vvEPA
United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Office of Municipal
Pollution Control
Washington DC 20460
Research and Development
EPA/600/M-86/023
September 1986
Design Information
Report
Centrifuges
The U.S. Environmental Protection Agency has undertaken a program to help municipalities and engineers
avoid problems in wastewater treatment facility design and operation. A series of Design Information
Reports is being produced that identifies frequently occurring process design and operational problems
and describes remedial measures and design approaches used to solve these problems. The intent is not to
establish new design practices, but to concisely document improved design and operational procedures
that have been developed and successfully demonstrated in field experiences.
With an increased emphasis being placed on environmental concerns associated with the disposal of
sludges from wastewater treatment facilities, there has been a growing awareness of the need for
improved efficiency and reliability in the performance of in-plant sludge treatment processes. Because of
advances in centrifuge technology that have improved their efficiency and reliability, centrifuges are now a
frequently considered alternative for the thickening and dewatering of wastewater treatment sludges.
Introduction
Centrifuges, recognized as potential machines for de-
watering sludges as early as 1902 in Germany, were
introduced to the United States during the 1920s and
1930s, but were subsequently abandoned when it
was found that they required excessive operation and
maintenance attention. In the 1960s, manufacturers
began to design centrifuges specifically for waste-
water sludge applications. In addition, sludge thicken-
ing and dewatering processes were improved with
the introduction of polyelectrolytes for chemical
sludge conditioning.
This report contains a brief description of the major
components and operational principles of solid bowl,
imperforate basket, and disc-nozzle centrifuges,
followed by a discussion of centrifuge application to
sludge thickening and dewatering. Common problems
experienced at centrifuge installations are identified,
including causes of problems, significant cost and
plant performance impacts, and appropriate remedial
measures. Finally, the relative advantages and dis-
advantages of each centrifuge and recommended
approaches for design and operation are summarized.
Description of Centrifuges
All centrifuges operate on the principle of centrifugal
force created by the angular velocity of a rotating
device. The resulting centrifugal acceleration is
measured with respect to the acceleration of gravity
(commonly referred to as "G"). When sludge is
subjected to these centrifugal forces, particles will
separate according to their specific gravity. Solids,
being the heaviest sludge constituent, are forced to
the periphery, and liquid, or centrate, remains nearer
the axis of rotation.
Two of the three centrifuge types, solid bowl and
imperforate basket, have applications to thicken or
dewater various types of sludge, whereas disc-nozzle
centrifuges are only applicable to waste activated
sludge thickening.
Solid Bowl Centrifuge
The solid bowl centrifuge consists of eight major com-
ponents: base, cover, bowl, scroll, feed pipe, mam
bearings, gear unit, and backdrive. Revolving around
a horizontal axis of rotation, the bowl and scroll make
up the rotating assembly. The scroll rotates at a
higher or lower speed than the bowl, known as the
differential speed, reported to range from 5 to 80
revolutions per minute (rpm). A planetary or cyclo-
gear-typegear unit works with a mechanical, hydrau-
lic, or electrical backdrive assembly to produce the
bowl/scroll differential speed.
Bowl design is generally in a compound cylindrical
and conical shape, the relative proportions of which
vary by manufacturer and specific application. Typical
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bowl length-to-diameter ratios vary from 2.5:1 to 4:1
(2). The scroll is a helical screw conveyor, fitted
concentrically into the bowl. Both the bowl and the
scroll are typically constructed of carbon steel or 300
series stainless steel. The outer edge of the scroll is
constructed of replaceable abrasion-resistant tiles
manufactured of ceramic, tungsten carbide, or
another type of hard facing (2,4,7). Sludge feed slurry
enters the centrifuge through a feed pipe in the
central core of the scroll. Feed nozzles, projecting
through the scroll, deliver sludge into the bowl.
The centrifuge base provides a structure on which to
mount and support the rotating assembly. Vibration
isolators are installed between the base and its
foundation, typically consisting of multiple springs
designed to dampen the vibration produced by the
centrifuge. A case serves as a guard and complete
enclosure of the rotating assembly, thereby reducing
noise levels and often directing solid and liquid
discharge flow. Main bearings guide the bowl and
scroll rotation and require a constant lubrication
system. Most installations use a water cooling system
to maintain safe bearing temperatures (2,9).
Solid bowl centrifuges are available in a variety of
sizes. The largest units available for municipal sludge
applications are 25 feet in length including the
rotating assembly and backdrive and 14 feet in width
including the rotating assembly and adjacent motor.
The throughput capacity of these units vary with
application. Feed rates of 100 to 200 gallons per
minute (gpm) for dewatering and 400 to 600 gpm for
thickening can be achieved for most municipal
sludges. Two configurations for the solid bowl
centrifuge, concurrent and countercurrent, are
shown in Figure 1(a)and 1(b). These two types vary in
their method of sludge feed and discharge. The
concurrent configuration introduces sludge feed into
the rotating assembly through the cylindrical end of
the bowl. The orientation of the scroll helix draws
settled solids in the direction of sludge feed toward
the conical "beach section" and shidge discharge.
The countercurrent configuration delivers sludge feed
through the conical end of the bowl, and the scroll is
oriented to draw solids against the direction of sludge
feed toward the beach and sludge discharge.
Two philosophies are upheld regarding the speed of
rotation of solid bowl centrifuges. Low-speed centri-
fuges operate at gravitational forces below 1,100 G,
whereas high-speed centrifuges operate above 1,100
G (16). Proponents of low-speed centrifuges argue
that operating at lower speed results in less energy
consumption, less noise, and lower maintenance
costs (5). High-speed centrifuge proponents suggest
that operating at higher speed increases perform-
ance, increases capacity, and reduces the need for
polymer conditioning. The introduction of improved
wear-resistant construction materials for rotating
assemblies has reduced the maintenance costs
associated with high speed centrifuges. Thus, high
speed centrifuges have increased in popularity for
municipal sludge thickening and dewatering.
Basic operation of the continuous feed, solid bowl
centrifuge consists of introducing sludge into the
rotating assembly. Centrifugal forces cause the liquid
portion to form a ring-shaped pool about the axis of
rotation. Pool depth is controlled by a circular weir
plate located at the cylindrical end of the bowl, and
clarified liquid, or centrate, is discharged by over-
flowing the weir plate. The differential speed creates
a net rotation of the scroll within the bowl. This
causes solid particles, settled along the inner wall of
the bowl, to be dragged in the direction of the scroll
helix orientation toward the conical "beach" section
of the bowl. As solids are drawn up the beach, they
emerge from the liquid poo! and drain off a portion of
free water before being discharged.
Figure 1. Solid bowl centrifuge schematic.
Feed Pipes
(Sludge and
Conditioning
Chemical
- Cover f
-Rotating
.Bowl /
Rotating Conveyor/Scroll
.Gear Reducer
Dewatering / Backdrwe
Beach/ / \
Centrate
Discharge
Differential
Speed Gear
Box "I
Centrate
Discharge
Port
(Adjustable)
Centrate
Withdrawal
(a) Concurrent
Cover
Sludge Cake
Discharge
Dewatering Beach
Mam Drive
/~ Sheave
Centrate
Discharge
Rotating.
Conveyor
(b) Countercurrent
Note Centrifuge Bases are Not Shown
Source' 15
Sludge
Cake
Discharge
Feed Pipes
(Sludge and
Conditioning
Chemical)
-------
Imperforate Basket Centrifuge
The imperforate basket centrifuge is composed of a
cylindrical basket rotating about a vertical axis. Figure
2 shows a simplified schematic of the top fed, bottom
driven basket centrifuge, illustrating the general
location of sludge and polymer feed pipes, centrate
discharge, skimmer, knife, and cake discharge (15).
Basket centrifuges are typically constructed of stain-
less steel to inhibit corrosion and wear of wetted
parts. The largest units are approximately four feet in
diameter and are capable of achieving rotational
velocities of up to 1,400 rpm, creating centrifugal
accelerations of 1,300 G. These units have an average
throughput capacity of 60 gpm.
The imperforate basket centrifuge process cycle
includes acceleration, sludge feed, skimming, decel-
eration, and unloading, followed by acceleration to
begin another cycle (6). Cycle times range from 15 to
30 minutes, during which sludge feed is discontinued
for up to three minutes for skimming and unloading
operations (9). When polymer sludge conditioning is
used, polymer is added directly to the basket.
Near the end of the cycle, sludge feed is discontinued,
triggering activation of the skimmer and deceleration
of the basket. The skimmer will remove the remaining
clarified liquid. Once the basket has decelerated to
the unloading speed (70 to 100 rpm), a knife will
slowly move horizontally toward the basket wall,
Figure 2. Imperforate basket centrifuge schematic.
Sludge Feed
Polymer Feed
Skimming -^-
Knife
scraping off solids that subsequently drop through
the cake discharge at the bottom of the basket (15).
Upon completion of cake removal, the skimmer and
knife retract, allowing the basket to accelerate and
begin a new cycle.
Disc-Nozzle Centrifuge
The disc-nozzle centrifuge rotating mechanism is
comprised of several conical-shaped discs stacked
one upon another, enclosed within a rotor bowl that
rotates about a vertical axis. Around the periphery of
the rotor bowl are numerous cylindrical discharge
nozzles. The entire rotating mechanism is contained
within a housing assembly that is compartmentalized
to receive and direct sludge feed, sludge discharge,
recirculation, and effluent discharge. The discs and
rotor bowl are typically constructed of corrosion-
resistant stainless steel. The housing assembly is
either stainless steel or bronze. Disc-nozzle centri-
fuges are available in a variety of sizes. The largest
units have an average throughput capacity of 200 to
300 gpm and operate at rotor speeds of up to 4,200
rpm, creating centrifugal forces of 6,500 G. These
units have a rotor bowl over three feet in diameter
contained within a six foot diameter housing as-
sembly. The support assembly and drive train can
reach an overall height of ten feet. A top driven, top
fed machine configuration is shown in Figure 3.
Disc-nozzle centrifuge operation involves continuous
feed of sludge to the bottom of the spinning rotor,
where centrifugal forces move the heaviest solid
particles directly to the edge of the bowl. Lighter
particles will pass through the spaces between the
discs. The disc spacing, typically 1.3 mm (0.05 in),
acts to minimize settling distance (15). Particles will
settle on the underside of the discs and accumulate
Figure 3. Disc-nozzle centrifuge schematic.
Sludge Feed
Sludge Feed
Disc Stack
Concentrating
Chamber
Sludge
Discharge
Effluent
Discharge
Centrate Discharge
Cake
Discharge
Rotor Bowl
Discharge
Nozzle
. Sludge
Discharge
Recirculation Flow
Source: 15
Source: 15
-------
until their mass is great enough to force them to the
periphery of the disc and subsequently to the edge of
the rotor bowl. Clarified liquid flows between the
discs and overflows the top of the rotor bowl through
an effluent discharge. Solid particles collected along
the periphery of the bowl are discharged through
small cylindrical nozzles, usually 1.3 to 2.5 mm (0.05
to 0.10 in) in diameter (1,12). The nozzles act to
reduce the solids discharge rate, thereby increasing
the retention time and accumulation of particles in
the concentrating chamber of the rotor bowl. A
variable portion of the discharged sludge may be
recirculated to the rotor bowl, undergoing further
concentration. The recirculation rate can be con-
trolled manually by the operator or automatically by
external controls that respond to sludge viscosity or
density.
System Assessment
Advances in centrifuge technology, including im-
proved design and construction materials, have
increased centrifuge applications for thickening and
dewatering municipal sludges. Although centrifuges
have recently achieved greater acceptance for munic-
ipal installations, certain conditions may make the
selection of a centrifuge inappropriate. Variable
sludge characteristics make process control difficult
and a high level of operator skill is required to
maintain optimum centrifuge performance. Variable
sludge characteristics often result from seasonal
changes, industrial waste contributions, or flow and
load variations related to combined sewers. Some
sludge types limit centrifuge application. These
include septic or old sludges containing solid particles
that are less dense and more difficult to separate by
centrif ugation. In addition, literature sources indicate
that waste activated sludge with a sludge volume
index (SVI) greater than 150 significantly reduces
centrifuge performance in thickening applications
(9,10,17).
The selection among the three centrifuge types for
thickening and dewatering municipal sludges should
be based on desirable sludge feed types, integration
with other treatment plant process elements, and
suitable plant size. In general, solid bowl centrifuges
have the most widespread use, due to high through-
put capacity and applications for sludge thickening or
dewatering. Imperforate basket centrifuges are also
capable of thickening or dewatering, but have lower
throughput capacity. Disc-nozzle centrifuges have
limited municipal sludge thickening applications.
Table 1 lists typical thickening and dewatering
performance for solid bowl centrifuges on various
types of sludges. Many solid bowl centrifuge thicken-
ing installations operate without polymer sludge
conditioning, but all dewatering installations require
varying polymer dosage to achieve satisfactory cake
solids concentration and centrate quality. Solid bowl
manufacturers consider pretreatment of sludge feed
unnecessary in most situations, but field experience
has shown that screenings and grit should be reduced
for successful centrifuge operation. Considering the
variety of solid bowl centrifuge sizes and capacities
currently manufactured, their application is well
suited for both small and large treatment plants.
Typical thickening and dewatering performance of
imperforate basket centrifuge on various types of
sludges is shown in Table 2. Generally, pretreatment
of the basket centrifuge feed sludge for removal of grit
and screenings is not required. Polymer sludge
conditioning may not always be necessary to achieve
desired cake solids concentration and centrate qual-
ity. Although basket centrifuges are versatile in
municipal wastewater applications, their use is
generally limited to smaller treatment plants for two
reasons. First, their relatively low basket speed limits
clarification and compaction of sludge solids. Second,
the maximum average capacity (including "no flow"
time periods of skimming and unloading) of the
largest units seldom exceeds 60 gpm (8,9).
Disc-nozzle centrifuges are used exclusively for
thickening waste activated sludge in municipal sludge
processing. Attempts to thicken mixed sludges re-
sulted in excessive discharge nozzle wear and
plugging (9). Table 3 lists the typical waste activated
sludge thickening performance of disc-nozzle centri-
fuges. Sludge feed pretreatment is generally required
to reduce the frequency of maintenance downtime for
rotor bowl and discharge nozzle cleaning. This
pretreatment train typically consists of rotary screens,
grit cyclones, and in-line strainers. Although high
throughput capacity (200-300 gpm) makes disc-noz-
zle centrifuges suitable for both small and large
treatment plants, the overall system complexity and
the advancement of solid bowl centrifuge technology
has virtually eliminated disc-nozzle centrifuges from
the municipal sludge thickening market.
Problem Assessment
The problems associated with the use of centrifuges
for thickening and dewateri.ng municipal wastewater
treatment plant sludge may be categorized as equip-
ment quality/design problems, system integration
problems, and process operation and maintenance
(O&M) problems. Each problem has been evaluated to
determine its cause, significant impacts on plant
performance and O&M costs, and remedial measures
that may be taken to correct the problem.
Equipment Quality/Design Problems
Problems in this category relate to centrifuge defects
and malfunctions, fatique and wear of centrifuge
-------
Table 1 . Typical Performance of Solid Bowl Centrifuge*
Solid Bowl
Centrifuge
Application
Thickening
Thickening
Dewatering
Dewatering
Dewatering
Dewatering
Dewatering
Dewatering
Dewatering
Dewatering
Dewatering
Sludge Type
Raw waste activated
Raw primary plus waste
activated (60:40)
Raw primary
Anaerobically digested
primary
Anaerobically digested
primary irradiated at
400 kilorads
Waste activated
Aerobically digested
waste activated
Thermal conditioned
primary & waste activated
Primary & trickling filter
High lime
Raw primary & waste activated
Feed Solids
Concentration
(%)
0.5-1.5
0.5-3.0
5-8
5-8
2-5
9-12
9-12
2-5
0.5-3
1-3
9-14
13-15
7-10
10-12
4-5
Average
Cake Solids
Concentration (%)
3-8
4-8
25-36
28-36
28-35
30-35
25-30
28-35
8-12
8-10
35-40
29-35
35-40
30-35
30-50
18-25
Dry Polymer
Per Dry Feed
(mg/kg)
0
0
500-2,500
0
3,000-5,000
0
500-1 ,500
3,000-5,000
5,000-7,500
1 ,500-3,000
0
500-2,000
0
1 ,000-2,000
0
1,500-3,500
Used
Solids
(Ib/ton)
0
0
(1-5)
(0)
(6-10)
(0)
(1-3)
(6-10)
(10-15)
(3-6)
(0)
(1-4)
(0)
(2-4)
(0)
(3-7)
Solids
Recovery
(%)
80-90
85-95
90-95
70-90
98+
65-80
82-92
95+
85-90
90-95
75-85
90-95
60-70
98+
90-95
90-95
*Data obtained from plant contacts and Reference 15.
parts, and high maintenance items associated with
centrifuges. These problems may relate to equipment
quality (defects), or to equipment design (selection of
materials that cannot withstand normal wear). Prob-
lems associated with ancillary equipment are dis-
cussed in the section addressing system integration
problems.
Excessive scroll wear—Predominantly due to abra-
sive grit, this has historically been a serious problem
for solid bowl centrifuges; but with the introduction of
hard surfacing, such as sintered tungsten carbide
tiles, scroll wear has been significantly reduced.
Annual maintenance costs are on the order of 1
percent to 10 percent of initial capital cost for scroll
rebalancing, resurfacing, and servicing the gear unit.
In older installations, a retrofit of the scroll with
improved hard surfacing can reduce scroll wear.
Centrifuge downtime associated with scroll rebuild-
ing may be minimized by obtaining a spare rotating
assembly. Scroll wear can also be reduced by
installing an automatic backdrive control that re-
sponds to changing torque load on the scroll, effec-
tively limiting the abrasive wearing forces.
Excessive vibration and noise—This problem, al-
though not particularly widespread in existing instal-
lations, is an important design consideration. Im-
properly designed mounting platforms, inadequate
vibration isolators, improper building construction or
material selection, and an unbalanced rotating as-
sembly are causes of vibration and noise. Excessive
vibration can reduce the operating life of bearings and
rotating assemblies, and high noise levels are con-
sidered operator health concerns. Vibration and noise
are best controlled by adhering to manufacturers'
specifications for mounting platform and vibration
isolator design and by using sound-absorbing, acous-
tical construction materials in centrifuge rooms. Use
of disposable ear plugs is a low-cost method of
mitigating noise problems.
Inadequate instrumentation for lubrication sys-
tem—The main bearings of solid bowl centrifuges
require constant lubrication, typically provided by a
pressurized or splash-type lubrication system. Inade-
quate instrumentatiQn for monitoring oil flow may
preclude proper maintenance of lubrication systems
and result in excessive bearing wear. The capability to
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Table 2. Typical Performance of Imperf orate Basket Centrifuge*
Imperforate
Basket
Centrifuge
Application
Thickening
Thickening
Thickening
Thickening
Dewatermg
Dewatermg
Dewatermg
Dewatermg
Dewatermg
Dewatermg
Dewatermg
Dewatermg
Sludge Type
Raw waste activated
Aerobically digested
Raw trickling filter
(rock & plastic media)
Anaerobically digested
primary & rock trickling
filter sludge (70:30)
Raw primary
Raw trickling filter
(rock or plastic media)
Raw waste activated
Raw primary plus rock
trickling filter (70.30)
Raw primary plus waste
activated (50 50)
Raw primary plus rotating
biological contactor (60:40)
Anaerobically digested
primary plus waste
activated (50 50)
Aerobically digested
Feed Solids
Concentration
(%)
0.5-1.5
0.5-1.5
1-3
1-3
2-3
2-3
2-3
2-3
4-5
2-3
2-3
0.5-1 5
0.5-1.5
2-3
2-3
2-3
2-3
2-3
1-2
1-2
1-2
1-3
Average
Cake Solids
Concentration (%)
8-10
8-10
8-10
8-10
8-9
9-11
8-10
7-9
25-30
9-10
10-12
8-10
12-14
9-11
7-9
12-14
20-24
17-20
12-14
10-12
8-10
8-11
12-14
Dry Polymer Used
Per Dry Feed Solids
(mg/kg)
0
500-1 ,500
0
500-1 ,500
0
750-1 ,500
0
750-1,500
1 ,000-1 ,500
0
750-1,500
0
500-1 ,500
0
750-1,500
500-1 ,500
0
2,000-3,000
0
750-1 ,500
2,000-3,000
0
500-1 ,500
(Ib/ton)
(0)
(1 0-3.0)
(0)
(1 0-3.0)
(0)
(1.5-3.0)
(0)
(1.5-3.0)
(2-3)
(0)
(1.5-3.0)
(0)
(1.0-3.0)
(0)
(1.5-3.0)
(1.0-3.0)
(0)
(4-6)
(0)
(1.5-3.0)
(4-6)
(0)
(1.0-3.0)
Solids
Recovery
(%)
85-90
90-95
80-90
90-95
90-95
95-97
95-97
94-97
95-97
90-95
95-97
85-90
90-95
95-97
94-97
93-95
85-90
98+
75-80
85-90
93-95
80-95
90-95
'Data provided in Reference 15.
monitor both oil flow and pressure is recommended to
assist operators in lubrication system maintenance.
An in-line sight glass may be installed to permit visual
checks of oil flow.
Erosion or failure of feed pipes and feed nozzles—
This solid bowl centrifuge problem results from
abrasive grit wear and rag accumulation. Replace-
ment costs range from $400 per stainless steel feed
nozzle to $1,400 for a stainless steel feed pipe. In
addition, at least one day of labor is required to
replace either part. The use of hard facing material
such as tungsten carbide or ceramic liners for nozzles
and stainless steel feed pipes cost-effectively reduces
wear. Improving feed grinder performance may
reduce the frequency of plugging in the feed pipes.
System Integration Problems
Problems in this category relate to the ancillary
equipment required for a complete centrifuge instal-
lation, such as instrumentation, pumps, grinders, or
conveyors.
Insufficient area for equipment maintenance—It is
an important design consideration to provide ade-
quate maintenance area for centrifuge systems. Poor
layouts are difficult to remedy and often result in
longer centrifuge downtime for even routine mainte-
nance. Solid bowl centrifuge system design should
include considerations for removal of the rotating
assembly and for in-house maintenance on the scroll
including the routine replacement of wearing sur-
faces (tiles). In addition, provisions should be made
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Table 3. Typical Waste Activated Sludge Thickening Per-
formance of Disc-Nozzle Centrifuge*
Underflow
Feed Solids Solids Solids
Capacity Concentration Concentration Recovery
l/min
570
1,510
1 90-300
230-1,020
250
760
(gpm)
(150)
(400)
(50-80)
(60-270)
(66)
(200)
(%)
0.75-1.0
—
0,7
0.7
1.5
0.75
(%)
5-5.5
4.0
5-7
6.1
6.5-7.5
5.0
(%)
90+
80
87-93
80-97
87-97
90
*Data provided in Reference 15.
Note: Indicated performance is without polymer sludge condi-
tioning.
for direct access by monorail or bridge crane to any
centrifuge mounting or maintenance area.
Inadequate mixing system in sludge holding
tanks—Failure to provide adequate mixing can result
in highly variable sludge characteristics, making
performance optimization difficult. In addition, un-
mixed holding tanks may freeze during periods of cold
weather. The design of holding tank mixing systems
will vary with sludge type. For aerobic sludges,
satisfying the aeration requirements may be more
critical than mixing requirements. Holding tank
covers should be considered for cold regions.
Lack of sampling locations—Regardless of the
mixing system used it is important to monitor, by
sampling, solids concentration of sludge feed, sludge
cake, and centrate. Difficulty in optimizing centrifuge
performance is often a direct result of insufficient
sampling locations. Existing facilities with this prob-
lem should consider retrofitting their sludge feed
lines, centrate discharge piping, and sludge discharge
conveyance equipment with sampling taps.
Performance limitations of polymer feed systems—
Performance limitations associated with polymer feed
systems include insufficient polymer dilution capacity
to compensate for lower demand during periods of
lowfeed solids concentration and insufficient number
of polymer injection points for complete mixing with
feed sludge. Generally, these situations result in
excessive polymer consumption. In-line dilution of
polymer is recommended after initial batch mixing to
ensure optimum mixing and dilution. Polymer manu-
facturers' product data sheets provide in-line dilution
recommendations for their products. At least three
points of injection for polymer mixing with sludge
feed are recommended. These injection points are
just before the centrifuge inlet or directly into the
basket centrifuge, ahead of the sludge feed pump,
and immediately after the sludge feed pump. These
locations allow operators to optimize polymer mixing
with sludge feed (10).
Progressive cavity feed pump wear—This problem
is commonly experienced in facilities with high grit
levels. Uncontrolled wear can reduce pump capacity
and diminish the accuracy of flow measurement and
control devices that are based on pump motor speed.
Alternative feed pumps, centrifugal and rotary lobe,
are less susceptible to grit-related wear.
Insufficient system interlocks, instrumentation, or
controls—This design problem is more prevalent in
older installations lacking adequate interlock or
control packages. Interlocks should provide total
centrifuge system shutdown with the occurrence of
any system component failure, including pretreat-
ment devices, polymer feed system, the centrifuge
unit, sludge cake conveyor, and centrate return.
Advances in manufacturers' instrumentation and
control packages and the introduction of magnetic
and sonic flow meters have improved process control,
simplifying centrifuge performance optimization.
Poor grinder performance—Typically a result of
overloading with sticks, grit, and rags, poor grinder
performance causes several other related centrifuge
problems, including feed pipe plugging and scroll
binding. Grinder problems may be abated by regularly
scheduled servicing, including blade sharpening.
Sludge discharge and conveyor problems—Centri-
fuge system designers should avoid belt conveyor
layouts that allow a free fall of sludge from the
centrifuge discharge at heights greater than two feet
above the conveyor, or that allow belt conveyors to be
designed at an incline. Allowing a free fall discharge
of sludge may result in sludge splatter and spills.
Sludge may accumulate at the base of an inclined
conveyor resulting in sludge spills and conveyor
overloading. An additional maintenance problem
associated with belt conveyors is the grit-related
wear of moving parts. Screw conveyor problems are
commonly caused by undersizing and by clogging
with rags. Several remedial measures include re-
ducing centrifuge throughput to stay within conveyor
capacity, adding baffles to the discharge chute to
minimize splatter, and replacing undersized convey-
ors.
Bowl/scroll binding—This problem occurs in solid
bowl centrifuge installations having deficiencies in
the removal of rags, fibers, and scum. These materials
may accumulate between the scroll tip and the bowl
wall until the allowable torque on the scroll is
exceeded, triggering an automatic shutdown switch.
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Extensive labor is required to disassemble and clean
the rotating assembly. Remedial measures include
adding grinders to the sludge feed, routine f lushi ng of
the bowl, discontinuing the disposal of scum with
waste activated sludge, and installing an automatic
backdrive control that appropriately adjusts differ-
ential speed to prevent excessive scroll torque
variations.
Frequent nozzle plugging—This problem, common
to disc-nozzle installations, is directly attributed to
grit and scum accumulation and indirectly attributed
to insufficient pretreatment of the waste activated
sludge feed. Nozzle cleaning is the most significant
cause of maintenance downtime. Disc-nozzle instal-
lations generally require extensive sludge feed pre-
treatment units, adding considerable cost to the
system and increasing plant recycle loads. Regular
steam cleaning of the disc stack and nozzles can
reduce the frequency of plugged nozzles.
Odor problems—Processing odorous sludge, includ-
ing anaerobically digested or chlorine oxidized
sludges, warrants additional air handling considera-
tions for odor control in centrifuge installations.
Minimum ventilation rates on the order of six air
changes per hour during summer and three air
changes per hour in the winter should be considered.
Higher rates and treatment of the exhaust air may be
needed if particularly odorous sludges are to be
handled. It may be more cost-effective to treat exhaust
air from the centrate discharge chute instead of
allowing odors to escape into the room and then
treating room air.
Process O&M Problems
Process O&M problems relate to improper operating
procedures, inadequate routine maintenance, and
lack of knowledge and understanding of the centri-
fuge system.
Operator training—An incomplete understanding of
centrifuge operation and maintenance makes it
difficult to optimize centrifuge performance. Opera-
tors often favor one or two variables for process
control which may limit sludge processing capacity
and increase polymer consumption. High operator
turnover and lack of adequate instrumentation to
monitor centrifuge performance are typical causes of
O&M problems. The machine characteristics and
process variables and their effects on centrifuge
performance are summarized in Table 4. Machine
characteristics vary between manufacturers and their
limitations on process flexibility should be considered
in centrifuge selection and specification.
Polymer selection and dosage optimization—Indis-
criminate polymer selection will not provide the most
efficient use of polymer, since various polymers do
not produce the same results. Polymer selection
Table 4. Machine Characteristics and Process Variables
Affecting Centrifuge Performance
Centrate Cake Solids
Adjustment Quality Concentration
Solid Bowl Centrifuge (6)
1. Machine Characteristics*
—increase bowl length
—increase bowl diameter
—increase beach angle
—increase bowl speed
—increase conveyor pitch
—move inlet closer to beach
2. Process Variables
—increase feed rate
—increase temperature of
feed sludge
—add polymer
—increase differential speed
—increase pool depth
Imperforate Basket Centrifuge
1. Machine Characteristics*
—increase basket speed
2. Process Variables
—increase feed rate
—increase temperature of
feed sludge
—add polymer
—increase cycle time
—increase skimming
Disc Nozzle Centrifuge
1. Machine Characteristics*
—increase rotor speed
—increase disc size
—increase disc spacing
2. Process Variables
—increase feed rate
—increase recirculation
rate
—increase temperature of
feed sludge
Improve
Improve
Reduce
Improve
Reduce
Improve
Cannot predict
Reduce
Improve
Improve
Improve
Reduce
Reduce Improve
Improve
Improve
Reduce
Improve
Improve
Improve
Improve
Reduce
Improve Improve
Reduce Improve
Improve Improve
Improve Improve
Reduce Improve
Reduce Improve
Improve Improve
Improve Improve
Reduce Cannot predict
Reduce Improve
Cannot
Predict Improve
Improve Improve
'Fixed by manufacturers' designs.
should begin with a review of manufacturers' product
data sheets. These product data sheets will provide a
general indication of the polymer's suitability for
centrif ugation. Generally, cationic polymers are used
for municipal wastewater sludges, especially for
waste activated sludges. Anionic polymers are fre-
quently used for alum sludges and high pH sludges.
However, there is no definitive way to select a
polymer without conducting bench scale testing with
samples of the plant sludge. The laboratory jar test is
a simple method of polymer performance evaluation
and comparison. Polymer dosage rates may be
optimized by the jar test or the capillary suction test.
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The ultimate test is within the operating centrifuge.
Sludge characteristics are frequently subject to
change including seasonal variation. Therefore, it is
recommended that weekly laboratory testing be
conducted to optimize polymer dosage and selection,
or as frequently as determined through operating
experience (10).
Polymer foaming—This problem is related to exces-
sive polymer dosage (3,13). In severe cases, polymer
foam can be forced through the discharge chutes,
creating polymer spills that are difficult to clean up
and present potential safety hazards from slippery
surfaces. Polymer foaming can be associated with a
lack of sludge conditioning testing (jar tests or
capillary suction tests) and with inadequate dosage
control in the polymer feed system. Dosage control
can be improved by the use of dry polymer feed
systems with automatic batch mixing.
Scale formation and struvite deposition in the
centrifuge—Scale formation can result from the
centrifugation of lime stabilized sludge, or from the
presence of other mineral salts in the sludge feed.
Struvite is a crystalline ammonium-magnesium-
phosphate associated with anaerobically digested
sludge and sludge having high levels of soluble
phosphorus. Identified causes of scale and struvite
build up include inadequate bowl cleaning and batch
mixing of alkaline polymer solution with plant efflu-
ent. The accumulation of scale and struvite decreases
centrifuge solids capture efficiency and requires
extensive labor to remove the deposits from pipe and
bowl surfaces. Several cleaning methods are avail-
able including regular hot flushing of the centrifuge
and piping with scale-removing solutions, using
hydrated lime in place of quicklime for stabilization of
sludge, selecting non-alkaline polymer solutions, and
adding chelating agents to prevent scale and struvite
deposition. Laboratory and performance testing
should be conducted prior to selection and bulk
purchases of non-alkaline (low pH) polymers.
Hydraulic overloading of solid bowl centrifuges—In
addition to potential performance deterioration, hy-
draulic overloading may result in rotating assembly
seal failure and lubrication system contamination.
Seal replacement requires considerable labor and
material expense, compounded when the bearing
and lubrication systems are contaminated. The most
effective remedial measure for this problem is to
educate operators on the hydraulic capacity limita-
tions of their centrifuges.
Solid bowl centrifuge backdrive seizure—This prob-
lem is experienced in facilities that infrequently use
their centrifuges. The lubricant can drain from the
bearing assemblies during extended downtime, re-
sulting in bearing corrosion and seizure. A regular
maintenance program of lubrication and manually
turning the rotating assembly during idle periods
prevents seizure of the backdrive unit.
Excessive sludge feed period in basket centri-
fuges—The sludge feed period per cycle is a process
variable controlled by the operator. Longer feed
periods generally improve the sludge solids concen-
tration, but may result in harder cake formation on the
inner wall of the basket. Hard cake solids have caused
the knife to chatter during the unloading phase of the
process cycle. In addition, excessive chatter may
result in unloading knife stress failure. The remedial
measure for this problem is to impose an operational
limit on the sludge feed period.
Summary
The selection of centrifuge systems for use in
municipal sludge treatment requires careful con-
sideration of their application to either thickening or
dewatering. Solid bowl centrifuges are most widely
used because of their high throughput capacity and
applications for thickening or dewatering. Imperforate
basket centrifuges are also capable of thickening or
dewatering, but they have relatively low throughput
capacity. Disc-nozzle centrifuges are limited to thick-
ening waste activated sludge. Each centrifuge type
has, by nature of design or operation, relative
advantages and disadvantages for municipal sludge
thickening and dewatering as summarized in Table 5.
In order to minimize the impact of design and
operating problems and to improve centrifuge per-
formance, the following approaches to design, opera-
tion, and training are recommended.
Design Approaches
• Provide adequate instrumentation to monitor oil
flow as well as oil pressure in hydraulic bearing
lubrication systems to prevent loss of lubricant
flow to the main bearings.
• Provide hard facing materials on centrifuge parts
that are normally subjected to high wear, such as
scrolls, feed ports, and feed tubes.
• Investigate the applicability of centrifuges, possibly
including pilot studies and/or laboratory analyses
of sludges, early in the design process. Centrifuges
may not be the dewatering device to select if the
sludge feed characteristics are highly variable, or if
the sludge is expected to contain a significant
amount of grit.
• Provide adequate floor space and hoisting equip-
ment for proper equipment maintenance, removal,
and installation.
• Provide appropriate equipment mounting compo-
nents and foundation design to minimize centri-
fuge vibration.
-------
Table 5. Advantages and Disadvantages of Disc-Nozzle,
Imperforate Basket, and Solid Bowl Centrifuges
Disc Nozzle
Advantages
Relatively high throughoput
in a limited area.
Polymer is not required for
satisfactory performance
Odors can be easily contained,
and a relatively small air
volume treated, if needed.
Disadvantages
Thickens waste activated
sludge only.
A complex pretreatment system
is required
Recycle loadings from
pretreatment systems are high.
Skilled operations and
maintenance staff are required
for this relatively complex
operation.
Imperforate Basket
Advantages
Thickening and dewatenng of
waste activated and mixed
sludge.
Especially well suited to
handling difficult sludges
(e.g., aerobically digested).
Resistant to grit and clogging,
requiring no sludge
pretreatment.
Provides maximum flexibility
for handling variable sludge
characteristics.
Disadvantages
Batch operation limits unit
capacities.
Special structural support is
required to support the
centrifuge, due to its bottom
discharge.
Sludge conveying options are
limited due to bottom discharge
and batch operation.
Operators must depend on the
result of a previous batch run for
process optimization
Solid Bowl
Advantages
Thickening and dewatenng of
waste activated and mixed
sludge
Relatively high throughput in
a limited area.
Odors can be easily contained,
and a relatively small air
volume treated, if needed
Somewhat adaptable to
varying sludge characteristics.
Disadvantages
Potentially high maintenance
costs.
Operating costs may be higher
due to polymer.
Adequate grinding of the sludge
feed must be provided to prevent
plugging.
Relatively skilled operators are
required for optimum results.
Reference: 14
• Provide polymer systems that are capable of
delivering an adequate polymer dose at low solids
concentrations (0.5 percent) in the sludge feed.
• Consider alternatives to progressive cavity sludge
feed pumps, including centrifugal or rotary lobe
pumps.
• Provide interlocks between the centrifuge and
ancillary equipment, such as feed pumps, grinders,
polymer systems, sludge conveying devices, and
centrate pumps, to ensure system shutdown in the
event that a system component fails.
• Provide instrumentation to allow plant staff to
determine centrifuge process variables, such as
sludge and polymer feed rates and the bowl-scroll
differential speed.
• Design sludge conveying systems to avoid steep
inclines, and to ensure adequate conveyor capac-
ity. Effort should be made to provide the most
simple, direct discharge from the centrifuge.
Ideally, dewatering centrifuges should discharge
directly into a disposal container or vehicle, and
thickened sludge to a blending or holding tank.
• Provide adequate ventilation systems for odor
control.
• Consider specification of acoustical construction
materials for centrifuge buildings and rooms to
reduce noise levels.
• Provide standby ancillary equipment, such as
grinders and pumps, to reduce centrifuge down-
time when this equipment fails.
• Consider the feasibility of providing temperature
control of sludge feed as a process variable.
Increasing sludge temperature improves centri-
fuge performance, but may create odor problems.
Operational Approaches
• Consider purchasing a spare rotating assembly to
minimize downtime during routine scroll main-
tenance, such as replacement of worn tiles.
• Perform grinder and degritter maintenance to
minimize centrifuge plugging by rags and grit-
related wear. Facilities that stabilize sludge with
quicklime should maintain optimum performance
of grit removal devices in lime slaking systems.
• Conduct polymer testing regularly, especially prior
to bulk purchases of polymer. Jar tests, capillary
suction tests, and full-scale operating tests should
be used where possible. This testing should
optimize polymer dosage and application points to
achieve the most efficient, economical operation
possible.
• Monitor centrifuges for accumulation of scale,
especially during initial startup period. If scale
formation occurs, actions should be taken to
remove the scale and prevent its accumulation.
w
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• Keep maintenance records for tracking chronic
maintenance problems and for scheduling pre-
ventive maintenance.
• Perform routine centrifuge flushing to prevent
problems related to solids and rags accumulation.
Training
Proper operator training will improve process effi-
ciency and reduce costs. Training in the following
areas should be provided to all personnel who are
involved in the operation and maintenance of cen-
trifuges:
• Process and machine control variables, their
impacts on solids throughput and centrate quality,
and how each variable is monitored and adjusted.
• Process data collection and evaluation.
• Process troubleshooting procedures.
• Preventive maintenance requirements and proce-
dures, including record keeping.
• Process and system economic considerations.
• Operation, maintenance, control theory, and
troubleshooting procedures for the overall centri-
fuge system, encompassing support equipment.
Training should be provided in the above areas during
plant start-up, and as an ongoing program, especially
when new personnel are placed in charge of the
centrifuge installation.
Acknowledgments
This report was prepared for the U.S. Environmental
Protection Agency by Metcalf & Eddy, Inc., Wakef ield,
Massachusetts, under Contract No. 68-03-3208.
Mr. Francis L. Evans, III, EPA Project Officer, was
responsible for overall project direction. Other EPA
staff who contributed to this work included:
Dr. Harry E. Bostian, Technical Project Monitor,
Water Engineering Research Laboratory
Mr. Walter Gilbert, Office of Municipal Pollution
Control
Dr. Joseph B. Farrell, Water Engineering Research
Laboratory
Metcalf & Eddy staff participating in this project
included:
Mr. Allan F. Goulart, Project Director
Mr. Thomas K. Walsh, Project Manager
Mr. Robert A. Witzgall, Project Engineer
Mr. Scott Cowburn, Staff Engineer
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11
* U S GOVERNMENT PRINTING OFFICE, 1986 — 646-017/47158
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