NordForsk: Off-flavor removal by CDI technology

Maintaining water quality in recirculating aquaculture systems (RAS) is essential using mechanical filters and biofilters that reduce fish toxic nitrogenous wastes through nitrification and denitrification. Full fish lifecycle until slaughter in RAS production have the problem of the accumulation of so called off-flavour compounds (OFC), that gives the fish a muddy taste. OFC are produced by microorganisms in the system. To remove the muddy flavour the fish needs to purged without feeding in a separate RAS. In a RAS purging unit, the system is used without biofilters that could house OF microorganisms. Not having a biofilter means you cannot feed the fish during purging which means weight loss. In this NordForsk project with Swedish and Danish partners we tested a capacitive deionization (CDI) technology for potential use in RAS. In addition, we tested surface coating possibilities to reduce OF microorganism growth. CDI is a popular desalination and hardness removal technique based on an electrochemical method that removes ions from water, which is energy efficient, environmentally friendly, cheap and sustainable. Every charged molecule can be targeted and removed. A CDI unit can remove fish and bacterial metabolites like ammonia, nitrite, nitrate, off flavour compounds and pathogens. The CDI system works based on the adsorption of ions through the application of an electrical voltage across the electrodes forming an electrode double layer which attracts the oppositely charged ions. In the project experiments in Denmark and Norway had been done. In the fish experiment at Nofima, we tried testing the CDI system in RAS to determine the possibility of the system against major nitrogen compounds and different physical water parameters. In this experiment, which lasted for two months, two different organic loads were created in three replicate RAS of 0.5 m3 tank where each tank was filled with 83 juvenile Atlantic Salmon smolts to create culture like conditions. Having a high organic loading treatment should favour the growth of OF microorganism to test anti OF coatings that been placed in the system. That coatings had been analysed by the university of Alborg to compare the microbial communities. The CDI technology for treating the fish production water was tested every 15th day of the experiment for 8 hours. In the experiment, a considerable reduction in alkalinity, conductivity and nitrate was observed and a lower removal of ammonia was noticed. Especially the significant nitrate removal can create possibilities for further application of this technology for RAS denitrification. The nitrate removal should be tested further as it seems like a very promising option to reduce RAS water exchange. The production of nitrite throughout the experiment through the CDI unit was not predicted. The nitrite production was surprising and was also observed in the Arhus experiment with trout in Denmark. However, that issue was solved by using a different current to collect and flush ions on the electrodes. There is still a huge potential for fine tuning the CDI unit to target different molecules by using different currents to treat the water. The little OF analysis been done in the project showed a similar increasing behaviour. Meaning that technically OFC were higher in the treated cleaned water. The Danish partners explain this through that OFC are release by the lysis of microorganism under electrical current. The OF water analysis should be done in Sweden, but the analysis was never functioning. We establish OCF measurements now at Nofima by an internal funded project and we hope that we can still analyse samples from this project. In the experiment at the University of Aarhus, with rainbow trout, the CDI unit was tested as a substitute for the biofilter. In this experiment, which was used at a lower voltage than in Sunndalsøra, the application of CDI controlled ammonia, reduced nitrate and kept the nitrite production low. The experiment showed that a CDI unit could be used in purging facilities as nitrogen compound removal method so that feeding under purging conditions would be possible. Something very valuable. The tested coatings to prevent OF microorganism growth were not significant different. However, there is still potential to find better surface coatings to reduce OF in fish farming. In the high organics load treatment, there was far too much biofilm on the settling plates that a potential reduction was difficult to determine. The CDI unit has big potential for water intake harness, nitrate and pathogen removal for RAS facilities. The effluent water of the unit that accumulates ions can make the system valuable for aquaponic systems. The CDI unit concentrates nutrients and minerals for the plants that not been needed in the fish production water anymore. CDI can be an electrical biofilter that could replace the traditional biological filter in freshwater RAS and/or purging units.

Actual outcomes: A internal funding project in Nofima (GB) that target to measure Geosmin and 2-Methylisoborneol for follow up projects. We still intend to measure the stored water samples that our project partners where not able to analyze during the project period. The CDI unit worked as an electrical "bio"filter and harness removal in aquaculture. Especially Nitrate removal showed a significant benefit to conventual denitrification processes that needs chemicals and bacteria to convert Nitrate to Nitrogen. The impact though conferences was that the salmon industry things more about technical purging and ion removal solutions in close containment aquaculture systems. In the anticipated publication we review all innovative electrical filtration units. The biggest impact was to set a starting point for electricidal applications unused in fish farming. This impact was successfully done though networking and talks on international conferences. Potential outcome: Possibly two follow up project with SWT (https://www.stockholmwater.com) and the Freshwater institute in Virginia (https://www.conservationfund.org/our-work/freshwater-institute). Another option for electrical "bio"filtration could be an industry project with the Norwegian based company Matkuling https://matkuling.no/. The Nofima GB project can help to finish a publication of the CDI Nano results in a special issue of Fishes which Kevin Stiller is a guest editor (https://www.mdpi.com/journal/fishes/special_issues/09MP69487J).

Fin fish aquaculture is currently experiencing significant pressure to improve its environmental sustainability to allow it to continue its rapid growth. As a result of these pressures, the industry is increasingly adopting land-based recirculating aquaculture systems (RAS), where water usage is minimised and the risks of release of inbred strains with associated diseases into wild populations is eliminated as are the associated nutrients that cause local environmental problems. Perhaps the biggest barrier to this industry migration to RAS, is the development of off-taste in these systems caused by the accumulation of geosmin and other chemicals to which the human palate is particularly sensitive. These chemicals are produced by microbial communities in biofilms and in the water column. The current solution to this off-taste development is to purge fasting fish for up to 14 days in clean water with associated significant weight and value loss. Traditional antimicrobial techniques to combat the off-taste micro-organisms are unavailable in RAS due to their negative impact on biofilters as well as their toxic effects on the growing fish. This consortium consists of a strong combination of University, RTD and Industrial partners from Denmark, Sweden and Norway with expertise in RAS systems, biofilm development and microbial off-flavour chemical production, as well as nanomaterials and water quality control. The CDI-NANO-RAS project aims at testing novel technologies for the control of off chemicals in RAS as well as purging systems. We hypothesise that the combined use of novel capacitive deionization (CDI) technology and photocatalytic nanotechnological surface coatings can provide an energetically efficient method for the control of off-taste development in RAS as well as in reducing the duration and water usage in pre-harvest purging. The overall aim is to develop cost effective solutions applicable for the modern RAS industry.