TM 5-813-3/AFM 88-10, Vol 3
2-13. Special Processes.
esses to assist in their removal. Three basic approaches
to control THMs are:
In some cases it will be necessary to use raw water sup-
(a) Use of a disinfectant that does not generate
plies containing unacceptably large concentrations of
THMs in water. (Ozone, chlorine dioxide)
constituents that cannot be removed by conventional
(b) Treatment to reduce the concentrations of
treatment processes. The most common of these objec-
precursor material prior to chlorination (coagulation,
tionable constituents are mineral salts, such as
flocculation, filtration),
sulfates and chlorides, and volatile organic com-
(c) Treatment to reduce THM concentrations
pounds, (VOCs). Special treatment processes are neces-
subsequent to their formation (aeration, carbon ad-
sary to remove these materials,
sorption).
a. Demineralization. The presence of excessively
These three methods have been presented throughout
high concentrations of dissolved minerals in water is
this technical manual.
2-
indicated by high chloride (Cl-), sulfate (S04 ), and
(2) Synthetic Organic Compounds (SOCs), are
total dissolved solids (TDS) levels. The recommended
VOCs some of which have been found in many ground-
limits for these substances are 250 mg/L, 250 mg/L,
water sources used for potable water supplies. SOCs
and 500 mg/L, respectively. These limits are based on
are found in groundwater due to improper disposal of
esthetic considerations and considerably higher con-
spent industrial-type solvents, paint thinners, cleaning
centrations, while not desirable, can be tolerated.
agents and some household chemicals, Two common
Where demineralization is required, processes com-
SOCs are trichloroethylene (TCE) and tetrachlo-
monly employed are electrodialysis, reverse osmosis,
rethylene. Some VOCs are rather soluable and have
distillation, and ion exchange. Disposal of waste brine
little affinity for soil materials, and therefore can
solutions derived from these processes often poses a
travel great distances to an aquifier from an industrial
serious problem and must be carefully considered at an
waste lagoon, industrial, commercial on domestic sep-
early stage in project development. All demineraliza-
tic system, landfill, accidental spill or illegal disposal.
tion processes are energy intensive, and alternative
(3) Removal Technologies for VOCs. Three differ-
water sources should be thoroughly investigated
ent technologies are available for the removal of
before a commitment to a demineralization project is
VOCs: aeration, carbon adsorption, or resin absorp-
made. If the demineralization process selected requires
tion. All of these methods have been presented in pre-
large inputs of electricity, consideration should be
vious sections of this technical manual, with the excep-
given to its operation principally during "off-peak"
tion of resin absorption. Resin absorption involves the
hours with storage of desalted water until needed,
physical separation of the organic compounds from
b. Removal of Volatile Organic Compounds. VOCs
water by using a polymeric absorbent or resin filled
can be either halgonated naturally occurring organic
unit. The resin is specific to the VOC it will remove,
substances (trihalomethanes), or synthetic organic
therefore great care must be taken in the selection of
compounds (SOCs).
the resin. The resin-filled units also require frequent
(1) Trihalomethanes. Naturally occurring organic
regeneration with a low pressure backwash and an
substances (precursors), such as humic and fulvic acids
alcohol-wash. The waste from the backwash will con-
are derived from leaf and soil extract and are not
tain high concentrations of VOCs and may be classi-
themselves volatile. When the precursors (usually
fied as hazardous waste.
found in surface waters) enter the treatment facility in
(4) Selection of a removal technology. Important
the raw water they react with the free available chlo-
parameters for removing VOCs are the concentrations
rine injected for purposes of disinfection. These halgo-
concerned, the type of VOC, and the cost of the
nated organic compounds are known as trihalo-
removal method.
methanes (THMs). Other THMs can be produced by ex-
(a) The higher the concentration of VOCs the
posing precursors to other halogens, such as bromine
more expensive removal will become, Higher concen-
or iodine. This grouping of total trihalomethanes
trations of VOCs will normally require larger equip-
(TTHMs) is generally comprised of four primary con-
ment, e.g. counter-current packed column aeration
stituents: trichloromethane (chloroform), bromodi-
towers must increase in either volume or blower and
chlormethane, chlorodibromomethane, and tribromo-
pump horse power for increased removal of VOCs,
methane (bromoform), Monitoring and analytical re-
Low TTHM concentrations may be handled by simply
quirements imposed by the EPA for THMs are to be
changing the point of chlorination and allowing coagu-
found in Title 40 CFR 141 Subpart C, sections 141.12
lation and flocculation to remove THM precursors.
(c), 141.30 and Appendix C. These sections of Title 40
High TTHM concentrations may require the addition
include MCL's, monitoring frequencies and the ap-
of an aeration tower or a GAC contactor and at the ex-
proved method for measuring TTHM's. THMs are
treme an alternate disinfectant such as ozone.
difficult to remove, hence the need for special proc-
(b) The type of VOC to be removed may dictate
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