TM 5-814-8
(b) Treatment and recycle. Laundry waste-
--Evaporative recirculating systems.
waters may be treated in commercially available
(a) In once-through systems, the cooling
physical-chemical units with the possibility of
water is obtained from a lake or stream and
recycling the effluent. One system involves chemi-
returned to the same stream with little or no
cal precipitation with alum, followed by sand
treatment. Periodic additions of biocides are
filtration, carbon adsorption and ion exchange.
sometimes required to prevent fouling of the
Another system consists of chemical precipitation
cooling water equipment. Chlorine is the most
and diatomaceous earth filtration. About 94 per-
commonly used biocide. In some instances, the
cent phosphate removal, 90 to 98 percent ABS
water may require de-chlorination prior to return
removal, 60 to 80 percent COD reduction and 60
to the stream.
to 70 percent BOD reduction can be obtained (35).
(b) Closed cooling systems are used where
g. Other generators. Other wastewaters typical
the composition of the cooling water is critical,
of some military facilities include hospitals dis-
such as in the cooling of high temperature
charges, boiler water blowdown, cooling water
surfaces. The cooling water rejects heat to an
system blowdown, blowdown from boiler flue gas-
air-cooled radiator or through a heat exchanger to
scrubber systems and vehicle washing facilities.
a once-through or evaporative recirculating sys-
tem. Blowdown or other losses from a closed
(1) Hospitals. Hospital wastewaters require
special attention because of several factors. The
system are small but contaminated. Corrosion
diurnal peaks and minimums of both flow and
inhibitors sometimes contain chromate, zinc, so-
concentration may be different from those nor-
dium nitrate, and borax which must be removed
mally associated with domestic wastewaters due
prior to biological treatment or stream disposal.
to the unique hospital patterns of activity. Patho-
(c) The evaporative recirculating system
genic organisms will probably be present in
uses a cooling tower or spray pond to dissipate
higher than normal concentrations; however, mod-
heat by evaporation of a part of the flow.
ern biological or physical-chemical secondary
Although limited by blowdown, this results in an
treatment plants with post-chlorination should
increase in the concentration of dissolved solids
eliminate excess pathogens in the effluent. Con-
to a level of 3 to 5 times that found in the
servative design of chlorination facilities is en-
makeup water. To avoid corrosion, scale and
couraged. Operating personnel must exercise spe-
biological problems, acid, inhibitors and biocides
cial care to reduce the possibility of infection.
are added to the system. Treatment of the
Ample design and maintenance of screening
blowdown is sometimes necessary for removal of
equipment should be exercised to eliminate most
any chromate, zinc compounds or other materials
problems caused by excessive quantities of gauze,
used as an inhibitor.
rags and bandages in the wastewater. Average
(4) Scrubber systems. Scrubbers are used to
sewage flows from hospitals are estimated at
avoid air pollution. Airborne wastes, accumulated
about 100 gallons per resident per day in TM
by the recirculating liquid, require that the liquid
5-814-1, while other sources estimate as high as
be periodically or continuously treated for re-
200 gallons per bed per day. These values are
moval of wastewater constituents. In scrubbing
quite similar to those for normal domestic sew-
of boiler stack gases, fine ash and sulfur oxides
age. Resident population includes patients and
must be removed or neutralized. Other scrubbing
full time employees.
systems have similar treatment requirements.
(2) Boilers. This waste is normally hot, up to
h. Treatment methods. Special treatment pro-
210 degrees F, and contain phosphates (30 to 60
c e s s e s are required f o r some in d u s t r i a l
mg/L), sulfite (30 to 60 mg/L), organic matter and
wastewater constituents. These processes may be
some suspended material. Normally, blending this
employed to provide for pretreatment prior to
water with other wastes reduces various constitu-
mixing w i t h o t h e r w a s t e s f o r c o m p l e t e
ents to a level which will not inhibit subsequent
wastewater treatment and discharge, or for recov-
biological treatment. Direct discharge of blow-
ery of special constituents.
down to a receiving stream would require treat-
(1) pH control. For discharging wastewater
ment to reduce phosphate and sulfite concentra-
to a biological treatment process or directly to a
tions. In addition, cooling would be required for
receiving stream, pH must generally be main-
direct discharge.
tained in the range of 6.0 to 9.0; although limits
(3) Cooling water systems. Cooling water sys-
may be much closer in certain instances. Treat-
tems can be classified in these general categories:
ment processes to destroy cyanides, to reduce
Once-through systems.
hexavalent chromium and to precipitate heavy
Closed systems.
metals also require pH control.
6-27
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