**1. Introduction**

Supercritical water (SCW) refers to water whose temperature and pressure are above its critical point (374.15°C, 22.1 MPa). The physical properties of SCW such as density, viscosity, dielectric constant, hydrogen bonding, and ionic product is vastly different from those of normal water and steam. The density of SCW is about 1/3 of the normal state, the dielectric constant is reduced from 87.7 at 0°C to 2–30 of SCW, and the ion product is reduced 103 to 109 times than that of normal water. Supercritical water has a low dielectric constant, and thus is extremely soluble to nonpolar compounds such as organic matter and oxygen. In addition, SCW has a significantly lower viscosity than that of the normal water, the diffusion coefficient is improved, the mass transfer performance is enhanced, and thus it can be completely miscible with nonpolar gases and hydrocarbons. In view of the special properties of SCW, nonpolar molecules such as O2 and organic matter have strong solubility in supercritical aqueous systems, which can be mutually dissolved in any ratio to form a homogeneous phase, and the mass transfer resistance at the interface

is eliminated. There, SCW can be used as an excellent reaction medium for organic matter and oxygen [1].

Supercritical water oxidation (SCWO), which was firstly proposed by Modell in MIT in the middle of 1980s, is an effective and advanced oxidation technology to destruct organic matters by taking advantage of the unique properties of SCW under the typical operation conditions of 450–600°C, 24–28 MPa. A schematic of a typical SCWO process is displayed in **Figure 1**. In SCWO reactor, organic wastes can be thoroughly oxidized and decomposed into harmlessly small molecules, such as CO2, N2, water, etc., under excess oxidants in single-phase SCW. Hetero-atoms in organic matters are mineralized into corresponding acids or inorganic salts, and the formation of nitrous oxides is inhibited owing to the low reaction temperature. SCWO is particularly suitable for disposing organic wastewaters with high toxicity, high concentration, and bio-refractory components [2–4]. It can also recover energy and achieve heat self-sufficiency easily to ensure an economic advantage [5]. When a mass concentration of organic matters in feedstock is in the range of 3–4%, the whole reaction process does not require an extra energy input commonly. Furthermore, compared with incineration, SCWO does not have the problems of high cost, public resentment, and secondary pollutants like dioxins [5]. Hence, SCWO has attracted much attention in the past three decades. Brunner [1] has summarized SCWO results of real waste materials including textile wastewater, wastewater from terephthalic acids, food wastes, municipal excess sludge, and alcohol distillery wastewater. Furthermore, Veriansyah and Kim [5] have also systematically introduced SCWO experiments of toxic organic wastes such as pesticide, bacteria and dioxins, chlorophenol and chlorobenzene, pharmaceutical and biopharmaceutical wastes, solid rocket propellants, military wastes, etc. Some other feedstocks, including landfill leachate [6–9], industrial dyeing wastewater [10, 11], polychlorinated biphenyls (PCBs) [12], chlorinated wastes, oily wastes, etc., have also been disposed via SCWO efficiently.

Nowadays, SCWO plants have been commercialized by several famous companies and universities [14–16], such as General Atomics, Chematur AB, HydroProcessing, Supercritical Fluids International (SCFI), SuperWater Solution, Xi'an Jiaotong University (XJTU), etc. In 2001, two commercial-scale SCWO

**133**

respectively.

*Supercritical Water Oxidation for Environmentally Friendly Treatment of Organic Wastes*

plants were installed in Harlingen (Texas, USA) to treat sewage sludge with the solid-containing rates of 7–8 wt% [14]. Another SCWO plant with a treatment

deal with sewage sludge [17]. From 2009 to 2011, SuperWater Solution installed and successfully tested a 5 dry t/d SCWO system for the Iron Bridge Regional Water Reclamation Facility in Orlando, FL, USA [16]. In 2007, the SCFI Group acquired patented SCWO technology (AquaCritox®) from the Sweden Chematur Engineering AB. Further work directed by the SCFI group has emphasized the reduction of costs. In 2015, the world's first 100 t/d SCWO plants for industrial refractory wastewater treatment was designed and constructed under the guidance of the authors. Besides, there were some other pilots or full-scale SCWO plants for the harmless treatment of sewage sludge and organic wastewaters. Nevertheless, the three key problems concerning corrosion, plugging triggered by salt precipitation, and high running cost still exist [18], even make some commercial scale SCWO

The harsh conditions involved in SCWO process, such as high temperature, high pressure, excessive oxygen, corrosive ions, etc., easily induce severe reactor corrosion problems, meaning a shorter reactor life. On the other hand, inorganic salts will precipitate from SCW due to its extremely low dielectric constant, which may result in reactor plugging triggered by deposited salts and further cause expensive and frequent shutdowns of the whole SCWO plant. These two aspects, to a certain degree, are still not effectively solved and seriously hinder the extensive commer-

Corrosion is a key obstacle to limit the commercial application of SCWO, which not only shortens reactor life but also induces a bad treatment effect of feedstock due to the formations of corrosion products. Harsh operation conditions (high concentration of oxidants, extreme pH values, high temperature, and pressure) together with reaction intermediate/ultimate products (high concentrations of ionic species, free radicals, and acids) result in severe corrosion problem in SCWO reactors. Corrosion mainly occurs on reactor's inner wall; it also appears in heat exchanger and cooler on inlet and outlet pipelines of the reactor [19–21].

Materials serving for SCWO include stainless steels, nickel-based alloys, titanium, tantalum, noble metal ceramics, etc. [21–25]. A series of investigations on the corrosion resistances of these materials under supercritical and/or subcritical conditions [20, 24, 26–30], reflected that no one kind of material can withstand corrosion at all conditions, but some exhibit perfect corrosion resistance under specific conditions, as given in **Table 1**. Thus, appropriate reaction conditions such as heteroatom types in feedstock, reaction temperature, and pressure should be optimized in order to minimize corrosion rate for a chosen reactor construction material. Generally speaking, nickel-based alloys show a relatively good corrosion resistance among all the acids listed in **Table 1** under supercritical conditions. Titanium is fit to be employed under subcritical conditions, and is a potential proper liner of preheater and cooler being installed before and after reactor,

Corrosion in SCWO circumstance also depends on parameters concerning both materials (alloy composition, surface condition, material purity, and heat

/h has also been built by Chematur Engineering AB (Sweden) to

*DOI: http://dx.doi.org/10.5772/intechopen.89591*

capacity of 7 m3

plants inactive.

cialization of SCWO.

**2.1 Corrosion**

**2. Key problems and status**

**Figure 1.** *A schematic of a typical SCWO process for sludge treatment [13].*

*Supercritical Water Oxidation for Environmentally Friendly Treatment of Organic Wastes DOI: http://dx.doi.org/10.5772/intechopen.89591*

plants were installed in Harlingen (Texas, USA) to treat sewage sludge with the solid-containing rates of 7–8 wt% [14]. Another SCWO plant with a treatment capacity of 7 m3 /h has also been built by Chematur Engineering AB (Sweden) to deal with sewage sludge [17]. From 2009 to 2011, SuperWater Solution installed and successfully tested a 5 dry t/d SCWO system for the Iron Bridge Regional Water Reclamation Facility in Orlando, FL, USA [16]. In 2007, the SCFI Group acquired patented SCWO technology (AquaCritox®) from the Sweden Chematur Engineering AB. Further work directed by the SCFI group has emphasized the reduction of costs. In 2015, the world's first 100 t/d SCWO plants for industrial refractory wastewater treatment was designed and constructed under the guidance of the authors. Besides, there were some other pilots or full-scale SCWO plants for the harmless treatment of sewage sludge and organic wastewaters. Nevertheless, the three key problems concerning corrosion, plugging triggered by salt precipitation, and high running cost still exist [18], even make some commercial scale SCWO plants inactive.
