Electrochemical disinfection and removal of ammonia nitrogen for the reclamation of wastewater treatment plant effluent

In situ generation of highly localized chlorine by laser-induced graphene electrodes during electrochemical disinfection

Laser-induced graphene (LIG) has gained popularity for electrochemical water disinfection due to its efficient antimicrobial activity when activated with low voltages. However, the antimicrobial mechanism of LIG electrodes is not yet fully understood. This study demonstrated an array of mechanisms working synergistically to inactivate bacteria during electrochemical treatment using LIG electrodes, including the generation of oxidants, changes in pH-specifically high alkalinity associated with the cathode, and electro-adsorption on the electrodes. All these mechanisms may contribute to the disinfection process when bacteria are close to the surface of the electrodes where inactivation was independent of the reactive chlorine species (RCS); however, RCS was likely responsible for the predominant cause of antibacterial effects in the bulk solution (i.e., ≥100 mL in our study). Furthermore, the concentration and diffusion kinetics of RCS in solution was voltage-dependent. At 6 V, RCS achieved a high concentration in water, while at 3 V, RCS was highly localized on the LIG surface but not measurable in water. Despite this, the LIG electrodes activated by 3 V achieved a 5.5-log reduction in Escherichia coli (E.coli) after 120-min electrolysis without detectable chlorine, chlorate, or perchlorate in the water, suggesting a promising system for efficient, energy-saving, and safe electro-disinfection.

From black water to flushing water: potential applications of chlorine-mediated indirect electrooxidation for ammonia removal

Removing ammonia from black water is one of the most urgent issues before it can be recycled as flushing water. In this study, an electrochemical oxidation (EO) process with commercial Ti/IrO₂-RuO₂ anodes to treat black water could remove 100% of different concentrations of ammonia by adjusting the dosage of chloride. Through the relationship between ammonia, chloride, and corresponding the pseudo-first-order degradation rate constant (K_obs), we could determine the chloride dosage and predict the kinetics of ammonia oxidation based on initial ammonia concentration in black water. The optimal N/Cl molar ratio was 1:1.8. The difference between black water and the model solution in terms of ammonia removal efficiency and oxidation products was explored. A higher chloride dosage was beneficial for removing ammonia and shortening the treatment cycle, but it also led to the generation of toxic by-products. Especially HClO and ClO₃^- generated in black water were 1.2 and 1.5 times more than the synthesized model solution under 40 mA cm^-2. Through SEM characterization of electrodes and repeated experiments, the electrodes always maintained a high treatment efficiency. These results demonstrated the potential of the electrochemical process as a treatment method for black water.

Electrokinetic geosynthetics restrained nitrogen release from sediment to overlying water through porewater drainage

Porewater is the primary carrier of sediment nitrogen and a crucial source of overlying water nitrogen; its separation thus is essential for restraining nitrogen release from sediment to overlying water. We developed a novel device using electrokinetic geosynthetics to drain porewater with nitrogen and restrain nitrogen release. A batch experiment lasted 1120 h (about 47 days) was conducted with 20 cm depth of overlying water under three conditions, i.e., undrained at 0 V/cm voltage gradient (control), drained at 0 V/cm, and drained at 0.5 V/cm. Under the pulsed direct current, once porewater drained, overlying water replenished sediment pore space and supplied porewater. Along with porewater drainage, sediment nitrogen concentration was reduced by 11%-30%, decreasing nitrogen release from sediment to overlying water from 83 mg/m² in the first 100 h to -95 mg/m² after about 600 h. Processes such as electroosmosis, electromigration, and redox reaction contributed to the restraint on nitrogen release. This research revealed the potentiality of applying electrokinetic geosynthetics to in-situ restraint on sediment nitrogen release in eutrophic waterbodies such as fishponds.

Electrochemical oxidation of landfill leachate using boron-doped diamond anodes: pollution degradation rate, energy efficiency and toxicity assessment

Electrochemical oxidation (EO), due to high efficiency and small carbon footprint, is regarded as an attractive option for on-site treatment of highly contaminated wastewater. This work shows the effectiveness of EO using three boron-doped diamond electrodes (BDDs) in sustainable management of landfill leachate (LL). The effect of the applied current density (25-100 mA cm^-2) and boron doping concentration (B/C ratio: 500 ppm, 10,000 ppm and 15,000 ppm) on the performance of EO was investigated. It was found that, of the electrodes used, the one most effective at COD, BOD₂₀ and ammonia removal (97.1%, 98.8% and 62%, respectively) was the electrode with the lowest boron doping. Then, to better elucidate the ecological role of LLs, before and after EO, cultivation of faecal bacteria and microscopic analysis of total (prokaryotic) cell number, together with ecotoxicity assay (Daphnia magna, Thamnocephalus platyurus and Artemia salina) were combined for the two better-performing electrodes. The EO process was very effective at bacterial cell inactivation using each of the two anodes, even within 2 h of contact time. In a complex matrix of LLs, this is probably a combined effect of electrogenerated oxidants (hydroxyl radicals, active chlorine and sulphate radicals), which may penetrate into the bacterial cells and/or react with cellular components. The toxicity of EO-treated LLs proved to be lower than that of raw ones. Since toxicity drops with increased boron doping, it is believed that appropriate electrolysis parameters can diminish the toxicity effect without compromising the nutrient-removal and disinfection capability, although salinity of LLs and related multistep-oxidation pathways needs to be further elucidated.

Decontamination of real urban sewage-comparison between Fenton and electrochemical oxidation

Advanced oxidation processes have been used for wastewater treatment due to their capacity to reduce the organic loading and for their fast reactions. In this paper, we explore the viability of isolated and sequential use of electrochemical oxidation and Fenton processes into treatment of real raw urban sewage. The electrochemical process was carried out using DSA®-Cl₂ electrodes and factorial planning in order to investigate the influence of pH, current density, and electrolyte. Fenton reaction was also used and H₂O₂ and Fe²⁺ concentration effects were investigated. The efficiency was estimated by chemical oxygen demand (COD) removal and in the optimized conditions the effluent was characterized by turbidity, suspended/dissolved/total solids, ammonia, chloride ions, free chlorine, nitrite, and potassium analysis and bioassays with Artemia ssp. and Lactuca sativa. The study demonstrated that the use of electrochemical technique followed by Fenton allowed an improvement in the degradation of organic matter and reduction of turbidity and solid content, reaching reductions of 86.8, 96.4, 99.4, 56.1, and 66.7% for COD, turbidity, SS, DS, and TS, respectively. The associated treatment also contributed to the reduction of energy consumption by 74.9%, from the 23.9 kWh m^-3 observed during the electrochemical treatment isolated to the 6 kWh m^-3 during the associated process. All the treatments presented toxicity reduction, with the electrochemical process achieving the best results.

Irreversible inactivation of carbapenem-resistant Klebsiella pneumoniae and its genes in water by photo-electro-oxidation and photo-electro-Fenton - Processes action modes

Carbapenem-resistant Klebsiella pneumoniae is a critical priority pathogen according to the World Health Organization's classification. Effluents of municipal wastewater treatment plants (EWWTP) may be a route for K. pneumoniae dissemination. Herein, the inactivation of this microorganism in simulated EWWTP by the photo-electro-oxidation (PEO) and photo-electro-Fenton (PEF) processes was evaluated. Firstly, the disinfecting ability and action pathways of these processes were established. PEO achieved faster K. pneumoniae inactivation (6 log units in 75 min of treatment) than the PEF process (6 log units in 105 min of treatment). PEO completely inactivated K. pneumoniae due to the simultaneous action of UVA light, electrogenerated H₂O_2, and anodic oxidation pathways. The slower inactivation of K. pneumoniae when using PEF was related to interfering screen effects of iron oxides on light penetration and the diffusion of the bacteria to the anode. However, both PEO and PEF avoided the recovery and regrowth of treated bacteria (with no detectable increase in the bacteria concentration after 24 h of incubation). In addition to the bacteria evolution, the effect of treatment processes on the resistance gene was examined. Despite inactivation of K. pneumoniae by PEF was slower than by PEO, the former process induced a stronger degrading action on the gene, conferring the resistance to carbapenems (PEF had a Ct value of 24.92 cycles after 105 min of treatment, while PEO presented a Ct of 19.97 cycles after 75 min). The results of this research indicate that electrochemical processes such as PEO and PEF are highly effective at dealing with resistant K. pneumoniae in the EWWTP matrix.

Effect of on-Site Sludge Reduction and Wastewater Treatment Based on Electrochemical-A/O Combined Process

Working on sludge with electrochemical oxidation is beneficial to promote the subsequent recessive growth of microorganisms in the sludge. To achieve the on-site sludge reduction, this study combined the anoxic/oxic (A/O) process with the electrochemical oxidation process based on the cell lysis-cryptic growth theory by determining the experimental conditions and mechanism of electrochemical cell lysis. The sludge reduction and effluent treatment of the combined process in practical operation were studied. The results showed that the cumulative sludge discharge had been reduced by 37.1% compared with that of the A/O process, and the apparent sludge yield had been reduced by 39.1% during the 30-day operation time, indicating that the electrochemical-A/O combined process could have a considerable sludge reduction effect. After the treatment, chemical oxygen demand (COD), ammonium nitrogen, and total nitrogen in the effluent of the combined process reached 33.02 mg/L, 0.83 mg/L, and 9.95 mg/L, respectively. Due to the limitation of the A/O process, the removal of total phosphorus was poor. As a result, poly aluminum chloride (PAC) was employed to achieve a chemical removal of phosphorus, by which the total phosphorus (TP) of the effluent was controlled to be lower than 0.5 mg/L.

Low voltage iron electrocoagulation as a tertiary treatment of municipal wastewater: removal of enteric pathogen indicators and antibiotic-resistant bacteria

In this paper we analyse the feasibility of low voltage iron electrocoagulation as a means of municipal secondary effluent treatment with a focus on removal of microbial indicators, Antibiotic Resistant Bacteria (ARB) and nutrients. A laboratory scale batch unit equipped with iron electrodes was used on synthetic and real secondary effluent from a municipal wastewater treatment plant. Synthetic secondary effluent was separately assayed with spiked Escherichia coli WR1 and with bacteriophage ΦX174, while real effluent samples were screened before and after treatment for E. coli, Extended Spectrum Betalactamase-producing E. coli, Enterococci, Vancomycin Resistant Enterococci, Clostridium perfringens spores and somatic coliphages. Charge dosage (CD) and charge dosage rate (CDR) were used as the main process control parameters. Experiments with synthetic secondary effluent showed >4log₁₀ and >5log₁₀ removal for phage ΦX174 and for E. coli WR1, respectively. In real effluents, bacterial indicator removal exceeded 3.5log₁₀, ARB were removed below detection limit (≥2.5log₁₀), virus removal reached 2.3log₁₀ and C. perfringens spore removal exceeded 2.5log₁₀. Experiments in both real and synthetic wastewater showed that bacterial removal increased with increasing CD and decreasing CDR. Virus removal increased with increasing CD but was irresponsive to CDR. C. perfringens spore removal increased with increasing CD yet reached a removal plateau, being also irresponsive to CDR. Phosphate removal exceeded 99%, while total nitrogen and chemical oxygen demand removal were below 15% and 58%, respectively. Operational cost estimates were made for power and iron plate consumption, and were found to be in the range of 0.01 to 0.24€/m³ for the different assayed configurations. In conclusion, low voltage Fe-EC is a promising technology for pathogen reduction of secondary municipal effluents, with log₁₀ removals comparable to those achieved by conventional disinfection methods such as chlorination, UV or ozonation.

Rapid and selective electrochemical transformation of ammonia to N2 by substoichiometric TiO2-based electrochemical system

In this study, we have developed a continuous-flow electrochemical system towards the rapid and selective conversion of ammonia to N₂, based on a tubular substoichiometric titanium dioxide (Ti₄O₇) anode and a Pd–Cu co-modified Ni foam (Pd–Cu/NF) cathode, both of which are indispensable. Under the action of a suitable anode potential, the Ti₄O₇ anode enables the conversion of Cl⁻ to chloride radicals (Cl˙), which could selectively react with ammonia to produce N₂. The anodic byproducts, e.g. NO₃⁻, were further reduced to N₂ at the Pd–Cu/NF cathode. EPR and scavenger experiments confirmed the dominant role of Cl˙ in ammonia conversion. Complete transformation of 30 mg L⁻¹ ammonia could be obtained over 40 min of continuous operation under optimal conditions. The proposed electrochemical system also exhibits enhanced oxidation kinetics compared to conventional batch systems. This study provides new insights into the rational design of a high-performance electrochemical system to address the challenging issue of ammonia pollution.

A continuous-flow electrochemical system for rapid and selective conversion of ammonia to N₂ was proposed. The system consists of a tubular substoichiometric titanium dioxide (Ti₄O₇) anode and a Pd–Cu co-modified Ni foam (Pd–Cu/NF) cathode.

Nitrification performance of high rate nitrifying trickling filters at low ammonia concentrations: does the aspect ratio matter?

Nitrifying trickling filters (NTFs) are often introduced to pre-treat waters before chlorination process, to reduce the ammonia-driven chlorine consumption in wastewater treatment. As a passive aerated system, the only power needed is to transport the water to the top of the filter for distribution. Thus, understanding the role of filter aspect ratio on ammonia oxidation might save energy cost. In the present study, a pilot-scale comparison NTF system was conducted on two filters with different aspect ratios (height/diameter) and the same specific surface area. The nitrification efficiencies of these two filters under relatively low influent ammonia-nitrogen concentrations (1.0-4.0 mg NH₄-N L^-1) were investigated. Results obtained from the present study indicated that the constructional aspect ratio of NTF showed no significant effect on nitrification performance of NTFs. Additionally, the operational parameters showed similar effects on nitrification in NTFs with different aspect ratios. Our findings could provide important information for the construction design of future NTFs.

The Prospect of Electrochemical Technologies Advancing Worldwide Water Treatment

Growing worldwide population, climate change, and decaying water infrastructure have all contributed to a need for a better water treatment and conveyance model. Distributed water treatment is one possible solution, which relies on the local treatment of water from various sources to a degree dependent on its intended use and, finally, distribution to local consumers. This distributed, fit-for-purpose water treatment strategy requires the development of new modular point-of-use and point-of-entry technologies to bring this idea to fruition. Electrochemical technologies have the potential to contribute to this vision, as they have several advantages over established water treatment technologies. Electrochemical technologies have the ability to simultaneously treat multiple classes of contaminants through the in situ production of chemicals at the electrode surfaces with low power and energy demands, thereby allowing the construction of compact, modular water treatment technologies that require little maintenance and can be easily automated or remotely controlled. In addition, these technologies offer the opportunity for energy recovery through production of fuels at the cathode, which can further reduce their energy footprint. In spite of these advantages, there are several challenges that need to be overcome before widespread adoption of electrochemical water treatment technologies is possible. This Account will focus primarily on destructive electrolytic technologies that allow for removal of water contaminants without the need for residual treatment or management. Most important to the development of destructive electrochemical technologies is a need to fabricate nontoxic, inexpensive, high-surface-area electrodes that have a long operational life and can operate without the production of unwanted toxic byproducts. Overcoming these barriers will decrease the capital costs of water treatment and allow the development of the point-of-use and point-of-entry technologies that are necessary to promote more sustainable water treatment solutions. However, to accomplish this goal, a reprioritization of research is needed. Current research is primarily focused on investigating individual contaminant transformation pathways and mechanisms. While this research is important for understanding these technologies, additional work is needed in developing inexpensive, high-surface-area, stable electrode materials, minimizing toxic byproduct formation, and determining the life cycle and technoeconomic analyses necessary for commercialization. Better understanding of these critical research areas will allow for strategic deployment of electrochemical water treatment technologies to promote a more sustainable future.