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

Effects of salinity on anaerobic digestion: Performance, microbial physiology, and community dynamics

Anaerobic digestion (AD) is widely applied to treatment and energy recovery from organic wastewater/wastes, while the efficiency of AD can be limited by salinity stress. This paper reviews the effects of salinity on AD. First of all, the effects of salinity on AD performance were compared, revealing that methane production is more susceptible to salinity stress. Secondly, the influence of salinity on microbial physiology and intracellular molecules was examined, demonstrating that salinity stress reduces the activity of key enzymes and increases the concentration of extracellular polymeric substances during AD. Thirdly, variations in microbial community structure under salinity stress were discussed, with archaeal communities showing more significant restructuring, including reduced dominance of acetoclastic methanogens. At last, strategies to mitigate salinity inhibition were presented, along with prospects for future research directions. This review provides theoretical guidance for engineering applications and strategies for enhancing AD in treating saline substrates.

Selective oxidation of ammonium to nitrogen with VUV/UV/Cl⁻ process: Efficiency, pathway and mechanism

Conversion of chloride ions (Cl⁻) into reactive chlorine species (RCS) is an effective strategy for ammonium (NH4⁺-N) selective oxidation to nitrogen (N2) under high salinity conditions. Herein, vacuum ultraviolet (VUV) irradiation was introduced for NH4⁺-N removal in simulated recirculating mariculture systems (RMS) water treatment. Complete oxidation of NH4⁺-N and 88.3 % N2 selectivity were achieved for VUV/UV/Cl⁻ process. Mechanism analysis revealed that Cl⁻ were effectively converted into RCS under VUV irradiation and chlorine oxide radical (ClO•) was the predominant RCS responsible for NH4+-N removal. The pathway of NH4+-N oxidation was proposed as chlorination because chloramine was identified as the main intermediate. Influence factor investigation indicated that Cl⁻ and bicarbonate (HCO3⁻) could significantly promote the removal of NH4+-N in VUV/UV/Cl⁻ process due to acceleration of ClO• generation. Ultimately, the NH4+-N removal performance of VUV/UV/Cl⁻ process in practical application was also investigated. The results showed that not only NH4+-N in actual seawater or RMS could be converted effectively to N2, but also nitrite (NO2⁻-N) and partial nitrate (NO3⁻-N) could be removed efficiently by VUV/UV/Cl⁻ process. Hence, the VUV/UV/Cl⁻ process has promising potential in NH4+-N and total nitrogen (TN) removal for RMS water treatment.

Insight into pH-dependent synergistic mechanism of the enhanced photoelectro-chlorine conversion of ammonia (NH4+/NH3) to N2 coupled with H2 evolution on Pt-decorated black TiO2 nanotube arrays

Achieving efficient NH4+-N degradation into harmless N2 and simultaneous harvesting hydrogen energy is of great significance for protecting aquatic ecosystems and alleviating energy shortages. Herein, a photoelectrochemical-chlorine (PEC-Cl) system was developed aiming to selective converting ammonia to N2 with simultaneous H2 evolution in a wide pH range of 3-13. The Pt decorated black TiO2 nanotube arrays (Pt/BTNAs) with sufficient oxygen vacancies defects and the enhanced PEC properties were synthesized as the bifunctional electrode. The parameters optimization illustrated high N2 selectivity (88 %) and H2 yield rate (67.5 μmol cm-2 h-1) at pH 9. The ClO• derived from the cross reactions between active chlorine and HO• plays a predominant role in oxidizing ammonia. More importantly, the Gibbs free energy calculation indicated that Pt decorating can also significantly promote the direct electrochemical ammonia oxidation reaction (AOR) on Pt/BTNAs anode. Under alkaline conditions (pH 9-13), the efficient removal and conversion of ammonia into N2 in the PEC-Cl system was achieved primarily through the direct anodic AOR, with N2 selectivity (89 %-93 %) and H2 yield rate (60.9-68.6 μmol cm-2 h-1) comparable to those of the ClO•-mediated indirect AOR. This study presents a flexible method for treating high and medium-concentration NH4+-N and simultaneously producing H2 through the synergistic PEC-Cl and direct electrochemical AOR processes, across a wide pH range.

Optimization of electro-Fenton process applied to the treatment of codfish brines in a context of industrial symbiosis

The ancient method of preserving fish by salting is still widely practiced but generates two challenging waste streams: contaminated salt (solid) and brine (liquid). Conventional treatment methods are ineffective in reducing the organic content of brine due to its high salt content (≅ 25-30 % wt. NaCl). Although advanced oxidation processes are extensively used for treating certain wastewaters, their application to real saline effluents near saturation, such as food industry brines, remains underexplored. This study optimized the electro-Fenton (EF) process for treating real contaminated brines from the codfish industry, aiming to reuse the treated brines in the pickling stage of the tannery industry, thereby diverting waste streams from environment disposal. A central composite experimental design and response surface methodology were used to evaluate the effects of three EF process operating variables: (i) current density (76-429 A m-2), (ii) electrolysis time (1.0-10.0 min), and (iii) hydrogen peroxide concentration ([H2O2], 50-201 mM), using iron electrodes. The primary goal was to maximize total organic carbon (TOC) removal from codfish brine. Additionally, the specific roles of reactive oxygen and chlorine species responsible for TOC removal (such as HO•, Cl•, ClO, O2•-/HO2• and HClO/OCl-) were investigated using scavengers. The results revealed that O2•-, HO2• were the main active species. The optimal EF operating conditions were determined to be a current density of 275 A m-2, electrolysis time of 5.2 min, and [H2O2] of 91 mM, resulting in a 70 % removal of TOC. The treated brines, diluted to ≅ 7.5-8.0 % wt. NaCl, were tested in hide pickling trials to assess their impact on the quality of the wet-blue leathers. Results showed that the treated brines did not affect leather quality; instead, they enhanced shrinkage temperature from 103 °C to 112 °C. This increase of shrinkage temperature broadens the potential applications of the leather, making it suitable for a wider range of markets and products. Furthermore, the chromium oxide content fixed in the leather increased from 4.1 % to 5.3 %, reducing chromium in the industrial wastewater generated at the end of the process. This valorisation of codfish brines presents a promising opportunity for industrial symbiosis between the codfish and the tannery industries.

A novel magnetic adsorption and capacitive deionization coupled technology for industrial saline wastewater recycling

The cost-effective desalination technologies were urgent needed to recycle industrial saline wastewater, desalinate seawater and brackish water. Deionisation techniques based on the adsorption principle usually suffer from low adsorption capacity of the adsorbent, susceptibility to contamination, regeneration difficulties and secondary contamination. In this paper, the magnetic reduced graphene oxide (mrGO) was successfully prepared as magnetic media, and a novel magnetic adsorption deionization and capacitive deionization coupled system (MDI-CDI) was constructed, in which a superposition magnetic field with consistent direction was formed by the internal additional magnetic field of magnetic media and the external magnetic field. The relationship between various salt solutions, initial concentration, operation patterner and deionization effect were investigated by KCl solution to optimize the MDI system. The actual petrochemical circulating wastewater (0.933 mS/cm), were adopted to investigate the magneto-electric coupling effect of MDI-CDI system, the average desalination rate and COD removal were 96.9 % and 84.8 %, respectively. In addition, the three-stage tandem MDI system was adopted to investigate the enhanced magnetic adsorption deionization effect, which was 79.3 % of catalytic cracking wastewater (37.4 mS/cm) and 84.0 % of petrochemical wastewater (3.68 mS/cm), respectively. The results indicate that the main deionization mechanisms of MDI system were enhanced by a superimposed magnetic field, including physical adsorption, magnetic attraction, electrostatic attraction, and surface complexation/deposition effects. The MDI-CDI coupled deionisation system can mitigate membrane contamination, regenerate online without secondary pollution under low-consumption, high-efficiency and stable state, providing a new technological idea for the regeneration and utilization of saline wastewater.

Space heterogeneity of nitrogen removal functional genes, nitrogen transformation pathways and mechanisms in MEBR treating mariculture wastewater

Membrane Electro-Bioreactor (MEBR), exhibited excellent nitrogen removal in mariculture wastewater treatment. However, the differences of microbial community and nitrogen transformation pathways on spatial scale caused by the mass transfer of reactive chlorine species (RCS) generated by electrooxidation were unclear. This study provided new insights into the space heterogeneity of ammonia transformation pathways and mechanisms. The results demonstrated an increase in the reduction of nitrate to nitrite through partial denitrification on the membrane. Coupling with dissimilatory nitrate reduction to ammonium and RCS, higher nitrate (0.3 mgN/L) and TN removal (2.6 mgN/L) was obtained in MEBR. Higher relative abundance of narGHI and nirB were obtained. Ammonia oxidation by biofilters was enhanced in MEBR, that ammonia removal contributed by biofilters was 32.0 %, 5.2 times higher than the membrane. Relative abundance of amoABC and Nitrosomonas in biofilters were the highest. The results will provide theoretical basis for reactor configuration and operation optimization.

Efficient electrochemical removal of ammoniacal nitrogen from livestock wastewater: The role of the electrode material

Wastewater from livestock farms contains high concentrations of suspended solids, organic contaminants, and nitrogen compounds, such as ammoniacal nitrogen. Discharging livestock effluents into water bodies without appropriate treatment leads to severe environmental pollution. Compared to conventional treatment methods, electrochemical oxidation exhibits higher nitrogen removal efficiencies. In the present work, the electrochemical removal of ammoniacal nitrogen from real livestock wastewater was investigated through a lab-scale reactor. Preliminary experiments were carried out to investigate the effects of different anode materials, including boron-doped diamond and iridium/ruthenium-coated titanium, on the total nitrogen removal efficiency using synthetic wastewater. Boron-doped diamond, a well-known non-active electrode, allowed to obtain 63.7 ± 1.21 % of total nitrogen degradation efficiency. However, the iridium/ruthenium-coated titanium electrode, belonging to the class of active anodes, showed a higher performance, achieving 78.8 ± 0.76 % contaminant degradation. Coupling iridium/ruthenium-coated titanium anode with a stainless-steel cathode improved the performance of the system, achieving even 96.2 ± 2.73 % of total nitrogen removal. The optimized cell configuration was used to treat livestock wastewater, resulting in the degradation of 67.0 ± 2.25 % of total nitrogen and 37.3 ± 0.68 % of total organic carbon when sodium chloride was added. At the end of the process, the ammonium content was completely removed, and only 17.7 ± 0.51 % of the initial nitrogen turned into nitrate. The results show that the proposed system is a promising approach to treating livestock wastewater by coupling high contaminant removal efficiencies with low operational costs. Anyway, further studies on process optimization with an emphasis on power requirements and electrode costs need to be carried out.

In situ electro-generated Ni(OH)2 synergistic with Cu cathode to promote direct ammonia oxidation to nitrogen

To solve the problem of low removal rate and poor N2 selectivity in direct electrochemical ammonia oxidation (EAO), commercial Ni foam and Cu foam were used as anode and cathode of the EAO system, respectively. The coupling effect between the cathode and anode promoted nitrogen cycling during the reaction process, which improved N2 selectivity of the reaction system and promoted it to achieve a high ammonia removal rate. This study showed that the thin Ni(OH)2 with oxygen vacancy formed on the surface of Ni foam anode played an effective role in the dimerization of intermediate products in ammonia oxidation to form N2. This electrochemical system was used to treat real goose wastewater containing 422.5 mg/L NH4+-N and 94.5 mg/L total organic carbon (TOC). After treatment, this electrochemical system achieved good performance with an ammonia removal rate of 87%, N2 selectivity of 77%, and TOC removal rate of 72%. Therefore, this simple and efficient system with Ni foam anode and Cu foam cathode is a promising method for treating ammonia nitrogen wastewater.

Electro-oxidation of ammonia nitrogen using W, Ti-doped IrO2 DSA as a treatment method for mariculture and livestock wastewater

Animal farming wastewater is one of the most important sources of ammonia nitrogen (NH4+-N) emissions. Electro-oxidation can be a viable solution for removing NH4+-N in wastewater. Compared with other treatment methods, electro-oxidation has the advantages of i) high removal efficiency, ii) smaller size of treatment facilities, and iii) complete removal of contaminant. In this study, a previously prepared DSA (W, Ti-doped IrO2) was used for electro-oxidation of synthetic mariculture and livestock wastewater. The DSA was tested for chlorine evolution reaction (CER) activity, and the reaction kinetics was investigated. CER current efficiency reaches 60-80% in mariculture wastewater and less than 20% in livestock wastewater. In the absence of NH4+-N, the generation of active chlorine follows zero-order kinetics and its consumption follows first-order kinetics, with cathodic reduction being its main consumption pathway, rather than escape or conversion to ClO3-. Cyclic voltammetry experiments show that NH4+-N in the form of NH3 can be oxidized directly on the anode surface. In addition, the generated active chlorine combines with NH4+-N at a fast rate near the anode, rather than in the bulk solution. In electrolysis experiments, the NH4+-N removal rate in synthetic mariculture wastewater (30-40 mg/L NH4+-N) and livestock wastewater (~ 450 mg/L NH4+-N) is 112.9 g NH4+-N/(m2·d) and 186.5 g NH4+-N/(m2·d), respectively, which is much more efficient than biological treatment. The specific energy consumption (SEC) in synthetic mariculture wastewater is 31.5 kWh/kg NH4+-N, comparable to other modified electro-catalysts reported in the literature. However, in synthetic livestock wastewater, the SEC is as high as 260 kWh/kg NH4+-N, mainly due to the suppression of active chlorine generation by HCO3- and the generation of NO3- as a by-product. Therefore, we conclude that electro-oxidation is suitable for mariculture wastewater treatment, but is not recommended for livestock wastewater. Electrolysis prior to urea hydrolysis may enhance the treatment efficiency in livestock wastewater.

Ammonia oxidation by in-situ chloride electrolysis in etching wastewater of semiconductor manufacturing using RuSnOx/Ti electrode: Effect of plating mode and metal ratio

The indirect chloride-mediated ammonia oxidation encounters challenges in maintaining the effectiveness of metal oxide anodes when treating wastewaters with complex compositions. This study aims to develop a highly stable anode with RuO2-SnO2 coatings for treating an etching effluent from semiconductor manufacturing, which majorly contains NH3 and organic compounds. The RuSnOx/Ti electrode was synthesized using wet impregnation and calcination processes. The metal oxide configuration on Ti plate substrate was tuned by varying the step-dipping process in RuCl3 and SnCl4 baths. A 10-day continuous-flow electrolysis was conducted for studying the ammonia removal and chlorine yield under variable conditions, including detention, pH, current density, and initial ammonia and chloride concentrations. In the RuSnOx coatings, the configuration comprising RuO2 nanorods as the surface layer and an intermediate layer of SnO2 crystallites (by plating Ru3+ for three times to cover one Sn4+ layer, denoted as the Ru3Sn/Ti electrode) exhibited the best durability for acid washing, along with relatively high Faradaic efficiency and low energy consumption. To further improve the treatability of real wastewater (NH3-N = 634 mg L-1, chemical oxygen demand (COD) = 6700 mg L-1, Cl- = 2000 mg L-1, pH 11), the duel-cell electrolyzers were constructed in series under a current density of 30 mA cm-2 and 45 min detention. Ultimately, removals of NH3 and COD reached 95.8% and 76.3%, respectively, with successful limitation of chloramine formation.

Comparison of moving bed biofilm reactor and bio-contact oxidation reactor start-up with heterotrophic nitrification-aerobic denitrification bacteria and activated sludge inoculation under high ammonia nitrogen conditions

To overcome poor ammonia tolerance and removal performance of bio-contact oxidation (BCO) reactor inoculated with activated sludge for high-ammonia nitrogen (NH4+-N) chemical wastewater treatment, this study compared inoculating heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria in moving bed biofilm reactor (MBBR) with activated sludge inoculation in BCO reactor under simulated high NH4+-N conditions. Results revealed that MBBR achieved faster biofilm formation (20 days vs. 100 days for BCO) with notable advantages: 27.6 % higher total nitrogen (TN) and 29.9 % higher NH4+-N removal efficiency than BCO. Microbial analysis indicated optimal enrichment of the key nitrogen removal (NR) bacterium Alcaligenes, leading to increased expression of NR enzymes hydroxylamine reductase, ensuring the superior NR efficiency of the MBBR. Additionally, functional enzymes and genes analysis speculated that the NR pathway in MBBR was: NH4+-N → NH2OH → NO3--N → NO2--N → NO → N2O → N2. This research offers a practical and theoretical foundation for extending HN-AD bacteria-inoculated MBBR processes.

Boosted Electrocatalytic Degradation of Levofloxacin by Chloride Ions: Performances Evaluation and Mechanism Insight with Different Anodes

As chloride (Cl-) is a commonly found anion in natural water, it has a significant impact on electrocatalytic oxidation processes; yet, the mechanism of radical transformation on different types of anodes remains unexplored. Therefore, this study aims to investigate the influence of chlorine-containing environments on the electrocatalytic degradation performance of levofloxacin using BDD, Ti4O7, and Ru-Ti electrodes. The comparative analysis of the electrode performance demonstrated that the presence of Cl- improved the removal and mineralization efficiency of levofloxacin on all the electrodes. The enhancement was the most pronounced on the Ti4O7 electrode and the least significant on the Ru-Ti electrode. The evaluation experiments and EPR characterization revealed that the increased generation of hydroxyl radicals and active chlorine played a major role in the degradation process, particularly on the Ti4O7 anode. The electrochemical performance tests indicated that the concentration of Cl- affected the oxygen evolution potentials of the electrode and consequently influenced the formation of hydroxyl radicals. This study elucidates the mechanism of Cl- participation in the electrocatalytic degradation of chlorine-containing organic wastewater. Therefore, the highly chlorine-resistant electrocatalytic anode materials hold great potential for the promotion of the practical application of the electrocatalytic treatment of antibiotic wastewater.

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.

Detection and removal technologies for ammonium and antibiotics in agricultural wastewater: Recent advances and prospective

With the extensive development of industrial livestock and poultry production, a considerable part of agricultural wastewater containing tremendous ammonium and antibiotics have been indiscriminately released into the aquatic systems, causing serious harms to ecosystem and human health. In this review, ammonium detection technologies, including spectroscopy and fluorescence methods, and sensors were systematically summarized. Antibiotics analysis methodologies were critically reviewed, including chromatographic methods coupled with mass spectrometry, electrochemical sensors, fluorescence sensors, and biosensors. Current progress in remediation methods for ammonium removal were discussed and analyzed, including chemical precipitation, breakpoint chlorination, air stripping, reverse osmosis, adsorption, advanced oxidation processes (AOPs), and biological methods. Antibiotics removal approaches were comprehensively reviewed, including physical, AOPs, and biological processes. Furthermore, the simultaneous removal strategies for ammonium and antibiotics were reviewed and discussed, including physical adsorption processes, AOPs, biological processes. Finally, research gaps and the future perspectives were discussed. Through conducting comprehensive review, future research priorities include: (1) to improve the stabilities and adaptabilities of detection and analysis techniques for ammonium and antibiotics, (2) to develop innovative, efficient, and low cost approaches for simultaneous removal of ammonium and antibiotics, and (3) to explore the underlying mechanisms that governs the simultaneous removal of ammonium and antibiotics. This review could facilitate the evolution of innovative and efficient technologies for ammonium and antibiotics treatment in agricultural wastewater.

Device Testing: High-Efficiency and High-Uniformity Microwave Water Treatment System Based on Horn Antennas

Microwave heating has excellent potential for applications in wastewater treatment. This study proposes a highly efficient continuous liquid-phase microwave heating system to overcome the problems of low treatment capacity, low dynamic range of loads, and insufficient heating uniformity of the existing equipment. First, a quarter-wavelength impedance-matching layer improves heating efficiency, and the heating uniformity has been enhanced by horn antennas. Second, an experimental system is developed. The simulation and experimental results are consistent, with the microwave system achieving over 90% energy utilization for different thicknesses and concentrations of salt water. Finally, simulations are performed to analyze microwave efficiency and heating uniformity at different flow rates, salinities, dielectric properties, and sawtooth structures. The system can efficiently heat loads with a wide range of dielectric properties, including saline water. Generally, when the permittivity varies from 10 to 80, and the loss tangent varies dynamically from 0.15 to 0.6, more than 90% of microwave efficiency and excellent temperature distribution (The coefficient of temperature variation COV < 0.5) can be achieved. The system’s modular design enables scaling up to further boost processing capacity. Overall, the system provides high-throughput, high-efficiency, high-uniformity, and large-dynamic-range microwave water treatment, which has promising applications in industrial water treatment.

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.

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L^-1 resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L^-1 at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L^-1. The nitrogen degradation mechanism proposed assumes a series-parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃^- was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater.

Study and actual application of the electrochemical reactor in flow-through mode based on channel confinement

The method of enhancing mass transfer and improving reaction efficiency by confinement has attracted much attention in the electrochemical research field. In this research, to make low diffusion-limited electrochemical reactors fieldable, a new electrochemical reactor in flow-through mode was established with the mass-produced Ti/RuO₂-IrO₂ felt fibers as the electrodes. The effects of voltage, current, and electrode thickness were explored in this study. When the flow mode was switched from flow-by to flow-through, the single-pass degradation effect of rhodamine B rose from 4.4% to 74.8% under the same operating conditions. Meanwhile, a mass transfer model was established based on the results of removal efficiency and electrode channel parameters. The model was in good agreement with the new electrode parameters verification (R² > 0.970). With this model, it could derive specific results on the effect of pore size change on the treatment effect. The impact of enhancing mass transfer by confining the pore sizes is most clearly gained at a certain range (less than 100 μm). Furthermore, a pilot-scale electrochemical reactor in flow-through mode was built, and excellent performance was shown in the treatment of actual waste leachate. The removal efficiencies of total nitrogen, ammonia, and nitrate were 80.9%, 88.6%, and 64.5% in 30 min, respectively. It will be a promising technology with good prospect.

Simultaneous removal of organics and ammonia using a novel composite magnetic anode in the electro-hybrid ozonation-coagulation (E-HOC) process toward leachate treatment

To achieve simultaneous organics and ammonia (NH₄⁺-N) removal toward leachate treatment, this study designed a composite anode (CA⁺), in which iron powders were attracted to RuO₂-IrO₂/Ti tube surface by an inserted magnet and utilized in electro-hybrid ozonation-coagulation (E-HOC). The E-HOC (CA⁺) resulted in higher chemical oxygen demand (COD) and NH₄⁺-N removal with most content of CO₂/H₂O and gaseous N in product compared with E-HOC (Fe⁺), electrolysis ozonation and single ozonation. Reactive chlorine species (RCS) and coagulants were co-produced by compositing RuO₂-IrO₂/Ti and Fe powders, resulting in multiple reactions including electrocoagulation, ozone oxidation, synergistic between ozone and coagulants (SOC), electrolytic chloride and synergistic oxidation between active chlorine and ozone (SCO) occurred. Hydroxyl radical (•OH) generated through SOC reaction was promoted due the RCS generation in E-HOC. The interaction between •OH and Cl^-/ClO^- also contributed to enhanced Cl•/ClO• production. Consequently, synergy of chlorine, coagulants and ozone enhanced reactive species generation which contributed to favorable organics and NH₄⁺-N removal. Enhanced •OH and RCS are also attributed to conversion of bio-refractory organics like polyphenol, polycyclic aromatics and S-containing to biodegradable ones, e.g., aliphatic compounds and CHO. This study provides an easily operating strategy for leachate treatment with high content organics and NH₄⁺-N.

Performance enhancement, membrane fouling mitigation and eco-friendly strategy by electric field coupled membrane bioreactor for treating mariculture wastewater

An eco-friendly strategy for mariculture wastewater treatment using an electric field attached membrane bioreactor (E-MBR) was evaluated and compared with a conventional membrane bioreactor (C-MBR). The removal efficiencies of total nitrogen (TN) and chemical oxygen demand (COD) increased significantly and the membrane fouling rate reduced by 44.8% in the E-MBR. The underlying mechanisms included the enriched nitrifiers and denitrifiers, the enhanced salinity-resistance, the increased activities and upregulated genes of key enzymes involved in nitrification and denitrification for improving the performance of mariculture wastewater treatment, and the enriched extracellular polymeric substance (EPS)-degrading genera, the downregulated EPS biosynthesis genes, the repressed biofilm-forming bacteria, the enhanced zeta potential absolute value and the generated H₂O₂ for membrane fouling mitigation by electrical stimulation. Compared with the C-MBR, the energy consumption, carbon emissions, and nitrogen footprint were reduced. These findings provide novel insights into mariculture wastewater treatment using an applied electric field.

A novel electrocatalytic system with high reactive chlorine species utilization capacity to degrade tetracycline in marine aquaculture wastewater

The problems of high salinity and coexistence of antibiotics in mariculture wastewater pose a great challenge to the traditional wastewater treatment technology. Herein, an electrocatalytic system based on cathodes to sustain reactive chlorine species (RCS) in a high chlorine environment was proposed. The results show that the content of RCS is affected by cathodes. The electrocatalytic system with FeNi/NF as cathode has the largest RCS retention capacity when compared with other cathode systems (carbon felt, nickel foam, copper foam, stainless steel, and nickel-iron foam). This is related to FeNi/NF's higher hydrogen production activity, which inhibits the reduction reaction of RCS. Furthermore, the degradation of tetracycline by the proposed FeNi/NF system maintained long-term effective performance across 20 cycles. Thus, the application of high chlorine resistance electrocatalysis system provides a possibility for practical electrocatalysis treatment of mariculture wastewater.