In recirculating water systems for aquaculture, biological treatment methods are considered the primary means of removing toxic metabolites from the water.
Electrochemical water treatment for recirculating aquaculture systems (RAS) is a promising approach to replace biological treatment methods and usher in a new generation of RAS with higher profitability, lower environmental footprint, and no startup periods.
A team of researchers from the Faculty of Civil and Environmental Engineering published a scientific review providing an overview of the incentives for implementing electrochemical methods in Recirculating Aquaculture Systems (RAS).
The study covers the relevant electrochemical principles for aquaculture applications, the effects of physical and chemical parameters, as well as design considerations. Additionally, the researchers review the research conducted to date on integrating electrochemical methods in RAS operation and describe the variety of designs and operational configurations.
Electro-oxidation of Ammonia
The electrochemical conversion of ammonia to N2 requires an exchange of only three electrons per NH3 molecule and can be achieved with very high faradaic efficiency, resulting in a lower theoretical energy consumption compared to conventional nitrification-denitrification biological systems.
Electrochemical water treatment is considered more beneficial than biological water treatment, especially in cold saltwater aquaculture (e.g., Atlantic salmon) where large nitrification reactors are required and significant water consumption for purging processes can be reduced. It is also beneficial for the cultivation of species sensitive to nitrates (e.g., Litopenaeus vannamei).
Potential Benefits of the Electrooxidation Process
Among the advantages of electrochemical water treatment, direct oxidation of ammonia into N2 is highlighted. Furthermore, electrochemical oxidation results in effective disinfection and removal of organic matter, including specific organic components such as off-flavor agents.
Ammonia Electrooxidation
The researchers classify ammonia electrooxidation processes into three categories based on the oxidation mechanisms and/or the method of introducing the oxidant:
- Direct anodic oxidation;
- Indirect electrooxidation of ammonia; and
- Breakpoint chlorination (also known as chemical chloramination).
The performance of the electrochemical reactor for ammonia electrooxidation depends on electrode potentials, electrode materials, current density, distance between electrodes, composition of the electrolyzed solution, temperature, and other parameters.
Electrolytic Disinfection
The most common disinfection methods in aquaculture are ozone-based treatments and ultraviolet light. However, the capital and operational costs of these treatments are relatively high.
Unlike ozone and ultraviolet light, the disinfection effect achieved by an electrochemical reactor results from a combination of various different disinfection effects.
“When an electrolytic solution (the water from the RAS) passes through an electrochemical reactor, it is exposed to an electric field, extreme redox conditions, and a variety of radical species,” they reported.
However, electrochemical disinfection in recirculating aquaculture systems is primarily mentioned as an advantageous secondary effect of ammonia electrooxidation, as water is disinfected in parallel with the ammonia oxidation action.
Electrochemical Oxidation of 2-Methylisoborneol and Geosmin
Among the wide spectrum of organic molecules that accumulate in RAS water, 2-methylisoborneol (MIB) and geosmin are of utmost interest to fish farmers because they are responsible for the earthy flavor in fish.
Studies addressing the electrochemical decomposition of MIB and/or geosmin are scarce; however, there is evidence that efficient oxidation of these compounds requires the presence of hydroxyl radicals.
The authors of the study cite research suggesting that UV-assisted photoelectrochemical oxidation can enhance the decomposition efficiency of both 2-methylisoborneol and geosmin, providing a better and more cost-effective outcome.
Conclusions
“This review presents ammonia electrooxidation as a powerful water treatment method and presents it as a viable and cost-effective alternative to biological processes,” they conclude.
They indicate that they do not suggest electrochemical methods as a categorical substitute for biological water treatment, but they demonstrate that it may be preferable for various applications and under certain conditions, such as saltwater aquaculture.
They also detail that, “The disinfection characteristic of electrooxidation has not yet been sufficiently defined in the context of aquaculture applications. The weight of each participating disinfection mechanism (electric field, presence of radicals, presence of chlorine, extreme redox and pH conditions) within the total disinfection effect, and the operational conditions required to achieve and maintain an effective disinfection effect, are not fully elucidated.“
Reference (open access)
Ben-Asher, R, Gendel, Y, Lahav, O. Electrochemical applications in RAS: A review. Rev Aquac. 2023; 1- 20. doi:10.1111/raq.12822
