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)

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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

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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

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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.

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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
References
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                                                            * U S GOVERNMENT PRINTING OFFICE, 1986 — 646-017/47158

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