A Review of Challenges and Recent Progress in Supercritical Water Oxidation of Wastewater

Comprehensive analysis (aerobic/anaerobic, molecular recognitions, band-position and degradation-mechanism) of undoped and Co-doped anatase-brookite - An experimental/theoretical evaluation of the less-studied TiO2 mixed phase

The molecular recognition (MRec) effect is required in the initial phase of organic reactions. The second stage involves molecular-orientations and molecular-orbitals energy-levels (MOrbE). The components of a reaction must be compatible in terms MRec and MOrbE. Therefore, the comprehension of photocatalytic systems applied in wastewater treatment will be improved if the MRec effect is also considered as an important factor. The purpose of this study is to provide a comprehensive understanding of the less studied anatase-brookite mixed-phase (doped and undoped). Anatase/brookite photocatalytic systems were evaluated utilizing experimental/theoretical approaches in H₂O (aerobic/anaerobic) environments with Vis-light and the organic pollutant (OrPo) methyl orange (MO). The compatibility of MRec and MOrbE of anatase-brookite mixed-phase (with the different reactive system components) confirmed this is the optimal combination for photocatalytic application. Using the sol-gel method, AM-TiO₂NP (amorphous), TiO₂NP (crystalline), and TiO₂NP-Co0.1 at% (crystalline Co-doped) anatase-brookite mixed-phase photocatalysts were obtained. The morphology and surface were characterized using XRD, BET, SEM, HR-TEM, FT-IR and XPS. Employing UV-vis DRS and PL, photo-response and electron-hole recombination were studied. LVS and Mott-Schottky plot were employed to determine photo-electrochemical activity. The results of TiO₂NP photocatalytic degradation in both aerobic and anaerobic environments are remarkable. The results of molecular dynamics (MD) simulation and Fukui Function (FF) based on density functional theory (DFT) validate the remarkable photocatalytic MO degradation.

Fabrication and Supercritical Water Corrosion Resistance of TiN/TiNC/Al2O3 Multilayer Coating on Inconel 625 Alloy by CVD Method

A TiN/TiNC/Al₂O₃ multilayer coating was deposited on an Inconel 625 alloy by the chemical vapor deposition method as a protective barrier to improve the corrosion resistance in supercritical water. The corrosion characteristics were evaluated in a reactor at 500 °C and 25 MPa for 72 h. The surface morphology of the coated samples was relatively dense with no obvious cracks or pores observed. The XRD analysis revealed that the coatings were composed of TiN, TiNC and α-Al₂O₃ phases. After exposure to supercritical water, the surface morphology of the coatings was still dense and kept integrity. The phase composition of the coatings was also not changed, with no obvious corrosion scales detected. This result demonstrates the effectiveness of TiN/TiNC/Al₂O₃ coatings as a protective coating under harsh supercritical water environments.

Characteristics, mechanisms and measurement methods of dissolution and deposition of inorganic salts in sub-/supercritical water

The efficient and harmless treatment of hypersaline organic wastes has become an urgent environmental problem. Compared to traditional thermochemical methods, supercritical water oxidation has been proven to be an efficient organic waste treatment technology due to the advantages of low cost, high degradation rate, no secondary pollutants, etc. However, the solubilities of inorganic salts drop rapidly near the critical point of water, and some sticky salts form easily agglomerates and then adhere to internal surfaces of reactor and pipeline, causing plugging and inhibition of heat transfer. Hence, the characteristics, mechanisms and measurement methods of the dissolution and deposition of inorganic salts in sub-/supercritical water are summarized and analyzed systematically and comprehensively in this work, intending to provide a valuable guide for salt deposition prevention and subsequent research directions. Firstly, a new classification form of inorganic salt is put forward based on melting point. The phase equilibriums of brine systems are then analyzed in detail. Six theories concerning dissolution mechanisms are discussed deeply and various measurement methods of salt solubility are also supplemented. Furthermore, salt deposition characteristics and related measurement technologies are summarized. Notably, a new idea "hydrothermal molten salt" system is reviewed which may provide a solution for salt deposition in sub/supercritical water. Finally, an outlook for the follow-up researches is prospected and some suggestions are proposed.

Removal of contaminants of emerging concern from secondary-effluent reverse osmosis retentates by continuous supercritical water oxidation- parametric study and conceptual design

The continuous removal of TOC and the degradation efficiency of carbamazepine and 17β-estradiol were investigated using actual secondary municipal-effluent RO-retentate solutions. A specific set of operating parameters were applied within the supercritical water oxidizing conditions: temperature range 420-480 °C, 25.1 MPa, hydraulic retention time (HRT) of 1-2 min, excess oxidant molar-ratio of 3-10 and presence of a homogenous catalyst (IPA) at 50-100 mg/L. > 99% organic carbon mineralization, along with complete degradation of model pollutants, was observed at 450 °C/1 min/OC= 5-10 and 100 mgIPA/L. The outlet estrone concentration, 1.03 ± 1.14 ng/L, representing estrogenic pollutants, dropped to the "no effect" range. A model for a SCWO plant treating secondary-municipal-effluent-RO-retentate for a city of 100,000 capita-equivalent was developed, based on a shell & tube SCWO flow reactor, showing > 75% energy-efficiency. The model yielded that for the extreme case of a zero caloric-value feed-solution, the total OPEX and CAPEX would be < $6.0 ± 2.5 per m³ of secondary effluents, i.e., two orders of magnitude lower than the reported environmental shadow-price associated with CECs (contaminants of emerging concern). Further work is required on the continuous and efficient separation of the salt-matrix, which can lead to higher overall heat transfer coefficients and enable further reduction in capital costs.

Analysis of the Energy Flow in a Municipal Wastewater Treatment Plant Based on a Supercritical Water Oxidation Reactor Coupled to a Gas Turbine

Biological municipal wastewater treatments lead to high sludge generation and long retention times, and the possibilities for recovery of the energy content of the input waste stream are very limited due to the low operating temperature. As an alternative, we propose a sequence of exclusively physicochemical, non-biological stages that avoid sludge production, while producing high-grade energy outflows favoring recovery, all in shorter times. Ultrafiltration and evaporation units provide a front-end concentration block, while a supercritical water oxidation reactor serves as the main treatment unit. A new approach for energy recovery from the effluent of the reactor is proposed, based on its injection in a gas turbine, which presents advantages over simpler direct utilization methods from operational and efficiency points of view. A process layout and a numerical simulation to assess this proposal have been developed. Results show that the model process, characterized with proven operating parameters, found a range of feasible solutions to the treatment problem with similar energy costs, at a fast speed, without sludge production, while co-generating the municipality’s average electricity consumption.

Subcritical and supercritical water oxidation for dye decomposition

The total annual output of synthetic dyes exceeds 7 × 10⁵ tons. About 1,000 tons of non-biodegradable synthetic dyes are released every year into the natural streams and water sources from textile wastes. The release of these colored wastewater exerts negative impact on aquatic ecology and human beings because of the poisonous and carcinogenic repercussions of dyes involved in coloration production. Therefore, with a growing interest in the environment, efficient technologies need to be developed to eliminate dyes from local and industrial wastewater. Supercritical water oxidation as a promising wastewater treatment technology has many advantages, such as a rapid reaction and pollution-free products. However, due to corrosion, salt precipitation and operational problems, supercritical water oxidation process did not gain expected industrial development. These technical difficulties can be overcome by application of non-corrosive subcritical water as a reaction medium. This work summarizes the negative impacts of dyes and role of subcritical and supercritical water and their efficiencies in dye oxidation processes.

Thermodynamics and Dynamics of Supercritical Water Pseudo-Boiling

Supercritical fluid pseudo-boiling (PB), recently brought to the attention of the scientific community, is the phenomenon occurring when fluid changes its structure from liquid-like (LL) to gas-like (GL) states across the Widom line. This work provides the first quantitative analysis on the thermodynamics and the dynamics of water's PB, since the understanding of this phase transition is mandatory for the successful implementation of technologies using supercritical water (scH2O) for environmental, energy, and nanomaterial applications. The study combines computational techniques with in situ neutron imaging measurements. The results demonstrate that, during isobaric heating close to the critical point, while water density drops by a factor of three in the PB transitional region, the system needs >16 times less energy to increase its temperature by 1 K than to change its structure from LL to GL phase. Above the PB-Widom line, the structure of LL water consists mainly of tetramers and trimers, while below the line mostly dimers and monomers form in the GL phase. At atomic level, the PB dynamics are similar to those of the subcritical water vaporization. This fundamental knowledge has great impact on water science, as it helps to establish the structure-properties relationship of scH2O.

Complete oxidation of organic waste under mild supercritical water oxidation by combining effluent recirculation and membrane filtration

Supercritical water oxidation (SCWO) is a technology that can oxidize various organic (wet) wastes into CO₂. Complete oxidation of specific organics with SCWO goes in tandem with tailored conditions, typically involving elevated operating temperatures, long residence times, high oxidizer-to-waste ratios, or a combination of those, which promote difficulties, e.g., corrosion. These challenges hamper the practical implementation of SCWO, albeit SCWO offers excellent oxidation efficiencies. This work proposes a novel process combining mild supercritical water oxidation (SCWO) with membrane filtration to enhance the oxidation of organics. The modified SCWO works at mild reaction conditions (i.e., 380 °C, 25 MPa and oxidizer equivalence ratios as low as 1.5) to potentially decrease the risks. The membrane filtration discards clean effluent and recycles the retentate (containing incomplete oxidized organics) back to the mild SCWO process for further oxidation, thereafter resulting in near-complete removal of organics. Fresh feed is continuously added, as in the conventional process, along with recycled retentate to guarantee the throughput of the modified SCWO process. A mixture of SCWO-resistant volatile fatty acids (TOC = 4000 mg·L^-1) was studied to validate the proposed process. The proposed process in this study enhances the organic decomposition from 43.2% to 100% at mild conditions with only 10% capacity loss. CO₂ was the dominant gas product with traces of CO and H₂. Carbon output in the gas products increased with recirculation and got close to the carbon input of the freshly added feed ultimately. The results indicated that the proposed process maximized the benefits of both technologies, which allows the development of a technological process for supercritical water oxidation, as well as a new stratagem for waste treatment.

Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes

Supercritical water oxidation (SCWO) is a promising wastewater treatment technology owing to its various advantages such as rapid reactions and non-polluting products. However, problems like corrosion and salt decomposition set obstacles to its commercialization. To address these problems, researchers have been developing the optimal reactor design and strengthening measures based on sufficient understandings of the degradation kinetics. The essence of the SCWO process and the roles of oxygen and hydrogen peroxide are summarized in this work. Then, the research status and progress of empirical models, semi-empirical models, and detailed chemical kinetic models (DCKMs) are systematically reviewed. Additionally, this paper is the first to summarize the research progress of quantum chemistry and molecular dynamics simulation. The challenge and further development of kinetics models for the optimization of reactors and the directional transformation of pollutants are pointed out.

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.

A novel experimental device for electrochemical measurements in supercritical fluids up to 700 °C/1000 bar and its application in the corrosion study of superalloy Inconel 740H

The design of the device enables in situ electrochemical measurements in ultra-supercritical fluids up to 700 °C/1000 bar.