Introduction
Maintaining low concentrations of nitrogen compounds (either ammonium, nitrate or nitrite) in recirculating aquaculture systems (RAS) is extremely important for a bigger and healthier fish production, as well as for discharge purposes. Due to high toxicity of ammonia to aquatic species, nitrifying treatment units are usually installed, transforming it into less toxic nitrate, which may accumulate to concentrations as high as 500 mg/L NO3- - N (Honda et al. 1993). More recently, the installation of denitrification units is also being suggested in RAS in order to decrease water exchange commonly done for nitrate control (Martins et al. 2010).
Available technologies for nitrate removal (denitrification) commonly rely on heterotrophic microorganisms where addition of carbon source (e.g. methanol) is usually necessary. This can be avoided with the use of (1) autotrophic bio-reactors, using H2 as an electron donor (Grommen et al. 2006), or (2) bioelectrochemical systems (BES), in which electrons can also be directly transferred from an inert graphite electrode (cathode) to microorganisms performing nitrate reduction to dinitrogen gas (Virdis et al. 2008). In BES, electrons can be abiotically generated by splitting water at the counter (anode) electrode, which simultaneously leads to anodic oxygen generation (Pous et al. 2015).
Although BES technology for denitrification is already proven, nitrate removal rates reported to date are still low and further improvements are necessary before this technology can be considered commercially attractive. Thus, this work focuses on the development of an easy-to-operate system which is able to perform simultaneous cathodic (autotrophic) nitrate removal and oxygen generation in aquaculture streams.
Material and Methods
In order to enable the maintenance of low concentrations of nitrate within the recirculation aquaculture tanks, a single-chamber cylinder reactor with total volume capacity of 1 Liter is currently being tested. Graphite granules were used as electrode material for both cathode (with denityifying biofilm growing attached) and anode (abiotically operated by splitting water and generating oxygen) electrodes. The synthetic aquaculture feed containing 20 mg/L NO3- -N flows firstly through cathodic region at the bottom of the reactor and then towards anodic zone. The net cathodic compartment (NCC) volume, in which denitrification is effectively happening, is 80 mL and cathode potential is controlled at -0.7 V vs Standard Hydrogen Electrode (SHE) with the use of a potentiostat (VMP3, Bio-Logic, France).
Results and Discussion
No oxygen was detected at the top of the cathodic region, showing that oxygen back-diffusion from the anode is not happening and is unlikely to interfere on denitrification under the conditions tested. Furthermore, the system was able to reduce up to 0.43 kg NO3- -N m-3 NCC d-1 at 88% Coulombic Efficiency, which indicates that most electrons transferred to the cathodic biofilm as current are being used for nitrate reduction. These values fall within the highest range of removal rates observed in the literature for BES (Virdis et al. 2008), despite the lack of membrane. Calculated energy expenditure is 0.15 kWh m-3 (22 kWh kg N-1), which is close to that required for nitrogen removal in activated sludge systems (Tchobanoglous et al. 2003). Thus, the system can represent a valid alternative to heterotrophic denitrification with the following advantages: (1) no need for added carbon source (which avoids on-site storage of chemicals such as methanol); (2) concomitant removal of toxic nitrite (Puig et al. 2011) and (3) potential for simultaneous oxygen production (partially restocking dissolved oxygen levels required by cultured species). Additional tests are currently being done under different experimental conditions and will enable a better understanding of the potentials and drawbacks of the technology for aquaculture application.
Conclusion
The proposed technology presents technical potential for simultaneous denitrification and partial re-oxygenation of recirculation aquaculture waters, which will improve quality of recirculated water (and thus produced fish) and reduce the volume of water exchange from these recirculation systems.
References
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Martins, C.I.M., et al., New developments in recirculating aquaculture systems in Europe: A perspective on environmental sustainability. 2010. Aquacultural Engineering 43(3): 83-93.
Pous, N., et al. 2015. Cathode potential and anode electron donor evaluation for a suitable treatment of nitrate-contaminated groundwater in bioelectrochemical systems. Chemical Engineering Journal 263(0): 151-159.
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