BIOLOGICAL TREATMENT OF WASTEWATER 151
present in suspended or colloidal form. The suspended
BOD is rapidly absorbed in a short period, ½ to 1½
hours, by the well-activated organisms and a part of
soluble BOD is metabolized. In the activation tank, the
sludge is reaerated for bio-oxidation and stabilization of
adsorbed food; and when returned to the aeration tank,
it is activated for higher BOD removal as compared to
the conventional plant where sludge has become lean
and hungry in the absence of a food supply. The addi-
tional advantage of this process is the reduced overall
tank volume required as compared to the conventional
system. However, the operation of such plants is more
complex and less flexible than conventional ones.D. Completely Mixed Activated Sludge
“Complex mix” approach is with respect to combining
the return sludge and wastewater in order to maintain the
entire contents of the aeration chamber in essentially a
homogenous state. Wastewater introduced into the aera-
tion basin is dispersed rapidly throughout the mass and is
subjected to immediate attack by fully developed organ-
isms throughout the aeration basin. Biological stability
and efficiency of the aeration basin is enhanced by this
design. Layout of a completely-mixed activated sludge
plant and variation in BOD are shown in Figure 17.
In this mathematical analysis, McKinney^5 considered
the complete mixing activated sludge process as the one
in which the untreated wastes are instantaneously mixedthroughout the aeration tank. In effect, the organic load
on the aeration tank is uniform from one end to the other
end and consequently a uniform oxygen demand and a
uniform biological growth are produced. It is assumed
to reduce the effect of variations in organic loads that
produce shock loads on conventional units, retain a
more biological population and hence, produce a more
uniform effluent, and be able to treat organic wastes of
any concentration and produce an effluent of any desired
concentration.^5 Using Treatment Model II, Figure 11, as
an example of a completely mixed system, Lawrence
and McCarty^11 have shown analytically that although
the complete-mixing will reduce the shock loads due to
variations in organic loads, plug flow type conventional
units, Treatment Model III, are more efficient.
Assuming that Eq. 13 is applicable for BOD removal
rate, and since the BOD in a completely mixed aera-
tor, S, is equal to the effluent BOD, S e , therefore under
steady state conditions:d
dS 0
tSS
t= e qXSe−
=or(^)
S
SqXt
e
0
1
1
.
(28)
SECONDARY
SETTLING
TANK
EFFLUENT
AERATION
(SORPTION)
BASIN-I
ACTIVATION
TANK-II
EXCESS SLUDGE RETURN SLUDGE
PRIMARY
SETTLING
TANK
INFLOW
SLUDGE DISPOSAL
BOD OF SETTLED MIXED LIQUOR BOD OF SETTLED MIXED LIQUOR
TIME IN I TIME IN II
FIGURE 16 Biosorption (contact stabilization) activated sludge.
C002_001_r03.indd 151C002_001_r03.indd 151 11/18/2005 10:15:54 AM11/18/2005 10:15:54 AM