The development of innovative and environmental-friendly food cultivation methods is required to face the near future (Godfray et al., 2010). One of the fastest-growing food-producing sectors is aquaculture (FAO, 2018). World production has increased from 3 to 80 million tonnes of fish from the 1970s to 2017.
Thus, it accounts for about 50% of the world’s fish consumption. Aquaculture could decrease the pressure on the endangered aquatic wildlife, but its development needs revision. Aquaculture impacts the environment by producing fish feed (usually produced from fish oils/flours) and nitrogen/antibiotics discharges (Read and Fernandes, 2003). At the same time, industrial agriculture is also being scrutinized.
The expansion of agriculture causes increasing land use, higher freshwater consumption, as well as nitrogen, phosphorus, and pesticides, overloads (Tilman et al., 2001). In this perspective, hydroponics (soilless plant cultivation) is considered as an alternative to conventional agriculture as it decreases the demand for land, water, nutrients, and pesticide dosing (Gwynn-Jones et al., 2018).
If nutrient-rich effluents coming from aquaculture are used in hydroponics and vice versa, a virtuous loop is generated, i.e., this is aquaponics. Aquaponics allows the production of both fish and edible plants while minimizing the environmental impact compared to conventional fishing and agriculture (FAO, 2014, Tyson et al., 2011), closing urban bicycles (Venkata Mohan et al., 2020).
From the conceptual point of view, aquaponics is a win-win situation, but its real-world implementation requires the correct management of the nitrogen cycle inside the system (Wongkiew et al., 2017). On the one hand, aquaculture effluents are usually characterized by high ammonium content, since about 60–70% of the feed is excreted as ammonia (Kissil and Lupatsch, 2004).
On the other hand, hydroponics requires almost ammonium-free water (<0.8 mgN-NH4+·L−1) but with a certain amount of nitrate (1–34 mgN-NO3−·L−1) as nitrogen source of cultured plants (FAO, 2014). In consequence, conventional nitrification-denitrification processes, usually focusing on full nitrogen removal, need to be adapted to the specific requirements of aquaponics.
Firstly, ammonium generated in the aquaculture pond should be converted into nitrate followed by controlled denitrification in order to avoid high nitrate accumulation (<90 mgN-NO3−·L−1) that could affect fish and plants growth (FAO, 2014, van Rijn et al., 2006), and to ensure no nitrite presence (<0.3 mgN-NO2−·L−1) due to its toxicity for plants and fish (Colt, 2006, FAO, 2014). Aquaculture recirculating systems can be easily adapted to aquaponics as they are already equipped with, e.g., biotrickling filters being characterized by a good nitrification performance.
However, the denitrification performance of such systems is poor due to the lack of organic matter (C/N < 3) (Mook et al., 2012, van Rijn et al., 2006). Thus, an externally added electron donor is needed to control and adjust the nitrate content. The most common external electron donor is organic matter, but it introduces additional cost factors (i.e. chemical dosage and sludge disposal). By finding a solution for the treatment of aquaponics, a solution for the treatment of other wastewaters with low C/N ratio wastewaters (e.g. some urban wastewater) could be also found (Mook et al., 2012).
Primary microbial electrochemical technologies (MET) have emerged as a biotechnological alternative for directly supplying an electron donor/acceptor to electroactive microorganisms by means of an electron conductor termed electrode (Schröder et al., 2015). Integrating primary MET in aquaponics could result in a considerable improvement thereof, as they were demonstrated to drive both nitrification (Vilajeliu-Pons et al., 2018) and denitrification (Gregory et al., 2004).
Still little is known about the recently discovered electricity-linked ammonium removal (Shaw et al., 2020), thus ammonium is usually oxidized into nitrate aerobically (He et al., 2016, Virdis et al., 2008). Microbial electrochemical denitrification has been widely tested in different waters such as wastewater (Virdis et al., 2008), groundwater (Pous et al., 2015a), or aquaculture effluents (Marx Sander et al., 2018).
The microbial structure and activity of denitrifying MET rapidly change with the mode of operation (Pous et al., 2015b) allowing better control of denitrification by fine-tuning different operational parameters, e.g., cathode potential (Virdis et al., 2009), current density (Park et al., 2005), pH (Clauwaert et al., 2009), or the hydraulic retention time (HRT) (Pous et al., 2017).
Besides MET implementation in aquaponics could be effective at low operational expenditures, the complexity and capital expenditures associated with its conventional configuration represent a matter of concern (for instance, usage of electrodes, membranes, potentiostats, etc.) (Sleutels et al., 2012).
Sustainable electrification of biotrickling filters was achieved by combining an aerobic zone (filled with a non-conductive material) with an anoxic electrified zone (filled with a conductive material).
However, the development of MET-based treatment concepts such as snorkels (Hoareau et al., 2019, Viggi et al., 2015) or METlands (Aguirre-Sierra et al., 2020, Prado et al., 2020) highlights the importance of the microbial ecology function over reactor materials and engineering (Koch et al., 2018). In consequence, only two components might be needed to reach an improvement of bioremediation activities: the appropriate microbiome inhering electroactive microorganisms and conductive support serving as an electrode.
Conventional technologies currently used in aquaculture and aquaponics (e.g., biofilters) are based on microbial degradation at non-conductive supports (Crab et al., 2007). Yet, it can be hypothesized that conductive support integrated into the effluent treatment site will enhance nitrification and denitrification due to the activity of electroactive microorganisms.
For this reason, this work explored the potential of biotrickling filters to be electrified for improving nitrification/denitrification rates and the efficient control of the nitrate content in the effluent. Consequently, a sustainable system was developed to improve aquaponics water recirculation by a controlled optimization of the nitrogen content in the aquaculture effluent according to the actual requirements of hydroponics. This technology could be used for the treatment of other wastewaters containing ammonium at a low C/N ratio.
Sustainable electrification of biotrickling filters was achieved by combining an aerobic zone (filled with a non-conductive material) with an anoxic electrified zone (filled with a conductive material). Relevant ammonium and nitrate removal rates were obtained (94 gN·m−3·d−1 and 43 gN·m−3·d−1, respectively) and the effluent quality criteria for an aquaponics application was reached. The reactor design developed in this study is a promising alternative for aquaponics but also for the treatment of organic carbon-deficient ammonium-contaminated waters.
Source: Pous, N., Korth, B., Osset-Álvarez, M., Balaguer, M. D., Harnisch, F., & Puig, S. (2021). Electrifying biotrickling filters for the treatment of aquaponics wastewater. Bioresource technology, 319, 124221.