TM 5-814-8
tions, the military services operate many photo-
stripping operation, resulting in a water usage of
processing facilities. The size of such facilities
only four gallons per gallon of stripper. Also,
varies greatly, with waste flows of 10,000 to
squeegees are used to remove the paint-stripper
1,000,000 gallons per month. Liquid wastes origi-
and paint skins onto plastic sheets which are
nate from the discharge of spent processing
disposed of at a sanitary landfill (29).
(2) Gaging and sampling. Care must be taken
solutions and associated rinse or washwaters.
Approximately 90 percent of the liquid waste
when sampling wastewaters with high oil con-
produced is from the rinse operations.
tents, such as washrack and paint-stripping
(1) Waste reduction practices. Waste reduc-
wastes, to ensure that a representative sample is
tion practices include recovery of silver, regenera-
obtained (15 1). The precaution is required due to
tion of ferrocyanide and other chemicals, chemical
the tendency of oil to float on the water surface.
bath reuse and the use of squeegees to reduce the
(3) Environmental impact. Washrack and
carryover, or dragout, of chemicals from one step
paint-stripping wastewaters containing high con-
to another.
centrations of phenols, organic solvents, chro-
(a) Silver recovery. Because of the high
mium, oils and surfactants are extremely toxic to
market value of silver, it can be economically
aquatic life. Failure to properly contain and treat
recovered from the spent bleach and fixer solu-
these wastes can result in fish kills, stream
tions as well as from the final washwater. Such
purification inhibition and odors. All of these are
recovery reduces the impact of silver as a pollut-
unacceptable by any water quality standards
ant and in some cases allows the fixer solution to
(26)(29)(1 13). Oils from machining operations can
be reused, reducing chemical replacement costs.
be toxic and may impose a high oxygen demand
Silver recovery is most often accomplished by
on the receiving body of water.
passing the waste effluent through a proprietary
(4) Treatment. Unless highly diluted, the raw
wastewaters from machining and paint-stripping
steel-wool-filled canister where silver is exchanged
operations and washracks utilizing solvents are
for iron. Silver can also be removed by precipita-
highly toxic to the microorganisms of biological
tion with sodium sulfide or by electrolysis.
treatment plants, interfering with both aeration
(b) Bleach regeneration. The bleach solution
and sludge digestion processes. Paint-stripping
may also be reused by regenerating ferrocyanide
wastes are particularly toxic. A typical pretreat-
from the spent ferrocyanide using oxidizing
ment system for a major facility would include
agents such as persulfate and ozone. One manu-
the following steps:
facturer offers a packaged bleach regenerator
Gravity separation tank equipped to re-
material (123). Regeneration provides a cost sav-
move floating oils and settleable solids.
ings as well as pollutant reduction. Methods of
Detention tanks with mixing to provide
complete cyanide destruction are discussed later
equalization of flow and waste strength
in this chapter.
as well as to permit evaporation of vola-
(c) Equalization. Equalization is very im-
tile solvents.
portant if photographic wastes are treated biolog-
Chemical addition in a rapid mix tank
ically, particularly when the photographic pro-
followed by slow mixing in a separate
cessing operation occurs during only part of the
tank to promote flocculation, break emul-
day. Daily variations in flow and concentration
sions and agglomerate solids.
can cause serious operating difficulties at the
Final treatment in an air flotation unit to
treatment plant.
remove flocculated particles.
(2) Gaging and sampling. To define waste-
For smaller facilities, where washrack wastes are
water quality and quantity for a new installation,
only a small part of the total waste flow, an
sampling and gaging data from a similar operat-
alternate approach can be used. A storage tank,
ing facility is valuable. The presence of a large
arranged to receive this waste and equipped with
amount of free silver metal will inhibit biological
air mixing and adequate air emission controls,
action and yield unreliable BOD test data. Large
would provide for evaporation of a part of the
amounts of thiosulfates from the fixing bath will
volatile solvents and permit pumping to the main
exert an oxygen demand. Care must be taken to
sewer at a controlled rate. At the main treatment
plant, the primary settling tank preceding biologi-
prepare appropriate waste dilutions to avoid these
interferences with the BOD tests.
cal treatment will have adequate oil and solids
(3) Environmental impact. The environmental
removal capacity.
e. Photographic processing. Because of the
impact of discharging improperly treated photo-
graphic waste can be severe due to high concen-
widespread use of photography in military opera-
6-25
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