Optimized energy consumption in electrochemical-based regeneration of RAS water

Synergistic mechanisms between chlorine-mediated electrochemical advanced oxidation and ultraviolet light for ammonia removal

The combination of chlorine-mediated electrochemical advanced oxidation (Cl-EAO) and ultraviolet (UV) radiation (UV-E/Cl) can efficiently remove ammonia from wastewater. However, the synergistic mechanisms between Cl-EAO and UV need to be explored in more detail. Thus, in this study, the ammonia oxidation performance of Cl-EAO and UV-E/Cl systems were compared, while the synergistic mechanisms were identified by the performance of UV/chlorine oxidation (UV-ClO) system and the results of electron paramagnetic resonance (EPR) analysis, free radical inhibition assays, and determination of steady-state concentration of free radicals. It was found that, compared with the Cl-EAO system, UV increased the ammonia removal rate by 42.85% and reduced the active chlorine concentration (56.64%) and nitrate yield (53.61%). In the Cl-EAO, and UV-E/Cl systems, Cl• were detected, and the free radical inhibition assays and determination of steady-state concentration of free radicals suggested that UV increased the concentration of Cl• by 51.47%, resulting in Cl• becoming the major contributor to ammonia oxidation in the UV-E/Cl system. Besides, UV also increase the concentrations of HO• and Cl2•-, which further promoted the organic matter removal in the real domestic wastewater. This study also discussed the ammonia oxidation performance of the UV-E/Cl system in real domestic wastewater, even with the presence of significant levels of organic and inorganic anions in the wastewater, UV increased the ammonia oxidation by 21.95%. The results of this study thus clarify the mechanisms and potential applications of UV-E/Cl technology.

Chlorine-mediated electrochemical advanced oxidation process for ammonia removal: Mechanisms, characteristics and expectation

Chlorine-Mediated Electrochemical Advanced Oxidation (Cl-EAO) technology is a promising approach for ammonia removal from wastewater due to its numerous advantages, including small infrastructure, short processing time, easy operation, high security, and high nitrogen selectivity. This paper provides a review of the ammonia oxidation mechanisms, characteristics, and anticipated applications of Cl-EAO technology. The mechanisms of ammonia oxidation encompass breakpoint chlorination and chlorine radical oxidation, although the contributions of active chlorine, Cl, and ClO remain uncertain. This study critically examines the limitations of existing research and suggests that a combination of determining free radical concentration and simulating a kinetic model would help elucidate the contributions of active chlorine, Cl, and ClO to ammonia oxidation. Furthermore, this review comprehensively summarizes the characteristics of ammonia oxidation, including kinetic properties, influencing factors, products, and electrodes. The amalgamation of Cl-EAO technology with photocatalytic and concentration technologies has the potential to enhance ammonia oxidation efficiency. Future research should concentrate on clarifying the contributions of active chlorine, Cl, and ClO to ammonia oxidation, the production of chloramines and other byproducts, and the development of more efficient anodes for the Cl-EAO process. The main objective of this review is to enhance the understanding of the Cl-EAO process. The findings presented herein contribute to the advancement of Cl-EAO technology and provide a foundation for future studies in this field.

Recent advances and applicable flexibility potential of electrochemical processes for wastewater treatment

This study examined >140 relevant publications from the last few years (2018-2021). In this study, classification was reviewed depending on the operation's progress. Electrocoagulation (EC), electrooxidation (EO), electroflotation (EF), electrodialysis (ED), and electro-Fenton (EFN) processes have received considerable attention. The type of action (individual or hybrid) for each electrochemical procedure was evaluated, and statistical analysis was performed to compare them as a new manner of reviewing cited papers providing a massive amount of information efficiently to the readers. Individual or hybrid operation progress of the electrochemical techniques is critical issues. Their design, operation, and maintenance costs vary depending on the in-situ conditions, as evidenced by surveyed articles and statistical analyses. This work also examines the variables affecting the elimination efficacy, such as the applied current, reaction time, pH, type of electrolyte, initial pollutant concentration, and energy consumption. In addition, owing to its efficacy in removing toxins, the hybrid activity showed a good percentage among the studies reviewed. The promise of each wastewater treatment technology depends on the type of contamination. In some cases, EO requires additives to oxidise the pollutants. EF and EFN eliminated lightweight organic pollutants. ED has been used to treat saline water. Compared to other methods, EC has been extensively employed to remove a wide variety of contaminants.

Electrochemical removal of ammonium nitrogen in high efficiency and N2 selectivity using non-noble single-atomic iron catalyst

Ammonia nitrogen (NH₄⁺-N) is a ubiquitous environmental pollutant, especially in offshore aquaculture systems. Electrochemical oxidation is very promising to remove NH₄⁺-N, but suffers from the use of precious metals anodes. In this work, a robust and cheap electrocatalyst, iron single-atoms distributed in nitrogen-doped carbon (Fe-SAs/N-C), was developed for electrochemical removal of NH₄⁺-N from in wastewater containing chloride. The Fe-SAs/N-C catalyst exhibited superior activity than that of iron nanoparticles loaded carbon (Fe-NPs/N-C), unmodified carbon and conventional Ti/IrO₂-TiO₂-RuO₂ electrodes. And high removal efficiency (> 99%) could be achieved as well as high N₂ selectivity (99.5%) at low current density. Further experiments and density functional theory (DFT) calculations demonstrated the indispensable role of single-atom iron in the promoted generation of chloride derived species for efficient removal of NH₄⁺-N. This study provides promising inexpensive catalysts for NH₄⁺-N removal in aquaculture wastewater.

Intensified fish farming. Performance of electrochemical remediation of marine RAS waters

Aquaculture has been the fastest growing agricultural sector in the past few decades and currently supplies about half of the fish market. A range of environmental and management concerns including limited land and water availability have led to intensifying fish production by recirculating aquaculture systems (RAS). Fish's diet contains 30-60 % protein and about 4-10 % nitrogen (N). As fish assimilate only 20-30 % of the feed to produce body mass, the unassimilated N is released in the form of toxic ammonium that deteriorates water quality and compels its degradation. Widely extended biological nitrification is not efficient in the removal of nitrites nor other chemicals and pharmaceuticals used during fish culture. Electrochemical oxidation, a less developed alternative, reports several advantages such as, i) simultaneous degradation of ammonia‑nitrogen (TAN) and water disinfection in the same step with considerable simplification of the whole process, ii) easy adaptability to different production scales and periods of fish growth, and iii) no generation of harmful by-products and no use of chemicals, among others. Besides, in the case of marine aquaculture, the technology benefits from the high conductivity of seawater; thus, electrochemical oxidation is positioned in a very good place to satisfy the water treatment needs of the increasing production rate of marine aquaculture fish. Here, we report the analysis of the performance of a RAS demonstration plant aimed at farming gilthead sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax) and provided with electrochemical remediation of culture water. The performance of the plant, with 20 m³ of seawater operating at a recirculation rate of 0.9-1.4 h^-1, has been analysed in terms of TAN removal, water disinfection, make-up water intake and energy consumption and compared to data of conventional RAS provided with biofilters. The benefits and advantages of the innovative electrochemical remediation of RAS water are highlighted.

IrOx Nanoclusters Modified by BaCO3 Enable ″Two Birds with One Stone″ in Solar-Driven Direct Unbuffered Seawater Electrolysis

Direct seawater electrolysis (DSE) coupled with renewable energy can maximize the sustainability of hydrogen energy acquisition by effectively alleviating the dependence on pure water resources. In a practical sense, the parallel chlorine evolution reaction (CER) of DSE opens up an opportunity to hit ″two birds with one stone″ by the dual values of anode and cathode. However, the biggest challenge is how to control the selectivity of CER to balance its values and drawbacks. Here, we use the different nucleation rates of iridium and barium ions in a weak basic solution and subsequent acid etching to devise an IrO_x nanocluster (IrO_x-Cs) supported BaCO₃. The catalyst-support interaction between IrO_x-Cs and BaCO₃ enables repelling the Cl^- near the electrode interface layer to achieve a controlled CER selectivity. Additionally, the mass activity of the prepared IrO_x-Cs@BaCO₃ is as high as 1402 A g^-1_Ir, which is 7.12 times higher than that of IrO₂ oxides in unbuffered seawater. The photovoltaic-electrolysis device paired by IrO_x-Cs@BaCO₃ with controlled CER activity and Pt demonstrated that valuable active chlorine and H₂ can be simultaneously obtained, with the flexibility to bind to different ion exchange membranes.

Electro-enhanced chlorine-mediated ammonium nitrogen removal triggered by an optimized catalytic anode for sustainable saline wastewater treatment

Electrochemical technology has unique superiorities in chlorine-mediated pollutant oxidation, but has limited application in saline wastewater treatment due to inadequate efficiency and high energy consumption. To promote electrochemical oxidation capacity, a novel but low-cost electrode containing TiO₂/Co-WO₃/SiC was prepared and optimized, achieving highly efficient chlorine-mediated ammonium nitrogen oxidation (98.3 ± 2.2% in 120 min, with initial NH₄⁺-N of 10.2 ± 0.5 mg L^-1) in a simple electrochemical system with supplied current density only at 1.00 mA cm^-2. Comparing with unmodified carbon fiber cloth, the catalytic anode achieved 96.0% nitrogen selectivity, enhanced the system current efficiency by 20.6% and reduced the energy consumption by 54.4%, making the treatment of simulated mariculture wastewater both energy-saving (36.5 ± 2.8 kWh kg^-1 NH₄⁺-N) and cost-effective (1.45 US$ m^-3), comparing with previously reported electrochemical processes (54-622 kWh kg^-1 NH₄⁺-N). The nitrogen content (<1 mg L^-1) in the treated wastewater, containing only 0.18 mg L^-1 NH₄⁺-N, meets the discharge standard of mariculture wastewater. The promoted electrochemical oxidation should be attributed to the chloride derived species (HOCl and ClO^-) and related active species (Cl, ClO, OH, etc.). This easily prepared and reusable catalytic electrode is a promising alternative to conventional anode materials in sustainable electrochemical treatment of saline wastewater.