Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 140

technology that has shown some promise in the treatment of PCBs and other sta-
ble compounds that resist chemical reaction. Wet air oxidation has generally been
limited to conditioning of municipal wastewater sludges but can degrade hydro-
carbons (including PAHs), certain pesticides, phenolic compounds, cyanides, and
other organic compounds. Oxidation may benefit from catalysts.
Vitrification uses electricity to heat and destroy organic compounds and im-
mobilize inert contaminants. A vitrification unit has a reaction chamber divided
into two sections: the upper section to introduce the feed material, contain-
ing gases and pyrolysis products, and the lower section consisting of a two-layer
molten zone for the metal and siliceous components of the waste. Electrodes are
inserted into the waste solids, and graphite is applied to the surface to enhance
its electrical conductivity. A large current is applied, resulting in rapid heating
of the solids and causing the siliceous components of the material to melt as
temperatures reach about 1600 ◦ C. The end product is a solid, glasslike material
that is very resistant to leaching.
All of these methods are energy intensive. This is another reason to avoid
generating wastes in the first place.
Indirect Pollution
In addition to direct treatment, air pollution is a concern for other means of
treating hazardous wastes, especially when these wastes are stored or treated more
passively, such as in a landfill or aeration pond. Leachate collection systems (see
Fig. 3.13) provide a way to collect wastes which can then be treated. However,
such pump-and-treat systems can produce air pollutants. Actually, this often inten-
tional. For example, groundwater is treated by drilling recovery wells to pump
contaminated groundwater to the surface. Commonly used groundwater treat-
ment approaches include air stripping, filtering with granulated activated carbon
(GAC), and air sparging. Air stripping transfers volatile compounds from water
to air (see Fig. 3.14). Ground water is allowed to drip downward in a tower filled
with a permeable material through which a stream of air flows upward. Another
method bubbles pressurized air through contaminated water in a tank. The air
leaving the tank (i.e., the off-gas) is treated by removing gaseous pollutants. Fil-
tering groundwater with GAC entails pumping the water through the GAC to
trap the contaminants. In air sparging, air is pumped into groundwater to aerate
the water. Most often, a soil venting system is combined with an air sparging
system for vapor extraction, with the gaseous pollutants treated as in air stripping.
Regulatory agencies often require two or three pairs of these systems as design
redundancies to protect the integrity of a hazardous waste storage or treatment
facility. A primary leachate collection and treatment system must be designed like
the bottom of a landfill bathtub. This leachate collection system must be graded