providing a very long hydraulic retention time, allowing the mixing of incoming
fluids with those in the rest in the aeration tank with little or no chance of short
circuiting. Wasting of sludge is done only periodically to avoid building up inert
solids as well as to avoid excessive carryover of solids in the effluent.
The extended aeration time and the near-complete return of all solids impose an
endogenous growth condition on the system. Volumetric loading is very low, and the
sludge normally has an inferior settling characteristic. One can expect on the average
a BOD removal efficiency of only 85% [32–34].
Since a long aeration time is employed, the process is generally applicable only to
small treatment plants of less than 3785 m
3 /d (1.0 MGD) capacity. Prefabricated
package plants commercially available for housing subdivisions, small communities,
institutions, schools, military bases, etc. generally use the expended aeration process.
No primary sedimentation is provided and no exercise of return sludge control is
attempted. The objective is to simplify the process in construction and in operation.
Sludge production is very small because of prolonged endogenous oxidation, which
minimizes the problem of sludge treatment and disposal. Needless to say, the
oversimplified approach does not guarantee good treatment efficiency. In fact,
more often than not, the process yields an effluent of lower quality. Most state
water pollution control agencies are reluctant to approve the installation of such
treatment plants, at least not as secondary treatment facilities.
3.5 Contact Stabilization Process
When activated sludge is contacted with wastewater in the aeration tank, there is an
initial removal of organic matter that is often considered an adsorption phenomenon;
this initial removal is rapid. The contact stabilization process takes advantage of the
rapid adsorption characteristics of the activated sludge. The wastewater influent is
allowed 20–40 min contact time with the return sludge in the aeration tank. During
this period, much colloidal and suspended organic matter and some dissolved
organics are adsorbed by the sludge flocs to some extent. Although very little
oxidation occurs in this brief period, the adsorbed organics are nonetheless removed
as the sludge is separated from the treated effluent in the final clarifier. The settled
sludge is then allowed 1.5–5 h of aeration in a sludge stabilization tank in which the
adsorbed organics are synthesized and oxidized. After the stabilization step, the
sludge is partially returned to the incoming wastewater in the aeration tank
(Fig. 3.10e).
The advantage of the process is evident in that the shorter aeration time reduces
the aeration tank volume considerably. Although the settled sludge needs aeration as
well, the volume is small, whereas in the other processes, the entire volume of the
mixed liquor is aerated to achieve sludge stabilization. Aeration tank volume
reduction by 50% over the conventional and step aeration processes is feasible.
Many existing conventional plants are too small to accommodate the increasing
volume and strength of the wastewater that they are converted to contact stabilization
102
L. K. Wang et al.
fluids with those in the rest in the aeration tank with little or no chance of short
circuiting. Wasting of sludge is done only periodically to avoid building up inert
solids as well as to avoid excessive carryover of solids in the effluent.
The extended aeration time and the near-complete return of all solids impose an
endogenous growth condition on the system. Volumetric loading is very low, and the
sludge normally has an inferior settling characteristic. One can expect on the average
a BOD removal efficiency of only 85% [32–34].
Since a long aeration time is employed, the process is generally applicable only to
small treatment plants of less than 3785 m
3 /d (1.0 MGD) capacity. Prefabricated
package plants commercially available for housing subdivisions, small communities,
institutions, schools, military bases, etc. generally use the expended aeration process.
No primary sedimentation is provided and no exercise of return sludge control is
attempted. The objective is to simplify the process in construction and in operation.
Sludge production is very small because of prolonged endogenous oxidation, which
minimizes the problem of sludge treatment and disposal. Needless to say, the
oversimplified approach does not guarantee good treatment efficiency. In fact,
more often than not, the process yields an effluent of lower quality. Most state
water pollution control agencies are reluctant to approve the installation of such
treatment plants, at least not as secondary treatment facilities.
3.5 Contact Stabilization Process
When activated sludge is contacted with wastewater in the aeration tank, there is an
initial removal of organic matter that is often considered an adsorption phenomenon;
this initial removal is rapid. The contact stabilization process takes advantage of the
rapid adsorption characteristics of the activated sludge. The wastewater influent is
allowed 20–40 min contact time with the return sludge in the aeration tank. During
this period, much colloidal and suspended organic matter and some dissolved
organics are adsorbed by the sludge flocs to some extent. Although very little
oxidation occurs in this brief period, the adsorbed organics are nonetheless removed
as the sludge is separated from the treated effluent in the final clarifier. The settled
sludge is then allowed 1.5–5 h of aeration in a sludge stabilization tank in which the
adsorbed organics are synthesized and oxidized. After the stabilization step, the
sludge is partially returned to the incoming wastewater in the aeration tank
(Fig. 3.10e).
The advantage of the process is evident in that the shorter aeration time reduces
the aeration tank volume considerably. Although the settled sludge needs aeration as
well, the volume is small, whereas in the other processes, the entire volume of the
mixed liquor is aerated to achieve sludge stabilization. Aeration tank volume
reduction by 50% over the conventional and step aeration processes is feasible.
Many existing conventional plants are too small to accommodate the increasing
volume and strength of the wastewater that they are converted to contact stabilization
102
L. K. Wang et al.