|Fig. 1: Overview of conventional SCWO process, after Marrone et al. |
Water is said supercritical when it is heated above its critical temperature (374°C) and compressed above its critical pressure (22.1 MPa).  In that state, the fluid has a density between that of water vapor and liquid, and shows high diffusion rates as well as high collision rates. Also, water becomes more like a nonpolar solvent when supercritical, so when water, organic compound (waste), and oxygen are mixed together and given high heat and pressure, both organic compounds and oxygen are soluble.  And high temperature allows oxidation reactions to occur from dissolved oxidizer, and the organic wastes oxidize rapidly to CO2 and H2O.  Also, hazardous air pollutants (NOx) are produced much less because high temperature discourages NOx to be catalyzed, but favors the production of N2 instead.  Some of the applications of supercritical water oxidation (SCWO) include treating hazardous waste. The conventional process of SCWO is shown in Fig. 1.
The first company to employ SCWO for commercial use of waste treatment was MODAR, Inc in Japan in the 1980s.  It developed a large-scale reactor system for treating industrial wastes, specifically semiconductor manufacture wastes.  The problem with the initial commercialization of SCWO was the high operating pressure, salt formation that can deter function of the reactor, and high processing cost before commercialization.  But due to ongoing research to solve these issues, waste treatment using SCWO was implemented in other countries like United States, United Kingdom, and Korea.
SCWO was applied to treat many different types of industrial wastes, including sewage sludge, pharmaceutical wastes, and chemical explosives and weapons in the military. First, SCWO has also been used to treat pharmaceutical and biopharmaceutical wastes. Aki and Abraham (1999) looked at the case of pyridine, and found that supercritical water effectively oxidized pyridine in aqueous waste system.  Qi et al. did it for the case of aniline.  Another field where SCWO has been employed is in the military for the destruction of the chemical weapons stockpile. Because of the health concerns in the neighborhood of the destruction cites, SCWO has been selected as alternative technologies to simply incinerating military wastes.  Also SCWO is used to destroy chemical warfare agents such as smokes, dyes, and pyrotechnics, and has been implemented by the US military. 
But what is the future of SCWO? Is it sustainable enough to be continuously commercialized? If so, what are some of the new fields where SCWO can be implemented? Although SCWO technology respects environment and eliminate toxic organic wastes from aqueous wastes by adding oxidant, heat, and pressure, its usage is limited to industrial scale. One suggestion to advance the commercial development of SCWO is to choose the most suitable reactor concept, the correct operational method, and the appropriate technical solution so that the SCWO can be applied in a more customized and effective way.  More research on these issues might expand the current usage of SCWO in large-scale, industrial level to municipal level that treats non-industrial wastewater. Also, I think more research should be done to ultimately find a method of treating organic waste, possibly at a lower cost and less limitation.
© Hailey Kwon. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 S. N. V. K Aki and M. A Abraham, "Catalytic Supercritical Water Oxidation of Pyridine: Kinetics and Mass Transfer," Chem. Eng. Sci. 54 3533 (1999).
 X.-H. Qi et al., "Decomposition of Aniline in Supercritical Water," J. Hazard. Mater. 90 51 (2002).