No abstract

Aquaponics is a solution for sustainable production of fish and plants in a single semi-closed system, where nutrient-rich water from the aquaculture provides nutrients for plant growth. We examined the microbial communities within an experimental aquaponics system. Whereas the fish feces contained a separate community dominated by bacteria of the genus Cetobacterium, the samples from plant roots, biofilter, and periphyton were more similar to each other, while the communities were more diverse. Detailed examination of the data gave the first indications to functional groups of organisms in the different compartments of the aquaponic system. As other nitrifiers other than members of the genus Nitrospira were only present at low numbers, it was anticipated that Nitrospirae may perform the nitrification process in the biofilm.

Hydrothermal carbonization (HTC) of stabilized and non-stabilized sewage sludge was investigated in a 25 L vessel as a pretreatment for sewage sludge before incineration, and the composition and properties of the obtained HTC coal and process water were studied. The observed values for H/C and O/C in HTC coal from stabilized and non-stabilized sewage sludge were shown to be higher than in natural coal and rather close to typical values for cellulose. The upper heating value of the stabilized sewage sludge was increased from 3.4 to 6.5%, and the upper heating value of the non-stabilized sludge was increased from 5.8 to 11.0%, after carbonization. The carbon efficiency ranged from 62 to 71% for stabilized sewage sludge and from 60 to 66% for non-stabilized sewage sludge, and the dry matter (DM) loss after carbonization was 31 and 42% for stabilized sludge and 34 and 44% for non-stabilized sludge. After carbonization, the mechanical dewaterability was increased from 30 to 70% DM content for non-stabilized sludge and from 37 to 52% for stabilized sludge. The drying process of sewage sludge including HTC needs a clearly lower energy input than established drying techniques to produce a fuel. For the drying process of 1 ton of nonstabilized sewage sludge with 9% DM, the calculated energy consumption was lowered by 99.6 kWh of thermal energy and 8.5 kWh of electric energy by introducing HTC. The results of these experiments show the feasibility of the HTC process as part of the drying process of sewage sludge and the fate of key elements in the process on a laboratory scale. However, the process has to be further optimized and developed on an industrial scale. Further important development steps include recovery steps for the carbon in the process water and adapted process water treatments.

Global environmental, social and economic challenges drive the need for new and improved solutions for food production and consumption. Food production within a sustainability corridor requires innovations exceeding traditional paradigms, acknowledging the complexity arising from sustainability. However, there is a lack of knowledge about how to direct further activities, to develop technologies as potential solutions for questions related to climate change, loss of soil fertility and biodiversity, scarcity of resources, and shortage of drinking water. One approach that promises to address these problems is controlled environment agriculture. Aquaponics (AP) combines two technologies: recirculation aquaculture systems (RAS) and hydroponics (plant production in water, without soil) in a closed-loop system. One challenge to the development of this technology is the conversion of the toxic ammonium produced by the fish into nitrate, via bacteria in a biofilter, to provide nitrogen to the plants. However, as this Special Issue shows, there are many other challenges that need to be addressed if the goal of the technology is to contribute to more sustainable food production systems.

The aquaponic principle is the coupling of animal aquaculture (e.g. fish) with plant production (e.g. vegetables) for saving resources. At present, various definitions of aquaponics exist, some bearing the risk of misinterpretation by dismissing the original meaning or being contradictory. In addition, there is no standard terminology for the aspects of coupling between the aquaponic subsystems. In this study, we addressed both issues. (1) We developed new or revised definitions that are summarised by: Aquaponic farming comprises aquaponics (which couples tank‐based animal aquaculture with hydroponics) and trans‐aquaponics, which extends aquaponics to tankless aquaculture as well as non‐hydroponics plant cultivation methods. Within our conceptual system, the term aquaponics corresponds to the definitions of FAO and EU. (2) A system analysis approach was utilised to explore different aquaponic setups aiming to better describe the way aquaponic subsystems are connected. We introduced the new terms ‘coupling type’ and ‘coupling degree’, where the former qualitatively characterises the water‐mediated connections of aquaponic subsystems. A system with on‐demand nutrient water supply for the independent operating plant cultivation is an ‘on‐demand coupled system’ and we propose to deprecate the counterintuitive term ‘decoupled system’ for this coupling type. The coupling degree comprises a set of parameters to quantitatively determine the coupling's efficiency of internal streams, for example, water and nutrients. This new framework forms a basis for improved communication, provides a uniform metric for comparing aquaponic facilities, and offers criteria for facility optimisation. In future system descriptions, it will simplify evaluation of the coupling's contribution to sustainability of aquaponics.