TM 5-814-8
withdrawal or vacuum withdrawal over the entire
but no large scale or long term demonstrations of
tank bottom. Circular clarifiers are of three gen-
the process have been undertaken. The common
eral types. With the center feed type, the waste is
method of removal is by chemical treatment
fed into a center well and the effluent is pulled off
usually employing alkaline precipitation with lime
at the weir along the outside. With a peripheral
or precipitation using minerals (iron or aluminum
feed tank, the effluent is pulled off at the tank
salts). The process can be accomplished in numer-
center. With a rim-flow clarifier, the peripheral
ous ways either in the primary system, secondary
feed and effluent discharge are also along the
system or as a separate system. The particular
clarifier rim, but this type is usually used for
method to employ at a given installation is a
larger clarifiers. The circular clarifier usually
matter of numerous constraints. The two predom-
inant methods are mineral addition to the pri-
gives the optimal performance. Rectangular tanks
may be desired where construction space is lim-
mary clarifier and lime clarification after second-
ited. The circular clarifier can be designed for
ary treatment, although addition of minerals or
lime to the final clarifier of trickling filter sys-
center sludge withdrawal or vacuum withdrawal
over the entire tank bottom. Center sludge with-
tems has been successful. Mineral additions to
the primary or secondary clarifier will not usually
drawal requires a minimum bottom slope of 1
in/ft. The flow of sludge to the center well is
provide quite as low a phosphorus level as lime
precipitation. All precipitation processes increase
largely hydraulically motivated by the collection
sludge quantities w h i c h must be handled.
mechanism, which serves to overcome inertia and
avoid sludge adherence to the tank bottom. The
Recalcination of lime will not be economical at
most military facilities. Design considerations for
vacuum drawoff is particularly adaptable to sec-
ondary clarification and thickening of activated
the various phosphorus removal alternatives are
presented in TM 5-814-3 and the U.S. EPA
sludge. The mechanisms can be of the plow type
Process Design Manual for Phosphorus Removal.
or the rotary-hoe type. The plow-type mechanism
employs staggered plows attached to two oppos-
(2) Sedimentation.
(a) Process description. Sedimentation is
ing arms that move about 10 ft/min. The rotary-
hoe mechanism consists of a series of short
the separation of suspended particles that are
heavier than water from water by gravitational
scrapers suspended from a rotating supporting
means. It is one of the most widely used unit
bridge on endless chains that make contact with
operations in wastewater treatment. This opera-
the tank bottom at the periphery and move to the
center of the tank.
tion is used for grit removal; particulate-matter
removal in the primary settling basin; biological-
(3) Microscreening. The use of microscreening
floc removal in the activated sludge settling
or microstraining in advanced wastewater treat-
ment is chiefly as a polishing step for removal of
basin; chemical-floe removal when the chemical
additional suspended solids (and associated BOD)
coagulation process is used; and for solids concen-
tration in sludge thickeners. Although in most
from secondary effluents. The system consists of
a rotating drum with a peripheral screen. Influent
cases the primary purpose is to produce a clari-
fied effluent, it is also necessary to produce
wastewater enters the drum internally and passes
radially outward through the screen, with deposi-
sludge with a solids concentration that can be
tion of solids on the inner surface of the drum
easily handled and treated. In other processes,
screen. The deposited solids are removed by
such as sludge thickening, the primary purpose is
pressure jets located at the top of the drum. The
to produce a concentrated sludge that can be
backwash water is then collected and returned to
treated more economically. In the design of
the plant. The screen openings range from about
sedimentation basins, due consideration should be
23 to 60 microns depending upon manufacturer
given to production of both a clarified effluent
type and material. However, the small openings
and a concentrated sludge (125).
themselves do not account for the removal effi-
(b) Clarifier design. Clarifiers may either be
ciency of the unit. Performance is dependent on
rectangular or circular. In most rectangular clari-
fiers, scraper flights extending the width of the
the mat of previously trapped solids which pro-
vide the fine filtration. Thus an important factor
tank move the settled sludge toward the inlet end
in design is the nature of the solids applied to the
of the tank at a speed of about 1 ft/min. Some
system. The strong biological floes are better for
designs move the sludge toward the effluent end
microscreening; weak chemical floe particles are
of the tank, corresponding to the direction of flow
not efficiently removed. Depending upon the in-
of the density current. Circular clarifiers may
fluent wastewater characteristics and the
employ either a center feed well or a peripheral
microfabric, suspended solids removals have
inlet. The tank can be designed for center sludge
6-18
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