5-814-8
is mixed at the head end of the aera-
are available (152). A number of variations of the
tion tanks.
process were devel-
conventional activated
-- T h e sludge is susceptible to slugs or
y, to minimize
oped to achieve greater
spills of acidic, caustic or toxic materi-
capital
operating costs or to correct a
problem. While not all of the variations are
als.
mentioned herein, the following should be evalu-
Completely mixed. In the completely
ated when considering a new facility, or upgrad-
mixed process, influent wastewater and recycled
ing an existing primary or secondary facility:
sludge are introduced uniformly throughout the
Completely-mixed.
aeration tank. This flow distribution results in a
--Step aeration.
uniform oxygen demand throughout the aeration
--Contact stabilization.
tank which adds some operational stability. This
--Extended aeration.
process may be loaded to levels comparable to
--Pure oxygen system.
those of the step aeration and contact stabiliza-
tion processes with only slight reductions com-
Summary characteristics on design criteria, re-
moval efficiencies and basic applications of the
pared to the removal efficiencies of those pro-
modifications are described in table 6-2. Based on
cesses. The reduced efficiency occurs because
there is a small amount of short circuiting in the
the overall BOD removal efficiency reported,
most variations are able to achieve a high degree
completely mixed aeration tank.
of treatment. The extended aeration system is a
(c) Step aeration. The step aeration process
flexible system, but is more cost-effective for
is a modification of the conventional activated
populations. Extended aeration and contact
sludge process in which influent wastewater is
introduced at several points in the aeration tank
stabilization are most applicable as package
plants and are described under that heading.
to equalize the F/M, thus lowering the peak
Activated sludge systems are commonly designed
oxygen demand. The typical step aeration system
to accomplish two or more of the operating modes
would have return activated sludge entering the
to accommodate flexible operational requirements.
tank at the head end. A portion of the influent
An example is the completely-mixed and step
also enters near the front. The influent piping is
arranged so that an increment of wastewater is
aeration systems. From the data in table 6-2, it
can be seen that depending upon volumetric
introduced into the aeration tank at locations
loading, F/M or detention time, selection of one
down the length of the basin. Flexibilityof opera-
variation over another can result in significant
tion is one of the important features of this
differences in the size of the aeration basins. The
system (125). In addition, the multiple-point intro-
information presented in table 6-2 covers the
duction of wastewater maintains an activated
range which has been experienced.
sludge with high absorptive properties. This al-
Conventional. The conventional acti-
lows the soluble organics to be removed within a
vated sludge process employs long rectangular
shorter period of time. Higher BOD loadings are
ft of aeration tank
aeration tanks which approximate plug-flow al-
therefore possible per 1000
volume.
though some longitudinal mixing occurs. This
process is primarily employed for the treatment
Contact stabilization. The contact stabi-
lization process is applicable to wastewaters con-
Return sludge is mixed
of domestic
taining a high proportion of the BOD in sus-
with the wastewater prior to discharge into the
aeration tank. The mixed liquor flows through the
pended or colloidal form. Since bio-adsorption and
and suspended solids occur
aeration tank during which removal of organics
flocculation of
very rapidly, only short retention periods (15-30
occurs. The oxygen utilization rate is high at the
minutes) are generally required. After the contact
entrance to the tank and decreases toward the
discharge end. The oxygen utilization rate will
period the activated sludge is separated in a
approach the endogenous level toward the end of
clarifier. A sludge reaeration or stabilization pe-
the tank. Principle disadvantages of conventional
removed
riod is required to stabilize the
activated
treatment in industrial applica-
in the contact tank. The retention period in the
tion are:
stabilization tank is dependent on the time re-
--The oxygen utilization rate varies with
quired to assimilate the soluble and colloidal
tank length and requires irregular spac-
material removed from the wastewater in the
ing of the aeration equipment or a
contact tank. Effective removal in the contact
modulated air supply.
period requires s u f f i c i e n t a c t i v a t e d s l u d g e t o
--Load variation may have a deleterious
remove the colloidal and suspended matter and a
the activated sludge when it
effect
The retention
portion of the soluble
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