62 JUNE 2023 • WORLD AQUACULTURE • WWW.WAS.ORG RAS technologies that rely on electrochemical water treatment for transforming ammonia excreted by fish into innocuous nitrogen gas have been suggested (Gendel and Lahav 2012) and shown to generally provide good water quality, satisfactory growth rate and cost competitiveness. The main incentive to apply electrooxidation processes in aquaculture is the one-step ammonia oxidation directly to N2(g). The process in saline-water RAS (Fig. 1) is based on maintaining relatively high NH4 + concentration in the rearing water, along with neutral pH, calculated to maintain NH3 below the chronic toxicity threshold, commonly 0.05 mg N/L for fish. Water flowing out of the fish tank is collected in two treatment tanks (Figs. 1A and 1B). When a treatment tank is full, it is disconnected from the fish tanks and undergoes batch-mode electrolysis in which total ammonia-nitrogen (TAN) is oxidized completely by the Cl2(aq) species formed on the anode due to chloride (Cl−) electrooxidation. Chloride is the major anion present in any saline water. During the electrolysis period in the first treatment tank, the flow from the fish tank is directed to the second treatment tank. Once the electrolysis step is finished in the first treatment tank, the TAN-devoid and disinfected water undergoes a dechlorination step to ensure that no residual chlorine or chloramine are sent to the fish tanks. The electrolysis step is operated to remove the exact daily mass of TAN released by the fish, thereby maintaining a constant TAN concentration in the fish tank. The process necessitates an efficient solids separation step to ensure that the solids retention time in the fish tanks would result in minimum growth of autotrophic (i.e., nitrifying) bacteria in the fish tank water. The process concept is based on a mechanism called indirect ammonia electrooxidation, or in simple terms, a direct oxidation of NH4 + into N2(g) via its reaction with electrochemically generated active chlorine (Eq. 1), which renders nitrification and denitrification unnecessary. Upon its reaction with NH4 + (Eq. 2), active chlorine A typical modern RAS is comprised of nine water treatment units: solids separation, CO2 degasification, nitrification reactor (biofilter) and an oxygen enrichment system, with additional water treatment steps commonly required to reduce water consumption up to “zero discharge”: foam fractionator, denitrification reactor, dosing pump to add chemicals for alkalinity compensation, disinfection unit and a sludge thickener. These components, each with its own operational requirements and specifications, ideally should be synchronized with the others to achieve the required water quality at a reasonable cost. To date, available RAS technologies suffer from several limiting factors restricting their wide application. Biological nitrogen removal is often identified as the weakest chain link in the RAS water treatment process. This is evident in operation of nitrification and denitrification bioreactors, which often tends to be unpredictable or unstable, thereby raising uncertainties over performance reliability over time (Graham et al. 2007). In addition, nitrification reactors are the main source of offflavor agents, which lead to one of the major challenges faced by near-zero-discharge RAS (Lindholm-Lehto and Vielma 2019). Further, nitrification biofilters have a high oxygen demand, require long start-up times, especially in cold water, and are a potential source for the proliferation of pathogenic bacteria. High capital costs are particularly apparent in RAS focused on coldwater fish, which requires large biofilter surface areas. Consequently, robust, environmentally friendly and economically feasible alternatives for biological reactors are crucially sought for zero-discharge RAS (Ben-Asher and Lahav 2016). Ammonia Electrooxidation Electrooxidation of dissolved ammonia has been documented since the early 1960s. Marinčić and Leitz (1978) were the first to propose this process in the context of wastewater treatment. A Novel Approach for Recirculating Aquaculture Systems (RAS) Igal Magen FIGURE 1. Schematic description of the suggested TAN operation approach. A and B are intermediate tanks, operating alternately as electrolysis or water receiving tanks (Ben Asher 2016). FIGURE 2. Defense mucous cell size. Group 1: BioFishency ELXTM and Group 2: Reference biofilter solution.