Environmental Engineering Reference
In-Depth Information
1.1.1.4.11 Water Separator
Water separators have been used in solvent distillation stills for dry cleaners and vapor degreasers
since the early 1900s. The distilled solvent and water mixture sits in a gravity separator unit. Heavier
perchloroethylene drains from the bottom of the unit to the solvent tank, and excess water is drained
off the top of the separator. In earlier decades, separated water was drained to l oor drains or onto the
ground; later it was routed to the sanitary sewer system (Morrison, 2002a). In the past, it was legal
to discharge wastewater to the sanitary sewer system, and high levels of perchloroethylene in the
separator water were routinely so discharged. For example, levels of perchloroethylene up to
1120 mg/L, with an average of 152 mg/L, were detected in samples of separator water; as much as
30% of the samples were pure solvent (Cohen and Izzo, 1992). Among older dry-cleaning machines
(ca. 1986), discharges from water separators commonly caused soil and groundwater contamination
(Hoenig, 2002). Leaking sewer lines also facilitated perchloroethylene release to the subsurface
(Cohen and Izzo, 1992).
1.1.1.5 Off-Site Solvent Recycling
Industrial plant managers and industrial engineers found various ways of dealing with the costs of
solvent consumption and disposal of waste solvent. Improvements in degreasing and dry-cleaning
equipment signii cantly reduced consumption. As solvent costs rose, operators switched to more
efi cient equipment. When regulations prohibited land disposal of waste solvent, the cost of permit-
ted disposal was a further incentive for improving the efi ciency of solvent use in various operations
and to use on-site solvent-recovery operations. Table 1.12 summarizes the relative ranks of solvents
recycled in the United States in 1981.
Prohibitions of land disposal of waste solvent redirected most of this waste stream to the solvent-
recycling industry, which collected still bottoms from degreasing operations for distillation and solvent
reclamation. Solvent recycling was already a viable business before the adoption of land disposal prohi-
bitions because it offered attractive economic incentives for the generators of waste solvent. Recovered
solvents were sold, and distillation residues were discarded or treated as hazardous waste. Prior to the
adoption and enforcement of the Resource Conservation and Recovery Act in 1985, still bottoms and
other waste solvent from recycling operations were generally sent off-site; the waste went into landi lls
and deep injection wells or was burned as fuel for blast furnaces in the steel industry or as fuel for
cement kilns. In 1985, because steel mills abruptly stopped taking waste solvent to avoid the burden of
obtaining permits as hazardous waste storage facilities, cement kilns became a primary destination
for waste solvent (Douthitt, 1990). The oil residue in waste solvent provided enough heat energy
(8000-9000 Btu) to make the waste attractive as fuel for kilns and boilers, provided the metal and chlo-
TABLE 1.12
Ranks of Chlorinated Solvents by Relative Quantity Recycled in 1981
Solvent
Solvent Recycling Rank
Methyl chloroform
2
Dichloromethane
5
Trichloroethylene
7
Perchloroethylene
8
Source:
Douthitt, C.A., 1990, Resource recovery through solvent reclamation. Presentation to
the U.S. Department of Energy Industrial Solvent Recycling Conference October 15,
p2pays.org/ref/26/25692.pdf
(accessed February, 2006).
Note:
Other recycled solvent ranks: methyl ethyl ketone (#1), acetone (#3), xylene (#4), and
toluene (#6).
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