HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 176

produce additional hydroxyl radical, which, in turn, oxidizes pollutants to
more readily biodegradable compounds.
• Chlorine . Chlorine and its various compounds are used extensively in water
and wastewater treatment. Chlorine is the principal chemical involved in
disinfection. However, when combined with organic material, chlorine
forms trihalomethane (THM), which is carcinogenic. Chlorine is evap-
orated to a gas and mixed with water to provide a hypochlorous acid
(HOCl) solution. Hypochlorous acid is then converted to hypochlorite ion.
The reaction is pH dependent and as pH increases the oxidation power
increases as well. One of the most commonly used hazardous waste treat-
ment process is oxidation of cyanide by alkaline chlorination process. The
purpose is to add chlorine and convert cyanide to cyanate. However, pH
must be greater than 10 to prevent the formation of cyanogen chloride,
which is a very toxic gas and can exist at lower pH levels. Cyanate pro-
duced is less toxic and hydrolyze under acidic conditions in lakes and
streams to ammonia and carbon dioxide. In this reaction, a pH of 8.5
is used to simplify operation to permit the last two reactions to occur
sequentially in a single treatment unit (Weber 1972).
2.3.2 Supercritical Water Oxidation
Supercritical water oxidation (SCWO) is an emerging technology in which dilute
concentrations of organic and inorganic wastewaters are oxidized under high
temperature and pressure. The temperature and pressure conditions of the water
are elevated to supercritical conditions that exert properties between those of
gas and liquid. When air and contaminated water are brought together above
the critical point of water, complete oxidation of organic contaminants occur
rapidly. As a result, organic contaminants dissolve and become soluble in the
high temperature and pressure environment exerted by the supercritical water,
and then they become oxidized.
Major design considerations of SCWO include the reactor and heat exchanger.
Design involves considerations of residence time, temperature, and materials of
construction. Precipitated inorganic salts may adhere to the reactor walls and
decrease effective volume available for the reaction and reduce residence time in
the reactor. In addition, the reactors must be resistant to corrosion because they
are exposed to high temperatures and high pressure. In the SCWO treatment, one
advantage is that organic compounds are destroyed rather than being removed,
and the chemical reactions are carried out in a closed system where there is
physical control and maintenance. The process is capable of generating all power
required for air compression and feed pumping.
Supercritical water oxidation has been proven to be effective technology for the
removal of organic compounds such as trichloroethylene (TCE), trichloroethane
(TCA), PCBs, organochlorine insecticides, dyes, pulp and paper wastes, and
chlorinated dioxins (Jensen 1994; Woodside, 1999; Watts 1997).