Environmental Engineering Reference
In-Depth Information
6.2.2 Types of sedimentation Tanks
Sedimentation equipment includes septic tanks, two-story tanks,
and plain settling tanks or clarifiers. All three devices may be used for
primary treatment, but plain settling tanks are normally used for sec-
ondary or advanced wastewater treatment processes.
6.2.2.1 Septic Tanks
Septic tanks are prefabricated tanks that serve as combined set-
tling and skimming tanks and as unheated, unmixed anaerobic digest-
ers. Septic tanks provide long settling times (6 to 8 hr or more) but do
not separate decomposing solids from the wastewater flow. When the
tank becomes full, solids will be discharged with the flow. The process
is suitable for small facilities (e.g., schools, motels, homes) but, due to
the long detention times and lack of control, it is not suitable for larger
applications.
6.2.2.2 Two-Story (Imhoff) Tank
The two-story or Imhoff tank is similar to a septic tank with regard
to the removal of settleable solids and the anaerobic digestion of solids.
The difference is that the two-story tank consists of a settling compart-
ment where sedimentation is accomplished, a lower compartment where
settled solids digestion takes place, and gas vents. Solids removed from
the wastewater by settling pass from the settling compartment into
the digestion compartment through a slot in the bottom of the settling
compartment. The design of the slot prevents solids from returning to
the settling compartment. Solids decompose anaerobically in the diges-
tion section. Gases produced as a result of the solids decomposition are
released through the gas vents running along each side of the settling
compartment.
6.2.2.3 Plain Settling Tanks (Clarifiers)
The plain settling tank or clarifier optimizes the settling process.
Sludge is removed from the tank for processing in other downstream
treatment units. Flow enters the tank, is slowed and distributed evenly
across the width and depth of the unit, passes through the unit, and
leaves over the effluent weir. Detention time within the primary settling
tank is from 1 to 3 hr (2 hr average).
Sludge removal is accomplished frequently on either a continuous
or intermittent basis. Continuous removal requires additional sludge
treatment processes to remove the excess water resulting from removal
of sludge containing less than 2 to 3% solids. Intermittent sludge removal
requires that the sludge be pumped from the tank on a schedule fre-
quent enough to prevent large clumps of solids rising to the surface but
infrequent enough to obtain 4 to 8% solids in the sludge withdrawn.
