HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 187

organics that can serve as additional substrates for the bacteria that degrade haz-
ardous contaminants. The plant roots can serve as adsorption sites and can provide
more oxygen into the soil. The plants can also uptake organics and degrade them
by using their enzymes or can respire them into the air, called phytovolatiliza-
tion , which helps remove compounds such as trichloroethylene (TCE), ben-
zene, toluene, ethyl benzene, xylene (BTEX), and chlorinated benzenes (LaGrega
et al. 2001).
Plants can uptake metals and transform them to less toxic forms by chang-
ing their redox states, such as in the case of conversion of toxic hexavalent
chromium to trivalent chromium. Plants can also accumulate metals in their roots
and above-ground portions and remove them from contaminated soils. Several
applications of phytoremediation include in situ cleanup, where plants uptake
contaminants from soil or groundwater; and ex situ cleanup, where plants are
grown in contaminated water, which is also referred to as hydroponics.
The treatment efficiency is depends on the ability of plants' uptake of the
constituents by the root system through the plant stems and leaves. Success also
depends on the contaminant of concern, existing vegetation, and potential risk to
humans.

4 THERMAL TREATMENT SYSTEMS — INCINERATION

Incineration is the process of controlled burning at high temperatures of solid,
liquid, or gaseous wastes. It reduces the volume and weight of hazardous waste
to a fraction of its original size. This process also converts hazardous organic
compounds to ash. The three most important operating conditions for proper
incineration are temperature, residence time, and turbulence, which are called
the three Ts of incineration. Incineration of waste materials converts hydrogen
to water vapor, chloride or fluoride to hydrochloric acid (HCl), or hydrofluoric
acid (HF), carbon to carbon dioxide (CO 2 ), sulfur to sulfur dioxide (SO 2 ), alkali
metals to hydroxides, and nonalkali metals to oxides. Waste consistency depends
on the type of waste produced and can be categorized into the following:
• Sludge materials are too viscous, abrasive, or varying in consistency.
• Wastes undergo partial or complete phase change during incineration.
• High residue materials, high ash liquids, and sludge and materials contain
salts or metals.

4.1 Gaseous Waste Incinerators

Incineration for gaseous wastes may be divided into three classifications: direct
flame, thermal, and catalytic. Direct-flame incineration is typically applied where
the gas stream has sufficient energy value to maintain a flame without the need
to provide a supplemental fuel source. A common application of direct-flame