Table 8
Characteristics of Package Biological Treatment Units
Approximate
Population
Unit Processes
Return
Served
Included in
Activated
and Flow (1)
Application
Process
Package
Solids Disposal
Aeration Method
Sludge
Most advantageous for
Diffusion.
By airlift
Up to 1,000 per
Digested solids
Aeration and
Extended
low population load
or pump.
unit. 0.12 mgd
Mechanical
to holding tank
aeration(2)
final settling.
and where only
(0.45 ML/d) per
for truck
surface
Chlorine contact
(activated
unit.
periodic attention
aerator.
tank may also be
disposal or to
sludge)
can be given to
dewatering
included.
equipment.
facilities.
Used in larger
By airlift
Up to 10,000 per
To digesters if
Diffusion.
Complete mix
Primary settling,
or pump.
unit. 1.2 mgd
plants.
Mechanical
not combined in
(activated
aeration and
surface
(4.5 ML/d) per
treatment unit.
sludge)
final settling.
unit.
aerator.
Digested solids
In smaller units,
to holding tank
Turbine
chlorine contact
aerator.
for disposal or
tank and aerobic
dewatering.
digester may be
included in
package.
Higher degree of
Same as
Diffusion.
By airlift.
Up to 5,000 per
Aeration, final
Step
treatment.
extended
unit. 0.6 mgd
settling and
aeration(2)
aeration.
(2.3 ML/d) per
aerobic digester.
(activated
unit.
Chlorine contact
sludge)
tank may also be
included.
Diffusion.
By airlift.
Up to 5,000 per
Do not use.
Same as step
Same as
Contact
aeration.
extended
unit. 0.6 mgd
stabilization
(2)(activated
aeration.
(2.3 ML/d) per
unit.
sludge)
Up to 500 per
Same as extended
Digested
Natural
Biofiltration
Primary and
unit. 0.06 mgd
aeration.
ventilation
secondary
anaerobically
or forced
(0.23 ML/d) per
in lower
settling, two-
ventilation.
unit.
compartment of
stage trickling
unit.
filtration, and
digestion.
Simplicity makes very
Disc rotation
None.
Up to 1,000 per
Rotating
RBC with final
To separate
unit. 0.12 mgd
advantageous for
settling tank.
digestion unit,
under partial
biological
or to primary
submergence.
(0.45 ML/d) per
small units. Entire
contactor(3)
package is operated
settling tank.
unit.
by one motor.
(1)Flow rates shown are based upon average 120-gpcd (450-Lpcd) flow rate.
(2)Separate primary settling generally not required.
(3)Primary settling generally installed separate from package unit.
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