Supercritical water oxidation of a pesticide wastewater

Supercritical water oxidation for the treatment and utilization of organic wastes: Factor effects, reaction enhancement, and novel process

Supercritical water oxidation (SCWO) has been regarded as a new and efficient technology for the harmless treatment and energy utilization of organic wastes, resulting in the quickly homogeneous oxidation between organics and oxidizers and the former being wholly degraded into small environment-friendly green molecules such as H2O and N2 and inorganic salts. This paper systematically analyzed the influencing behavior and mechanisms of the reaction factors, such as temperature, pressure, residence time, oxidant type, oxidation coefficient, and the concentration and pH values of the raw material, on the treatment effect of organic wastes. For most organic wastes, the SCWO conditions at 550 °C with a residence time of 1min and an oxidation coefficient of 100% can meet the removal rate of more than 99%. To further enhance the degradation rate of organics, the principles, implementation cases, and related equipment components of general enhancement technologies of supercritical water oxidation were discussed, such as fractional oxygen injection, auxiliary fuel co-oxidation, and hydrothermal flame-assisted degradation. This paper proposes a novel supercritical flame-assisted oxidation process in which the reactor performs preheating, corrosion protection, and desalination functions. The use of additive-enhanced oxidation, segmented oxidation, and supercritical hydrothermal flame-assisted oxidation has achieved good results in the complicated treatment process of brutal degradation of organic matter.

Red mud with enhanced dealkalization performance by supercritical water technology for efficient SO2 capture

The total de-alkalization treatment of industrial solid waste red mud (RM) has been a worldwide challenge. Removing the insoluble structural alkali fraction from RM is the key to enhancing the sustainable utilization of RM resources. In this paper, supercritical water (SCW) and leaching agents were used for the first time to de-alkalize the Bayer RM and to remove sulfur dioxide (SO2) from flue gas with the de-alkalized RM slurry. The results showed that the optimum alkali removal and Fe leaching rates of RM-CaO-SW slurry were 97.90 ± 0.88% and 82.70 ± 0.95%, respectively. Results confirmed that the SCW technique accelerated the disruption of (Al-O) and (Si-O) bonds and the structural disintegration of aluminosilicate minerals, facilitating the conversion of insoluble structural alkalis to soluble chemical alkalis. The exchangeable Ca2+ displaced Na+ in the remaining insoluble base, producing soluble sodium salts or alkalis. CaO consumed SiO2, which was tightly bound to Fe2O3 in RM, and released Fe2O3, which promoted Fe leaching. RM-SCW showed the best desulfurization performance, which maintained 88.99 ± 0.0020% at 450 min, followed by RM-CaO-SW (450 min, 60.75 ± 6.00%) and RM (180 min, 88.52% ± 0.00068). The neutralization of alkaline components, the redox of metal oxides, and the liquid-phase catalytic oxidation of Fe contributed to the excellent desulfurization performance of the RM-SCW slurry. A promising approach shown in this study is beneficial to RM waste use, SO2 pollution control, and sustainable growth of the aluminum industry.

Advanced oxidation process: a sustainable technology for treating refractory organic compounds present in industrial wastewater

The world faces tremendous challenges and environmental crises due to the rising strength of wastewater. The conventional technologies fail to achieve the quality water that can be reused after treatment means "zero effluent" discharge of the industrial effluent. Therefore, now the key challenge is to develop improved technologies which will have no contribution to secondary pollution and at the same time more efficient for the socio-economic growth of the environment. Sustainable technologies are needed for wastewater treatment, reducing footprint by recycling, reusing, and recovering resources. Advanced oxidation process (AOP) is one of the sustainable emerging technologies for treating refractory organic contaminants present in different industrial wastewaters like textile, paper and pulp, pharmaceuticals, petrochemicals, and refineries. This critical review emerges details of advanced oxidation processes (AOPs), mentioning all possible permutations and combinations of components like ozone, UV, the catalyst used in the process. Non-conventional AOP systems, microwave, ultrasound, and plasma pulse assisted are the future of the oxidation process. This review aims to enlighten the role of AOPs for the mineralization of refractory organic contaminants (ROC) to readily biodegradable organics that cannot be either possible by conventional treatment. The integrated AOPs can improve the biodegradability of recalcitrant organic compounds and reduce the toxicity of wastewater, making them suitable for further biological treatment.

Fate of phthalate esters in landfill leachate under subcritical and supercritical conditions and determination of transformation products

The hypothesis of this study is that the complex organic load of landfill leachate could be reduced by supercritical water oxidation (SCWO) in a single stage, but this operation could lead to the formation of some undesired by-products of phthalate esters (PAEs). In this context, the fate of selected PAEs, butyl benzyl phthalate (BBP), di-2-ethylhexyl phthalate (DEHP) and di-n-octyl phthalate (DNOP), was investigated during the oxidation of leachate under subcritical and supercritical conditions. Experiments were conducted at various temperatures (250-500 °C), pressures (10-35 MPa), residence times (2-18 min) and dimensionless oxidant doses (DOD: 0.2-2.3). The SCWO process decreased the leachate's chemical oxygen demand (COD) from 34,400 mg/L to 1,120 mg/L (97%). Removal efficiencies of DEHP and DNOP with longer chains were higher than BBP. The DEHP, DNOP and BBP compounds were removed in the range of -35 to 100%, -18 to 92%, and 28 to 36%, respectively, by the SCWO process. Many non-target PAEs were qualitatively detected in the raw leachate apart from the selected PAEs. Besides, 97% of total PAEs including both target and non-target PAEs was mineralized at 15 MPa, 300 °C and 5 min. Although PAEs were highly mineralized during SCWO of the leachate, aldehyde, ester, amide and amine-based phthalic substances were frequently detected as by-products. These by-products have transformed into higher molecular weight by-products with binding reactions as a result of complex SCWO process chemistry. It has also been determined that some non-target PAEs such as 1,2-benzenedicarboxylic acid bis(2-methylpropyl)ester and bis(2-ethylhexyl) isophthalate can transform to the DEHP. Therefore, the suggested pathway in this study for PAEs degradation during the SCWO of the leachate includes substitution and binding reactions as well as an oxidation reaction.

Review: Hydrothermal treatment of per- and polyfluoroalkyl substances (PFAS)

PER: and polyfluoroalkyl substances (PFAS) are a concerning and unique class of environmentally persistent contaminants with biotoxic effects. Decades of PFAS discharge into water and soil resulted in PFAS bioaccumulation in plants, animals, and humans. PFAS are very stable, and their treatment has become a global environmental challenge. Significant efforts have been made to achieve efficient and complete PFAS mineralization using existing and emerging technologies. Hydrothermal treatments in subcritical and supercritical water have emerged as promising end-of-life PFAS destruction technologies, attracting the attention of scholars, industry, and key stakeholders. This paper reviews the state-of-the-art research on the behavior of PFAS, PFAS precursors, PFAS alternatives, and PFAS-containing waste in hydrothermal processes, including the destruction and defluorination efficiency, the proposed reaction mechanisms, and the environmental impact of these treatments. Scientific literature shows that >99% degradation and >60% defluorination of PFAS can be achieved through subcritical and supercritical water processing. The limitations of current research are evaluated, special considerations are given to the challenges of technology maturation and scale-up from laboratory studies to large-scale industrial application, and potential future technological developments are proposed.

Supercritical water co-oxidation behavior in the different monohydric alcohol-ammonia reaction environment

The degradation of ammonia is a key rate-limiting step during the supercritical water oxidation of nitrogen-containing organics. This paper studied the co-oxidation behavior between different ammonia-alcohol environments, including the influence of reaction parameters and the co-oxidation mechanism. The results showed that increasing temperature, oxidation coefficient, residence time, and alcohol concentration significantly promoted the degradation of NH₃-N and TOC, while rising the ammonia concentration enhanced the NH₃-N destruction but inhibited the TOC degradation. Alcohols were oxidized first in the co-oxidation system to produce more OH* and HO₂* radicals. Ethanol generated the highest concentration of HO₂* in the shortest time, leading to more significant ammonia removal than isopropanol and methanol; however, the produced intermediate products like aldehydes and ketones reacted with residual ammonia to generate a small amount of organics at lower temperatures, inhibiting the degradation of alcohols slightly, and combined catalyst or nitrate in the batch reactor or used continuous supercritical water oxidation or supercritical hydrothermal combustion system without controlling the exotherm of fuels could improve this.

Novel tapered variable diameter biological fluidized bed for treating pesticide wastewater with high nitrogen removal efficiency and a small footprint

In this study, the removal efficiency of nitrogen, specific nitrification rate (SNR), specific denitrification rate (SDNR) and compliance rate of the novel tapered variable diameter biological fluidized bed (TVDBFB) and anoxic/oxic (AO) process were compared at different temperatures. The results showed that the optimal TN, NH₄⁺-N, and TKN removal efficiencies of the TVDBFB were 76%, 89% and 88%, respectively, and those of AO were 65%, 67% and 69%, respectively. The SNR and SDNR of the TVDBFB were significantly higher than those of AO. The TVDBFB had a smaller footprint than AO. The alkalinity/NH₄⁺-N, BOD₅/TN and temperature play important roles in the compliance rate. Increasing the carrier packing rate has emerged as a new strategy for enhancing the compliance rate. Mathematical models were developed and determined to be well-fitted with the experimental values, which can be employed to predict the SNR and SDNR of the TVDBFB.

Treatment of hospital wastewater by supercritical water oxidation process

Hospital wastewater contains several micro and macro pollutants that cannot be removed efficiently by conventional treatment processes. Thus, generally hybrid and multistage treatment methods are suggested for the treatment of hospital wastewater. Supercritical water oxidation (SCWO) is a promising method for the removal of emerging organic pollutants from hospital wastewater in one step and a very short reaction time. In this study, supercritical water oxidation (SCWO) process was used for the removal of pharmaceuticals in addition to conventional pollutants from real hospital wastewater. As a result of a series of preliminary studies, the optimum conditions were selected as 450 °C, 60 s, and 1:1 for temperature, reaction time, and oxidant ratio (H₂O₂/COD), respectively, for the treatment of hospital wastewater at 25 ± 1 MPa. The removal rates were determined above 90% for COD, BOD, TOC, TN, and SS from hospital wastewater. Phosphorus removal was greater than 90%, while the removal rates were around 80% for phenol, AOX, and surfactants in hospital wastewater. A total of 9 pharmaceuticals were observed in the real hospital wastewater samples. The highest removal rate was obtained for Paracetamol as 99.9%, while the lowest removal rate was obtained for Warfarin as 72% after SCWO treatment of hospital wastewater. As a result, it can be concluded that SCWO process is sufficient for the treatment of hospital wastewater without the need of additional treatment steps, with high removal rates in a short reaction time.

Supercritical water oxidation and process enhancement of nitrogen-containing organics and ammonia

Supercritical water oxidation (SCWO), as a promising technology for treating organic wastewater and sludge, has attracted the attention of many scholars. Nitrogen-containing organics are refractory substances that widely exist in industrial waste, and their effective degradation is of great significance to the environment. In this paper, the treatment effects, reaction kinetics, and migration and transformation pathways of various nitrogen-containing organics (amino group, nitro group, mixed group, and nitrogen heteroatom) under SCWO conditions are summarized, and the influences of the reaction temperature, oxidant type and concentration, residence time, and initial concentration of organics on the degradation of organics are also discussed. NH₃-N is the primary intermediate product produced during the oxidation process of the amino group and nitrogen heteroatom organics, and the further degradation of NH₃-N is the limiting step for the whole reaction. This paper focuses on the relevant strengthening technologies used to enhance the degradation of NH₃-N, including heterogeneous catalytic oxidation with reactor wall or metal oxides; co-oxidation with auxiliary fuels such as methanol, ethanol, isopropanol, and glycol; strong oxidation with NO₃^- or NO₂^-; and segmented oxidation by multi-injection of oxidants or fuels. In addition, in order to achieve the complete removal of NH₃-N and COD synergistically under relatively mild SCWO conditions, avoid the formation of NO_x, NO₃^-, and NO₂^-, and convert organic nitrogen into environmentally friendly products such as N₂ and N₂O, further research requirements and challenges are introduced.

Recent Strategies for Environmental Remediation of Organochlorine Pesticides

The amount of organochlorine pesticides in soil and water continues to increase; their presence has surpassed maximum acceptable concentrations. Thus, the development of different removal strategies has stimulated a new research drive in environmental remediation. Different techniques such as adsorption, bioremediation, phytoremediation and ozonation have been explored. These techniques aim at either degrading or removal of the organochlorine pesticides from the environment but have different drawbacks. Heterogeneous photocatalysis is a relatively new technique that has become popular due to its ability to completely degrade different toxic pollutants—instead of transferring them from one medium to another. The process is driven by a renewable energy source, and semiconductor nanomaterials are used to construct the light energy harvesting assemblies due to their rich surface states, large surface areas and different morphologies compared to their corresponding bulk materials. These make it a green alternative that is cost-effective for organochlorine pesticides degradation. This has also opened up new ways to utilize semiconductors and solar energy for environmental remediation. Herein, the focus of this review is on environmental remediation of organochlorine pesticides, the different techniques of their removal from the environment, the advantages and disadvantages of the different techniques and the use of specific semiconductors as photocatalysts.

Supercritical water oxidation of semi-coke wastewater: Effects of operating parameters, reaction mechanism and process enhancement

Semi-coke wastewater is a kind of industrial wastewater with complex composition, high concentration of organic pollutants and high chroma, seriously threatening the ecological environment and requiring to be effectively degraded. Supercritical water oxidation (SCWO), as for a promising environmental technology, was applied to treat semi-coke wastewater in this work. The influences of key operating parameters such as reaction temperature (400-600 °C), oxidation coefficient (1.0-4.0) and residence time (0.5-10 min), the reaction mechanism for organics in semi-coke wastewater and the process enhancement methods like catalytic oxidation and segmented oxidation were systematically investigated. Experimental results showed that the removal efficiency of COD and NH₃-N both significantly increased with the increasing of temperature, oxidation coefficient and residence time, the COD removal efficiency and NH₃-N removal efficiency could be 99.02% and 63.94% obtained under the condition of 600 °C, 25 MPa, 1.3 times oxidation coefficient and 10 min. The residual organics in liquid products were mainly phenols, ketones, imidazoles, esters and pyridines, which produced from the cyclization and esterification reaction between intermediate products such as alcohols, aldehydes, acids and NH₃-N, etc. What's more, NH₃-N was proved to have inhibitory effect on the degradation of phenol by generating more stubborn nitrogen-containing compounds with that. Besides, compared with single catalyst, the composite catalyst of MnO₂/CeO₂ exhibited the highest catalytic activity, which could synergistically degrade 98.52% COD and 67.18% NH₃-N under a relatively mild reaction condition (550 °C, 25 MPa, 1.3 times oxidation coefficient, 2 min). Moreover, the segmented oxidation, combining the pre-oxidation in preheater and oxidation in reactor, was firstly observed and analyzed here, could achieve a higher COD removal efficiency with a shorter length of the reactor. The results obtained in this paper proved the technical feasibility and could provide basic data support for the industrialization of semi-coke wastewater treatment by SCWO.

Assessment of carbon recovery from solid organic wastes by supercritical water oxidation for a regenerative life support system

The carbon recovery from organic space waste by supercritical water oxidation (SCWO) was studied to support resource recovery in a regenerative life support system. Resource recovery is of utmost importance in such systems which only have a limited total amount of mass. However, the practical waste treatment strategies for solid space wastes employed today are only storing and disposal without further recovery. This work assesses the performance of SCWO at recovering organic wastes as CO₂ and water, to discuss the superiority of SCWO over most present strategies, and to evaluate the different SCWO reactor systems for space application. Experiments were carried out with a batch and a continuous reactor at different reaction conditions. The liquid and gas products distribution were analyzed to understand the conversion of organics in SCWO. Up to 97% and 93% of the feed carbon were recovered as CO₂ in the continuous and the batch reactor, respectively. Residual carbon was mostly found as soluble organics in the effluent. Compared with the batch reactor, the continuous reactor system demonstrated a ten times higher capacity within the same reactor volume, while the batch reactor system was capable of handling feeds that contained particulate matter though suffering from poor heat integration (hence low-energy efficiency) and inter-batch variability. It was concluded that SCWO could be a promising technology to treat solid wastes for space applications. A continuous reactor would be more suitable for a regenerative life support system.

Experimental and Numerical Simulation Study on Co-Incineration of Solid and Liquid Wastes for Green Production of Pesticides

A large amount of solid and liquid waste is produced in pesticide production. It is necessary to adopt appropriate disposal processes to reduce pollutant emissions. A co-incineration scheme for mixing multi-component wastes in a rotary kiln was proposed for waste disposal from pesticide production. According to the daily output of solid and liquid wastes, the proportion of mixing was determined. An experiment of the co-incineration of solid and liquid wastes was established. Experimental results showed that the mixed waste could be completely disposed at 850 °C, and the residence time in the kiln exceeded 1 h. A model method for mixture and diesel oil-assisted combustion was proposed. Numerical simulation was performed to predict the granular motion and reveal the combustion interactions of the co-incineration of mixed wastes in the rotary kiln. Simulation results reproduced movements, such as rolling and cascading, and obtained the optimum rotational speed and diesel oil flow for the rotary kiln incineration operation. The simulation showed that the temperature in the kiln was maintained at 850 °C, and the mass fraction of CO and O2 at the outlet reached the standard for the complete combustion of the waste. Finally, the rotary kiln incineration and flue gas treatment processes were successfully applied in engineering for green production of pesticides.

Fenton's reagent-enhanced supercritical water oxidation of wastewater released from 3-hydroxypyridine production

A study on Fenton's reagent-enhanced supercritical water oxidation (SCFO) of wastewater released from 3-hydroxypyridine production was carried out in this paper. The effects of temperature, oxidant multiple, residence time, Fe2+ concentration, and pH on the degradation efficiency of wastewater were investigated. The Plackett-Burman test was designed to evaluate various factors, namely, temperature, oxidant multiple, and pH, which were found to significantly affect degradation efficiency. Response surface analysis was performed to optimize the parameter levels of the main influencing factors. The results indicated that the optimal conditions required for the oxidative degradation of wastewater in the SCFO systems were pH of 3, temperature of 473 °C, oxidant multiple of 7, Fe2+ concentration of 0.5 mg L-1, and residence time of 262.6 s (flow rate: 1.5 mL min-1). Under these conditions, the total organic carbon removal rate of the wastewater could reach 98.1%. The activation energy of the wastewater under SCFO conditions was 55.3 kJ mol-1, and the pre-exponential factor A was 52.8 s-1.

Novel design concept for a commercial-scale plant for supercritical water oxidation of industrial and sewage sludge

Supercritical water oxidation (SCWO) is a promising chemical technology for organic waste water and sludge treatment. Our team has successfully constructed the first pilot-scale SCWO plant in China, and the design concept for our first commercial-scale plant is reported in this paper. The challenges that hinder the commercial development of SCWO are introduced, including corrosion, plugging, high investment and operating costs. Some important lab-scale and pilot-scale experimental results are shown, and some key design parameters for the commercial plant are proposed. The technological process, specialized equipment design and new system flowsheet are described objectively. Moreover, an estimate of the equipment investment and operating costs of this commercial plant is carried out, and a comparison is made with other commercial sludge SCWO plants. This information is valuable for guiding how to best design commercial SCWO plants for the treatment of sludge and other feedstocks including solid particles.

Removal of organic matter from wastewater using M/Al-pillared clays (M = Fe or Mn) as coagulants

This work is about organic matter removal from Sidi Bel Abbes wastewater plant (Algeria) by coagulation on pillared clays (PILCs) under pH and PZC (point of zero charge), conditions. Two pillared clays, M/Al-PILCs (M = Fe or Mn), were synthesized, characterized, and studied as coagulants. Results showed that Fe/Al-pillared clay exhibits superior efficiency, with 18% higher removal rate than the common coagulants alum (AS) and ferric chloride (FCl), and that sedimentation time has positive effect on turbidity removal, with 95.85% removal rate during 30 min. Moreover PILCs will not cause pH go down too low, which is an advantage for achieving the best overall treatment. The IR and UV bands' reduction reveals the breakdown fragmentation of high molecular weight organic substances into smaller units. The highest total organic carbon (supercritical water oxidation analysis) and chemical oxygen demand adsorption capacities (48.52% and 61.85% respectively) obtained for Fe/Al-PILC can be related to increased basal spacing between adjacent layers, creating favorable adsorption sites in the microporous system. The suggested adsorption mechanism involves strong interactions between pollutants and PILCs leading to PILC-pollutant complex formation.

Sub- and supercritical water oxidation of anaerobic fermentation sludge for carbon and nitrogen recovery in a regenerative life support system

Sub- and supercritical water oxidation was applied to recover carbon as CO₂, while maintaining nitrogen as NH₄⁺ or NO₃^-, from sludge obtained from an anaerobic fermenter running on a model waste composed of plant residues and human fecal matter. The objective was to fully convert carbon in the organic waste to CO₂ while maintaining nutrients (specifically N) in the liquid effluent. In regenerative life support systems, CO₂ and nutrients could then be further used in plant production; thus creating a closed carbon and nutrient cycle. The effect of the operational parameters in water oxidation on carbon recovery (C-to-CO₂) and nitrogen conversion (to NH₄⁺, NO₃^-) was investigated. A batch micro-autoclave reactor was used, at pressures ranging between 110 and 300 bar and at temperatures of 300-500 °C using hydrogen peroxide as oxidizer. Residence times of 1, 5 and 10 min were tested. Oxidation efficiency increased as temperature increased, with marginal improvements beyond the critical temperature of water. Prolonging the residence time improved only slightly the carbon oxidation efficiency. Adequate oxygen supply, i.e., exceeding the stoichiometrically required amount, resulted in high carbon conversion efficiencies (>85%) and an odorless, clear liquid effluent. However, the corresponding oxidizer use efficiency was low, up to 50.2% of the supplied oxygen was recovered as O₂ in the effluent gas and did not take part in the oxidation. Volatile fatty acids (VFAs) were found as the major soluble organic compounds remaining in the effluent liquid. Nitrogen recovery was high at 1 min residence time (>94.5%) and decreased for longer residence times (down to 36.4% at 10 min). Nitrogen in the liquid effluent was mostly in the form of ammonium.

Enhanced anaerobic degradability of highly polluted pesticides-bearing wastewater under thermophilic conditions

This work presents a sustainable and cost-competitive solution for hardly biodegradable pesticides-bearing wastewater treatment in an anaerobic expanded granular sludge bed (EGSB) reactor at mesophilic (35°C) and thermophilic (55°C). The reactor was operated in continuous mode during 160days, achieving an average COD removal of 33 and 44% under mesophilic and thermophilic conditions, respectively. The increase of temperature improved the biomass activity and the production of methane by 35%. Around 96% of pesticides identified in raw wastewater were not detected in both mesophilic and thermophilic effluents. A dramatic selection of the microbial population in anaerobic granules was caused by the presence of pesticides, which also changed significantly when the temperature was increased. Pesticides caused a significant inhibition on methanogenesis, especially over acetoclastic methanogens. Aerobic biodegradability tests of the resulting anaerobic effluents revealed that aerobic post-treatment is also a feasible and effective option, yielding more than 60% COD reduction.