Capacitive deionization (CDI) is an environmentally sustainable technology that utilizes low electric voltage to adsorb ions onto electrical double layer region around high surface area electrodes through electric double layer capacitor mechanism at the electrodes-solution interface that is known to be energy-efficient and environmentally sustainable. Capacitive deionization (CDI) is an upcoming technology, positioned to transform the field of cost-effective, low carbon footprint water desalination. The technology has also been examined with municipal wastewater for removing and recovery of nitrogenous and even off flavor compounds. This also gives the option for fish purging and aquaponics applications. It is possible to engineer capacitive electrodes to target specific charged molecules in the input water stream. Our test in RAS water was the first attempt with salmon to exam its usability as a potential bypass biofilter and possible purging applications.
The capacitive deionization system used was from Stockholm Water Technology AB (Sweden) model STROM. Six individual 1000 L MicroRASs (Landing Aquaculture, the Netherlands) at Nofima Sunndalsøra were used for the experiment with Atlantic salmon. It was designed to have a long-term exposure (60 days) of Atlantic salmon to two different organic loads in a RAS experiment defined by using 2 drum filter mesh sizes. The water treatment used 100 µm drum filter for dirty water and the clean water was cleaned using 20 µm drum filters. Technically this should create an environment with 2 different water bodies that do not harm the fish but should facilitate off flavor compound production in the dirty treatment. The portable CDI unit was used 8 hours in each tank every 20 days. Water was pumped out of the fish tank, and treated for 19 cycles in 8 hours using the STROM system. Clean treated deionized water was sent back to the RAS system and effluent (waste, loaded with ions) water was discarded. During the experiment, 494 L of water was flown through the CDI systems resulting in 342 L being recirculated back to the tank generating only 152 L of reject water. Main components analyzed were nitrogen compounds (total ammonia, nitrite, and nitrate). Off-flavor compounds analysis is still ongoing. Water samples were collected in the beginning of the 8 hours treatment period from the fish tank, deionized water- and effluent water outflow. The same was done at the last cycle after 8 hours. Morning and evening fish tank values of the same molecules were compared to test the effect of continuous CDI usage over 8 hours in the RAS systems.
The results concerning the different nitrogen compounds showed that using CDI technology in aquaculture system needs more tuning of the CDI unit treatment variables, like used Voltage on the electrodes. The unit was adjusted for salinification and could do more to reduce the low concentrations of nitrogenous products of the fish and bacterial metabolism. Performance of the CDI system changes over the day so that the TAN removal tends to be better after running for longer hours. Something unexpected was that in all measurements of nitrite concentration after the CDI unit increased, in the treated and effluent water. Nitrate showed the most promising removal efficiency. Mass balancing of the system was not possible since there was always mixing of inflowing and outflowing water during deionization cycles. We will present the development of the testing procedures and learning experiences by doing different experiments. The latest experiments done at the University of Aarhus with Rainbow trout demonstrate finally that the CDI can remove higher N compound concentration much better than lower. Also, it could be shown that at least under low stocking densities the CDI unit can be used as a biofilter substitute.
Since this was after our knowledge the first try to use the system in salmon RAS application, the CDI unit still has a high potential as an ion removal tool. It could be off flavor purging or especially accumulating of nutrients for aquaponic systems. This used voltage could have led to some reactions for oxidizing Ammonia into Nitrite or reduction of Nitrates into Nitrites at the electrode surface. It could also lead to hydrolysis of bacteria and release of off-flavor compounds which we still try to investigate. In summary we will present results for using a CDI system as something like a “electrical biofiltration” unit and present attempts for further applications based on our findings. As the first experiment of its kind for salmon RAS applications, CDI holds significant potential as an ion removal tool for tasks like off-flavor purging and nutrient accumulation in aquaponic systems.
Nordforsk project link