TM 5 - 8 1 4 - 8
CHAPTER 6
WASTEWATER TREATMENT PROCESSES
further classified as manually or mechanically
6-1. Preliminary and Primary
cleaned. Coarse screens are used in preliminary
water Treatment Processes
treatment, while fine screens are used in lieu of
a. Introduction. Preliminary treatment of
sedimentation preceding secondary treatment or
wastewater generally includes those processes
as a step in advanced wastewater treatment. Fine
that remove debris and coarse biodegradable
screens as a preliminary or primary treatment are
material from the waste stream and/or stabilize
more applicable to process or industrial wastes.
the wastewater by equalization or chemical addi-
TM 5-814-3 provides detailed descriptions of
tion. Primary treatment generally refers to a
these units and design considerations.
sedimentation process ahead of the main system
(b) Comminution. A comminutor acts as
or secondary treatment. In domestic wastewater
both a cutter and a screen. Its purpose is not to
treatment, preliminary and primary processes will
remove but to shred (comminute) the solids.
remove approximately 25 percent of the organic
Solids must be accounted for in subsequent
load and virtually all of the nonorganic solids. In
sludge handling facilities. Comminutors, like most
industrial waste treatment, preliminary or pri-
screens, are mounted in a channel and the
mary treatment may include flow equalization,
wastewater flows through them. The rags and
pH adjustment or chemical addition that is ex-
other debris are shredded by cutting teeth until
tremely important to the overall treatment pro-
they can pass through the openings. Some units
cess. Table 6-1 liss the typical effluent levels by
require specially shaped channels for proper hy-
degree of treatment. This section of the manual
draulic conditions, resulting in more expensive
will discuss the various types of preliminary and
construction. Treatment. plant design manuals,
primary treatment processes available.
textbooks, and manufacturer's bulletins provide
b. Preliminary treatment. An important part of
detailed information on these units. A bypass
any wastewater treatment plant is the equipment
channel is required for all comminutors to permit
and facilities used to remove items such as rags,
maintenance of equipment.
grit, sticks, other debris, and foreign objects.
(2) Grit removal. Grit represents the heavier
These interfere with the operation of the facility
inert matter in wastewater which will not decom-
and often cause severe problems. Methods of
pose in treatment processes. It is identified with
.
removing these materials prior to primary and
matter having a specific gravity of about 2.65,
subsequent treatment are part of a pretreatment
and design of grit chambers is based on the
or preliminary treatment. While a summary dis-
removal of all particles of about 0.011 inch or
cussion of the commonly employed unit opera-
larger (65 mesh). For some sludge handling pro-
tions follows, a more complete description of
cesses, it may be necessary to remove, as a
design criteria which must be used is contained in
minimum, grit of 0.007 inch or larger (100 mesh).
TM 5-814-3.
Grit removal, compared to other unit treatment
(1) Screening and comminution. Screening
processes, is quite economical and employed to
and comminution are preliminary treatment pro-
achieve the following results:
cesses utilized to protect mechanical equipment in
Prevent excessive abrasive wear of equip-
the treatment works, to aid downstream treat-
ment such as pumps and sludge scrapers.
ment processes by intercepting unacceptable sol-
Prevent deposition and subsequent oper-
ids, and to alter the physical form of solids so
ating problems in channels, pipes, and
they are acceptable for treatment. Screening or
basins.
comminution shall always be used for military
Prevent reduction of capacity in sludge
domestic wastewaters.
handling facilities.
(a) Screening. Screening devices remove
Grit removal facilities shall be used for combined
materials which would damage equipment or
sewer systems or separate sanitary systems
interefere with a process or piece of equipment.
which may have excessive inert material. Grit
Screening devices have varied applications at
removal equipment should be located after bar
wastewater treatment facilities, but most often
screens and comminutors and ahead of raw sew-
are employed as a preliminary treatment step.
age pumps. Sometimes it is not practical to locate
Screens are classified as fine or coarse and then
6-1
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