An innovative suction filter device reduces nitrogen loss in double recirculating aquaponic systems
Introduction
Climate change, water and fossil fuel scarcity as well as food shortage is linked to an increasing demand of water, food and energy due to growing world´s population (Godfray et al., 2010). In a life cycle assessment study for tomato production it was demonstrated that fertilisers cause by far the greatest environmental impact (Torrellas et al., 2012). Especially during the manufacturing of nitrogen fertiliser, high emissions of CO2, N2O, NO, and NO2 occur (Wood and Cowie, 2004), which have major impacts on global warming, eutrophication, and air acidification (Antón et al., 2004; Torrellas et al., 2012). Therefore any attempt to make use of nitrogen within value added chains of fertilisers supports sustainability by reducing environmental impacts. Furthermore, agriculture uses 70% of the global fresh water ressources (FAO, 2011) and thus freshwater availability is one of the most important issues for maintaining food security. As such, the fresh water consumption by, for instance, tomato production is an issue as well. Depending on the season and production systems the water footprint of tomato production in greenhouses can rise to 122.6 L kg−1 produced tomatoes (Almeida et al., 2014). In order to achieve a high food quantity without adverse impacts on the environment, innovative and sustainable food production systems are urgently needed (Godfray et al., 2010).
Aquaponics, which is the combined production of aquaculture and plants in hydroponics is a sustainable option to reduce fertiliser and water use (Maucieri et al., 2017). The main advantage of aquaponics is the double use of resources like water and nutrients, used firstly for fish and secondly for plant production (Rakocy et al., 2006; Tyson et al., 2011). Due to metabolic processes, fish excrete mainly ammonia (NH3) which is usually protonated immediately to ammonium (NH4+) after it enters the water. A good overview of the biological background is given by Wongkiew et al. (2017). While NH3 is highly fish toxic, nitrate (NO3−) is much less harmful to fish (Timmons et al., 2010). For intensive fish production in recirculating aquaculture systems (RAS) NH4+ is converted into NO3− in a biofilter by nitrifying bacteria. Nitrate nitrogen (NO3-N) concentrations in fish cultures can be up to 500 mg L-1 (Honda et al., 1993) and are commonly controlled by daily water exchange (Timmons et al., 2010). The release of such nitrogen rich fish waste water into the environment can result in a massive environmental pollution (d’Orbcastel et al., 2009). In order to reduce the overall emissions of aquaculture and fertiliser manufacturing it is absolutely appropriate and sustainable to use the nutrients from fish water for plant nutrition in aquaponic systems, thus creating value added chains.
Conventional single recirculating aquaponic systems (SRAPS) are mainly operating as a one loop low-tech system. Due to the different environmental requirements of fishes and of plants, compromises in terms of water quality have to be made and reveal reduced yields compared to separate single production systems for fishes and plants, respectively (Graber and Junge, 2009; Vergote and Vermeulen, 2012; Wortman, 2015). Therefore, an innovative approach to overcome these problems was recently published by Kloas et al. (2015). They developed a high-tech double recirculating aquaponic system (DRAPS), where two independently operating systems, a RAS and a recirculating hydroponic system, are coupled. The connection of both is realised by a one-way-valve creating a unidirectional water flow (Kloas et al., 2015). This means that the main specific characteristics of DRAPS are the separation, the independent operation of both productions cycles (fish and plant), and the unidirectional water transfer from fish to plants. Latter means that the water is not transferred back from the hydroponic system to the RAS as known from SRAPS. Thus, the growth conditions can be optimised for each species separately and allows an intensive production of fish and plants comparable to single production units (Kloas et al., 2015; Suhl et al., 2016). Furthermore an indirect water transfer from the hydroponic unit back to RAS can be implemented for the full version of DRAPS (Kloas et al., 2015) by regaining the evapotranspirated water in the greenhouse by an active cooling unit in order to close the water loop. A good overview about the advantages and disadvantages of DRAPS compared to SRAPS is given by Suhl et al. (2016). Since both, the RAS and the hydroponic system, are working independently from each other in DRAPS, the water quality can be optimised for each individual cycle in order to produce fish and, for example, tomatoes in an intensive way. So far, only tilapia, a common aquaculture fish, combined with tomato plants have been produced using DRAPS (Kloas et al., 2015; Monsees et al., 2017; Suhl et al., 2016). Delaide et al. (2016) also used the idea of DRAPS to produce lettuce with waste water from a tilapia rearing system.
According to the fish species chosen and the respective rearing conditions, the waste water quality of RAS can vary widely in terms of nutrient concentrations and composition. In order to evaluate the use of DRAPS for further fish species, the present study focused on the first combined production of the African catfish and tomatoes. However, the main focus of the recent study was the replacement of the yet used sedimentation unit by an innovative filter suction device in the RAS part of the DRAPS to improve its efficiency and sustainability. Thus in the already established DRAPS (Suhl et al., 2016) it has been investigated whether the innovative filter suction device can provide fish waste water with a higher nitrogen concentration for intensive tomato production and thus reduce the environmental impacts. Yield quantities in terms of fish and tomatoes, the fertiliser use efficiency and the fresh water use per unit biomass of fish and tomatoes were determined for a system comparison considering DRAPS and a conventional separate hydroponic unit serving as control. In this context the reduction of emissions caused by saving nitrogen fertiliser due to the use of the modified DRAPS compared to the original DRAPS and the control was estimated. To evaluate the prospects of fish and fruit productivities, the optimum plant to fish ratio and its potential total output were calculated.
Section snippets
Experimental set-up
The DRAPS used in the present study was described in detail by Suhl et al. (2016). A spatially separated RAS contained four identical glass fiber tanks with a total net production volume of 7.2 m³, a mechanical sedimentation filter (1.3 m³) to clean the fish water from solids, a trickling biofilter with pump sump (2.3 m³) and a reception tank. The latter collected the water after passing the biofilter, before it was pumped back to the fish tanks. The RAS had a total water volume of about 12.0 m3
Fish and tomato fruit production
It was already stated by Kloas et al. (2015) and Suhl et al. (2016) that double recirculating aquaponic systems (DRAPS) can compete with separated tilapia or tomato productions. In comparison to conventional low-tech single recirculating aquaponic systems (SRAPS), DRAPS are developed for intensive large scale and sustainable food production. As such, high productivities in both units, fish and plants, are required.
The evaluation of fish productivity in the used DRAPS, producing African catfish
Conclusion
The modification of the already existing double recirculating aquaponic systems (DRAPS) by the implementation of a new suction filter device successfully prevented nitrogen loss and increased the fertiliser saving potential. As such, the monthly mean fertiliser input to the conventional hydroponic unit was reduced up to 39.1% using the original DRAPS. After modification to reduce nitrogen loss caused by the implemented 3-chamber pit, the fertiliser input was even reduced by 77.7%, compared to
Declarations of interest
None.
Acknowledgements
This work was supported by the European Commission via INAPRO under Grant agreement number 619137. We gratefully acknowledge M. Schramm, B. Lehmann, and J. Lusch for excellent technical support.
Johanna Suhl is writing her doctoral thesis at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries focused on double recirculating aquaponic systems. She has experience in controlled environement horticulture and her focal point is the plant production within aquaponic systems and how the fish waste water can become optimised for plant growth. Currently, she is working at the Humboldt Universität zu Berlin as scientist and deals with controlled environment horticulture.
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Johanna Suhl is writing her doctoral thesis at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries focused on double recirculating aquaponic systems. She has experience in controlled environement horticulture and her focal point is the plant production within aquaponic systems and how the fish waste water can become optimised for plant growth. Currently, she is working at the Humboldt Universität zu Berlin as scientist and deals with controlled environment horticulture.
Dennis Dannehl has worked at different research centers in Germany and New Zealand and is currently employed as postdoc at the Humboldt Universität zu Berlin. He is an expert in controlled environment horticulture and especially focused on the development of technical systems and control methods applied in greenhouses to optimise plant growth and health related plant compounds in vegetables.
Daniela Baganz is currently working at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries as the scientific coordinator of the EU- Project INAPRO dealing with the optimisation of the aquaponic technology. She has many years of experience in fish behaviour, fish physiology, fish health monitoring, chronobiology, water quality monitoring and aquaculture/aquaponics.
Uwe Schmidt is working as head of the Biosystems Engineering Division and Director of the Thaer-Institute at the Humboldt Universität zu Berlin. In the last 20 years, he has been active in the fields of greenhouse technology, process automation as well as irrigation and fertilization systems, software development and intelligent sensor technology. His expertise is in the field of energetic system analyses, thermodynamic models for estimating plant physiology and technical systems performance. A kind of strength is his collaboration with industry in the field of technology transfer. Since 1995, Dr. Schmidt has been the head of the Steinbeis Technology Transfer Centre Energy-Environment-Information.
Werner Kloas is head of the Department Ecophysiology and Aquaculture at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries and professor for Endocrinology at the Humboldt University, Berlin. He is a basic animal physiologist specialised in comparative endocrinology of aquatic vertebrates focused on stress, reproduction, and nutrition. Main research interests are ecotoxicology assessing anthropogenic impacts on aquatic organisms, e.g. fishes and amphibians, such as endocrine disruptors, pharmaceuticals, microplastics or artificial light at night, as well as sustainable aquaculture. The latter one is dealing especially with the development and improvement of innovative aquaponic systems as well as alternative sources for fish feed.