pays out needs to be explored in a crop and fish dependent economic assessment.
Hybrid Backyard Approach
The hybrid decoupled system is a combination of the one-loop and the decoupled approach (Fig. 4). Home and backyard growers who still want to get one’s foot into decoupled aquaponics, might want to try this approach. Resizing an existing system would be obsolete, as the remineralized sludge would serve as a nutrient source for the additional growbeds.
We believe that decoupled aquaponic systems have the potential to achieve performances similar, or even higher than hydroponic production. We know it is a bold statement, but current observation support these assumptions. Yet, one still has to determine whether these growth advantages of DAPS compared with hydroponics are still observed under perfect growth conditions (i.e. optimal climate control, light intensity, and CO2 supplementation). The key advantage, however, is the sustainable approach which strives at recycling everything that enters the system. This aspect alone is a full justification for decoupled aquaponics.
With respect to the remineralization component, further research is needed with respect to its remineralization performance depending on different hydraulic retention time (HRT) and sludge retention time (SRT). In conclusion, it can be said that while technical research in this area is important, additional geographically dependent follow-up studies are needed, dealing with the economically viable size of DAPS as well as the comparison with equivalent hydroponic systems.