Sammendrag
Capacitive deionization (CDI) is a technology that works based on an electrochemical method that has been known to be energy-efficient and environmentally sustainable. This technique is becoming popular because of its ability to remove ions from water cheaply and energy efficient compared to reverse osmosis. 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 a CDI unit in a way to target specific charged molecules in the water. Our test in RAS water was the first attempt with salmon to exam its usability as an electronic bypass biofilter and possible purging applications.
The capacitive deionization machine used was Stockholm Water Technology’s (Sweden) STROM unit. Six individual 1000 L MicroRASs (Landing Aquaculture, the Netherlands) at Nofima Sunndalsøra were used for the experiment. 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 dirty treatment was fitted with 100 µm and the clean treatments were fitted with 20 µm drum filter meshes. Technically this should create an environment with 2 different water bodies that do not harm the fish but should facilitate off flavor production in the dirty treatment. The portable CDI unit was used 8 hours in each tank every 20 days. Water was sucked out of the fish tank, treated 19 cycles in 8 hours. Clean treated unionized water was sent back to the RAS system and effluent (waste, loaded with ions) water was discarded. Total water treated during the experimental period = 494 L. Total water returning to the tank = 342 L. Total water leaving as effluent = 152 L. Main components that been analyzed were nitrogen compounds (total ammonia nitrogen, nitrite, and nitrate). Off-flavor compounds analysis are still ongoing. Water samples were taken in the beginning of the 8 hours treatment period from the fish tank, unionized 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 CDU 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 aways 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 Ahus 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.
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