Treatment of coking wastewater by aeration assisted electrochemical oxidation process at controlled and uncontrolled initial pH conditions

Performance of a polymerization-based electrochemically assisted persulfate process on a real coking wastewater treatment

Industrial wastewater should be treated with caution due to its potential environmental risks. In this study, a polymerization-based cathode/Fe3+/peroxydisulfate (PDS) process was employed for the first time to treat a raw coking wastewater, which can achieve simultaneous organics abatement and recovery by converting organic contaminants into separable solid organic-polymers. The results confirm that several dominant organic contaminants in coking wastewater such as phenol, cresols, quinoline and indole can be induced to polymerize by self-coupling or cross-coupling. The total chemical oxygen demand (COD) abatement from coking wastewater is 46.8% and the separable organic-polymer formed from organic contaminants accounts for 62.8% of the abated COD. Dissolved organic carbon (DOC) abatement of 41.9% is achieved with about 89% less PDS consumption than conventional degradation-based process. Operating conditions such as PDS concentration, Fe3+ concentration and current density can affect the COD/DOC abatement and organic-polymer yield by regulating the generation of reactive radicals. ESI-MS result shows that some organic-polymers are substituted by inorganic ions such as Cl-, Br-, I-, NH4+, SCN- and CN-, suggesting that these inorganic ions may be involved in the polymerization. The specific consumption of this coking wastewater treatment is 27 kWh/kg COD and 95 kWh/kg DOC. The values are much lower than those of the degradation-based processes in treating the same coking wastewater, and also are lower than those of most processes previously reported for coking wastewater treatment.

Origin, types, and contribution of emerging pollutants to environmental degradation and their remediation by physical and chemical techniques

The growing presence of emerging pollutants (EPs) in aquatic environments, as well as their harmful impacts on the biosphere and humans, has become a global concern. Recent developments and advancements in pharmaceuticals, agricultural practices, industrial activities, and human personal care substances have paved the way for drastic changes in EP concentrations and impacts on the ecosystem. As a result, it is critical to mitigate EP's harmful effects before they jeopardize the ecological equilibrium of the overall ecosystem and the sustainable existence of life on Earth. This review comprehensively documented the types, origins, and remediation strategies of EPs, and underscored the significance of this study in the current context. We briefly stated the major classification of EPs based on their organic and inorganic nature. Furthermore, this review systematically evaluates the occurrence of EPs due to the fast-changing ecological scenarios and their impact on human health. Recent studies have critically discussed the emerging physical and chemical processes for EP removal, highlighting the limitations of conventional remediation technologies. We reviewed and presented the challenges associated with EP remediation and degradation using several methods, including physical and chemical methods, with the application of recent technologies. The EP types and various methods discussed in this review help the researchers understand the nature of present-day EPs and utilize an efficient method of choice for EP removal and management in the future for sustainable life and development activities on the planet.

Treatment of tomato paste wastewater by electrochemical and membrane processes: process optimization and cost calculation

This study investigated the treatment of wastewater from tomato paste (TP) production using electrocoagulation (EC) and electrooxidation (EO). The effectiveness of water recovery from the pretreated water was then investigated using the membrane process. For this purpose, the effects of independent control variables, including electrode type (aluminum, iron, graphite, and stainless steel), current density (25-75 A/m2), and electrolysis time (15-120 min) on chemical oxygen demand (COD) and color removal were investigated. The results showed that 81.0% of COD and 100% of the color removal were achieved by EC at a current density of 75 A/m2, a pH of 6.84 and a reaction time of 120 min aluminum electrodes. In comparison, EO with graphite electrodes achieved 55.6% of COD and 100% of the color removal under similar conditions. The operating cost was calculated to be in the range of $0.56-30.62/m3. Overall, the results indicate that EO with graphite electrodes is a promising pretreatment process for the removal of various organics. In the membrane process, NP030, NP010, and NF90 membranes were used at a volume of 250 mL and 5 bar. A significant COD removal rate of 94% was achieved with the membrane. The combination of EC and the membrane process demonstrated the feasibility of water recovery from TP wastewater.

Removal of chemical oxygen demand and ammonia nitrogen from high salinity tungsten smelting wastewater by one-step electrochemical oxidation: From bench-scale test, pilot-scale test, to industrial test

In recent years, electrochemical oxidation (EO) shows the characteristics of green and high efficiency in removing chemical oxygen demand (COD) and ammonia nitrogen (NH₃-N) from wastewater, which has been favored by researchers. However, at present, most of current studies on EO remain in laboratory stage, reports about pilot-scale or even industrial tests with large treatment capacity are few, which slowing down the use of the advanced technology to practical application. In this study, bench-scale tests, pilot-scale tests (treatment capacity 200-500 L/h), and industrial tests (treatment capacity 100 m³/h) were carried out by EO technology in view of the characteristics of tungsten smelting wastewater (TSW) with high salinity (NaCl), COD, and NH₃-N. Results showed that the removal of COD and NH₃-N was a competitive reaction in the EO process, and COD could be removed more preferentially than NH₃-N. When NH₃-N content was low, the influent pH had a minimal effect on its removal, and when NH₃-N content was high, increasing the influent pH was beneficial to its removal. Industrial tests showed that the one-step removal of COD and NH₃-N in TSW met the standard, and the power consumption per cubic meter of wastewater was only 4.2 kW h, and the treatment cost was much lower than the two-step process of "breaking point chlorination to remove NH₃-N and adding oxidant to remove COD". This study has successfully realized industrial application of EO technology in TSW treatment for the first time and provided a successful case, which is helpful to accelerate the popularization and application of this technology in the field of high salinity organic ammonia nitrogen wastewater treatment.

Strategic analysis on development of simultaneous adsorption and catalytic biodegradation over advanced bio-carriers for zero-liquid discharge of industrial wastewater

With rapid industrial development, millions of tons of industrial wastewater are produced that contain highly toxic, carcinogenic, mutagenic compounds. These compounds may consist of high concentration of refractory organics with plentiful carbon and nitrogen. To date, a substantial proportion of industrial wastewater is discharged directly to precious water bodies due to the high operational costs associated with selective treatment methods. For example, many existing treatment processes rely on activated sludge-based treatments that only target readily available carbon using conventional microbes, with limited capacity for nitrogen and other nutrient removal. Therefore, an additional set-up is often required in the treatment chain to address residual nitrogen, but even after treatment, refractory organics persist in the effluents due to their low biodegradability. With the advancements in nanotechnology and biotechnology, novel processes such as adsorption and biodegradation have been developed, and one promising approach is integration of adsorption and biodegradation over porous substrates (bio-carriers). Regardless of recent focus in a few applied researches, the process assessment and critical analysis of this approach is still missing, and it highlights the urgency and importance of this review. This review paper discussed the development of the simultaneous adsorption and catalytic biodegradation (SACB) over a bio-carrier for the sustainable treatment of refractory organics. It provides insights into the physico-chemical characteristics of the bio-carrier, the development mechanism of SACB, stabilization techniques, and process optimization strategies. Furthermore, the most efficient treatment chain is proposed, and its technical aspects are critically analysed based on updated research. It is anticipated that this review will contribute to the knowledge of academia and industrialist for sustainable upgradation of existing industrial wastewater treatment plants.

Challenges and engineering application of landfill leachate concentrate treatment

Landfill leachate concentrate (LLC) is a concentrated waste stream from landfill leachate treatment systems and has been recognized as a key challenge due to its high concentration of salts, heavy metals, organic matters, etc. Improper management of LLC (e.g. reinjection) would exacerbate the performance of upstream treatment processes and pose risks to the surrounding environments near landfill sites. Addressing the challenge and recovering resources from LLC have thus been attracting considerable attention. Although many LLC treatment technologies have been developed, a comprehensive discussion about the challenges still lacks. This review critically evaluates mainstream LLC treatment technologies, namely incineration, coagulation, advanced oxidation, evaporation and solidification/stabilization. We then introduce a geopolymer-based solidification (GS) process as a promising technology owning to its simple casting process and reusable final product and summarizes engineering applications in China. Finally, we suggest investigating hybrid systems to minimize LLC production and achieve the on-site reuse of LLC. Collectively, this review provides useful information to guide the selection of LLC treatment technologies and suggests a sustainable alternative for large-scale application, while also highlighting the need of joint efforts in the industry to achieve efficient, ecofriendly and economical on-site management of landfill waste streams.

Treatment of thiocyanate-containing wastewater: a critical review of thiocyanate destruction in industrial effluents

Thiocyanate is a common pollutant in gold mine, textile, printing, dyeing, coking and other industries. Therefore, thiocyanate in industrial wastewater is an urgent problem to be solved. This paper reviews the chemical properties, applications, sources and toxicity of thiocyanate, as well as the various treatment methods for thiocyanate in wastewater and their advantages and disadvantages. It is emphasized that biological systems, ranging from laboratory to full-scale, are able to successfully remove thiocyanate from factories. Thiocyanate-degrading microorganisms degrade thiocyanate in autotrophic manner for energy, while other biodegrading microorganisms use thiocyanate as a carbon or nitrogen source, and the biochemical pathways and enzymes involved in thiocyanate metabolism by different bacteria are discussed in detail. In the future, degradation mechanisms should be investigated at the molecular level, with further research aiming to improve the biochemical understanding of thiocyanate metabolism and scaling up thiocyanate degradation technologies from the laboratory to a full-scale.

Electrochemical oxidation process on palm oil mill effluent waste activated sludge: optimization by response surface methodology

Biological-based treatment as the conventional treatment for palm oil mill effluent (POME) in open-ponding system face a well known rate-limiting step which is hydrolysis. In this study, electrochemical oxidation (EO) by a ruthenium oxide-coated titanium (Ti/RuO₂) electrode was introduced as a pre-treatment for POME waste activated sludge (WAS). Surface morphology and elemental analysis were investigated using field emission scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Response surface methodology type central composite design was used in this study to understand the relationship between the independent and dependent variables. Analysis of variance (ANOVA) was used to validate the model of the studied variables. The correlation coefficients (R²) indicated a close agreement between the experimental results and the predicted values, with high R² values of 0.9044-0.9773. Multiple response optimization suggested that the range of current density (17-27 mA/cm²) and electrolysis time (55-75 min) at a fixed concentration of sodium chloride (10 g/L), resulted in mixed liquor volatile suspended solids (MLVSS) removal >20%, capillary suction timer (CST) reduction >43%, extracellular polymeric substances (EPS) increment <19% and soluble chemical oxygen demand (sCOD) increment >25%. EO appears to be an efficient pre-treatment as well as practical way to improve the POME WAS disintegration and dewaterability.

Treatment of contaminants by a cathode/FeIII/peroxydisulfate process: Formation of suspended solid organic-polymers

Treatment of highly contaminated wastewaters containing refractory or toxic organic contaminants (e.g. industrial wastewaters) is becoming a global challenge. Most technologies focus on efficient degradation of organic contaminants. Here we improve the cathode/Fe^III/peroxydisulfate (PDS) technology by turning down the current density and develop an innovative mechanism for organic contaminants abatement, namely polymerization rather than degradation, which allows simultaneous contaminants removal and resource recovery from wastewater. This polymerization leads to organic-particles (suspended solid organic-polymers) formation in bulk solution, which is demonstrated by eight kinds of representative organic contaminants. Taking phenol as a representative, 83% of PDS is saved compared to degradation process, with 87.2% of DOC removal. The formed suspended solid organic-polymers occupy 59.2% of COD of the original organics in solution, and can be easily separated from aqueous solution by sedimentation or filtration. The separated organic-polymers are a series of polymers coupled by phenolic monomers, as confirmed by FTIR and ESI-MS analyzes. The energy contained in the recovered organic polymers (4.76 × 10^-5 kWh for 100 mL of 1 mM phenol solution in this study) can fully compensate the consumed electrical energy (2.8 × 10^-5 kWh) in the treatment process. A representative polymerization model for this process is established, in which the SO₄^•- and HO^• generated from PDS activation initiate the polymerization and improve the polymerization degree by the production of oligomer intermediates. A practical coking wastewater treatment is carried out to verify the research results and get positive feedback, with 56.0% of DOC abatement and the suspended solid organic-polymers accounts for 42.5% of the total COD in the raw wastewater. The energy consumption (47 kWh/kg COD, including electricity and PDS cost) is lower than the values in previous reports. This study provides a novel method for industrial wastewater treatment based on polymerization mechanism, which is expected to recover resources while removing pollutants with low consumption.

Variation of current density with time as a novel method for efficient electrochemical treatment of real dyeing wastewater with energy savings

Electro-oxidation is a promising technology for wastewater treatment with biorefractory organic and nitrogen pollutants; however, the high energy demand hinders its wide application. In this study, a novel method by regulating significant parameter during the electro-oxidation process in a timely manner for real dyeing wastewater treatment with energy savings was studied. Operating factors (i.e., flow rate, initial pH value, electrode distance, and current density) were investigated for chemical oxygen demand (COD) and ammonia removal, and the results indicated that current density was the key factor that obviously influenced the electrochemical performance. Indirect oxidation by active chlorine was then confirmed as the main reaction pathway for pollutant oxidation, and the relationship between the current density and the generation of active chlorine was established, suggesting that a large part of the generated active chlorine was not utilized effectively. Subsequently, a novel method of varying the current density in a timely manner based on the reaction mechanism was proposed; the results indicated that, with similar pollutant removal efficiencies, energy consumption could be reduced from 31.6 to 20.5 kWh/m3. Additionally, the novel system was further optimized by Box-Behnken design: The COD removal efficiency could reach 71.8%, and the energy demand could be reduced by 45.6%.

Treatment of aquatic medium containing common and emerging contaminants using an aero-electrochemical process based on graphite cathode and three metal oxides alloy as anode: Central composite design and photo/sono-enhancement

An aero-electrochemical advanced oxidation process (aero-EAOP) equipped with graphite cathode and dimensionally stable anodes was utilized for the treatment of aquatic media containing common and emerging contaminants. Among various anode materials, the application of Ti/RuO₂/IrO₂/SnO₂ anode resulted in the highest effectiveness. Central composite experimental design (CCED) was used to attain the optimum operational parameters in terms of chlorine generation. Simultaneous decolorization and ammonium removal by the aero-EAOP process were investigated. Accordingly, the decolorization efficiency of 94%, along with the ammonium removal of 65.2%, was obtained within 30 min. Implementation of ultrasound and UV irradiation resulted in the complete decolorization within 25 and 20 min, respectively. In comparison, the influence of ultrasound and UV irradiation on the ammonium removal by the aero-EAOP reactor was not remarkable. Mineralization efficiency of 75.1% was obtained during the short reaction time of 30 min. With increasing hydraulic retention time (HRT) from 2 to 20 min, decolorization efficiency increased from 12.0 to 55.7% and ammonium removal efficiency increased from 16.6 to 37.8%, respectively. The complete degradation of amoxicillin (AMX) and tetracycline (TC) antibiotics were achieved within 25 and 30 min, respectively. The degradation efficiencies of ibuprofen (IBP), acetaminophen (APAP) and endocrine disrupting compound of bisphenol A (BPA) were obtained to be 58, 66 and 78% within 30 min, respectively. Photo-assisted aero-EAOP was more efficient than the aero-EAOP in degrading target emerging pollutants.

Functionalization of MXene-based nanomaterials for the treatment of micropollutants in aquatic system: A review

The increased industrialization and urbanization generate a larger quantity of effluent that is discharged into the environment regularly. Based on the effluent composition produced from various industries, the number of hazardous substances such as heavy metals, hydrocarbons, volatile organic compounds, organic chemicals, microorganisms introduced into the aquatic systems vary. The conventional wastewater treatment systems do not meet the effluent standards before discharge and require a different treatment system before reuse. Adsorption is an eco-friendly technique that uses selective adsorbents to remove hazardous pollutants even at microscale levels. MXene, a 2-Dimensional nanomaterial with resplendent properties like conductivity, hydrophilicity, stability, and functionalized surface characteristics, is found as a potential candidate for pollutant removal systems. This review discusses the fabrication, characterization, and application of MXene based nanoparticles to remove many pollutants in water treatment systems. The improvement in surface properties and adsorption capacity of MXene based NPs, when modified using different modification agents, has also been discussed. Their feasibility in terms of economic and environmental aspects has been evaluated to understand their scope for practical application in large-scale industries. The challenges towards the synthesis and toxicity's importance have been discussed, with the appropriate recommendations.

Nitrogen and organic load removal from anaerobically digested leachate using a hybrid electro-oxidation and electro-coagulation process

This study evaluated the performance of an integrated electrochemical process, which simultaneously utilizes electro-oxidation (EO) and electro-coagulation (EC) methods while removing organic and nitrogen loads from high-strength leachate obtained from anaerobic digesters. A bipolar arrangement of the aluminum electrode, sandwiched between a monopolar boron-doped diamond anode and stainless-steel cathode, integrates EC and EO into a single reactor. This arrangement demonstrated an enhancement of 33%, 27%, and 24% in removal capacity for ammonia nitrogen (AN), total Kjeldahl nitrogen (TKN), and total nitrogen, respectively, when compared to just EO at 0.8 A current intensity after 24 h. Increasing the current intensity from 0.4 A to 1.0 A enhanced the organic nitrogen and AN removal. Chemical oxygen demand (COD) exhibited initial faster removal kinetics with higher current intensities and eventually reached 95%-98% removal for intensities of 0.6 A or higher. Additional removal for AN, TKN were also observed with increasing current intensity. Lowering the pH further expedited the COD removal kinetics. Reducing and maintaining the pH at 4, 6, and 8 by dosing of hydrochloric acid (HCl) resulted in the 100% removal of AN and TKN from the integrated system in 6, 8, and 20 h, respectively. Accelerated removal of COD and the enhanced removal of AN and TKN through pH control could be linked to the formation of active chlorine species in bulk solution. The integrated system offered lower energy consumption than EO due to oxidation on the additional anodic surface of the bipolar electrode, as well as the adsorption-precipitation of contaminants in aluminum flocs.

An electro-oxidation reactor for treatment of nanofiltration concentrate towards zero liquid discharge

Nanofiltration (NF) concentrate generated from the secondary wastewater treatment contains high concentration of ammonium nitrogen and refractory organics, thus having great environmental risks. In this study, an electro-oxidation (EO) reactor built up with a boron-doped diamond (BDD) anode is utilized to treat the NF concentrate. To reach "zero liquid discharge", a mixture of the electrolytic effluent and the raw secondary wastewater was collected and transported back to the NF module. Results show that under the current density of 30 mA·cm^-2, most of ammonia nitrogen was decomposed into N-gases within 30 min due to the active chlorine radicals generated in the electrochemical process. Moreover, the EO reactor completely eliminated antibiotics, humic acids and bacteria in the NF concentrate under long electrolysis time of 60 min. In particular, the organic pollutants removal rate was kept at a stable value in the EO reactor for a long-term operation of up to 120 h. In addition, the NF membrane remained a constant permeate flux without being affected by the membrane biofouling caused by organic components in wastewater. Our study highlights the potential of the NF-EO process as a "zero liquid discharge" approach for treatment of the secondary wastewater.

Recent developments in physical, biological, chemical, and hybrid treatment techniques for removing emerging contaminants from wastewater

Emerging contaminants (ECs) in wastewater have recently attracted the attention of researchers as they pose significant risks to human health and wildlife. This paper presents the state-of-art technologies used to remove ECs from wastewater through a comprehensive review. It also highlights the challenges faced by existing EC removal technologies in wastewater treatment plants and provides future research directions. Many treatment technologies like biological, chemical, and physical approaches have been advanced for removing various ECs. However, currently, no individual technology can effectively remove ECs, whereas hybrid systems have often been found to be more efficient. A hybrid technique of ozonation accompanied by activated carbon was found significantly effective in removing some ECs, particularly pharmaceuticals and pesticides. Despite the lack of extensive research, nanotechnology may be a promising approach as nanomaterial incorporated technologies have shown potential in removing different contaminants from wastewater. Nevertheless, most existing technologies are highly energy and resource-intensive as well as costly to maintain and operate. Besides, most proposed advanced treatment technologies are yet to be evaluated for large-scale practicality. Complemented with techno-economic feasibility studies of the treatment techniques, comprehensive research and development are therefore necessary to achieve a full and effective removal of ECs by wastewater treatment plants.

Understanding the toxicity effect and mineralization efficiency of in-situ electrogenerated chlorine dioxide for the treatment of priority pollutants of coking wastewater

Oxidation of phenol, cyanide and aniline have been analyzed by the enhanced electro-oxidation process in which sodium chlorite was used as an electrolyte and results were validated using statistical tool based on Box-Behnken design. The mineralization efficiency of 78.4%, and 98.18% were predicted at optimized variables condition for phenol, and aniline respectively, whereas complete mineralization has been observed for the cyanide at the optimized conditions, which describes the significance of the design model approach.The process mineralizes the higher phenol concentration revealing a drastic reduction in power consumption in comparison of direct oxidation, i.e., 799.36 kWh/kg to 138.18 kWh/kg for more than 90% mineralization of phenol even at a higher current density of 13.63 mA/cm². The kinetic modelling approach justified that higher current density has also played a role in higher mineralization of pollutants at the specific operating conditions. The by-product formation and toxicity effect on microalgae in wastewater were assessed by the full scan mass spectrometry and microalgae pigment inhibition test after the electro-oxidation of coking wastewater. The pigment growth inhibition rate of Chlorella sp. NCQ and Micractinium sp. NCS2 suggests that sodium chlorite as an electrolyte aid can also effectively used as an oxidizing agent and algal inhibiter in the coking wastewater.