Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 16

water in the separator should be monitored. If operators found the water layer to be acidic, it was
replaced or buffered with an addition of a small amount of sodium bicarbonate to maintain the pH
above 7. Some operations l ushed the water layer weekly (Dow Chemical Company, 1999b). Operators
were also directed to open the valve that drained the water off the top of the water separator fre-
quently to allow accumulated water to drain out (ASTM, 1962). Wastewater from degreasers can be
expected to contain solvents at concentrations equal to the solvent's solubility in warm water.
The volume of water condensed in a vapor degreaser is largely a function of the humidity at the
degreaser location. The fate of water removed from degreasers varied with regulations in effect
while the degreaser was operated and with the degree of operator compliance, as well as regulatory
outreach and enforcement. Prior to regulation, degreaser wastewater was discharged to sewers, dry
wells, storm drains, evaporation ponds, or the ground, or placed in drums and sent to unregulated
landi lls. Under actively enforced regulation, degreaser wastewater is managed through pretreat-
ment using carbon i ltration or on-site distillation, and waste discharges are subjected to inspection
and sampling by local agency industrial wastewater pretreatment inspectors. After the mid-1980s,
solvent-contaminated wastewater was managed as a RCRA (Resource Conservation and Recovery
Act) waste, and discharge to sewer lines was prohibited. However, a common release mechanism for
solvents from degreasing operations before and after RCRA implementation was through spills onto
cracked concrete on shop l oors or from poorly maintained sumps beneath the degreasers.

1.1.1.3.5 Heating Solvents
Vapor degreasers heat solvents to their boiling points with electric, steam, or gas heaters.
Accumulation of sludge and metal i nes on the heating elements may cause “hot spots” that result in
solvent decomposition and burnout of the heating elements (Campbell, 1962). Dow Chemical
advised operators to remove degreasing sludge when the solvent reached boiling temperatures listed
in Table 1.10. To remove sludge, operators boiled solvent down to within 2 inches of the heating
coils while diverting the solvent return line to a clean drum. The solvent sludge mixture was then
drained and placed in drums for further distillation or disposal. Solvent sludge mixtures approach-
ing 60-70% oil are not recoverable by further distillation (Dow Chemical Company, 1999b).
In addition to avoiding hot-spot formation in the boiling solvent, accumulation of oil presented
problems for critical cleaning applications. Most cutting oils have low vapor pressures; nonetheless,
parts per million concentrations have been found in the vapor phase. The range of vapor pressures
for oily wastes is small; solvent boiling temperature controls the quantity of oil in the vapor phase.
Only a few parts per million oil could be found in vapors of dichloromethane, which has a low boil-
ing point, but up to 800 ppm oil could be found in vapors of perchloroethylene, which has a high
boiling point (Dow Chemical Company, 1999b).

1.1.1.3.6 On-Site Solvent Recovery in Degreasing Operations
The primary costs for operating vapor degreasers include solvent, energy for heating, and labor.
New designs of degreaser covers and fume hoods to recover solvents from vapors were motivated by

TABLE 1.10
Solvent Boiling Temperature for Oil-Laden Solvent

Normal Boiling Temperature
[°C (°F)]

Boiling Temperature at 25%
Oil [°C (°F)]

Solvent

Dichloromethane 39.7 (104) 42.2 (108)

Methyl chloroform 73.8 (165) 78.8 (174)

Trichloroethylene 87 (189) 89.4 (194)

Perchloroethylene 121 (250) 125 (257)

Sources: ASTM (1962), Johnson and Wedmore (1983), and Dow Chemical Company (1999b).