TM 5-814-8
ing some of those from military industrial manu-
the grit removal system ahead of the raw sewage
facturing processes as discussed later in this
pumps because of the depth of the influent line.
chapter. Equalization reduces fluctuations of the
Therefore, i t may be required to pump the
influent to levels compatible with subsequent
wastewater containing grit. If this mode is se-
biological or physical-chemical processes. A prop-
lected, pumps capable of handling grit should be
erly designed facility dampens the wide swings of
employed.
flow, pH, BOD, and other parameters to levels
(a) Horizontal flow grit chambers. This
such that downstream systems operate more
t y p e of grit chamber is designed to allow
efficiently and economically, and can be con-
wastewater to pass through channels or tanks at
structed at a reduced capital investment. Proper
a horizontal velocity of about one foot per second.
This velocity will allow grit to settle in the
more consistently provide a better quality efflu-
channel or tank bottom, while keeping the lighter
ent. A graphical example of how an equalization
organic solids in suspension. Velocity control and
facility can stabilize a wastewater having signifi-
other design features are covered in TM 5-814-3.
cant cyclic pH variations is illustrated in figure
(b) Detritus tanks. A grit chamber can be
6-1. While there are definite primary benefits for
designed with a lower velocity to allow organic
equalization, a facility can also be designed to
matter to settle with the grit. This grit-organic
yield secondary benefits by taking advantage of
matter mixture is referred to as detritus and the
physical, chemical, and biological reactions which
removal devices are known as detritus tanks.
might occur during retention in the equalization
When detritus tanks are employed, the organic
basin. For example, supplemental means of aera-
matter is separated from the grit by either gentle
tion are often employed with an equalization
aeration or washing the removal detritus to
basin to provide:
re-suspend the organic matter. Several propri-
--Better mixing.
etary systems are available to accomplish this,
--Chemical oxidation of reduced com-
and the advantage over other types is that the
pounds.
configuration of the tank is simple and the
--Some degree of biological oxidation.
system allows for continuous removal of grit.
(c) Aerated grit chambers. As the name
from settling.
implies, diffused air can be used to separate grit
If aeration is not provided, baffles or mechanical
from other matter. A secondary benefit to the
mixers must be provided to avoid stratification
aeration method is that is also freshens the
and short circuiting in equalization basins. The
wastewater prior to further treatment; quite often
size and shape of an equalization facility will vary
it is used in conjunction with a preaeration
with the quantity of waste and the patterns of
facility. The different types of grit removal facili-
waste discharge. Basins should be designed to
ties employed are described in TM 5-814-3.
provide adequate capacity to accommodate the
(3) Preaeration. Methods of introducing sup-
total volume of periodic variation from the
plemental oxygen to the raw wastewater are
wastewater source (125) (130).
sometimes used in preliminary treatment. This
(5) pH control. Similarly to equalization, the
process is known as preaeration and the objec-
use of pH control as a preliminary treatment step
tives are to:
is usually limited to treatment of industrial
--Improve wastewater treatability.
process wastes. It is necessary to regulate pH
--Provide grease separation, odor control,
since treatment processes can be harmed by
and flocculation.
excessively acidic or basic wastes. Regulation of
--Promote uniform distribution of sus-
this parameter may be necessary to meet effluent
pended and floating solids to treatment
levels specified for secondary treatment. Control
units.
--Increase BOD removals in primary sedi-
of the pH at elevated levels is usually required to
mentation.
precipitate certain heavy metals and/or alleviate
This is generally provided by either separate
an odor producing potential.
aeration or increased detention time in an aerated
(6) Flotation. In preliminary treatment, flota-
grit chamber. Provisions for grit removal are
tion is sometimes used for wastes which have
heavy loads of grease and finely divided sus-
provided in only the first portion of the tank
(125).
pended solids. These are mainly systems having
large industrial discharges and may apply to
military installations with significant oil and
plication for domestic wastes, but should be
grease quantities from manufacturing or laundry
employed for many industrial discharges includ-
6-3