HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 171

The emergence of the Resource Conservation and Recovery Act in 1976 and
its amendments in 1984 led to state-of-the-art technologies for the treatment of
hazardous waste. The main purpose was to clean up air, water, land, and ground-
water. The different hazardous waste technologies covered in this chapter are
grouped under physicochemical treatment, biological treatment, thermal treat-
ment, and land treatment categories.

2 PHYSICAL/CHEMICAL TREATMENT SYSTEMS

2.1 Stripping

2.1.1 Air Stripping
Air stripping removes volatile contaminants (VOCs), dissolved gasses, and
semivolatile organic contaminants from the waste stream. It is mostly suitable
for removing wastewaters with VOC concentrations less than 200 mg/l
(LaGrega et al. 2001). VOCs are chemical compounds that have a tendency to
evaporate faster. Some examples of VOCs can be treated with air stripping,
including benzene, toluene, ethylbenzene, xylene (BTEX), and solvents such as
trichloroethylene (TCE), dichloroethylene, chlorobenzene, and vinyl chloride.
Air stripping takes place in a stripping tower. The tower contains a packing
medium. The wastewater contaminated with VOCs is introduced from the top
of a packing media located in the tower. Air is introduced from the bottom
of the tower. The contaminated water flows by forming a thin film over the
medium. Stripping occurs when the wastewater and the air stream contact with
each other and the dissolved molecules are transferred from a liquid into flowing
gas. The governing equilibrium between the dissolved water phase and a gas
phase (air) is based on Henry's law. According to Henry's law, partial pressure
of a gas in the air above a dilute aqueous solution is proportional to its con-
centration in the solution. If Henry's law constant is high, the contaminant has
low solubility in water and can be stripped easily. In addition, there is a direct
correlation between Henry's constant and temperature: As temperature increases,
air stripping efficiency increases as well.
Upon contact, the VOCs are vaporized and are collected in the air stream
leaving the tower. The air stream exiting the tower needs to be further treated
by using processes such as activated carbon. The different types of stripping
towers include tray towers, packed towers, spray towers, diffused towers, and
mechanical aeration towers.

2.1.2 Steam Stripping
Steam stripping is a process used to remove higher concentrations, up to several
percent by weight of volatile organic compounds (VOCs), in an aqueous waste
stream. It is also used to treat compounds that are not readily air-strippable, such
as acetone, methanol, and pentachlorophenol. Nonaqueous wastes (such as spent