Environmental Engineering Reference
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2.5. Supercritical Water Oxidation (SCWO)
Supercritical Water Oxidation, SCWO, also referred to as hydrothermal oxidation, is a
process that oxidizes organic solutes in an aqueous medium using oxygen/air or hydrogen
peroxide as oxidants, at temperatures and pressures above the critical point of water, i.e.,
374 o C and 22MPa, respectively (Bermejo et al., 2006). The SCWO process has been under
development since the early 1980's when the well known process of wet oxidation was
developed at MIT (Modell, 1982). SCW is a superior reaction medium with a high diffusivity,
a low viscosity, and relatively high-density therefore rapid oxidation reactions are expected.
Moreover, the low temperature of the SCWO process in comparison to conventional
combustion can lead to a greatly reduced NOx and SO 2 formation. In addition, water is not
only the reaction medium but it participates directly in the reaction through the formation of
free radicals (Griffith and Raymond, 2002). Since water is utilized in the reaction there is no
requirement to dewater the sludge before processing. Sludge can be processed at 10% solids
by weight or even less (Mahmood and Elliott, 2006).
Figure 8 outlines a schematic of a SCWO process. Pressurized sludge (25.5MPa) and
pressurized oxygen are fed into the preheater reactor at 25 degrees Celsius. In the preheater
the mixture of sludge and oxygen are heated up to approximately 300 to 400 o C, to achieve the
supercritical state of water. Water at its supercritical state can dissolve organics and hydrolyze
even polymers and hence prevent the formation of char (Perry and Green 1999; Fang and
KoziƄski, 2000; Mahmood and Elliot, 2006). The reaction mixture enters the main reactor
where the remaining portion of the organics is oxidized in short hydraulic residence time of 5-
10 min at the maximum process temperature of around 600 o C (Mahmood and Elliott, 2006).
After reaction, the effluent is cooled and energy is recovered. According to Svanstrom et al.
(2004), about half of the heating value of the sludge can be recovered in the studied process.
The solid and liquid products are separated and the wet inorganic solids can be sent to a
landfill or spread on dedicated land while the water can be redirected to the wastewater
treatment plant.
Figure 8. Schematic of a SCWO system (modified from Mahmood and Elliott, 2006).
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