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UCRL-21008 Vol. 8
Approved for public release
Distribution unlimited
Evaluation of Military Field-Water Quality
Volume 8. Performance of Mobile Water-Purification Unit (MWPU)
and Pretreatment Components of the 600-GPH Reverse Osmosis Water
Purification Unit (ROWPU), and Consideration of Reverse Osmosis (RO)
Bypass, Potable-Water Disinfection, and Water-Quality Analysis Techniques
R, E. Selleck
Z. Ungun
G. Chester
V. Diyamandoglu
J. I. Daniels
B. Mariflas
May 1990
m
m .
immM
Supported by
U.S. Army Medical Research and Development Command
Fort Detrick, Frederick, MD 21701
Project Order 82PP2817
Project Officer: Dr. Stephen A. Schaub
astir
X JL U
x Vi O
ii a'jjuagmaMaifeiaai
REPORT DOCUMENTATION PAGE
Form Approved
OM»Ho.on*4il»
ia. report security classification
UNCLASSIFIED
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4 FIRFORMING ORGANIZATION RCPORT NUMEER(S)
UCRL-21008 Vol. 8
a. NAMI OF FIRFORMING ORGANIZATION
Lawrence Uvarmora
National Laboratory
<c AOORIJS (Gty, Statt, and ilf Coda)
Environmantal Sciancaa Division
P.0. Box 5507, L-453
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a. NAMI OF FUNOING/ SPONSORING
ORGANIZATION U.S. Army Medical
lasearch and Davelopmant Command
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Fradarick, MD 21701-5012
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Davalopmant Laboratory
AOORESS (C/ty, srata, TnSTFcaSSJ
ATTNi SGRD-UBZ-C
Fort Datrick (Building 568)
5HMfM3E3in G nnarTfniTTT inaRrmiTJVTZM.'i’iRrr.i tti
Army Project Ordar 82PP2817
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IFWTOT*
NAL AUTHORS)
R.E. Sallack. Z. Ungun. G. Chastar. V. Diyamandoglu, J.I. Daniala, and B. Marina*
tl. SUPPLEMENTARY NOTATION
I 11*. TYPE OP RIPORT |1JD. TIME COVIRIO 114. DATE OP REPORT ( Y$tr, Month, My)
FINAL _ I FROM 11-86 TO 5-90 I May 1990
ATI COOES
GROUP I 5UI*GROUP
24
07
06
11
II. SUIJICT TCRMS (Contmua on ravarj* i) n«c****ry and NFamiFy
Mobil* Water Purification Unit; Revnroa Osmosis Water
Purification Unit; Pretraatmant; Filtration; Diainfection;
Water-Quality Analysis: Reverse Osmoaia Bvpaaa _
19, ABSTRACT (Cohdnua on rtvam ,p nataiMry and Fdatwiy by Mock number) "" " r " truwi .
A comparison is made between the performances of the 600-gph Ravarse Osmosis Water
Purification Unit (ROWPU) operated in the bypass mode and the Mobile Water Purification Unit
(MWPU, also referred to as an ERDLAT0R). Generally, the performance of the MWPU is
significantly better than the pretreatment units of the ROWPU in terms of removing both
turbidity and pathogenic organisms. It is recommended that the practice of bypassing ths
reverse osmosis (RO) components of the ROWPU be avoided unless it can J>a dsmonstratad claarly
that the cartridge filters will remove the cysts of infectious organisms effectively and
reliably. If the ROWPU must be operated in the bypass mode, it is recommended that the
dose of disinfectant used be made equal to that currently employed in the field for untreated
raw water.
The analytical methods used to determine total dissolved solids (TDS) and residual
free chlorine wilfh the new Water-Quality Monitor (WQM) are also reviewed briefly. The
limitations of ths methods used to calibrate the TDS and f rae— chlorine probes of ths new
20. OIJ TRIIUTION / AVAIIAIILIT Y OP AISTRACT
□ UNCLASSIRHOAJNUMITEO gj SAMI AS RPT
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Mary Frances Bostian
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UCRL-21008Vol.8
Evaluation of Military Field-Water Quality
Volume 8. Performance of Mobile Water-Purification Unit (MWPU)
and Pretreatment Components of the 600-GPH Reverse Osmosis Water
Purification Unit (ROWPU), and Consideration of Reverse Osmosis (RO)
Bypass, Potable-Water Disinfection, and Water-Quality Analysis Techniques
R. E. Selleck,* Z. Ungun,41 G. Chester,* V. Diyamandoglu*
J. I. Daniels, and B. Marinas*
Environmental Sciences Division
Lawrence Livermore National Laboratory
University of California
P. O. Box 5507
Livermore, CA 94550
Ac<M*l*sil»a For
"NT If? UVUfcl
me ui<
UuMnooojio ert
Just if I.0&1. loa
n
!'l
May 1990
*f . .
ms'., rtbul luai/
Supported by
U.S. Army Medical Research and Development Command
Fort Detrick, Frederick, MD 21701
Project Order 82PP2817
Av*LL'U>U 1 1,»
th
Dlat
A»hM. uno/nr
Kp#o 1;\ I
Project Officer: S. A. Schaub
Principal Investigators at Lawrence Livermore National Laboratory:
L. R. Anspaugh, J. I. Daniels, and D. W. Layton
Approved for public release; distribution unlimited
The findings of this report are not to be construed as an official Department
of the Army position unless so designated by other authorized documents.
* Sanitary Engineering and Environmental Health Research Laboratory [Building 112],
University of California, Richmond Field Station, Richmond, CA 94804.
FOREWORD
This report is the eighth volume of a nine-volume study entitled Evaluation of
Military Field-Water Quality. Titles of the other volumes are as follows: Vol. 1, Executive
Summary: Vol. 2, Constituents of Military Concern from Natural and Anthropogenic
Sources: Vol. 3, Opportunity Poisons: Vol. 4, Health Criteria and Recommendations for
Standards: Vol. 5, Infectious Organisms of Military Concern Associated with Consumption:
Assessment of Health Risks, and Recommendations for -Establishing_Relat£cL Standards;
Vol. 6, Infectious Organisms of Military.. Concern Associated with Nonconsumptive
Exposure; Assessment of Health Risks, and Recommendations _fQr _Establisliing. Related
Standards; Vol. 7, Performance. Evaluation of the 600-GPH . Reverse -Osmosis Water
Purification Unit (ROWPU); Reverse Osmosis (RO) Components; and Vol. 9, Rata ..for
Assessing Health Risks in Potential Theaters of Operation for U.S, Military. Forces.
The nine volumes of this study contain a comprehensive assessment of the
chemical/ radiological, and biological constituents of field-water supplies that could pose
health risks to military personnel as well as a detailed evaluation of the field-water-
treatment capability of the U.S. Armed Forces. The scientific expertise for performing the
analyses in this study came from the University of California Lawrence Livermore
National Laboratory (LLNL) in Livermore, CA; the University of California campuses
located in Berkeley (UCB) and Davis (UCD), CA; the University of Illinois campus in
Champaign-Urbana, IL; and the consulting firms of IWG Corporation in San Diego, CA,
and V.J, Ciccone & Associates (VJCA), Inc., in Woodbridge, VA. Additionally a Department
of Defense (DoD) Multiservice Steering Group (MSG), consisting of both military and
civilian representatives from the Armed Forces of the United States (Army, Navy, Air
Force, and Marines), as well as representatives from the U.S. Department of Defense, and
the U.S. Environmental Protection Agency provided guidance, and critical reviews to the
researchers. The reports addressing chemical, radiological, and biological constituents of
field-water supplies were also reviewed by scientists at Oak Ridge National Laboratory in
Oak Ridge, TN, at the request of the U.S. Army. Furthermore, personnel at several research
laboratories, military installations, and agencies of the U.S. Army and the other Armed
Forces provided technical assistance and information to the research on topics related to
field water and the U.S. military community.
iii
ACKNOWLEDGMENTS
The authors extend their appreciation to the scientists and staff of the
Environmental Sciences Division at the Lawrence Livermore National Laboratory (LLNL),
as well as to Dr. Stephen A. Schaub and his colleagues at the U.S. Army Biomedical
Research and Development Laboratory (USABRDL), for their cooperation and assistance. A
special thank you is extended to the scientists and staff of the Sanitary Engineering and
Environmental Health Research Laboratory at the University of California, Richmond
Field Station, and at the University of California, Berkeley, for their efforts in preparing this
document. The authors also express their gratitude to those individuals and organizations
in the reverse osmosis industry who provided information about reverse osmosis
technology that was intrinsic to our research, and to Dr. G.C. White for his review of the
material related to disinfection procedures and analytical methods for the assessment of
field-water quality.
i v
TABLE OF CONTENTS
Foreword . iii
Acknowledgments. . iv
List of Tables . v i
List of Figures . ix
Preface . x
Abstract . 1
Introduction . .. . 1
Concepts of Deep-Bed Filtration . 2
Theory . 5
Particle Transport . 5
Particle Attachment . 12
Coagulation and Coagulents . 14
Jar Tests . 15
Initial Mixing and Mean Velocity Gradients..... . 16
Cartridge Filters . 18
i Diatomaceous-Earth (Precoat) Filters . 19
Description of the 600-gph ROWPU Pretreatment System . 21
Coagulation . 21
Multimedia Filter . 23
ROWPU Cartridge Filters . 23
Description of the Mobile Water Purification Unit . 24
Background . 25
Treatment System . 27
TheERDLATOR . 29
Conditioning Chemicals . 32
Precoating-Filter Operation . 33
Performance of Deep-Bed Filters Operated in the Direct-Filtration Mode . 34
Determination of Best Type and Optimum Dosage of Coagulent . 36
General Background Papers . 42
600-gph ROWPU Pretreatment Units . 52
Performance of Cartridge Filters . 58
Performance of Precoat Filters Operated in the Direct-Filtrations Mode . 59
Performance of the MWPU (ERDLATOR) . 68
Nuclear Warfare Agents . 73
v
Chemical Warfare Agent GB . 74
Disinfection . 74
Heating . 76
Iodine . 76
Chlorination . 78
Analytical Methods . 81
Comparison of WQAS and WQM Equipment . 82
Physical Characteristics . 82
Analytical Capabilities . . . 84
Summary, Conclusions, and Recommendations . 89
Direct Filtration . 89
Theoretical Removal of Particles in the 600-gph ROWPU Multimedia Filter . 89
Observed Removal of Microorganisms . 90
Limits of Process Applicability . 93
Filter-Bed Composition . 93
Cartridge Filters . 94
Diatomaceous-Earth (Precoat) Filtration . 95
The ROWPU Bypass . 96
Coagulation Control . 98
Determination of Optimal Coagulent Dose . 99
Initial Mixing . 100
Disinfection . 101
Analytical Methods . 101
Suggested Improvements . 102
References . 103
v i
Volume 8
LIST OF TABLES
1. Effect of coagulant dosage and type on the removal of Giardia
muris cysts and coliform bacteria by direct filtration with
preliminary flocculation .
2. Types of filter media used in the direct-filtration studies of
Virginia surface waters .
3. Details of the successful filter runs with Cat-Floe T1 coagulant for
raw-water turbidities exceeding 10 NTU .
4. Average removal of total coliform bacteria and particles in the 7-
to 12- urn size range for direct dual-media filtration without
chlorination .
5. Raw-water characteristics of Monocacy River .
6. Average removal of turbidity observed for a filtration system
similar to that in the 600-gph ROWPU pretreatment section.. .
7. Average removal of microorganisms observed for a filtration
system similar to that in the 600-gph ROWPU pretreatment section. . . .
8. Average removal of total aerobic bacteria observed for a filtration
system similar to that used in the 600-gph ROWPU pretreatment
section for various feed-water flows . . .
9. Average removal of microorganisms from a turbid water at three
pH values by a multimedia filter similar to that used in the
600-gph ROWPU .
1 0. Observed removal of total bacteria and fecal coliform bacteria by
the MWPU without chlorination in 1952 .
1 1. Observed concentration of the spores of B. subtilus var. niger in
the ERDLATOR floe and effluent, and the” filtered-water effluent
of the'MWPU . . . ,
i 2. Average daily removal of turbidity and total coliform bacteria
from Potomac River water in 1967 with the standard 3000-gph
MWPU .
13. U.S. Army field-water-quality standards and capabilities of Army
water-quality-analysis sets . .
14. Specifications for WQM parameters .
35
49
51
53
55
56
56
57
59
70
71
72
83
86
VM
Volume 8
15. Summary of recommended maximum limits on raw-water-quality
characteristics for direct filtration (gravity filters) . 94
600-gph ROWPU with that of the 3000-gph MWPU when operated
in accord with Army operation manuals— no prechlorination . . 97
viii
Volume 8
LIST OF FIGURES
1 . Classification of modes of filter operation . 4
2. The single-collector theory according to Yao e£ai. . 6
3. Comparison of numerical and analytical solutions of Eq. 7 . 9
4. Differential volume element in the filter bed . 9
5. Comparison of observed and theoretical removal in a deep-bed filter. ... 11
6. The effect of the mean velocity gradient, G, on the aggregation
rate of suspended particles for rapid-mix devices of various
geometries . 17
7. Water-processing block diagram for the 600-gph ROWPU . 22
8. ROWPU multimedia filter . 24
9. The backwash water system for the 600-gph ROWPU multimedia
filter . 25
10. Diagram of the 600-gph ROWPU cartridge filter . 26
11. The 1500-gph MWPU and supporting equipment in operating position. ... 28
1 12. Cutaway view of Mobile Water Purification Unit . 28
13. Cross-sectional flow diagram of ERDLATOR (original testing version). . . 30
14. Cross section of the 1500-gph MWPU (ERDLATOR) assembly . 31
15. Cross section of diatomite filter, final testing device for use in the
Mobile Water Purification Unit . 33
16. Comparison of jar-test results with the passage of kaolinlte
turbidity through rapid sand filters . 37
17. Effect of polyelectrolyte dose on the passage of 0.1- ym latex
beads through a deep-bed filter . 37
18. Removal of MS2 bacteriophage by uncoated and polyelectrolyte
(PE) coated diatomaceous-earth (DE) filter aid . 65
19. Theoretical removal of destabilized particles in the 600-gph
ROWPU multimedia filter . . . 91
i x
PREFACE
This work is Volume 8 of the series, Evaluation of Military Field-Water Quality, and
it is concerned primarily with the performance of the mobile water-purification unit
(MWPU) and the pretreatment components of the 600-GPH reverse osmosis water
purification unit (ROWPU) available in 1986. A performance evaluation of the reverse
osmosis (RO) components of the ROWPU appears in Volume 7, the companion volume to
Volume 8. Also considered in this volume are (1) the efficacy of the operation of the
ROWPU in a mode that bypasses the RO components, (2) the potable-water disinfection
processes available to the U.S. Armed Forces, and (3) the water-quality analysis techniques
applicable to field water. Together, Volumes 7 and 8 represent a complete assessment of the
treatment, disinfection, and water quality analysis procedures now used by U.S. military
forces.
Volume 8
ABSTRACT
A comparison is made between the performances of the 600-gph Reverse Osmosis
Water Purification Unit 'ROWPU) operated i i the bypass mode and the Mobile Water
Purification Unit (MWPU, frequently referred to as an ERDLATOR because the equipment
was developed at the Engineer Research and Development Laboratory at Fort Belvoir,
VA). Generally, the performance of the MWPU is significantly better than t
pretreatment units of the ROWPU in terms of removing both turbidity and pathogenic
organisms. It is recommended that the practice of bypassing the reverse osmosis (RO)
components of the ROWPU be avoided unless it can be demonstrated clearly that the
cartridge filters will remove the cysts of Infectious organisms effectively and reliably. If
the ROWPU must be operated in the bypass mode, it is recommended that the dose of
disinfectant used be made equal to that currently employed in the field for untreated raw
water.
The analytical methods used to determine total dissolved solids (TDS) and residual
free chlorine with the new Water-Quality Monitor (WQM) are also reviewed briefly. The
limitations of the methods used to calibrate the TDS and free-chlorine probes of the new
WQM are discussed.
INTRODUCTION
The U.S. Army is considering the use of the 600-gph Reverse Osmosis Water
Purification Unit (ROWPU) in an alternate configuration, here called the "bypass mode".
Only the multimedia and cartridge filters would be utilized, followed by chemical
disinfection. The advantage of this is that the loss of between 1/2 and 2/3 of the filtered
water as RO reject water would be avoided. A second benefit of this arrangement would
be cutting the power requirements in half, because roughly 50% of the power supplied to
the ROWPU is used in the reverse osmosis (RO) process.^
The treatment system in the bypass mode would include the following components:
(1) a garnet-sand-anthracite deep-bed multimedia pressure filter, (2) a 5-pm (nominal
rating) cartridge filtration unit, and (3) chlorination with a chlorine dose equivalent to 5
to 1C mg/L of free-chlorine residual, depending on the pH, temperature, and other
feed-water-quality parameters. Three criteria were suggested by the U.S. Army planners
2
to be considered in the treatment of raw waters in the bypass mode. These criteria were
as follows:
l
Volume 8
• The water must be free of "acute toxicity from Industrial, agricultural,
domestic, or natural resource contributions."
• The water must satisfy potability requirements regarding NBC (nuclear,
biological, and chemical) warfare agents.
• The water must meet palatability requirements.
Modifications to the treatment train have been considered by the U.S. Army
planners. Two of the alternatives under consideration are (1) reducing the pore size of
the cartridge filters from a nominal rating (defined by a military test) of 5 pm to an
absolute rating (the diameter of the largest spherical particle passing through a filter) of
3 pm, and (2) using alternative disinfectants that might have viricidal and cysticidal
properties superior to those of chlorine. A major concern expressed by the U.S. Army
planners was the health risk inherent in the failure to remove pathogenic viruses and cysts
when bypassing the RO section of the 600-gph ROWPU.
The objectives of this report are to review and summarize the state of knowledge
concerning primarily the removal of microorganisms achieved with multimedia deep-bed
filters operated in the direct-filtration mode equivalent to that proposed for the 600-gph
ROWPU bypass, and to compare the results with those that can be expected for the
current 3000-gph Mobile Water Purification Unit (MWPU). In this way the U.S. Army can
judge better the risks associated with bypassing the RO section of the ROWPU and can
compare them with the risks associated with the use of the MWPU.
CONCEPTS OF DEEP-BED FILTRATION
The pretreatment portion of the current 600-gph ROWPU contains a deep-bed
multimedia, filter. Deep-bed filtration or depth filtration. may be defined as a process in
which a fluid suspension is passed through a filter composed of granular or fibrous
materials. The suspended solids are deposited on the surfaces of the grains or fibers along
the entire filter depth as the water flows through the filter media.
A second general type of filtration is surface fiitration. In this case the solids are
strained from the water at the surface of the filter medium. Straining occurs if the sizes
4
of the particles in suspension are greater than about 0.2 times the filter grain size.
Precoat filters of the type used in the MWPU are commonly considered to be surface
filters. A filter may act both as a surface and deep-bed filter, depending primarily on the
physical and chemical characteristics of the suspension and the filter media, as well as
•j
the rate of filtration.
2
Volume 8
In water treatment, the process of destroying the stabilizing forces that keep the
3
colloidal particles apart is referred to as "chemical coagulation." According to Weber,
coagulation is the overall process of colloidal particle aggregation, Including both
destabilization and transport, whereas flocculation is just the particle transport that
occurs after destabilization. Following the mixing of the coagulating chemicals with the
colloidal suspension, several complex reactions take place. These reactions are fast,
taking place in less than one second. At this point in the process, the colloids are
destabilized, and any particle formed is very small.
The rate of agglomeration at which the destabilized particles form visible floes is
dependent on the number of opportunities for particle-particle contact. When the water
containing the destabilized particles is passed through a granular deep-bed filter column,
flocculation takes place at a greatly accelerated rate because of the large number of
opportunities for contact between the destabilized particles alforded by the passage of
the water in the tortuous flow pathways. The floe particles then become attached to or
adsorbed by the surface of the filter grains. As the filtration run progresses, the pores in
the upper section of the filter medium gradually fill with floe, and the particles move
down into the filter to find an available surface for attachment. Finally, the pressure
drop across the filter bed increases to an undesirable level or breakthrough occurs.
'Consequently, filter backwashing is required.
The terms direct and conventional water filtration apply to the mode of filter
operation. In direct filtration, all solids (those occurring naturally in the raw water and
those added during the treatment) must be removed and stored in the filter bed.5 On the
other hand, in conventional water filtration some of the solids are removed in a
sedimentation basin located upstream from the filter. According to the
Coagulation-Filtration Committee of the American Water Works Association (AWWA),
direct filtration refers to a treatment system In which filtration is not preceded by
sedimentation, and consequently those systems that do not use any chemicals for
pretreatment are not considered direct-filtration systems.6 The main steps in the
direct-filtration process are the following; (1) the addition of the destabilizing chemicals,
(2) complete or partial flocculation with no settling, and (3) filtration. The flocculation
step may be eliminated depending on the raw-water quality.5 The flow schemes shown in
Fig. 1 illustrate the differences between conventional and direct filtration, as well as the
differences between the various versions of direct filtration.
3
Volume 8
Typical Conventional Filter Planti
A.
Older
planti
B.
Recent
dailgn
trendi
Direct Filtration
C.
In-line
tiltrationi
D.
Direct
filtration
E.
In-line
with
contact
hniln
Nonlonlc polymer
0.08 to 0.5 mg/L
or activated silica
Nonlonlc polymer
0.05 to 0.5 mg/L
or activated silica
Figure I. Classification of modes of filter operation. Adapted from Culp.
U
Volume 8
THEORY
7
According to O'Melia and Stumm, filtration is similar to coagulation in the sense
that removal within a deep-bed filter column involves at least two separate steps:
(l) particle transport and (2) particle attachment. Particle transport, a physical-hydraulic
process, is principally affected by parameters governing the mass transfer. On the other
hand, particle attachment, basically a chemical process, is influenced by both chemical
and physical parameters.
Particle Transport
7 8
O'Melia and Stumm and Yao et al. studied deep-bed filtration from a microscopic
point of view. According to those authors, the suspended solids are removed by the
following two-step process: (1) the mass transport of the suspended solids to the
immediate vicinity of the solid-liquid interface of the filter media, and (2) the
attachment of particles to the media. A simplified mass-transport model was postulated
in which a single grain of the filter media (henceforth called the collector) is separated
from the surrounding grains and fixed in space. The water flows in the direction of the
gravitational force in a laminar regime. Stokes' equations were used to compute
laminar-flow velocities around the spherical collector. Coagulation theory was then used
to compute the number of contacts that would occur per unit time between a collector of
diameter d^ and a uniform suspension (i.e., ail particles having the same size) of spherical
particles of diameter d^, as shown in Fig. 2.
The transport mechanisms by which suspended particles * are collected are
interception, sedimentation, and diffusion. According to Fig. 2, a suspended particle will
touch or intercept the collector when the particle is contained in a flowing-fluid
streamline that passes within a distance of dp /2 of the collector (point A in Fig. 2).
Inertial and gravitational forces may also cause the particle to deviate from a streamline
(i.e., sedimentation) and collide with the collector (path B in Fig. 2). The third
mechanism, Brownian motion or diffusion, may also cause the particles to cut across the
streamlines and collide with the collector (path C in Fig. 2). In water suspension the
inertial effect on particle collection is very small and has been ignored by O'Melia and
7 8
Stumm and Yaoet al. in their works. Other possible forces such as electrostatic or van
der Waals forces have also been ignored.
A materials balance may be written about a differential volume to obtain the
temporal and spatial variation in n, the number of suspended particles per unit volume of
8
water:
5
Volume 8
Figure 2. The single-collector theory according to Yao et aL A a Interception,
B a sedimentation, C » diffusion, U a velocity (well removed from collector) at which
particles approach the collector. Adapted from Yao et al.8
4 n _ - _ „ 2
r-+ v • Vn = D V n +
o t p
J2H E _
3tr • y • d
3 n
lz'
(1)
in which
2 - 1
D s the particle Brownian-motion diffusion coefficient (I T )
— ^ _ I
v - streamline-flow velocity (a vector) (LT )
-2
g = gravitational acceleration (LT )
m = mass of a suspended particle (M)
-3
p and p = densities of the water and the suspended particles respectively (ML )
M = water dynamic viscosity (ML T )
z = coordinate in the direction of gravity (L)
t = time (T)
dp = diameter of spherical particle (L)
Vn = gradient in the number of particles, general (vector notation)
L,M,T = units of length, mass, time
Volume 8
The first term on the left-hand side of the equation gives the temporal variation
of n at any point (x,y,z). The second term, (v* Vn) describes the effect of advection on
the concentration at that point. The first term (DpV n) on the right-hand side of the
equation describes the effect of diffusion, and the second term the effect of gravitational
settling if the particles settle in accord with Stokes' law. The effect of interception on
particle collection is included in the boundary conditions used to integrate Eq. 1 .
Equation 1 has been solved analytically by considering the following assumptions:
<1 ) It = 0 <steady state); (2) all the suspended particles that touch the collector
disappear instantaneously from the system without increasing the size of the collector
(the collector is always clean); and (3) only one removal mechanism is predominant at a
specific suspended particle size d . The results obtained were expressed in terms of a
r
single-collector efficiency n, which is defined as
n = rg/UnAg ,
(2)
in which
r
g
n
= rate at which the suspended particles touch or strike the collector (number of
particles* T"^);
= velocity at which the particles approach the collector, well removed from the
collector (LT" 1 );
= particle concentration upstream from the collector (number of
particles • L"^); and
2 2
= cross-sectional area of the collector = trdg/4 (L ).
The analytical solutions based on the assumptions described previously are:
Interception only:
Sedimentation only:
n, = 3 «p/dg)2/2
(Pp - P) g <
nC= 18 yU
Diffusion only: = 4(Dp/Udg)^ ,
(3)
(4)
(5)
7
Volume 8
in which
D = kT /3 IT yd (6)
P a' H p
and
k = Boltzmann constant = 1.39 x 10"*^erg/K
Tfl = absolute temperature (K).
Equations 3, 4, and 5 are plotted In Fig. 3 for the special case of
2 3
U = 2 gal/(mln • ft ), d = 0.5 mm, p a 1,05 g/cm , at 25#C. A numerical solution of
O r
Eq, l (curved line shown in Fig. 3) demonstrates that the overall collector efficiency can
be obtained without significant error simply by adding Eqs. 3, 4, and 5;
n=VnG+nD * ' (7)
The results shown in Fig. 3 demonstrate that
• There exists a size of the suspended particles for which the removal efficiency
is a minimum. For conditions typical of conventional water-filtration practice,
this size is about l pm and particles of greater or lesser size than 1 pm are
removed more effectively.
• For example, for particles smaller than l pm, the collector efficiency
increases with decreasing particle size, and particle removal is accomplished
by diffusion.
• For particles greater than l pm, the collector efficiency Increases rapidly with
Increasing particle size, and removal is accomplished by interception and/or
sedimentation.
Conceptually, a single isolated collector is very different from a packed bed of
collectors. With the collectors in contact with each other, the flow streamlines are not as
pictured in Fig. 2. Even so, the single-collector theory has been applied directly to
deep-bed filtration with reasonably good results, as will be demonstrated subsequently.
The number of single collectors contained in a differential volume element (i.e.,
having dimensions so tiny that particle behavior can be described using differential
equations) of bed of thickness 6z (see Fig, 4) is:
8
Volume 8
Figure 3. Comparison of numerical and analytical solutions of Eq. 7.
(U ■ 2 gal/(min • ft2), d„ * 0.3 mm, p_ ■ 1.03 g/cm3 and T ■ 23‘C). Adapted from Yao
,8 8 p
et al.
Figure 4. Differential volume element in the filter bed. (Q = flow rate,
A = cross-sectional surface area).
9
Volume 8
Number of collectors = (1 - e) A6z/V^, in which e is the bed porosity, A is the total
bed area, and is the volume of a single collector. The rate at which all of the
collectors contained In the filter bed are removing particles from the water is rR (Eq. 2)
times the number of collectors. The rate of removal per unit volume of all of the
collectors, r , is
S
rg s rR x number of collectors/volume of all collectors
= 3nUn/2d .
O
Also, the rate at which the particles are removed from the water per unit volume of the
water, rL, Is
(8)
The change In the particle-number concentration with depth at any time may be
obtained from a material balance written about the differential element of bed volume of
thickness 6z shown in Fig. 4, or
(cA6z)‘ rL * Q6n
or
Urn rL = (U/e)(dn/dz) , (9)
6z+ dz
where Q = U *A. Substituting Eq. 8 into Eq. 9 gives
(10)
in which X is the filter coefficient.
Q
Yao et al. checked the validity of Eq. 7 with Eq. 10. The results of this comparison
are shown In Fig. 5. The observed and theoretical minimum efficiencies were both
located at a particle size of approximately I ym, but the observed collector efficiencies
were In general much greater than the theoretical. This means that the theory can be
used to obtain a quantitative estimate of the particle size most likely ;«.» pass through a
deep-bed filter, but only a qualitative estimate of the proportion removed.
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Volume 8
Figure 3. Comparison of observed and theoretical removals In a deep-bed filter,
(dg a 0.397 mm, e a 0.36, U> 2 gal/(mln • ft2 ), T * 23*C, z a 3.3 in., pp a 1.03 g/cm3,
clean filter). Adapted from Yao et ah8
It was assumed In the previous discussions that all particles that strike or touch a
collector will stick to the collector. This would be the case if all the suspended particles
were completely destabilized. In general this is not the case and a parameter called the
collision efficiency factor, o, is sometimes used to correct for a system of partially
destabilized particles, or
(11)
The collision efficiency factor represents the ratio of the particles that stick to the
collector to the total number of particles that strike the collector.
As stated in the assumptions, the single-collector theory was developed for the case
of absolutely clean collectors. Filtration, however, Is an Intrinsically transient process
because the particles deposited on the collectors change the geometry of the Interstitial
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4
spaces in the filter bed, as well as the nature of the collector surfaces. The deposited
particles first act as additional collectors, but they eventually restrict the flow through
the filter bed. Typically, an initial increase in the filter efficiency is observed, followed
by a monotonlc decrease.
The change in filter efficiency with the deposition of particles in the filter bed was
investigated by Ives, among others. The following semiempirical expression was proposed
for the filter coefficient, X (Eq. 1 0):
X = XQ[ 1 + ( 8po/e)]y • [1 - (o/e)]2 • [1 - (o/ou)]x , (1 2)
in which
X^ = filter coefficient for a ciean bed
0 e
3p = packing constant = ■pj
e a bed porosity for a clean filter
a a volume of particles deposited per unit bed volume
ou a ultimate value of a when the filter bed becomes ineffective at a specified
depth (X+ 0 as a+cu)
x,y,z a empirical constants.
In Eq. 12 the first term, [1 + (0pa/e)]y, gives the effect of the Increase in specific filter
surface (i.e., collector surface area per unit filter volume) available for deposition on the
filter coefficient, which results in an initial improvement period. The second term,
[1 - (o/e)]2, accounts for the decrease in the specific filter surface when the void spaces
start to be filled. The third term, [1 - ( o/ou)]x, gives the increase in interstitial-water
flow velocity caused by the restriction in the flow passageways. Thus, the performance of
a deep-bed filter will at first increase (the first term in Eq. 12 is predominant) and then
deteriorate (second and third terms are predominant) with time. The durations of the
initial Improvement and the subsequent deterioration periods depend upon the relative
magnitudes of the exponents x, y, and z. The pressure drop across the filter bed will
always Increase with time.
Particle Attachment
The transport model described in the previous section, albeit elegant, applies to the
case of completely or nearly completely destabilized particles. It has little significance if
the particles are not destabilized.
i 2
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Whether or not a particle will stick to a collector is controlled by the surface
7
properties of both the particle and the collector. According to O'Melia and Stumm,
there are two significant models that have been used to describe the interactions between
suspended particles and the filter media. The first model is based on the electrical
double-layer theory. According to this theory, the solid side of a solid-liquid interface
assumes an electrical charge, called the primary charge, which depends on the surface
chemistry of the solid. An equivalent number of charges of opposite sign (secondary
charges) form in the aqueous phase to counterbalance the primary charge. The counter
charges are provided by the ions dissolved in the water, and such ions are called
cn filter ions. Because most clayey colloids, bacteria, cysts of protozoa, viruses, etc.,
carry negative primary charges, the counterions are usually cations. Some of the
counterions are closely associated with the charged particles, and move with the particles
when an electrical potential is applied to the colloidal solution. The net charge on the
particle, Including that of the closely associated counterions, determines a parameter
called the zeta potential.
Theory demonstrates that if a particle with its associated zeta potential approaches
another particle (or filter collector) with a zeta potential of the same sign, then an
electrostatic repulsive force occurs between the particles that prevents a closer
approach. If a strong electrolyte Is added to the system, and especially if the electrolyte
contains cations of high charge density, then the zeta potential will be depressed because
the cations will be adsorbed on or near the solid surfaces. The particles may then collide
when carrying sufficient kinetic energy to overcome the residual electrostatic repulsive
forces. They will also stick together if the short-range van der Waals' attractive forces
are greater than the repulsive forces at this proximity.
O'Melia and Stumm state that particle attachment brought about by charged
synthetic or natural polymers (or polyelectrolytes), which have been successfully used as
filter conditioners or coagulating agents in water treatment, cannot be characterized by
the double-layer model. The model neglects the importance of chemical forces, such as
coulombic attraction, when the suspended particle and the filter collector are of opposite
charge, and it Is only valid for lyophobic surfaces and simple electrolytes/ The second,
multifaceted theory, called the bridging theory, has been developed In recent years to
explain Interactions such as ion exchange, hydrogen bonding, and the formation of
coordinative bonds and linkages that can outweigh electrostatic forces when polymer
coagulation is used.
13
Volume 8
COAGULATION AND COAGULANTS
As shown in Fig. 1, a coagulant (or coagulants) is always added ahead of a deep-bed
filter, regardless of the mode of operation. The coagulants most commonly used in the
United States are alum [Al^SO^)^ • UH^O], hydrated ferric chloride (FeCI^ • 61^0),
and various nonionic or cationic synthetic polymers. A polymer is called a polyelectrolyte
if the monomers from which it is synthesized contain ionizable groups. Cationic polymers
gain a positive charge upon ionization, and anionic polymers gain a negative charge.
Polymers that do not contain any ionizable groups are called nonionic. Research has
revealed that both cationic and anionic polymers are capable of destabilizing negatively
charged colloidal particles such as those found in natural waters, but anionic polymers
usually have to be used in conjunction with another electrolyte such as NaCl or CaClj, or
with a coagulant such as alum.
Early Investigations demonstrated that the double-layer theory alone did not
adequately explain the observed mechanisms of coagulation with polymers. The
3
chemical-bridging theory was then developed to explain the observed behavior.
According to this theory, a molecule of an organic polymer or an inorganic polymer
resulting from the hydrolysis of alum and ferric chloride will become attached to a
colloidal particle at one or more sites. The number of sites occupied by a polymer
molecule depends on the nature of the various electrostatic or chemical forces, and the
3
charge and structure of the polymer and the colloid.
A nonionic polymer, or even a polyelectrolyte having the same polarity as that of
the colloid, may be adsorbed by the surface of the colloid. Some segments of the polymer
molecule will be adsorbed by the surface, and some will extend into the solution phase.
According to the bridging theory, those segments that extend into the solution, called
loops and tails, form the bridges between the colloidal particles. There must be a
minimum polymer size (i.e., minimum molecular weight) required to bridge the
potential-energy barrier existing between two negatively charged colloidal particles.7
Cationic polymers can destabilize the negatively charged colloids commonly
encountered in natural waters and wastewaters by both bridge formation and charge
neutralization. As a result, cationic polymers do not require a minimum molecular weight
to be effective since they can be adsorbed specifically by negative colloids and have their
primary charge neutralized. Ghosh et £l. state that studies of polymer configuration on
colloidal surfaces by electron paramagnetic resonance have shown that cationic
polyelectrolyte segments are often adsorbed almost completely by the particle surface
without many loops or tails extending into the solution. This requires that a mechanism,
14
Volume 8
in addition to bridging, be operative, because the bridging mechanism depends on the
presence of the extending segments. Those investigators concluded that charge
neutralization plays an important role in cationic-polymer coagulation.
3AR TESTS
It appears from the foregoing that the destabilization of colloids with the cationic
polyelectrolytes is achieved primarily by charge neutralization and Interparticle bridging.
Both of these mechanisms imply a stoichiometric reaction between the colloid and the
coagulant, and both mechanisms may cause particle restablllzation with overdosing.^ The
theory of coagulation has not yet been sufficiently developed to determine the optimum
coagulant dose or coagulant type that should be used in direct filtration; therefore, these
determinations must be made experimentally.
The optimum coagulant dosage is most commonly determined by jar tests. The jar
test is a laboratory method used to determine the effectiveness of various coagulants,
optimum coagulant and coagulant-aid dosages, optimum pH for coagulation, and the most
effective order in which the various chemical coagulants should be added to the
suspension. The typical jar-test apparatus consists of six agitator paddles mechanically
1 coupled to operate at the same speed, which can be varied from 10 to 100 rpm. The
coagulation containers commonly are l- or 2-L beakers. The general jar-test procedure
consists of flash-mixing the chemicals by agitating at 100 rpm for 1 min, followed by
flocculation at a stirring rate of 20 to 70 rpm for 10 to 30 min. The residual turbidity of
the sample is measured after 30 min of quiescent settling.10
According to Benefield et al.1 1 the optimum dose, as determined by jar tests, often
does not match that observed in the treatment plant. One reason for this is that the jar
test is a batch test, whereas the static flash-mixing tanks used in many water-treatment
plants are continuou&-fiow devices that provide significant backmlxing. A batch reactor
is analogous to a plug-flow reactor and is Inherently more efficient than a backmlxing
reactor for most reactions (Vrale and lorden;1^ Benefield e£ al.1 ^ consequently, the dose
of the coagulant required for the flash mixers in the treatment plant may be greater than
that indicated by the jar tests.
v This situation applies to the type of coagulant mixing presently used in the
ERDLATOR of the MWPU. Agreement between jar tests and plant performance may be
better for plants utilizing non-backmixing devices, such as that used in 4fce 600-gph
ROWPU, where rapid mixing is initiated in a pipe elbow and continued at a relatively high
Reynolds number in the pipe leading to the multimedia filter.
15
Volume 8
Other tests used to ascertain the optimum coagulant type and dose include
electrophoretic-mobility measurements (zeta potential) and more recently, particle-size-
distribution analyses. Zeta potential is closely aligned with the simple double-layer
theory of coagulation and hence is disparaged by those who believe that bridging plays the
7
dominant role in cationic polyelectrolyte coagulation.
INITIAL MIXING AND MEAN VELOCITY GRADIENTS
The aluminum and ferric iron contained in alum and ferric chloride, respectively,
will generate positively charged poiymer-ilke chains when given sufficient time and the
appropriate water-quality characteristics. According to Weber, the hydrolysis and
polymerization of these salts of Al(III) and Pe(III) are very rapid and require a uniform pH
and coagulant concentration. The instantaneous blending of the alum and ferric-chloride
feed solutions with the water is thus essential. The instantaneous blending of the
high-molecular-weight organic polymers with the water is less essential because the
polymers do not have to be formed within the system, and their rates of adsorption by the
colloids are slower due to their larger size. Regardless of this distinction, it is apparent
that the rate at which a coagulant is mixed with the raw water is nearly as Important as
the optimum coagulant dose. For example, the poor mixing of a polyelectrolyte with the
water could result in an overdose on some of the suspended particles, and an underdose on
others. The result would be poor coagulation, even though the average dose is correct.
The mean velocity gradient G (defined mathematically as the square root of P/Cty),
where P is the mixing-power input, V is the volume of water being mixed such that P/V is
the power dissipation per unit volume, and u is the dynamic viscosity of water)^® is
commonly used to describe the intensity of mixing required in both chemical-mixing and
flocculation devices. The greater the value of G, the more rapid the mixing rate for a
device of a given geometry. This rule can not be extrapolated to devices of different
12
geometries, as demonstrated by the results of Vrale and 3orden.
Those investigators conducted some rapid-mixing experiments with different types
of reactors, using alum as the coagulant. They employed one completely backmixed,
stirred-tank reactor (CSTR) and four turbulent pipe-flow reactors of various
12
geometries. The efficiency of the coagulation process was determined by measuring
the residual turbidity subsequent to a period of slow flocculation and sedimentation. An
apparent turbidity-removal rate was then estimated from the observed turbidity values.
Some of the results of their study are presented in Fig. 6.
Volume 8
The performance of the CSTR mixer (Unit 2) was the poorest, with the rate of
aggregation actually decreasing when G was increased. It was also observed that the
minimum mixing speed had to be greater than 150 rpm (G s 350 s“b to achieve a uniform
mixture in the CSTR. For mixing speeds less than 150 rpm, the water was not mixed at
the bottom of the reactor. Unit 1, one of the pipe-flow reactors, provided a slow rate of
blending and was used to determine the effect of the background mixing in the piping
following the in-line turbulence inducers. The performance of Unit 1 is not depicted in
F ig. 6. Unit 3, which had a complicated design and no practical application, was intended
to serve as the standard of best performance, although this did not prove to be the case.
Unit 4, a combination of Units 1 and 3, had a similar performance to that of Unit 3. Unit
5, which performed the best of all the mixing devices tested, consisted of an annular ring
inserted in a round pipe. The ring had six holes for alum injection into the flow stream,
with the holes facing downstream. Significant turbulence was created by expansion of the
flow downstream from the annular ring.
[Mmh velocity gradient ) G(rU
Figure 6. The effect of the mean velocity gradient, G, on the aggregation rate of
suspended particles for rapid-mix devices of various geometries. Adapted from Vrale and
Jorden.* ^
Vrale and 3orden ^ concluded that the completely backmixed stirred-tank reactor
(CSTR) of the type often used in water-treatment plants provide the least -satisfactory
way of mixing two miscible fluids rapidly, and that G by itself Is not a sufficient criterion
for judging the rate of initial aggregation for devices with different geometries. The
location of coagulant-application points and the turbulence intensity at and immediately
17
Volume 8
downstream from the chemical-addition points also should be considered. These
researchers also indicated that an optimal design for colloid destabilization with alum
would be optimal for destabilization with cationic poiyelectroiytes because the particle-
destabilization mechanisms are similar. This, in turn, is dependent upon the local and
temporal concentrations of the coagulant and the colloids because the adsorption
reactions are rapid and irreversible.
CARTRIDGE FILTERS
13
Cole et al. define a cartridge filter as a cylindrical object consisting of a
disposable or cleanable porous medium, associated with an appropriate plastic or metal
structure. Individual cartridges or multiples of cartridges may be installed in a filter
housing. In most Instances, cartridge filters are required to remove particles of size
14
ranging from fractions of a micrometer to 40 ym (Warring gives this size range as 1 to
500 ym), and they are used in many industrial applications. Materials such as cotton,
wool, rayon, cellulose, fiberglass, polypropylene, acrylics, nylon, asbestos, etc., or various
combinations of these materials, are commonly used as filter media in the disposable
cartridge elements. Materials such as stainless steel, Monel alloys, and ceramics are used
in the reusable filter elements.
Cartridge filters are classified Into two categories, "depth" and "surface" filters.
Depth-type filters capture particles throughout the total thickness of the medium within
the interstices of the internal structure.* These filters normall^have a structure that
gradually increases in density toward the center of the element.1 Thin media, such as
paper or woven-wire cloths are described as surface-type filters because most of the
13
particle capture occurs at or near the filter surface. According to Cole et al., the terms
"surface" and "depth" are relative rather than absolute because removals vary
considerably with the size and chemical properties of the suspended particles, the rigidity
of the medium, the uniformity of the element pore sizes, and the thickness of the medium.
The absolute size rating of a cartridge filter Is defined as the largest hard spherical
particle that will pass through the filter under specified conditions, or sometimes the
largest opening in the filter. The "nominal" size rating is a term used by the military
and is defined by a test apparently used only by the military.*^ Increments of a fine test
dust are introduced upstream from the filter. Small portions of the filter effluent are
then examined microscopically for particle counts. The counts are-, converted
mathematically to weight values, and the size at which 98% of the particles by weight is
retained on the filter is called the "nominal size". The test does not give reproducible
1 3
results and has never been widely accepted.
18
Volume 8
DIATOMACEOUS-EARTH (PRECOAT) FILTERS
A precoat filter consists of a rigid, semiflexible, or flexible septum (or screen) on
14
which filter aid (or medium) is deposited. Usually a filter system consists of several
septa which are housed in a pressure vessel. Each of the septa supports a thin layer of
filter aid that has been deposited hydraulically on one side of the septa at the beginning of
the filtration cycle} this thin layer is called filter cake.
The screen or the septum on which the aid is formed is basically a strainer, and the
principal mechanism Involved in the removal of the solids is believed to be mechanical
straining or direct interception.1** The septum, the filter cake, and the particles which
are filtered from the raw water all contribute to the straining of additional suspended
particles from the water being filtered. In straining, particles larger than the pore size of
the filter cake are collected on the upstream surface of the filter. Adsorptive forces also
provide a small positive contribution to the removal of suspended solids. The adsorptive
forces arise from the nonuniformity of the upstream surface of the filter, which results
from the fact that the pores are not uniform in shape or direction and the surface is not
I L
perfectly smooth.
The desired properties of the precoating filter aid are that it be a finely divided
1 powder, light weight and insoluble in the liquid to be filtered. The aid should have no
effect upon the chemical properties of the filtrate, and should be fine enough to remove
essentially all of the suspended solids.
The filter aid used In most precoat filters is manufactured from
dlatomaceous earth, which consists of the siliceous fossil remains of dead aquatic algae
called diatoms. The diatoms are processed by crushing, calcining (heating to a high
temperature without fusing), and classification to yield a fine, porous, multishaped,
angular media, ranging in size from about 5 to 50 pm. Processed dlatomite filter aid has
the required characteristics for water filtration. The structure permits the formation of
a rigid, porous (>90% porosity) filter cake that will retain solids well as the water or other
liquid containing those solids passes through the filter. Perlite, which comes from a
siliceous rock, is another material which can be used in precoat filters.15
The clarifying ability is the parameter used to characterize the performance of
dlatomite filter aid. The number that quantifies this ability is called the "clarity index"}
it is inversely proportional to the permeability of the aid. Thus, as the permeability of
the diatomite increases to allow more throughput, its clarifying ability decreases and
larger suspended solids pass into the filtrate.
19
Volume 8
The precoat filtration run or cycle consists of four steps1 (l)a thin layer of filter
aid is applied to the septum to form the filter precoat} (2) the water is filtered until a
predetermined pressure drop across the filter is reached? (3) the fouled filter aid is
expelled from the filter elements by backwashing; and (4) the water In the filter shell
containing the backwash sludge is released to waste.
Precoat filter aid is applied in one of the two following ways: (1) the aid is applied
directly to the filter septum by filtering water containing the aid at the start of the run,
or (2) a previously prepared filter-aid slurry is recirculated through the filter until the
filter effluent returning to the slurry tank is clear. Generally this precoat has a thickness
of 1/16 to 1/8 In. The procedure to be used should be selected according to the porosity
of the septum and the fineness of the filter aid. The recirculating procedure is
advantageous when the septum openings are large and the filter aid is fine.16
As water containing suspended particles is filtered during a filtration run, the
precoat-cake surface gradually becomes fouled with a layer of particles. This Increase In
the thickness of the filter cake will result in clogging of the filter and an Increased
pressure drop across the filter. This effect can be mitigated to some extent by
continuously Injecting small amounts of filter aid into the raw water as it enters the
filter, which helps to maintain a highly porous filter cake. This additional filter aid is
called body feed.
The total filter-aid requirement Includes the amount of material used to precoat the
septa, as well as that added as body feed. The optimal amount of precoat material to be
used in any filter will be the minimum amount needed to protect the filter element from
clogging while producing an effluent of the desired quality. In general, body-feed
requirements vary in proportion to the raw-water turbidity, although the type of
suspended solids also affects the amount of body feed required.16
If the primary mechanism of suspended solids removal with diatomaceous earth
filtration is straining, then it follows that the removal of suspended solids Is a function of
the particle size of the filter aid. Tien and Payatakes** state that straining Is effective
when the size of the particles Is greater than 0.2 times the filter-grain size.
13
Cheremislnoff and Azbel state that dlatomaceous-earth filter aids have good efficiency
14
in retaining particles of sizes less than 1 ym. Warring does not give a quantitative
measure for particle size but states that dlatomaceous-earth filter aids can provide
extremely fine filtering and are used frequently when very high purity is required, such as
in the filtering of sugars, edible oils, or water. The particle size of commercially
available dlatomaceous-earth filter aids ranges from 5 to 50 ym.
20
Volume 8
Prior to filtration, the raw water to bo filtered may be coagulated, flocculated, and
allowed to settle. However, all, some, or none of these three processes may be used) the
choice will be made based on the raw-water quality.
The modes of filter operation shown in Fig. 1 for deep-bed filters apply equally well
to precoat filters. The term direct filtration is also used for precoat filters when the
coagulated water is not settled prior to filtration.
DESCRIPTION OF THE 600-GPH ROWPU PRETREATMENT SYSTEM
According to Carnahan et al.,17 a conventional clarification system with separate
coagulation-flocculation and sedimentation basins could not be used in the 600-gph
ROWPU pretreatment system because of restrictions Imposed by the U.S. Army on the
overall dimensions of the ROWPU. Instead, the pretreatment used consists of direct
filtration by a multimedia pressure filter with in-line polyelectrolyte addition, followed by
processing through a cartridge-filtration unit. This pretreatment, aside from the
cartridge unit, is somewhat in accord with scheme C In Fig. 1, except that a cationic
polyelectrolyte is used in lieu of the alum, activated silica, or nonionic polymer, and the
filtration rate is 7 gal/(mln * ft ), Instead of the 5 gai/(min • ft ) shown in the scheme.
As shown in Fig. 7, the multimedia filter removes solids that pass through the Input-
water strainer, as well as the smaller solids that are acted upon by the coagulant.1 A
cationic polyelectrolyte and sodium hexametaphosphate are added ahead of the
multimedia filter. Citric acid Is added after the multimedia filter and before the
18 19
cartridge-filter unit according to Fig. 7. * Sodium hexametaphosphate and citric acid
are added primarily to control the rate of RO membrane fouling. The citric acid is added
18
in quantities sufficient to adjust the water pH to a value between 5 and 8. Chlorine is
added after. the RO unit to prevent chlorine from damaging the membrane elements.
Carnahan et al.,17 state that the multimedia filter was designed to operate at rates
ranging from 5 to 10 gal/(mln • ft2) because this range provides the optimum trade-off in
1 8
size versus filtration efficiency. The nominal filtration rate Is stated to be
2
6.5 gal/(mln • ft ), which is equivalent to a total ROWPU feed-water flow of 32 gal/mln.
COAGULATION
The cationic polyelectrolyte, Cat-Floe Tl (manufactured by Caigon, Pittsburgh, PA)
is used In the ROWPU pretreatment system.17 It is pumped to the filter-feed line with an
American Lewa positive-displacement pump with a 200-mL/min capacity. According to
21
Volume 8
Product
water
*
Figure 7. Water-processing block diagram for the 600-gph ROWPU. From
' TM 5-4610-213-10.18
the 600-gph ROWPU operator's manual,*8 the chemical feed pump is calibrated to a flow
rate of 60 mL/min, which corresponds to a dosage of 5.2 mg/L for the nominal ROWPU
flow rate.
The chemical-feed flow rate is adjusted during operation to Improve the
performance of the multimedia filter if necessary. This Is achieved by using a turbidity
tube, the bottom of which contains a white bull's-eye set in a black background disk. A
600-mL flltered-water sample is drawn into the tube from the cartridge-filter drain. The
criterion for water clarity is that both the white bull's-eye and the black disk at the
bottom of the tube should be seen clearly. If both are not visible, the polymer dosage
should be altered. This is accomplished by changing the chemical-feed flow rate. The
U.S. Marines adjust the dosage in accord with the 600-gph ROWPU operator's manual.*8
It Is uncertain what operating procedure is followed by the U.S. Army.
Carnahan et al.,*^ state that the ROWPU chemical-feed system provides a mean
velocity gradient of 300 s“* for approximately 0.1 s and 800 s" for 2 s. This was
checked for a total-flow rate of 34.5 gal/min and a 2-in. i.d. multimedia filter feed-water
pipe 6 ft long with two right-angle elbows. The mean velocity gradient G was estimated
22
Volume 8
to be approximately 3000 s .or the elbow where the coagulant is injected, and
approximately 800 s’* for the pipe. The residence time for the pipe was about 2 s. These
numbers check weil with those reported by Carnahan jrt al.,* ' except for the discrepancy
in the G value at the elbow.
MULTIMEDIA FILTER
The multimedia filter in the 600-gph ROWPU is a pressure filter (Cuiligan mixed-
media filter) operated under a nominal pressure of 30 pslg.*2 The dimensions and
characteristics of the media installed in the filter are given in Fig. 8. The minimum
j 8
operating pressure is reported to be 23 psig.
The backwash-water system for the multimedia filter is shown schematically in
Fig. 9.* 8 Brine from the RO elements is used as the backwash water. The backwash
pump is a centrifugal pump with a maximum capacity of 120-gai/mln water flow and
70-psig pressure and is controlled by a gate valve as shown in Fig. 9. The backwash
system, which is provided with a timer, operates automatically. The backwash operation
lasts approximately 20 min after the backwash pump Is started. During backwash, the
water-flow rate varies automatically from 0 to between 70 and 120 gal/mln (0 to between
1 14 and 24 gal/[mln • ft2]) while washing and rinsing, which fully fluldlaes the multimedia
bed. 1,1 7,1 8 According to the operator's manual, the filter should be backwashed after
20 h of operation or when the filter pressure drop exceeds the Initial drop by 3 psi. The
Initial pressure drop is usually about 2 psi.*2
Carnahan et al., 7 give the results of some ROWPU multimedia filtration tests,
from which it was estimated that the filter retained approximately 3300 g of solids just
before backwashing was required. It was also observed that, by using two backwash cycles
of 15 and ?4.gal/(mln • ft2), the filter was cleaned sufficiently to regain the Initial
pressure drop of 2 psi.
ROWPU CARTRIDGE FILTERS
The effluent of the multimedia filter is polished with 5-ym nominal sire rating
cartridge filters,* (The definition of nominal Is given In the section entitled Cartridge
Filters). The purpose of this unit is to prevent the carry-over of solids or organics to the
reverse osmosis (RO) components.*2
Eight replaceable filter elements are housed in a single pressure vessel as Illustrated
in Fig, 10. The filter elements consist of polypropylene cord woven around stainless steel
cores. The water flows in a spiral motion around and Into the stainless steel core, and
23
volume 8
3 In.
14 In.
7 In.
3 In.
30 In. (i.d.)
Riw-w»ttr Inlet
=535^
1/8 X 1/8-In. Plastic pellets
Anthrioits ooal
Calolnad aluminum
illloats
Garnet tend
Small iliad garnet gravaT
Madlum-tiiad garnet gravel
Effective
diameter
(mm)
0.80
0.42
0.30
L>— Fllterad-watar outlet
Speeiflo
gravity
1.2
1.5
2.5
3.95
Density
Uniformity lb/ft*
coefficient (kg/m3)
— 45
(720)
2
52
(830)
1.74 —
1.2 135
(2100)
Figure 8. ROWPU multimedia filter. From Small etal..1
then out of the bottom of the pressure-containment vessel. The filter elements are
classified in the group called "depth" filters with high contaminant-holding capacity.20
The tubular elements are 40 in. long and 2-3/4 in. in diameter, which gives a surface area
of 2.4 ft2 per element, or a total of 19.2 ft2 per installation.21 The filtration rate is thus
1.8 gal/(min • ft2) (14.4 m/h) for a ROWPU flow rate of 34.5 gal/min.
According to the manufacturer of the cartridge-filter elements (Fllterite
Corporation, Timonium, MD), the polypropylene filter media is compatible for use wltn
strong acids and concentrated bases, and at temperatures up to 275°F (135°C).
Polypropylene, however, is affected by oxidizing agents such as free chlorine. This may
inhibit or exclude prechlorination of the raw water if the 600-gph ROWPU is to be
operated in the bypass mode (i.e,, by-passing the ROWPU RO section).
DESCRIPTION OF THE MOBILE WATER PURIFICATION UNIT
A Mobile Water Purification Unit (MWPU) was developed and tested by the
Engineering Research and Development Laboratory (ERDL) at Fort Beivoir, Virginia. The
development of this equipment was the U.S. Army's response to a need for a lightweight,
24
V WIUIIIV u
Baokwaih
watar
connection
Canvaa
brlna
hoaa
2-In. Swivel
adaptar
2-In. Doubla I
hoia nlppla
Figure 9. The backwash water system for the 600-gph ROWPU multimedia filter. From
TM 5-4610-213-10.18
mobile system that was reliable and capable of producing potable water for field troops in
a variety of hostile environments. Concern was focused on operation with cold water, and
the equipment was designed for freshwater point sources.
BACKGROUND
Water fit for drinking is now used by the field Army for drinking, bathing, kitchen
tasks and laundry. If local water-treatment facilities are not available, then raw fresh
waters must be treated by field equipment to provide for these needs. The equipment
must be reliable and capable of treating water from almost any surface source. While the
2.5
volume b
Figure 10. Diagram of the 600-gph ROWPU cartridge filter. From TM 3-4610-213-10.
18
water points are checked by the Medical Corps for suitability, easily treatable water Is
not always found, and the equipment may be called upon to purify waters that are turbid,
colored, polluted with natural biological pathogens, or have extreme pH values. Nuclear,
22
biological, and chemical warfare agent removal is also a concern.
The terrain and weather conditions In which the water treatment Is to take place
are unlikely to be kind to the equipment. For example, the equipment will be used in
Isolated locations where the only protection for the unit Is that which Is actually part of
the unit Itself. It must be capable of operating at high and low ambient temperatures, in
rough terrain, in rain, sleet, high winds, and at night. The equipment may be moved on
short notice, and should be self-contained so that it can be started up soon after arrival at
the next water point. Because the troops-often must operate the equipment under adverse
conditions, it should be simple to operate and rugged. Moreover, a minimum number of
troops should be required to operate it.
26
Volume 8
The development of the Mobile Water Purification Unh (MWPU) began in 1949, and
testing was completed in 1955. This work was conducted by the U.S. Army in conjunction
with special studies performed by institutions funded by the Army. All three processes
used for water treatment in the MWPU were studied: (l) coagulation and clarification, (2)
dlatomaceous-earth filtration, and (3) disinfection. Research projects were funded at
Harvard University on water-disinfecting agents, at New York University on
water-coagulation procedures, at the University of Illinois on filter septa and water
filterabllity, and at Johns Hopkins University on filter aids. The Sanitary Engineering
Branch of ERDL conducted basic research and development efforts on related studies.
Two prototype water-purification units were tested using the solids-contact
clarifier, filters, and disinfection processes. Field operations were carried out at sites at
Lake Michigan in Illinois, at several lake and river sites in southern Virginia, and at the
Potomac River site of ERDL at Fort Belvolr, VA. Fifty hours of operation were logged at
each site. The two test models used were similar, except for the solids-contact clarifier.
One had an Army-designed clarifier and the other an off-the-shelf commercial clarifier,
23
which could serve the same function as the Army modei.
The results of these tests gave U.S. Army designers the criteria to be used for
equipment having the required military characteristics. They decided what design
modifications should be made to accommodate the high and low ambient temperatures in
which the equipment would be operated, and they recommended that the "Water
Purification Unit, Mobile, Electrified, 1500-GPH Capacity" should be designed and
fielded. They also recommended the expansion of the project to include development of a
23
3000-gph capacity model. Most data on MWPU performance have been published for
the 3000-gph model.
TREATMENT SYSTEM
The MWPU with supporting equipment in place Is illustrated in Fig. 11. A cutaway
view of the processes contained in the truck-mounted van is shown in Fig. 12. The
treatment processes consist of the following: solids-contact clarification} disinfection
with calcium hypochlorite} external solids concentration} and diatomaceous-earth
filtration. The solids contact clarifier, designed at ERDL, is called the ERDLATOR. The
rate at which the water is treated by the MWPU is controlled by the hydraulic loading on
the ERDLATOR. The MWPU is fielded in three versions, 600, 1500, and 3000 gph.
27
T VIVIIM& g
Volume 8
The raw water is pumped to the clarifier with a self-priming, 2-hp centrifugal
pump. Large debris is excluded with a strainer installed on the suction hose as shown in
Fig. 11. The effluent from the ERDLATOR is pumped through two diatomaceous-earth
filters. The filter effluent then flows by gravity to two 3000-gal capacity storage tanks in
series. The treated water is pumped by a second self-priming, 2-hp pump from the second
storage tank to the distribution system. Wastewaters from the system, which include the
concentrated sludge derived from the ERDLATOR, as weli as the backwash of the filters,
are released by gravity to a discharge point located downstream from the raw water
intake. Normally, the equipment can be Installed in less than one hour by three soldiers
2ti
and routinely operated by one.
The van-type body used to hold the treatment units for the 3000-gph model is
25
258 in. long, 98 in. wide, 88 in. high, and Is carried on a 2.5-ton truck. The van interior
is insulated and heated. When the unit Is operational at temperatures between 0 and
26
-40°F, a gas-burning heater is used to warm the treatment system; other heating
devices may be used to prevent freezing of the effluent in the distribution system. The
shipping weight of the van is 9400 lb. The total weight is much greater when operating,
due to the weight of water being orocessed; the van is leveled and supported by four jacks
23
mounted on the van frame.
All the required power is supplied by a military standard 10-kW generator that
weighs 1500 lb. Fifty feet of insulated power cable is used so that the generator may be
operated remotely from the water-purification unit, thereby minimizing the noise to
which the operators are subjected. The generator is skid-mounted and transported on a
1.5-ton cargo trailer,
The ERDLATOR
Figures 13 and 14 present cross-sectional and three-dimensional views, respectively,
of the ERDLATOR used in the MWPU. The raw water is first metered and then
introduced into two small mixing launders located above the floccuiator. Dissolved gases
that may be supersaturated in the feed water are removed to a considerable extent in
these chambers by water forced through four aspirator nozzles, as shown in Fig. 14.
Solutions of calcium hypochlorite, ferric chloride, and pulverized limestone slurry
are all added to the water in the influent launders in the production model of the
ERDLATOR. (Figure 13 indicates the addition of chemicals at different points in the
apparatus; this was the mixing arrangement used in the prototype model.) The
arrangement used to add the ccagul&nt (ferric chloride) to the water makes calculation of
the mean velocity gradient G impossible.
29
Volume 8
Figure 13. Cross-sectional flow diagram of ERDLATOR (original testing version).
Adapted from Ruiz and Schmitt.^
From the launders the water then passes to the cylindrical flocculation chamber,
which is separated into three or more sections with horizontal, flat-metal disks. The disks
are attached to a vertical hollow shaft and are spaced equally along the shaft. The disks
serve to divide the chamber into compartments, thereby shortening the required mixing
time and avoiding major opportunity for short-circulating. The shaft is rotated at about
100 rpm to provide velocity gradients for the flocculation process. This agitation also
serves to detach gas bubbles from the solids. The gas can escape either through holes
drilled in the disks or in the hollow shaft. The mixing chamber contains approximately
40% of the total volume of 930 gal held in the ERDLATOR tank of the 3000-gph MWPU.
This gives a mean flocculation time of about 7-1/2 min. The flat-metal disks have an
edge velocity of 24 ft/sat 100 rpm.26
The water leaving the flocculation chamber is directed vertically upward in the
solids contact clarifier by shallow baffles that reverse the rotation of the water as it
leaves this chamber. A well-defined ferric chloride-limestone slurry sludge blanket is
formed in the clarifier, as indicated in Fig. 13. The excess sludge is drawn off at a point
located about 10 in. below the circular dual-edge coiiecxor weirs. The waste sludge is
allowed to concentrate by gravity in a separate sludge concentrator, as Jllustrated in
Figs. 13 and 14.
30
Volume 8
Influent launder
Figure 14. Cross section of the 1500-gph MWPU (ERDLATOR) assembly. Adapted from
TM 5-4610-218-1 2. 25
The vertical upflow water velocity in the clarification zone is about 3 ft/mln
(1.22 gal/fmin • ft ]) at the section of maximum horizontal area for the 3000-gph model.
It was recommended that this "rise velocity" should be reduced for water temperatures
below 40°F to allow for floe formation. The total mean residence time in the ERDLATOR
including the flocculation zone is about 20 min. The turbidity of the effluent typically
ranges in value from 0.5 to 2.0 turbidity units.22
3 i
Volume 8
The effluent from the ERDLATOR, together with the clear supernatant from the
sludge concentrator, flows to a wet well. The water is then pumped to the
dlatomaceous-earth filters. A float operated valve triggers a switch to notify the
operator if the wet-well level drops too low.
Conditioning Chemicals
Ferric chloride, the primary coagulant used in the ERDLATOR, is dissolved in water
and is pumped to the ERDLATOR with a dual-sided electrical diaphragm pump. The dose
applied is commonly in the range of 50 to 75 mg coagulant/L feedwater* ^
A slurry of the pulverized limestone is prepared in a small dual-compartment slurry
feeder. The tank contains agitators that revolve vertically. Measuring cups attached to
the agitators convey the chemical to a funnel leading into the slurry tank. The feed rate
is controlled by hand, and the amount of chemical to be added is determined by the soldier
on duty. Commonly, the limestone dose rate varies from 50 to 150 mg of limestone per
liter of influent feed water. The same slurry feeder is used to prepare the
diatomaceous-earth body coat for the filters. The slurries are pumped to the ERDLATOR
or the filters with a pump located Inside the slurry tank.
The pulverized-limestone particles tend to become enmeshed with and add weight to
the flee particles formed by the ferric chioride coagulant. In their research work, the
U.S. Army found in many different test situations that the use of pulverized limestone
brought stability to the coagulation process and provided water of relatively uniform
quality in the effluent. Limestone also provides some protection against low-pH waters.
The enhanced sludge-blanket stability also makes it possible to bring the MWPU on-line
quickly.^
A solution of calcium hypochlorite is prepared and pumped to the ERDLATOR wi !h
one side of the dual-sided diaphragm pump, which is also used to pump the ferric chloride
solution. This disinfectant also Increases the rate of coagulation. The dosage is adjusted
by the operator to give the same total-residual chlorine in the fully treated potable water
as that specified by medical personnel. Equipment and chemicals are available for
determination of either free-residual or total-residual chlorine. Typical doses of calcium
77
hypochlorite range from 3 to 7 mg/L.
Powdered activated carbon may also be added to control strong tastes and odors.
The carbon is applied in the mixing zone of ■•he ERDLATOR at the samejpoint as the
ferric chloride, calcium hypochlorite, and pulverized limestone. The activated-carbon
77
dose recommended for this treatment is 2 to 10 mg/L.
32
Volume 8
Air pump
Air blMd
External prtnt'r* gaga
Filter alamantt
( wirt ■wound with
diatomite filter-
aid caka)
Diatomite praooat funnal
Viawlng window
Coagulated water from
ERDLATGR wat wall
Internal
prMiure gaga
EXT'.""."! Baokwath diioharga
Filtered affluent
Figure 15. Cross section of diatomite-f .liter, final testing device for use in the Mobile
50
Water Purification Unit. From Donahew.
Precoat-F liter Operation
The water stored in the ERDLATOR wet well is pumped by a centrifugal pump to
50
the diatomaceous-earth filters, as illustrated in Fig. 15. The water enters the vertical
cylindrical shell of the filters under pressure and is forced through six diatomaceous-earth
filter elements. The filtered water then flows by gravity to two 3000-gal storage tanks
2
arranged in series. The total effective surface area of the filter elements Is 10 ft for
each of the two filters. ^
The wire-wound filter elements are precoated with a slurry of diatomaceous earth.
Diatomaceous earth is also added continuously to the influent water to give longer filter
25
runs. A controller installed on the filtered-water line is used to control the flow rate
through the filter element. The maximum filtration rate permitted by the. controller is
2 2
3.0 gal/(min • ft ) and the nominal rate is 2.5 gal/(mln • ft ) for a 3000-gph production
rate. ^
33
Volume 8
Eventually the pressure drop across the filter elements builds up to such an extent
that the filter cake and captured solids must be removed and the filter elements
recoated. This is accomplished by the "air-bump" method. Air is trapped In two places in
the filter housing during a filter run, and the pressure on the filter elements is relieved
almost instantaneously when the air-relief valve located on top of the filter is opened by
the operator to initiate backwash. The trapped air dislodges the filter cake and captured
solids in its rush to the air-relief valve. The expended filter aid is then washed to waste.
The loss of production of filtered water due to the time that it takes to backwash is
minimal, amounting to only 2% according to estimates for operation under combat
conditions.26
The section of the van that contains the two dlatomaceous-earth filters also
Includes the filter pumps. The pumps are used to provide water to the filters, as well as
to flush the filters after backwashing. Also included are pressure gauges, flow
controllers, air-relief valves, precoat tanks, and the necessary pipes and valves.
PERFORMANCE OF DEEP-BED FILTERS OPERATED IN THE DIRECT-FILTRATION
MODE
Filtration is a process intended to remove particulates such as bacteria, viruses,
algae, protozoan cysts, and clay from water and other liquids. Sand and its derivatives
are commonly used as deep-bed filter media, and garnet and coal have been used since the
1960s. The multimedia deep-bed filter used in the 600-gph ROWPU Is shown in detail in
Fig. 8.
The conventional steps for granular-media filtration Include coagulation,
flocculation, sedimentation, and filtration, as shown in schemes A and B in Fig. 1. No
provision Is included in the treatment train of the 600-gph ROWPU for flocculation or
sedimentation. This approach represents one form of the process called "direct
filtration". Many papers have been published about the proper operation of the
flocculation and sedimentation steps for filters operated in the conventional mode, but
because the focus of this report is the ROWPU, those papers have been omitted from
further consideration herein,
For good performance of a conventional filtration plant, the consensus is that proper
coagulation is the most Important of the three steps (coagulation, flocculation, and
29-31
sedimentation) that may precede filtration. Incomplete particle destabilization
occurs with coagulant doses that are either too high or too iowj this permits particles to
pass through the filter. An Environmental Protection Agency (EPA) study on the
removal of Giardia muris cysts and coliform bacteria gives evidence that although some
34
Volume 8
Table 1. Effect of coagulant dosage and type on the removal of Giardia muris cysts and
coliform bacteria by direct filtration with preliminary flocculation.* (20- to 30-mln
flocculation time, 20#C water temperature, and low-turbldity feed water.)
Coagulant
Coagulant dose
(mg/L feedwater)
Filtered-
water turbidity
(NTU)b
Cyst removal
(%)
None
—
0.35
59 to 94
Alum
1.8
0.60 to 0.65
94
Alum
1.9
0.77 to 0.79
23
Alum
2.1 to 2.2
0.46 to 0.63
63 to 88
Alum
7
0.32
>95
Alum
9.8
0.26
99.7
Alum nonionic polymer
11 4 0.01
0.08
99.75
Coliform removal
<%)
None
—
0.54 to 0.72
44 to 62
None
—
0.26 to 0.47
96.4 to 98.1
Alum
30
0.24 to 0.42
98.7 to 98.9
Alum
30
0.22
99.2
Cationic polymer
0.5
0.16 to 0.21
97.7 to 98.7
a - rs —
Adapted from Logsdon and Fox.
13 NTU s nephelometric turbidity unit.
removal is obtained with even Inadequate coagulation, the performance of the filter Is
much enhanced with properly coagulated water. Evidence of this enhancement Is given by
the data presented In Table l.
Because the coagulation step is so important, It behooves the equipment operator to
use the best type and optimum dosage of coagulant in order to achieve maximum filter
performance. A number of tests have been proposed that might be used to ludge the
appropriate coagulant dosage and type. These Include the far test, zeta-potentlal
measurements, and particle-size-distribution analysis. The advantages and limitations of
these tests will be discussed in detail subsequently.
35
Volume 8
One technique for monitoring filter performance involves the use of turbidity
measurements, and another is by particle-size-distribution analysis of the filter feed and
effluent waters. Early filter breakthrough can be detected with continuous or frequent
turbidity measurements. Logsdon and FoxJ* state that this is particularly Important for
filters required to remove viruses or Giardia cysts. To date particle-size-distribution
analysis has not been used much for the control of the filtration process, but turbidity has
been almost always measured since the early times of rapid sand filtration.
The impact of backwashing is also noteworthy. Backwashing is necessary when
32
turbidity rises. Backwashing should be initiated as soon as the operator notices rising
turbidity, even though the turbidity of the effluent is still < 1 NTU. In an EPA study to be
discussed in a later section, it was found that "a small increase in turbidity can be
associated with a dramatic increase in cyst concentration."
When the filter is returned to service after backwashing, poor effluent-water quality
may be experienced for a short time. Filtering to waste until the quality of the filtered
water improves is a commonly used strategy. Harris found that when a nonionic
polymer was added to the backwash water, low-turbidity filtered water was produced
immediately after the filter run began. The nonionic polymer was thought to Improve the
adsorption of particles.
The seemingly close association of turbidity removal with the removal of
microorganisms in deep-bed filtration made it necessary to review direct-filtration papers
dealing solely with turbidity. Some of those studies will be covered in detail in the
following sections of this report.
DETERMINATION OF BEST TYPE AND OPTIMUM DOSAGE OF COAGULANT
35
Adin qnd Rebhun injected a cationic poiyelectrolyte (Cat-Floe) into a 1/2-in. pipe
leading to two 2-1/4-in. l.d. sand-filter columns. The blending between the coagulant and
the water containing 20 mg/L of kaolinite clay was achieved by the turbulence created
between the jet derived from the coagulant-injection needle and the flow of the water
through the pipe and two 90-deg elbows. The two filters consisted of sand columns, 5 and
15 cm deep, with an effective grain size of 0.62 mm. The hydraulic loading was held
constant at 5 m/h (2.0 gal/[min • ft ]). 3ar tests were performed simultaneously with the
filter studies us shown in Fig. 16. The optimal jar-test coagulant dosage closely matched
the dosage giving the best filter performance, but the filter was much more tolerant to
off dosages.
36
volume s
Dot* (mg polymer/L fttd w*t*r) X10*
Figure 16. Comparison of jar-test results with the passage of kaolinlte turbidity through
rapid sand filters, where C/CQ is the ratio of the turbidity of the treated water to that of
the untreated water (kaolinlte * 20 mg/L, cationic polymer, filtration rate ■ 5 m/h).
i j
Adapted from Adin and Rebhun.
Filter run time (min)
Figure 17. Effect of polyelectrolyte dose on the passage of 0.1- pm latex beads through a
deep-bed filter. (Bead concentration = 9.7 mg/L, filtration rate a 2 gal/[min • ft4'].)
Adapted from Habibian and O'Melia.^
37
Volume 8
36
Habibian and O'Melia conducted a study to investigate the role of chemical
parameters in direct filtration. Suspensions of different sizes of latex beads were
coagulated with various cationic polymers and filtered through six test-filter columns
operating under the same conditions but receiving different polymer dosages. No
information was given on the type of filter media used or the kind of mixing employed.
One filter was operated as a control with no polymer} two were underdosed relative to the
results of jar tests; two were overdosed; and one was operated at the optimum jar-test
dosage. The filtration rate was kept constant at 2 gai/(mln • ft ), and all filter media
were precoated with polymer prior to the start of a filtration run.
The results of the experiments for the 0.1-pm latex particles and the Cat-Floe
polymer are shown in Fig. 17. The optimum jar-test coagulant dosage was determined to
be 0.07 mg/L. The control and the two test columns receiving gross underdoses of the
coagulant (runs 1, 2, and 3 in Fig. 17) gave constant concentrations of the latex particles
In the filtered water, as determined by turbidity soon after the start of the filter runs.
The passage of the particles through the filter remained constant for a while and then
increased rapidly to nearly 100%. The two filters receiving gross overdoses of the
coagulant (runs 5 and 6 in Fig. 17), performed poorly almost immediately after the start
of the filter runs. At the optimum dose as determined by the jar tests (run 4 In Fig, 17),
the passage of the particles through the filter attained a maximum value of approximately
40% soon after the Initiation of the filter run, and then decreased to a constant minimal
value of less than 10%. Filtration in this run was so effective that the 7.3-ft (of water
head) allowable pressure drop across the filter column was reached in less than 3 h of
continuous operation.
Jar-test results for the 1.099- ym latex particles using polyethyleneimlne coagulants
with molecular weights ranging from 600 to 100,000, but with almost equal charge
densities (as determined by acid-base titrations), indicated that the optimum coagulant
dosage was nearly the same for ail the polymers regardless of their molecular weights,
except for the lowest-moleculer-weight polymer that also had a higher charge density.
This indicated that charge neutralization plays a significant role In the coagulation of
latex particles by polyethyleneimlne. Subsequent electrophoretic-mobility measurements
Indicated that the optimum dosage occurred at negative zeta potentials. This finding
confirms the effect of charge neutralization, but since optimum dost^e did not occ.ut at
zero zeta potential (zero mobility), it also Indicates that complete charge neutralization
is not requited for effective coagulation.. From these results the investigators concluded
that charge neutralization plays a role In coagulation, although it Is not the sole
mechanism.®
38
Volume 8
As stated previously, Yao's theoretical and experimental studies showed that the
size of the suspended particles affects the removal efficiency of the filter markedly. In
36
order to evaluate the validity of this prediction, Habibian and O'Melia filtered
suspensions of three sizes of latex particles (0.109, 1.099, and 7.6 ym) through three
shallow filter beds 0.8 in. deep. The coagulant, PEI- 1 8 (poiyethyleneimine), was added
continuously to the filter Influents. The polymer dosages were 0.176, 0.032 and
0.024 mg/L, respectively. The Initial latex-particle concentrations in the the three filter
influents were approximately the same (30, 48 and 32 mg/L, respectively). The
particle-removal efficiency was greatest for the 7.6-ym particles. It was also quite high
for the 0.109 ym particles for which the maximum allowed pressure drop across the filter
bed (85 in.) was reached before particle breakthrough. Filtration of the 1.099- ym
particles was the least efficient, and breakthrough was observed at a filter pressure drop
of 5 In. of water head. The Investigators concluded that the trend In particle-removal
efficiency as a function of particle size was In qualitative agreement with Yao’s transport
model.
37
Stump and Novak Investigated the performance of various polymers In a process
consisting of rapid mechanical mixing, followed by 20 min of flocculation, and then
filtration through a multimedia filter (anthracite, sand, and garnet). The filtration rate
1 was 5 gal/(mln • ft2). The feed water, a 100-mg/L kaollnlte clay suspension, had a
turbidity of 80 formazln turbidity units (FTU). The optimum coagulant dosage obtained by
the jar tests provided the longest filter runs. However, the jar tests failed to predict
which polymer would perform the best in the filter runs. It was concluded that the jar test
could not be used to select the polymer, but It could be used to determine the optimal
coagulant dosage for a given coagulant.
37
Stump and Novak also concluded that the low-molecular-weight cationic polymers
(molecular weight <10,000) gave poor removals, whereas those with molecular weights in
excess of 1 x 10^ created excessive filter pressure drops. They recommended a polymer
molecular weight within the range of 10,000 to 200,000 for direct filtration. They also
recommended that the mean velocity gradient, G, In the flash mixer should be Increased
with the molecular weight of the cationic polyelectrolyte. A range of about 200 to 950 s'*
was recommended for the polyelectrolytes deemed useful for water coagulation.
Electrophoretic-mobility measurements (zeta potential) and more recently
partlcle-size-dlstrlbutlon analyses are also used to help ascertain the optimum coagulant
dose. According to O’Melia and Stumm/ zeta potential cannot provide an accurate
estimate of the forces affecting the particle attachment in water filtration, and
consequently its use as a monitoring tool for correct coagulant dose Is limited.
Volume 8
38
Letterman et al. performed a study to ascertain whether zeta potential could be
used to prejudge the performance of the cationic polyelectrolyte Cat-Floe T In the direct
filtration of water having a turbidity of 32 FTU derived from a mix of bentonlte-kaollnlte
clays. The coagulant was blended with the feed water through a pipe tee leading Into a
mechanically stirred flocculator containing four turbine Impellers. The mean velocity
gradient G was not estimated for the pipe mixer. The mean flocculation period was 2 to
10 min, depending on the feed flow, and the mean velocity gradient could be varied
between 0 and 700 s"1 in the flocculator. The water was filtered through 3-in. i.d.
dual-media filter columns (anthracite and sand). The filter rate was varied from 2.5 to
7.5 gal/(min • ft ) during the course of the study.
The polyelectroiyte dosage was adjusted in a filter run performed at a filtration rate
of 7.5 gal/(min • ft ) to obtain a minimum filter effluent turbidity. This corresponded to
particle zeta potentials in the range of -5 to +12 mV in the suspension leaving the
flocculator. The investigators stated that these results were in good agreement with the
principal author's previous studies on direct filtration of natural suspensions from Lake
IQ
Michigan, using cationic polyelectrolytes. In these former tests the particle zeta
potentials ranging from -4 to +13 mV corresponded to an Interval of minimum turbidity in
the filtered water. It was concluded that zeta-potential measurements at or near zero
Indicated the best coagulant dosage.
Yeh and Ghosh5 conducted studies with the objective of developing methods to
select polymers for direct filtration. They Investigated the influence of polymer
molecular weight and charge density on filtration efficiency, as well as the effects of
mixing energy on polymer-particle Interactions, by using zeta-potentlal measurements, jar
tests, colloid titration, and particle-size-distribution analysis. Cationic polymers
(molecular weights ranging from 1200 to 5x10^) were Investigated, Including Cat-Floe T
(molecular weight = 1x10 ). Studies were performed both in batch- and continuous-flow
modes. The batch tests consisted of the jar tests and zeta-potentlal measurements. In
the continuous-flow filtration studies a 3.7-L mixing chamber with a variable-speed mixer
was used ahead of a 2.34-crn (1-in.) i.d. filter. The filter contained silica sand with an
effective size of 1.0 mm and a uniformity coefficient of 1.2, packed to a depth of
15.2 cm.
It was found that the coagulant dosage that gave the minimum residual turbidity In
the jar tests also corresponded to a zero zeta potential. The partlcie-size-distrlbutlon
analysis conducted in the continuous-flow studies, on the other hand, - indicated a
substantially higher optimum polymer dose when measured after 3 min of rapid mixing.
The filtration runs indicated that the coagulant dose obtained from the
40
Volume 8
particle-size-distribution analysis, rather than that given by the jar tests, gave the best
filter performance. The investigators concluded that particle-size-distributlon-analysis
was the preferable method to determine the optimum polymer dosage for direct filtration.
Yeh and Ghosh^ concluded that low- to medium-molecular-weight cationic polymers
(molecular weights from 10,000 to 100,000) perform best in direct filtration, which is in
38
agreement with the results of Letterman et ah discussed previously. The investigators
recommended that rapid mixing should not be continued for prolonged periods of time
because it might cause fioc breakup, especially when hlgh-molecular-welght polymers are
used. They also concluded that for most direct filtration operations, slow flocculation
following rapid mixing is not necessary, especially if the suspended-solids concentration in
the raw water is 30 mg/L or higher.
9
Ghosh et al. Investigated the relationships between polymer molecular weight,
charge density and dosage, mixing conditions and the flocculated partlcle-size-
dlstributions in batch reactors. Three groups of commercial cationic polyelectrolytes
were tested, including the Cat-Floe group. The study results indicated the following)
(1) both charge neutralization and bridging play a role in coagulation with
polyelectrolytes, (2) the optimum polymer dosage Is independent of polymer molecular
weight In most cases, and (3) a strong correlation exists between the optimum polymer
dosage and its charge density. It was observed that the optimum coagulant dosage
decreased as a power function of the number of cationic charges per molecule, and the
power was close to unity for the type of suspensions coagulated In their study (silica and
bentonite clay).
o
Ghosh et al. also compared particle-size distribution and zeta potentials subsequent
to rapid mixing and flocculation in a mechanically stirred batch reactor. They found that
the polymer dosage yielding the largest mean size of the particles corresponded
approximately to a zero zeta potential, regardless of how the mean size was computed.
These findings tend to support the concept that charge neutralization plays an important
role in polyelectrolyte coagulation, as does the applicability of zeta-potential
measurements for the determination of the optimum polymer dose.
Particle-size-distribution analyses also Indicated that a significant amount of coagulation
occurred during the first phase of rapid or flash mixing. The optimum mean velocity
gradient G was 800 s"* for most of the cationic poiyelectrolytes tested, including the
Cat-Floe group. Increasing G beyond 800 s"1 failed to improve the coagulation rate.
The experimental data of Ghosh et al. also Indicated that the charge density of the
9
Cat-Floe group of polymers was independent of the pH of the suspension.
41
Volume 8
GENERAL BACKGROUND PAPERS
39
Robeck etai. observed removals of attenuated poliovirus Type 1 (Mahoney strain)
ranging from 1 to 50% when uncoagulated water was filtered through a dual-media filter
with filtration rates of 2 to 6 gal/(min • ft ). Properly coagulated water with no in-line
flocculation chambers and no settling tank gave rise to 90-99% removals of the virus, with
most removals being >98%. Two kinds of raw water were used: (1) a blend of hard ground
water with demineralized water and (2) water from the Little Miami River, OH. The
raw-water temperatures varied from 17 to 19°C; the pH varied from 8.1 to 7.7, and the
alkalinity ranged from 100 to 200 ppm. The feed-water virus concentration was
maintained in the region of 10,000 pfu/mL.*
The dual-media filter contained 16 in. of anthracite coal and 8 in. of Muscatine
sand, and the raw water was coagulated with 10 ppm of alum when the raw-water
+
turbidity was 10 3TU or less. The alum dose was increased for higher raw-water
turbidities (about 40 JTU). These higher turbidities and alum doses tended to shorten the
filter runs. Combinations of alum and polyelectroj ;tes were tested In a few of the runs.
One run lasted 52 h with a pressure drop across the filter reaching 7 ft of water head.
From their experiments, Robeck et al. concluded that (1) an increase In filtered
water turbidity of less than 0.5 JTU, indicating a floe breakthrough, was usually
accompanied by a breakthrough of virus, (2) a virus penetration of the filter could occur
without a turbidity increase, even though the turbidity might be as low as 0.5 3TU,
(3) poiyelectrolyte doses as low as 0.05 mg/L helped to increase the floe strength and
prevent virus breakthrough, and (4) more than 98% of the viruses could be removed in the
dual-filter media at filtration rates of 2 to 6 gal/(rnin • ft ) if a low but well-mixed dose
of alum was fed just ahead of the filters.
A task group of the American Water Works Association (AWWA)^® collected and
analyzed data available at the time (1980) relative to the performance of direct-filtration
water-treatment plants. The objectives of the study were the following: (1) to obtain
data on direct filtration, (2) to analyze the results and identify problems, (3) to suggest
feasible solutions to the problems identified, and (4) to recommend areas for further
research and development. Multimedia filters were installed in a few of the plants, but
most of the plants studied included just dual-media filters. Turbidity, color, algae types
* pfu = plaque-forming unit.
^ JTU = Jackson turbidity unit.
4 2
Volume 8
and concentration, and water temperature were judged to be the most important
feed-water quality parameters. Filter media, pressure drop, and filtration rate, as well as
the mode of operation and type and dosage of coagulants, were considered to be the most
Important operating parameters. From the results obtained in this study, it was difficult
to distinguish trends in the operating parameters that would point to an improvement in
the process performance.
Problems identified with the raw-water quality included the following: (l)a color
greater than about 30 to 40 Hazen units tended to move through the filter beds;
(2) turbidities greater than about 15 FTU wore poorly removed; (3) certain types of
diatoms tended to clog the filters; (4) coagulation became slow at low temperatures; and
(5) coagulant performance could be poor at the elevated pH values associated with algae
blooms. Other problems were related to the presence of iron and manganese. The AWWA
committee concluded that problems with direct filtration may be encountered with raw
waters containing more than 40 Hazen units of color, 5 FTU of turbidity, 2000 asu/mL
[(one areal standard unit (asu) equals 400 ym^)] of algae, 0.3 mg/L of iron, and 0.05 mg/L
of manganese. They also recommended that water-flow rates should be decreased at low
temperatures in order to permit the completion of the coagulation process; otherwise,
post-flocculation may occur in the filtered water.
The daia related to the types and doses of coagulants used in the direct filtration
plants surveyed varied widely, with no operational problems readily identified with the
single exception of backwashing. It was observed that the use of polyelectrolytes as
coagulants caused stickiness in the filter media, which made the cleaning of the filter bed
with backwashing difficult. The formation of mudballs on the surface of both the dual-
and multimedia filters was also noted.
Trends were not readily apparent in the information collected on flash-mixing and
in-line flocculation. Despite these inconclusive results, the Committee recommended that
in-line mixing of the coagulants with the water should be favored over mechanical mixing.
The size of the filter media used proved to be a significant parameter. Anthracite
media smaller than 0.8 mm decreased the length of the filter runs considerably and were
subject to algae binding. Based on this information, the Committee recommended an
effective size uf 1.1 mm with a low uniformity coefficient for the anthracite layer. The
depth of this layer should be in the range of 37.5 to 90 cm (15 to 36 in.).
It was found that the sand layer in sand-anthracite dual-media beds controlled the
filtered water quality. The depth of the sand bed should be about 20 to .30 cm (8 to
12 in.), and the effective size of the media of 0.45 mm, commonly used
43
Volume S
in practice, appeared adequate. A third layer of garnet sand (effective size of 0.2 to
0.3 mm) was also used in some of the plants surveyed. It appeared that those plants could
successfully treat waters containing turbidities up to 500 FTU, and color up to 1000
Hazen units, at the expense of shorter filter runs.
The filtration rates ranged f*-om about 1 to 6 gal/(min • ft ) in the plants surveyed.
The AWWA committee concluded that the higher rates could be applied with careful use
of mixing energy and coagulant aids, and recommended a filtration rate in the region of 4
to 6 gal/(min • ft ). The only microbiological data considered by the Committee were
those reported by Robeck et ai.^ in 1962? they concluded that waters derived from the
direct-filtration process must be disinfected. They also pointed out the increased need
for a good water-quaiity-monitoring program because the direct-filtration process utilizes
little detention time for coagulation ahead of the filters.
A review of the advantages and disadvantages of direct filtration was also prepared
by Trussell et al.^1 They decided that the quantity and physicochemical properties of the
particulate matter determined the success or failure of direct-filtration systems. Other
important factors considered included bacterial quality, tastes, odors, and color of the
raw water. Direct filtration should be viewed with extreme caution when the raw water
contains high levels of coliform bacteria because it does not provide as many barriers of
protection as conventional filtration systems. Moreover, heavy dosing of powdered
activated carbon to remove tastes and odors causes significant decreases In filtered-water
output. Other observations made by Trussell and co-workers included the increase in
performance resulting from the installation of in-line flocculation, and the significant
increase in filter pressure drop caused by a bottom layer of garnet without a
compensating increase in filtered-water quality. A very coarse top layer composed of
pumice or plastic media was also recommended.
The East Bay Municipal Utility District^ L (EBMUD) headquartered in Oakland, CA,
investigated the replacement of alum as a coagulant with cationic polymers in three
direct-filtration water-treatment plants. Six cationic polymers, four of which were
polymers of diallyldimethyl ammonium chloride and two of epichlorohydrindimethylamine,
were tested. The coagulated water was filtered through dual-media filters (3 in. of
0.9-mm effective size anthracite and 27 in. of 0.5-mm effective-size sand, both layers
with a uniformity coefficient of 1.4). The filtration rates utilized ranged from
2c,0 to 2.5 gal/(min • ft ) with alum coagulation and from 3.5 to 4.0 gal/(min • ft ) with
clay-polymer coagulation in two - of the treatment plants, -and from
44
Volume S
2 2
2.8 to 3.3 gal/(min • ft ) with alum coagulation and from 4.0 to 4.5 gal/(min • ft ) with
clay-polymer coagulation in the third plant.
It was found that the removals of turbidity achieved with alum and the
polyelectrolytes were virtually identical when the raw-water turbidity exceeded about 3.5
nephelometric turbidity units (NTU). The alum performed considerably better than the
polyelectrolytes for raw-water turbidities in the range of 2 to 3 NTU. The
polyelectrolytes gave acceptable turbidity removals when artificial turbidity (clay) was
added to the low-turbidity feed waters. In this case the typical filter-run length increased
from 23 h with alum coagulation to 47 h with clay-polymer coagulation.
The EBMUD Investigators emphasized the increase in the potential for the
unreacted polymer to pass through the filter if the clay dosage was low relative to the
polymer dosage. Quantitative data could not be obtained because the polyelectrolyte-
concentration detectability limits of the analytical methods used were too high.
Nevertheless, those investigators recommended the monitoring of polymer residuals in the
treated water for future applications. It was also noted that the coagulative properties of
the polyelectrolytes tested tended to change with age (storage). Changes in chlorine
demand and the growth of microorganisms were also noted.
High alum residuals in filtered water, inadequate plankton removals, and powdered
activated carbon breakthrough were the main operational problems encountered in 1978 in
Q
the 2 x 10 -gai/d-capacity Alfred Merrit Smith Water Treatment Facility of the Southern
43
Nevada Water System. Raw water derived from Lake Mead was filtered at a constant
2
rate of 5 gal/(min • ft ) through dual-media filters (20-in. depth of 0.6- to 0.7-mm
effective size anthracite and 10 in. of 0.45-mm effective size sand). Alum and, when
necessary, powdered activated carbon were mixed with the raw water in mechanical flash
mixers without in-line flocculation or sedimentation.
Pilot-plant tests demonstrated that the operational problems cited previously could
be ascribed to insufficient alum flocculation times. The carryover of alum was reduced
significantly, and activated carbon doses as high as 20 mg/L could be used for taste and
43
odor control when the water was preflocculated for a period of 15 to 20 min.
Additional pilot-plant tests were then performed by a consulting engineer. Both
direct filtration with and without flocculation and conventional filtration were
investigated, using the filter media installed in the existing plant. A few runs were also
made utilizing a larger-grain-size anthracite layer with an effective grain diameter of
0.95 mm and a uniformity coefficient of i.5. It was demonstrated that the conventional
treatment gave the best filter performance, but the improvement over the other schemes
did not warrant the additional expense of installing sedimentation basins for this
45
Volume S
particular lake water. Direct filtration with 30 min of preliminary flocculation yielded
longer filter runs} more consistent performance; better turbidity and plankton removals;
more even distribution of pressure drop through the filter bed; elimination of
activated-carbon-particle breakthrough at high filtration rates; and less coagulant usage
than direct filtration without flocculation. These results confirmed the findings of the
. 43
preliminary pilot-plant studies.
The plankton problems merit further discussion. It was found that the species or
type of plankton was much more important than the total plankton count. Specifically,
relatively large numbers of anabaena and anacystis species of bluegreen algae decreased
the filter performance significantly. Finally, the results of these studies demonstrated
that a 0.95-mm-effective-size anthracite layer was preferable to the 0.6- to 0.7-mm
anthracite used in the plant filters. The filter with the larger-size anthracite gave longer
filter runs with turbidity and plankton removals comparable to those of the
smaller-anthracite-slze filter.
32
Research reported by the University of Washington and the U.S. EPA shows a
relationship between the turbidity of the filtrate and the removal of cyst-sized particles
(5 to 13 ym). Filtrate-turbidity levels must be consistently lower as desk od removals of
cyst-sized particles increase. It was found that for removals >99.0%, an overwhelming
majority of the filtered water samples (88%) had turbidities less than 0.10 NTU. For
removals of 90.1 to 95.0% only about a third of the samples had turbidities of less than
0.10 NTU.
33
Logsdon et a). Investigated the removal of cyst models and the cysts of G. muris
in direct filtration with a dual-media filter (46 cm of anthracite with an effective size of
1.27 mm and 15 cm of sand with an effective size of 0.36 mm). Three sets of
experiments were run, each with a different type of coagulant. Alum, or alum plus a
cationic or .nonionic polymer, was blended with the feed water as It was fed through three
in-line static mixers in series. (No coagulant was added to the water in some of the initial
test runs.) The solution was then flocculated for about 20 to 30 min in a cascade of CSTR
reactors. The filtration rate on the dual-media filter was varied from about 2.7 to
7.4 mm/s [4 to 11 gal/(min • ft )] during the course of the experiments. The most
commonly used filtration rate was 2.7 mm/s [4 gal/(min • ft*)].
The water used in the experiments was obtained from a gravel pit. This water was
characterized by low turbidity, and experiments were conducted at a water temperature
of about 20#C and a pH of 8.2. The cysts of G. muris or cyst models (radioactive beads)
Volume 8
were added continuously to the in-line flash mixers to yield initial concentrations ranging
from about 470 to 190,000 cysts per liter. The coagulants, when used, were added in
amounts sufficient to maintain a filtered-water turbidity of less than 1 NTU.
The removal of the cysts ranged from 59 to 94% when no coagulant was added to the
raw water. The removals increased to 99.5% or more during periods of stable operation,
when alum was added in quantities sufficient to yield turbidities of <* 0.3 NTU in the
filtered water, and the initial feed-water cyst count was >10,000/L. Three out of 20
experimental runs yielded filtered-water cyst concentrations > 100/L at a filtration rate
of 2.7 mm/s [4 gal/(min • ft2)].
It was concluded that "a filter treating water dosed with an adequate amount of
coagulant and operated in a manner that prevents filtered-water turbidity Increases
should remove a very substantial portion of the Glardia cysts." It was also pointed out
by the investigators that the continuous passage of a small number of cysts through a
dual-media filter operated in the direct filtration mode cannot be ruled out.
Some other observations of interest were also made by Logsdon et aL Increasing
the filtration rate by 30, 100, or 150% in less than 10 s increased the passage of turbidity
and cysts through the filter. The simultaneous application of alum and a nonionic polymer
helped to mitigate this effect at 20#C, but not at 10#C. Sudden surges in flow rate also
dislodged the turbidity and cysts stored »n the filter. For example, increasing the
filtration rate abruptly from 1 1 to 27 m/h then back to 11 m/h [4.5 to 11 gal/(min • ft2),
then to 4.5 gal/(min • ft2)] for a period of 2 min increased the filtered-water turbidity
about 4-fold and the cyst concentration about 25-foid. Finally, initial tilter-rlpening
periods (allowing equipment to operate for a while to Improve performance) lasting about
0.5 h were evidenced in the experiments. The filtered-water cyst concentrations were
about 10 to 25 times greater than those during stable operation (immediate performance
monitoring) during these initial periods.
Rebhun et al.^ determined the removal of organic colloids by direct filtration. The
study apparatus used (for feeding, dosing, flowmeter, and flow-control arrangements) was
similar to that used in an earlier work discussed previously in the section on determination
of the best type and optimum dosage of coagulant.^ Two filter columns, one with a 5-cm
i.d. and 15-cm media depth, and the other with a 4-cm i.d. and 100-cm media depth, were
used. Both columns had the same dual media» A coarse quartz sand with a 1.21-mm
effective size and 1.17 uniformity coefficient on top of a medium-grade quartz sand with
a 0.62-mm effective size and 1.21 uniformity coefficient. The media porosities were 0.4
and 0.37, respectively.
47
Volume 8
The feed water contained 10 mg/L of a humic acid dispersion, and the coagulants
tested were alum, polyelectrolytes, and alum plus polyelectrolytes. The mean residence
time given for coagulation was only 2 to 3 min. It was found that effective filtration
could be achieved with a combination of alum and a cationic polymer if the doses of the
coagulants were carefully selected. Alum or cationic polymers (Magnafloc R-140 and
R- 139, Cat-Floe C), added singly, failed to give acceptable results.
McCormick and Klng^ determined the removal of turbidity, color, algae, and total
coliform bacteria from various surface waters located in the Commonwealth of Virginia
with a direct-filtration pilot plant. The results of this study are of special interest here
because the modes of operation, filtration rates, and type of coagulants used were often
similar to those used in the 600-gph ROWPU.
Alum and/or cationic polyelectrolytes were blended with the raw water in a
mechanical flash mixer. The blended water could then be Introduced Into a mechanically
stirred flocculation basin or bypassed to the filter-pump well. (The flocculation basin was
not utilized in most of the experiments.) The water was then pumped to one of the three
9.2-cm-i.d. gravity-filter columns that contained the types of filter media listed in
Table 2.
The mean residence times were 3, 28, and < 10 min in the flash mixer, flocculation
basin, and filter-pump well, respectively. The mean velocity gradients utilized in the
flash mixer were 1290 s"1 for alum coagulation and 1650 s”1 for polyelectrolyte
coagulation. The mean velocity gradient was varied between approximately 20 and 63 s“*
in the flocculation basin when the floe basin was used. The water was usually filtered at a
2
rate of 12.6 m/h [5 gal/(min • ft )] and backwashed at a rate of 50.4 m/h
2
[20 gal/(min • ft )] for 7 min. No mudball formation was observed. A successful filter
run was defined as that in which the filtered-water turbidity was always < 1 NTU for a
period of at least 8 h.
The quality of the surface waters tested varied considerably. For example, the
raw-water turbidity ranged from a minimum of 1.5 to a maximum of 24 NTU. The
observed ranges in the other raw-water quality constituents measured were the following!
0 to 42 APHA* color units, 185 to 20,000 algae clumps/ml, (not always determined), 1.5 to
12.5°C water temperature, and 0 to 2300 coliform bacteria/ 100 mL. The optimal doses of
the coagulants ranged from 5 to 40 mg/L for alum, and from l to 5 mg/L for Cat-Floe Tl.
Other polyeiectrolytes investigated were Cat-Floe 21, Cat-Floe T, and Magnafloc 572 C.
The polyeiectrolytes were at times mixed together with the alum in the flash mixer.
* APHA = American Public Health Association.
48
Volume 8
Table 2. Types of filter media used in the direct-filtration studies of Virginia surface
waters.®
Filter
number
Media
type
Media
depth (cm)
Effective
size (mm)
Uniformity
coefficient
l
coal
25
1.0 to 1.1
<1.7
sand
30
0.42 to 0.49
1.3 to 1.5
garnet
23
0.21 to 0.25
1.6 to 2.1
2
coal
‘ 51
1.3
1.35
sand
25
45
1.4
3
coal
51
1.7
l.l
sand
25
0.45
1.4
—
From McCormick and King.
Firm conclusions could not be drawn for all of the process variables investigated by
45
McCormick and King, but the following proved to be readily apparenti
• The raw-water turbidity was 10 NTU In 39 experimental runs. These waters
were filtered successfully 80% of the time. The failures were ascribed to
premature breakthrough of turbidity (13%) and failure to maintain a constant
coagulant dosage (3%).
• The raw-water turbidity exceeded 10 NTU In 1 4 experimental runs. The water
was filtered successfully In only three of those runs, and the results for the
three successful runs are summarized In Table 3. Failures were ascribed to
premature breakthrough of turbidity or a poor-quality effluent at all times.
• The polyelectrolytes tested commonly performed better than the alum,
although it was noted that the alum was superior for color removal. A mix of
alum and a cationic polymer worked the best of all. Premature turbidity
# . ..
breakthrough occurred in 43% of all the alum runs, 30% of the polymer runs,
49
Volume 8
and 28% of the runs in which both were applied to the raw water. It also
appeared that the polyelectrolytes would have been superior to alum in terms of
the projected length of a filter run.
Large algae concentrations (range of 7000 to 20,000 algae clumps/mL) tended
to clog all of the filters tested. The high pH associated with algae blooms also
increased the amount of alum that had to be added to the raw water. This, in
turn, resulted in a pressure drop across the filters.
A filtered-water turbidity of less than 0.10 NTU also gave total coliform
bacteria counts below the detectable limits of the analytical method used in the
Investigation.
It was determined that the effective size of the anthracite layer used in Filter
No. 3, 1.7 mm (see Table 2), was too large. Early breakthrough occurred in
most of the runs performed with that filter. The rate of Increase of
filter-pressure drop was significantly greater In the multimedia bed (Filter
No. 1) than the dual-media bed (Filter No. 2) In which the effective size of the
anthracite was 1.3 mm. It was concluded that the media contained In Filter
No. 2 represented the most effective combination of media sizes and bed
depths.
The effects of other parameters Investigated could not be judged from the
relatively limited amount of data taken In the study. These Included water
temperature (which was always lathei low, 1.5 to 12.5°C), an increase in
filtration rate from 12.6 m/h to 20.16 m/h (5 to 8 gal/[mln • ft^]), and direct
filtration with and without in-line flocculation. It did appear that preliminary
flocculation might have Improved the performance of the multimedia filter
because the 25 cm (10 in.) of anthracite coal placed in that filter was not
sufficient to provide the additional detention time required for good bed
flocculation.
The investigators concluded that the raw water should have a turbidity of less
than 10 NTU, a color of less than 15 APHA units, and an algae concentration of
less than 1000 algae clumps/mL in order to be treated reliably with direct
filtration at a filtration rate of 12.6 m/h [5 gal/(mln • ft^)].
Volume 8
Table 3. Details of the successful filter runs with Cat-Floe T! coagulant for raw-water
turbidities exceeding 10 NTU.a
Average turbidity (NTU)
Raw Filtered Filter Temp Coagulant
water water no.15 Modec (#C) dose (mg/L)
10.4
0.40
3
R
5.7
3.7
1 6.5
0.45
2
R
7
3.5
12.0
0.9
2
RF
4
5.0
a - ZT5 — -
Adapted from McCormick and King.
b See Table 2.
c R stands for rapid mixing followed by direct filtration and RF stands for rapid mixing
and flocculation followed by direct filtration.
Hand et al. conducted a bench-scale study to compare the suitability of direct
filtration versus conventional filtration for the upgrading and extension of the
pretreatment facilities of the Sweetwater Authority Water Treatment Plant in San Diego,
CA. The existing plant operating In the "in-line" filtration mode (see Fig. 1) had
experienced short filter runs, and product-water turbidities exceeding the 0,3-NTU
turbidity standard established by the State of California Department of Health Services.
The raw-water quality appeared to be good enough to be filtered successfully with
the direct-filtration mode. The raw-water turbidity was less than 5.3 NTU 50% of the
time, and less than 14 NTU 90% of the time. However, It was decided from the
bench-scale batch tests that changing from direct filtration to conventional treatment,
Including flocculation and sedimentation, would serve to improve the performance of the
filtration plant significantly.
Cleasby et al/*7 investigated the ability of a 10-cm-i.d. dual-media filter column
(40 cm of l.34-mm effective-size anthracite with a uniformity coefficient of 1.18, and
30 cm of 0.43-mm effective-size sand with a uniformity coefficient of 1.53) to remove
particles, using particle counts, turbidity, total collform bacteria, and standard plate
count as Indicators, The Influent water was taken from a gravel pit, which normally
would have supplied high-quality water, but during the testing period was subject to
several unanticipated algal blooms. Water pH was adjusted to 6.8 to 7.8,. during the
51
Volume 8
high-pH periods of algae blooms. Alum or Cat-Floe T was blended with the raw water in
static mixers, and no flocculation time was provided other than that occurring in the feed
lines. The coagulated water was filtered through the dual-media filter at filtration rates
ranging from 6.8 to 13.5 m/h [2.8 to 5.5 gal/(min • ft2)].
The raw-water temperature varied considerably during the period of testing
(October 1981 to November 1982) as shown in Table 4. The optimum alum dosage ranged
within the relatively narrow limits of 6.1 to 12.3 mg/L for raw-water turbidities varying
from about 0.4 to 8 NTU. The optimum Cat-Floe T dosage ranged widely from 0.09 to
0.84 mg/L. The results listed in Table 4 indicate that in general, alum coagulation is
superior to that achieved with cationic polyelectrolyte as far as effluent quality is
concerned.
Other conclusions of this study were as foliowss (1) poorer filtrate is found at the
beginning of the run, affirming the Importance of filtering to waste during ripening at the
start of a run for a period of time, especially where Glardia cysts or 7- to 12>um particles
are of concern; (2) removal of total coliform bacteria was >,86%; (3) water as cold as 2°C
did not seem to affect the removal efficiencies; (4) longer filter cycles, but
poorer-quality filtrate, were obtained using the cationic polymer; and (5) prechlorination
with alum was essential during periods of severe algae blooms (prechlorination with the
polyelectrolyte proved Ineffective)* Finally, Cleasby et al» recommended that the
raw-water turbidity should not exceed about 16 NTU with "small" algae populations or
1 1 NTU with "moderate" algae populations for the cationic polymer to be effective.
600-GPH ROWPU PRETREATMENT UNITS
Small et al.1 conducted a study on the removal of microorganisms by the 600-gph
ROWPU pretreatment system without chlorination in 1979-80. River water (Monocacy
River, MD) was first coagulated and then filtered through a multimedia pressure filter
(Culligan Model MD30 Mixed-Media Filter) similar to the one Installed in the current
600-gph ROWPU. The filtered water was then pumped under pressure through cartridge
filters. The raw water was coagulated with a cationic polymer (Cat-Floe T) at an average
dosage of 5.0 mg/L (4.3 to 5.6 mg/L). Three filtration rates of 6.5, 7.6, and
8.7 gal/(min • ft ) were investigated in nine experimental runs lasting 9 h each. These
filtration rates are equivalent to feed-water flows to the ROWPU of 30, 35, and
40 gal/min. The operation of these pieces’ of equipment was as described in the technical
manual for the 600-gph ROWPU. ^
52
Volume 8
Table 4. Average removal of total coliform bacteria and particles in the 7- to 12-ptn size
range for direct dual-media filtration without chlorination.® (Filtration rates were varied
from 6.8 to 13.5 m/h.)
Coagulant
Temp
(#c)
Coliform bacteria
Particles (7 to 12 um In size)
Influent
(number/
100 mL)
Removal after
filter ripening
(*)
Influent
(number/
100 mL)
Removal after
filter ripening
(%)
Alum
7 to 14
1300
91
2320
98.8
Cat-Floe T
4 to 5
8200
96.5
1170
96.7
Cat-Floe T
3 to 4
1500
89.7
370
87.0
Alum
2
1600
96
2190
99.0
Cat-Fioc T
3
640
89
1620
98.0
Alum
6 to 7
350
91.3
2860
94.0
Alum
17
90
86
13040
99.0
Cat-Floe T
21 to 23
50
86
1350
85.5
Alum
20 to 24
550
89
1640
96.5
Cat-Fioc T
23
170
86.5
340
87.5
a From Cleasby et al.^
The cartridge-filter elements had the same nominal size rating as those used in the
current 600-gph ROWPU (5 um)v but they had less surface area. Six instead of eight
polypropylene Fllterite elements were housed in one pressure vessel.
The ranges in the temperature, pH, turbidity, TDS concentration, and alkalinity of
the Monocacy River water are shown in Table 5. The water had a total organic carbon
(TOC) content of approximately 5.1 mg/L, and an immediate chlorine demand of about
1 .4 mg/L. Cultures of Bacillus Rlobigll spores and poliovirus I, LSc strain were well mixed
with the river water prior to adding the coagulant. The feed-water concentrations ranged
from 220 to 15,000 colony- forming units (cfu)/mL for B. globlgil, 14 to 8500 cfu/mL for jn
situ Escherichia coll, 440 to 1720 plaque-forming units (pfu)/mL for poliovirus, .354 to
24,290 cfu/mL for total ]n sjtu bacterial counts, and 204 to 11,190 cfu/mL for total ]n
situ enterics.
53
Volume 8
The average removals of turbidity are shown in Table 6 for various run times up to
9 h. As might be anticipated, most of the turbidity removal took place in the multimedia
deep-bed filter, and the removals increased with filter ripening. Table 7 lists the
observed removal of the microorganisms averaged over all run times, and Table 8 shows
the average removal of the total aerobic bacteria (determined by standard plate count)
for the three water flows investigated. Again the results support the contention that
most bacteria and viruses are removed In the multimedia filter and almost none in the
cartridge filters. The data shown in Table 8 are erratic, making it difficult to judge the
effect of feed-water flow on the removal of total aerobic bacteria, except possibly to say
that removal may have decreased at the highest feed-water flow tested (40 gal/mln). The
Investigators concluded that a 40-gal/min feed-water flow probably should be avoided, and
that operations at 35 gal/mln may be acceptable.
Small et al,1 also concluded that the removals of the microorganisms shown In
Tables 7 and 8 are not meaningful in terms of providing an effective barrier against the
transmission of infectious diseases. The major line of defense against pathogens would
have to be disinfection if the RO section of the ROWPU were to be bypassed. They also
pointed out that the results of this study cannot be used to predict the removal of
amoebic cysts. They did suspect that the cysts would be better removed than the bacteria
due to their larger size.
48
3. B. Duncan conducted tests on the removal of B. globlgil spores, E. coll,
Saccharomyces cervislae (a yeast) as a cyst simulant for Entamoeba histolytica. He made
total plate counts and total enteric bacteria (as defined by Small et al. ) with two 5.25-in.
i.d. filter columns packed with the multimedia used in the 600-gph ROWPU (see Fig. 8).
Soil from Frederick County, MD, was collected, ground, and sifted through 175-mesh
screens. The fines were then mixed with Fort Detrick, MD, tap water to yield feed-water
turbidity levels of 50 and 120 NTU. A particle-slze-dlstrlbution analysis Indicated that
most of the suspended particles had sizes of < 50 pm. The suspension pH was then adjusted
to 5.3, 7.3, and 9.3 with carbon dioxide or sodium carbonate, and then the water was
pumped through the filters at a filtration rate of 6.3 gal/(mln • ft^). The two filter
columns were arranged in parallel and the run lasted for 4 h. A filtration rate of
6.3 gal/(mln • ft ) is equivalent to a total ROWPU feed-water flow of 31 gal/min.
The turbid feed water was seeded first with the microorganisms and then a Cat-Floe
polyelectrolyte in a pipeline leading to the filters. The polyelectrolyte dosage was not
stated In the preliminary report. The flow was then divided and delivered equally to the
54
Volume 8
Table 5. Raw-water characteristics of Monocacy River.3
Run
Date time (h)
Turbidity
(NTU)
pH
TDS
(mg/L)
Alkalinity
(mg/L as CaCOj)
Temperature
(°C)
Plant flow
(gal/mln)
9/17
0
3.5
7.9
278
120.4
19.4 to 22.3
30
3
3.5
8.0
275
116.8
6
3.7
8.4
270
116.8
9
4.0
8.4
258
116.8
9/18
0
6.0
7.9
262
113.1
20.6 to 22.8
35
3
5.9
8.2
275
116.8
6
5.5
8.4
258
113.1
9
5.6
8.4
322
113.1
9/23
0
4.7
7.8
288
170.4
22.2 to 24.0
40
3
3.9
7.8
267
120.4
6
3.8
8.4
262
113.1
9
4.0
8.4
260
116.8
10/9
0
3.5
7.9
274
153.9
14.4 to 17.7
40
3
4.2
8.1
286
173.8
6
3.8
8.4
267
158.9
9
3.8
8.5
260
158.9
10/10
0
2.4
8.1
282
158.9
14.2 to 14.8
30
3
2.3
7.9
267
148.9
6
2.2
7.8
270
151.4
9
2.4
7.8
294
153.9
10/15
0
4.3
7.8
289
148.9
11.2 to 13.8
35
3
4.3
7.8
300
158.9
6
3.1
8.0
278
158.9
9
3.3
8.1
272
156.4
10/22
0
3.0
7.7
282
171.3
12.7 to 14.8
35
3 '
3.1
7.7
305
166.3
3.0
7.7
285
161.4
9
3.1
8.0
286
158.9
10/23
0
3.2
7.7
309
156.4
10.6 to 12.5
40
3
2.9
7.5
293
158.9
6
2.9
8.0
304
163.8
9
3.2
7.8
285
158.9
10/29
0
7,1
7.6
398
213.5
10.0 to 12.4
30
3
6.7
7.9
424
213.5
6
5.1
8.0
445
228.4
9
5.0
8.0
452'
248.2
““
a Adapted from Small etal.^
55
Volume 8
Table 6. Average removal of turbidity observed for a filtration system similar to that in
the 600-gph ROWPU pretreatment section.®
Elapsed time (h)
Turbidity removal (*)
Across multimedia filter
Across system
0
67.3
74.4
3
81.4
84.0
6
83.5
86.4
9
85.9
88.1
a Adapted from Small et al.*
Table 7. Average removal of microorganisms observed for a filtration system similar to
that in the 600-gph ROWPU pretreatment section.®
Organism
Microorganism removal (%)
Across
multimedia
filter
Across
cartridge
filters
Overall
B. Klobiftii spores
98
1
98
E. coll _
91
3
94
Poliovirus I
72
8
80
Total aerobic bacteria15
74
9
83
Total enteric bacteria*5
84
3
87
a Adapted from Small et al?
13 As defined by Small et al.1
56
Volume 8
Table 8. Average removal of total aerobic bacteria observed for a filtration system
similar to that used in the 600-gph ROWPU pretreatment section for various feed-water
flows.3
Total aerobic bacteria removal
Feed-water flow (gal/min)
Across
multimedia
filter
Across
cartridge
filter
Across
system
30
86
3
90
35
45
48
93
40
78
-45
33
a Adapted from Small etal.^
b Average of three runs per feed-water flow; across-system value would be sum of
across-multi media and across-cartridge filter values, only If perfect sampling and
recovery occurred.
two test-filter columns. Chlorine was added to the feed water entering one of the
columns while the other column was used as a control. The concentrations of the
microorganisms seeded to the feed water were determined after adding the
polyelectrolyte. Only the results of the control studies are discussed herein. (Much more
information is required to evaluate the disinfection studies than that which was presented
in the preliminary report.!
The initial concentrations in the filter influent of the organisms tested were
commonly in the range o* thousands of cfu/mL except for E. coil, which had arithmetic
mean initial counts of 68,000 cfu/100 mL at a pH of 5.5, none detectable at a pH of 7.5,
and 1200 cfu/100 mL at a pH of 9.5. The observed removals of the microorganisms are
summarized in Table 9. (No information on turbidity removal was given in the
preliminary report.; In all cases, removal was inconsequential, as far as providing an
effective secondary barrier against the transmission of waterborne diseases. Disinfection
(chlorination in this case) is the primary barrier. However, the coagulant dosage may
have been less than optimal for the large concentrations of suspended solids contained in
the feed water.
57
Volume 8
Follow-up tests were run by Duncan using river water obtained from the
Monocacy River, MD. The raw water had a turbidity of 19 NTU and a pH of 8.3.
6
Approximately 10 cfu/ml of B. globigli spores were added to the raw water. The
multimedia filter removed 99.86% of the spores on the average, and the cartridge filter
removed 70.79% of the remaining spores. The river-water pH was then decreased to 7.0
with carbon dioxide and the test was repeated. The multimedia filter removed nearly all
of the B. globigli spores (initial concentration = 10^ cfu/mL). Unfortunately, the
information made available to our investigators does not state whether the water was
prechlorinated or not. In view of the results shown in Table 9 for unchlorinated water,
one has to suspect that chlorine may have been used in these follow-up tests.
PERFORMANCE OF CARTRIDGE FILTERS
The current 600-gph ROWPU pretreatment system Includes eight Filterite
polypropylene cartridge filters, as shown in Fig. 10. These filters have a nominal size
rating of 5 urn. Most bacteria and all viruses are smaller than this size rating and are also
smaller than the 3*pm absolute rating now being considered as a replacement for the 5-pm
cartridge filters. An important contribution to pathogen removal to be made by the
cartridge filter could be in the area of cyst removal. Glardia lamblla cysts have a
minimum dimension of 7 pm, and E. histolytica cysts are commonly larger (6 to 18 pm).
It appears that very little research relevant to the purposes of our study has been
performed with cartridge filters. Long*7 evaluated 17 cartridge filters having various
size ratings for removal of a microsphere cyst model (5.7 pm in diameter). The
microspheres were filtered from a solution containing 40,000 to 65,000 microspheres/mL,
diluted with tap water. No further information was included in the article regarding pH,
temperature, etc. More than 99.99% of the microspheres was removed in 10 of the
17 filters tested. It is interesting to note, however, that filters with size ratings of 1.0,
2.0, and 5.0 pm did not satisfy the 99.99% removal criteria, whereas some filters with
ratings of 2.0, 3.0 and 10.0 pm did. In addition, two filters rated at the same size, 5 pm,
had widely varying particle removals (99.96 vs 21.4%).
It was not possible to determine from the article whether the filter used in the
600-gph ROWPU was tested. In view of the large discrepancies in the results, however, it
is clear that the performance of cartridge filters may be highly variable.
58
Volume 8
Table 9. Average removal of microorganisms from a turbid water for three pH values by
a multimedia filter similar to that used in the 600-gph ROWPU.a (Filtration rate = 6.3
gal/(min • ft^) and feed-water turbidities of 50 and 120 NTU.)
Microorganism removal b (%)
Organism
pH * 5.5
pH = 7.5
pH = 9.5
B. Rlobigii spores
81
83
90
E. coll
63
NDC
75
S< cerevisiae (6- to 18-ym elliptic yeast ceils)
84
NDC
94
Total aerobic bacteria*1
80
70
87
Total enteric bacteria*1
82
42
63
a Adapted from Duncan^
15 Arithmetic averages.
c None detected In the water supplied to the multimedia filters.
d As defined by Small et al.1
Carbon-impregnated cartridge-filter elements were tested during the early stages of
the development and design of the 600-gph ROWPU.^ Cartridge-filter elements ranging
from 3 to 20 yn, in size rating were tested. It was found that the filter elements with the
smaller size ratings gave better -quality effluents, but a trade-off had to be made between
filter life and effluent quality. The data showed that a filter element with a size rating
of 5ym produced a water with a turbidity of <0.5 FTU, and had an average useful life of
72 h. The useful life is apparently designated as the time required to yield a 20 psid1* or
25 psid pressure drop across a filter element.
Radoski50 investigated the removal of diesel fuel from tap water with carbon-
containing cartridge filters. It was claimed that the filtered water was free of any
objectionable taste or odor at a water temperature of 14 to 19°C when the feed water
contained 1 mg/L of the diesel fuel.
PERFORMANCE OF PRECOAT FILTERS OPERATED IN THE DIRECT-FILTRATION
MODE
In i very early literature review, Cummins^ 1 states that size distribution and
particle shape of the filter-aid media are very important factors in dlatomaceous-earth
filtration.
59
Volume 8
The performance of a diatomaceous-earth filter aid can be improved by coating it
with an electropositive coating medium. The filter-aid coating is attained either by
mixing the diatomite and coating material prior to application of the filter aid or by
filtering water with the coagulant in it through the filter aid after it has been applied. A
1936 patent by Cummins states that a filter aid coated with alum is more effective than
an uncoated aid in removing suspended solids from water. Diatomaceous earth coated
with alum is less permeable to water than uncoated diatomaceous earth of the same
grade, but the coarser grades coated with alum are more permeable than the uncoated
finer grades, although each may possess similar capacities for the removal of turbidity,
coliform organisms, and color. It was concluded that the use of alum coated filter aid was
52
advantageous due to the reduction in pressure drop.
The Department of Water and Power of Los Angeles, CA, carried out a study on
diatomaceous-earth filtration for the city's water supply in 1931. 33 It was found that
slimy materials, large concentrations of algae, and close-textured diatoms shortened the
filter runs. Turbidity resulting from more than 20 ppm of suspended silt particles, as veil
as any fine floe reaching the filter, were "troublesome." It was also found that
dissolved and entrained gases had no appreciable effect on the filter efficiency, provided
that sufficient backpressure was maintained to prevent the release or entrainment of
gases in the filter.
The openings in the septa used in the Los Angeles study varied from 25 to 250 ym.
The septum with the largest openings required a longer time to precoat, but the
precoating time was fairly constant for septa with openings finer than 160 ym. The
septum with the smallest openings had the largest initial pressure drop, tended to clog
rapidly, and was difficult to clean. The clogging in the precoated filter elements was
caused by the finer filter-aid particles contained in the precoat, and was more noticeable
with the thicker septa. It was concluded that the largest opening capable of retaining the
filter aid should be used to minimize pressure drop and clogging.
Microorganism removal in the tests agreed with the previously observed 0 to 96%
removal for bacteria derived from treatment of swimming-pool water, depending on how
long Into the filter run the sample was taken. The passage of bacteria through the filter
increased rapidly as the size of the filter aid increased. For the most economical
operating conditions employing the coarse filter aids and thin precoats, there was a
tendency for bacterial breakthrough, and this was especially evident at pressure drops
across the filter elements > 10 psi.
Volume 8
52
Oulman et al. conducted a study to determine and evaluate the effects of adding
cationic polyelectrolytes to the feed water continuously or during the precoating
operation of the diatomaceous-earth filters. The parameters investigated included
polymer dose, filter-cake permeability, pH, and removal of turbidity, bacteria, and color.
The effects of coating the filter aid with the polyelectrolytes were explained by the fact
that the coating increases the contact area between the suspended particles and filter aid,
and decreases the probability of desorption of an adsorbed particle. It was demonstrated
that filter aid coated with polyelectrolytes provides a more complete removal of
turbidity, bacteria, and color colloids with a lower pressure drop than the same filter aid
without a polyelectrolyte coat.
A 1965 American Water Works Association (AWWA) Task Group Report*^ stated
that many of the municipal diatomaceous-earth filtration systems functioning at the time
were inadequate for the following reasons: (1) the equipment for effective precoating,
filtering, and backwashing was inadequate, (2) the hydraulic loading rate of 6 to
8 gal/(mln • ft ) commonly used in practice was too high for continuous operation at
fixed installations, and (3) the equipment was unable to supply filter aid adequately as
body feed during the run.
The AWWA report also stated that most of the municipal experience with turbidity
removal had been restricted to waters low in turbidity and of good bacteriological quality
that required little pretreatment except prechlorination. There was no agreement as to
the maximum level of turbidity that could be removed without difficulty, but it was
agreed that direct diatomaceous-earth filtration could not be recommended for a grossly
or even moderately polluted supply. Appropriate pretreatment (conventional or chemical
pretreatment to increase filterability) could widen the application areas for
diatomaceous-earth filters. Finally, the AWWA Task Group recommended that the
filtration rate be held to 1 gai/(min • ft ) for public water supplies unless available data
justify an increase in the rate.
Syrotynski'*^ reviewed the performance of diatomaceous-earth filtration plants in
the State of New York. No coagulants were reported to be added to the filter-feed
waters. The results of their survey indicated that the total counts of bacteria in the
filtered water did not correlate with either initial count or the filtered-water turbidity.
In a subsequent report, Syrotynski and Stone^ reported that the total microscopic count
is decreased about 60 to 90% with direct diatomaceous-earth filtration with no
coagulation. Body feed was utilized and the filtration rate was held constant at
1.0 gal/(min • ft^).
61
Volume 8
56
Hunter et al. investigated the removal of total coliform bacteria with direct
dlatomaceous-earth filtration and no in-line coagulation, with and without coated filter
aids. The removal of the coliform bacteria increased from 90 to 99.86% as finer grades of
diatomaceous earth were tested. The initial feed-water count of the coliform bacteria
exceeded 19,000 organisms/ 100 mL for these runs. Various levels of body feed were
utilized with a precoat of 0.1 lb/ft2.
Those investigators concluded thatt (1) the greater the permeability of the filter
aid, the greater the number of coliform organisms that are able to penetrate it}
(2) "complete" coliform removal is possible using very fine filter aid for influent coliform
concentrations of several thousand organisms per 100 mL} (3) beyond that level, higher
influent coliform concentrations give higher effluent concentrations} (4) an increase from
1 to 2 gal/(min • ft2) in the filtration rate did not change the removal of the coliform
bacteria significantly} (5) increasing the body-feed filter-aid dosages from 7.3 to 60 ppm
did not change the removal of the bacteria significantly} and (6) coated filter aids
improved the removal of the bacteria, but the improvement was not sufficient to merit
the use of such aids.
Burns et al. coated three different grades of diatomlte filter aid (Hyflo Supercel,
Celite 543, and Ceiite 560 manufactured by 3ohns-Manvllle Products Corp., Manville, N3)
with a cationic polyelectrolyte (Purlfloc 601, manufactured by Dow Chemical Co.,
Midland, MI). The coating operation consisted of mixing the diatomlte for about 3 min in
57
a "dilute solution" of the polyelectrolyte and pumping the mixture through the filter.
The filter cake was then washed with distilled water to remove excess polyelectrolyte. A
1.5-in. diameter flat, horizontal septum (100-mesh stainless-steel wire screen) was used in
those experiments. The following three suspensions were fllteredi (1) a 1-g/L calcium
bentonite clay with particle sizes of 0.8 to 1.0 pm, (2) a 4-g/L Black Hills clay with
particle sizes of 0.45 to 0.8 pm, and (3) 30-mg/L Folger's Instant Coffee filtered through a
0.45-pm Millipore filter. The coffee solution was intended to serve as a source of color
colloids.
The uncoated diatomaceous-earth filter aids did not remove any of the clayey
suspended solids. The removal with coated filter aids Increased with the weight of
polyelectrolyte added per unit weight of the filter aid. Two different mechanisms were
postulated to explain the results of the study. First, the polyelectrolyte coating on the
dlatomaceous-earth filter aid ensures nearly 100% removal of the suspended clay particles
until its charge capacity is exhausted; and second, any subsequent removal is^chieved by
straining only. The effectiveness of the polyelectrolyte coating deteriorated for pH
values in excess of about 6 to 7.
62
Volume 8
The coated filter aid did not fully eliminate the color produced by the coffee
solution under any circumstances. It was concluded that the color was associated at least
in part with either nonreactive colloids or dissolved substances. The suspended-solids
removal was around 80% with the fully coated filter aids.
58
A study carried out at the University of Texas in 1974 reports on the removal of
an E. coll T2 bacteriophage by direct diatomaceous-earth filtration. The system used did
not include in-line flocculation, and the filter aids were coated with a cationic
polyelectrolyte. Filter aids were Hyflo, Hyflo B, Hyflo D, and Cellte 560 (manufactured
by Johns-Manville Products Corp.). Coatings were Purlfloc C-31 (manufactured by Dow
Chemical Co.), aluminum and ferric salt hydrates. The experiments were performed with
a 0.15-lb/ft2 precoat, 50-mg/L body feed, and a 1 gal/(min • ft2) filtration rate. The
bacteriophage was added to dechlorinated tap water in amounts sufficient to give a feed
water concentration of 1700 pfu/L.
From a series of 2-h runs two filter-aid configurations were selected for additional
study: (1) Hyflo filter aid with 0.07 mg/L of polyelectrolyte, and (2) Hyflo D alone (no
polyelectrolyte). Two 12-h runs were made. No observable breakthrough occurred after
12 h with the pretreated Hyflo, though breakthrough occurred after 2 h with Hyflo D
alone. It was concluded that a good removal of viruses (>99%) could be achieved only
with a polyelectrolyte coagulant plus an adequate precoat with certain of the tested aids.
Testing was also done to affirm that the polyelectrolyte had no viricidal effect on T2
phage after 4 h at a polyelectrolyte concentration of 0.081 mg/L.
A second stage of testing59 at the University of Texas was conducted with
procedures identical to those described previously for the T2 phage study. In this case
poliovirus, Mahoney Type I was added to tap water; some additional test runs with T2
phage are reported as well. A series of 2-h runs was made. With coated filter aids and
polyelectrolyte pretreatment >98 % removal of virus was obtained. The uncoated filter
aid, however, gave only 62% removal of poliovirus after 2 h, as opposed to 90% removal
of phage T2 after the same period of time.
Two 12-h runs were made with Hyflo filter aid and polyelectrolyte. After 12 h of
continuous operation, there was no significant breakthrough of poliovirus.
Two 4-h runs were made with two filter aids to Investigate the breakthrough
phenomenon. One of the filter aids was coated, the other one uncoated. After 4 h,
removal of poliovirus had dropped to 90% for the uncoated filter aid and to 98% for the
coated filter aid.
63
Volume 8
Finally, a series of 2-h runs using T2 phage with three kinds of filter aid at flow
rates of 1 and 2 gal/(min * ft ) and pH values of 6.7 and 9.5 were performed. From the
results obtained, It was concluded that fliter-ald grade and flow rate did not affect T2
phage removal. However, pH did affect removal, with >99.0% at pH 9.5 and 92.3% at pH
6.7 for one filter aid.
It was concluded that >98 % removal of phage T2 and poliovirus can be obtained
when a coagulant is added to the filter feed water. Filtration rate, body-feed rate, and
fiiter-aid grade did not appear to affect the removal of the viruses significantly.
Surprisingly, it was found that "the same virus removals were obtained either with or
without body feed}" hence the body-feed rate was set to maintain cake porosity.
Amlrhor and Engelbrecht60 investigated the removal of bacteriophage MS 2 using
direct dlatomaceous-earth filtration with Celite 560 (manufactured by Oohns-Manville
Products Corp.) without body feed. A 0.5-in. thick precoat of diatomaceous earth was
applied to the filter, and filtration took place at a filtration rate of 1 gal/(min • ft2).
The filter aid was coated with a cationic polymer, Purlfloc C-31 (manufactured by Dow
Chemical Co.), at quantities equivalent to 0.2 and 0.4 mg of polyelectrolyte per gram of
diatomaceous-earth filter aid. The virus was seeded to a dlstllled-deionized water
buffered to pH levels of 6, 7, and 8.
Virtually no virus was removed by the uncoated diatomaceous-earth filter aid as
shown in Fig. 18. The coated filter aid gave about 90% removal or better for a run time
of 7 h and a pH of 6.0. The results of other tests not given herein demonstrated that the
removal of the virus decreased with increasing pH, and breakthrough occurred when the
absorptive capacity of the polyelectrolyte was exhausted. A number of experiments were
also conducted with an uncoated filter aid and the poiyelectrolyte added to the filter feed
water. The results demonstrated that the dose of a coagulant must be tailored to the
character of the feed water in direct filtration. The lowest dose of poiyelectrolyte used
in this study (0.25 mg/L) gave the best removal of the virus for the operating conditions
used.
Recent studies by Lange et al.^ demonstrated a decrease in the removal of
total-collform bacteria with an increase in the filtration rate The authors performed the
studies on low-turbidity water (4.5 to 5.4 NTU) at filtration rates of 1, 2, and
4 gal/(min • ft ). Unlike the results reported by Hunter et al., the removal of both
total coliform bacteria and standard plate count bacteria increased significantly when the
diatomaceous earth was coated with alpm. For example,, the range in coliform bacteria
removal increased from about 30 to 70 % without alum coating to about 96 to 99.9% with
coating. The authors also concluded that the most Important factor in removal of
64
Volume 8
0 1 2 3 4 6 6 7 8 9 10 11 12 13 14 16
Filtration time (h)
Figure 18. Removal of MS 2 bacteriophage by uncoated and polyelectrolyte (PE) coated
diatomaceous-earth (DE) filter aid. [pH a 6.0{ virus concentration a 4 x 108 pfu/mLj
filtration rate = 1 (gal/min • ft2)]. Adapted from Amlrhor and Engelbrecht.60
bacteria is the grade of the diatomaceous earth. A coarse grade of diatomite gave a 28%
removal of collform bacteria and a 38% removal of standard plate count bacteria,
whereas a fine grade of diatomite gave a 99.9% removal of the conforms and a 99.8%
removal of the standard plate-count bacteria.
The cysts of E. histolytica and G. lamblia are similar in size and surface electrical
charge. They are dissimilar in that the pathogenicity of E. histolytica has long been
known, whereas only recently has the pathogenicity of G. lamblia been recognized. Since
the mld-70's, research has been conducted in removal and inactivation techniques for G.
lamblia. Both G. lamblia and l:. histolytica are protozoans that are pathogenic to
humans. They are similar in size, the cysts of G_. lamblia being ovoid in shape, 14 to
16 pm long and 6 to 12 pm wide. E. histolytica has a spherical cyst form 10 to 15 pm in
diameter. They are also similar in symptoms of disease, i.e., both cause diarrhea. Of
65
Volume 8
concern to the mobile field Army, the debilitating effects of this condition could render
military personnel incapable of action. Because water supply has been incriminated for
carriage of both protozoan cysts, the Army is concerned about removal of cysts from Its
water supply.
62
According to an early paper by Kominek, diatom aceous-earth filters are effective
in the removal of chlorine-resistant organisms, such as cysts, as well as suspended solids.
Kominek hypothesized that diatomaceous earth forms a porous cake with exceedingly fine
openings, thereby providing excellent filtration without the refinement in pretreatment
technique required for rapid sand filtration.
Leading investigators of diatomaceous-earth filtration for drinking-water treatment
were with the U.S. Army during World War I!.63 The data contained in an Army report6^
of Duly 1944 resulted from the combined efforts of the Office of the Surgeon General
(OTSG) and the National Institutes of Health. The experiments are described In the Army
33
publication and were summarized recently by Logsdon et al. The Intent of those studies
was to determine the removal of the cysts of E. histolytica with diatomaceous-earth
filtration. Several diatomaceous-earth filtering systems were tested, from small batch
systems (4-L volume) to large continuous-flow systems (73 L/mln). The precoating water
was contaminated in the only run out of a total of 13 where more than one cyst was
recovered from the filtered water. It was concluded by both the Army and Logsdon that
virtually all cysts of E. histolytica can be removed with direct diatomaceous-earth
filtration.
£3
Dones and Brady studied the removal of Schistosoma mansonl cercariae by
" 2
filtration through diatomaceous earth In a small model filter. A precoat of 0.15 lb/ft
was used without body feed, and either tap or raw river water was used In the
experimental runs. The removal of the cercariae was investigated in nine runs. Varying
filtration rates [0.3 to 11.1 gal/(min • ft2)] and applied pressures (0 to 17 psi) were used,
as well as three types of diatomaceous-earth filter aids (Sorbo-Cel 503, Speedplus, and
4200+kaolln). Referring to the filtered water quality, the authors state "...there were no
cercariae recovered in any of the samples examined."^ ^
Baumann and Babbitt^ also examined the removal of the cysts of E. histolytica
with porous filter septa both with and without filter aid. They were looking for a septum
that would remove cysts without filter aid, on the hypothesis that application of the filter
aid is the most likely procedure for error. One of the more interesting conclusions made
in that report is that "a septum that will effect a removal of roughly 75 to-£0% of the
applied bacteria will remove also 100% of the applied cysts. They also concluded that
septa with a mean pore size of 18.6 pm or less will remove all of the cysts.
66
Volume 8
r y /o
Logsdon and several associates0' and DeWalle et al. found diatomaceous-earth
filtration to be effective in removing G. latnblia cysts, G. muris cysts, and 9-ym
radioactive beads that served as models of G. lamblla.
The Drinking Water Research Division of the Environmental Protection Agency
(EPA) conducted a study in 1981 on the removal of Giardia cysts and cyst models from
drinking-water supplies by alternative filtration methods. 3 Radioactive microspheres
9 ym in diameter were used as Giardia cyst models in the first phase of the diatomaceous-
earth filtration study. Those spheres are very similar to Giardia cysts in size and are
/ n
easily traced. Logsdon et al. pointed out that data procured from the microsphere
studies gave conservative results because in some of their corroborative runs better
removals were obtained with G. lamblla than with the microspheres. G. muris cysts,
which are similar in size, shape, and surface electrical charge to G. lamblla cysts, were
also used later on In the diatomaceous-earth filtration studies because it proved difficult
to obtain viable G. lamblla cysts on a continuous basis.
Low-turbidity water was obtained from a local gravel pit on the premise that it
closely resembled the mountain streams through which giardiasis is commonly spread.
The diatomaceous-earth test filter had an area of 0.1 m* (1 ft^) and was operated as a
pressure filter with a slurry feeder for body feed. The hydraulic loading rates
investigated were 2.4 and 3.5 m/h [1.0 and 1.4 gal/(min • ft2)], and the
body-feed-to-turbidity (mg/L to NTU) ratios ranged from 2»1 to 35j1. The entire effluent
from the test filter was filtered through 5-um membrane sampling filters to determine the
removals of the cysts and cyst simulants.
The microspheres were dosed on a continuous basis, and the filter was operated for
periods ranging from 2 to 27 h, with a typical run lasting from 6 to 8 h. The G. muris
cysts were dosed in one slug of 20 x 106 to 40 x 106 cysts, just ahead of the
filter-pressure-containment vessel. Filtration was continued until at least 99% of the
cysts could be collected on or pass through the filter.33 After the run the membrane
filter was washed and the retained cysts were concentrated and counted.
The results of the radioactive microsphere tests indicated that in most cases more
than 99.9% of the microspheres were removed by the diatomaceous-earth filter. The
removal of the cysts appeared to be unrelated to the reduction in turbidity.33 The
passage of turbidity at a time in the run when cysts do not pass is attributed to the
difference in the sizes of the several kinds of particles. Cysts have a smallest dimension
of about 7 ymj clays and bacteria can be as small as 1 ym, and thus would be capable of
passing through the filter-cake pores, while the cysts would be strained out.
Volume 8
The microsphere-removal efficiencies were found to be related to certain operating
conditions. The removals increased with the application of the precoat up to 1.0 kg/m2.
Additional precoat did not improve performance. The removal consistently equaled or
exceeded 99.9% with a precoat of 1.0 kg/m2 and body feed. Finally, the test results also
indicated that the removal capability of the diatomaceous-earth filter usually improved
during a filter run. This was caused by the increase in cake depth resulting from the
collection of the body feed and suspended solids.^
The G. murls cyst studies gave good removal at both the 2.4 and 3.5 m/h [1.0 and
1.4 gal/(min • ft2)] filtration rates. Removal was greater than 99.8% with the exception
of one run at 99.36%. That particular run was also distinguished by an above-normal
amount of dlatomite in the filter effluent. The results of this test series also confirmed
the finding given previously that cyst removal does not correlate with turbidity removal.
It was concluded that diatomaceous earth filtration with body feed and a precoat of
1.0 kg/m2 removes Glardia cysts effectively and economically as long as the filters are
operated and maintained properly, and as long as the filtration rate does not exceed that
used in the study [3.5 m/h or 1.4 gai/(mln • ft2)].
DcWallc ct al. added Glardia cysts to unfiltered Seattle tap water with a turbidity
in the range of 0.6 to 0.9 NTU. The seeded water was not coagulated or disinfected. The
ranges in precoat and body feed investigated were 0.5 to 1.2 kg/m2 and 10 to 40 mg/L,
respectively. The Glardia cysts were added as a slug to the filter Influent (3.0 x 10^ cysts
in 10 s). Removal ranged from 99.03 to >99.87% In 12 separate determinations. The best
removal was obtained with the following operating conditions: a precoat of 1.0 kg/m2, a
body feed of 20 mg/L, an hydraulic loading rate of 3.8 L/min [0.09 gal/(mln * ft2)] and a
water pH of 6.7. The removal of the cysts increased as the bed matured during the course
of a filter run.
Lange et al.^ reported Glardia cyst removals of 99.9% for seven grades of
diatomaceous-earth filter aids subjected to filtration rates of 2.44 to 9.76 m/h [1 to
2
4 gal/(mln • ft )] in the water-temperature range of 5 to 19°C. Low-turbidity waters,
approximately 5 NTU, were used in the testing.
PERFORMANCE OF THE MWPU (ERDLATOR)
69
Black and Spaulding conducted research on the then new Army diatomaceous-
earth filtration technique. Half of their experimental runs were made with just filtration,
and the other half included coagulation with alum and sedimentation prior to filtration.
The concentrations of E. coli were determined with the multiple-tube dilution technique.
68
Volume 8
The untreated water when filtered "seldom produced negative tests for E_. coli in all five
tubes even when filtered turbidity was at a minimum." Better results were obtained with
the coagulated and settled water. Measurable turbidity (although fairly low) was found in
most filtrates obtained from the untreated water.
Special field studies were conducted in the 1950's to assess the performance of the
28
MWPU under closely controlled conditions. Table 10 shows results of one series of
biological testing on the prototype MWPU. The coagulation process alone provided good
reductions of bacteria. Of the six runs made without the addition of chlorine, fecal
coliform bacteria were detected in the filtered water of only one of the runs.
Three nonpathogenic organisms (Serratia marcescens, Bacillus subtills var. niger,
and T3 coliphage) were used in another series of MWPU tests. "Very substantial"
quantities of S. marcescens and T3 coliphage were removed by the MWPU without
disinfection. For example, an average of 99 .6 & T3 coliphage was removed by the
22
ERDLATOR, followed by dlatomaceous-earth filtration.
Bacterial counts showed removals in the ERDLATOR (the clarifier) of 81 to 92% in
yet another series of tests when contaminated water with 490 to 33,000 bacteria/ 100 mL
26
was treated. The test water was Potomac River water and standard additions of FeCl^
and pulverized limestone were used without prechlorination. The turbidities ranged from
25 to 150 ppm in the raw water, and from less than l to 3 ppm in the filtered water.
26
Testing was performed by the National Institutes of Health on cysts of
E. histolytica at a concentration of 5500 cysts/gal. Cysts were recovered in the
ERDLATOR (the clarifier) effluent, ranging from 0 to 8 cysts/gal.
B. subtllus was used as the test organism to assess the MWPU's ability to remove
bacterial spores in cold water (35 to 36°F). Table 1 1 shows the results obtained without
chlorination. As the floe bed matured from the beginning of the run through the sixth
hour, the removal of spores by coagulation alone increased from 95.45 to 97.36%. The
performance of the filter improved substantially when 20 ppm of dlatomite body feed was
added to the filter influent after 3 h of operation. Spore removal increased from about
86% without body feed to an average of 98.86% with body feed. The overall decrease in
3 4
the bacterial spores was from about 10 - to 10 -fold for the entire unit during the time
when body feed was added to the filter.
Lindsten and Schmitt^ conducted a series of tests In 1967 on the performance of a
"standard 3000-gal/h, ERDLATOR-type... water-purification unit." The tests were
performed to allow comparison of expanded perlite with dlatomaceous earth for use in
Army filters.
69
Volume 8
Table 10. Observed removal of total bacteria and fecal coliform bacteria by the MWPU
without chlorination in 1952, a (Raw-water source: Potomac River Water, temperature =
56 to 88°F, turbidity a 28 to 1 10 ppm during the testing period.")
Date
Sample identity
Total bacteria count
(number/mL)
Fecal coliform bacteria
(MPN/100 mL)
23 May
feed water
1050
4900
ERDLATOR effluent
6
NDC
29 May
feed water
18
610
ERDLATOR effluent
NDC
NDC
20 June
feed water
29
5400
ERDLATOR effluent
NDC
NDC
filtered water
NDC
NDC
1 July
feed water
1300
7900
ERDLATOR effluent
2
13
filtered water
NDC
NDC
10 July
feed water
1030
4900
ERDLATOR effluent
5
49
filtered water
1
1.8
7 July
feed water
160
1100
ERDLATOR effluent
3
46
a Adapted from Ruiz and Schmitt.23
b No further Information Is available "...the Potomac River Is subject to wide variations
in quality, typical of surface sources draining densely populated areas."23 Also, the
reporting of turbidity in ppm is as shown in the original report.
c NDb below the detectability limit of the analytical method used.
The removal of turbidity and total-coliform bacteria was determined for several
grades of dlatomite and one grade of perlite. The tests were performed daily for a total
of 50 d, and the individual test runs lasted 6 to 8 h on each day. The filtration rate was
held constant at 25 gal/min per filter or 2.5 gal/(min • ft ^). The Potomac River water
used as the water source in the teuts did not have to be fortified with either turbidity or
coliform bacteria. The general tangos in two of the quality characteristics (turbidity and
total-coliform organisms) of the raw water are given in Table i 2.
The raw water was coagulated but not chlorinated in the ERDLATOR in.most of the
test runs, but a few tests were made without the addition of any conditioning chemical.
The average daily results are shown in Table 12 for the particular test conditions of
70
Volume 8
Tible 11. Observed concentration of the spores of ft. subtilus var. niger in the
ERDLATOR floe and effluent, and the fiitered-water effluent of the MWPU.a
(Raw-water source: pond water, temperature = 35 to 36°F, pH 7.2 to 7.6, filtration
rate - 2.5 gai/(min • ft^), FeClj dose = 49 ppm, limestone dose = 227 ppm, body
feed = 20 ppm, feed concentration of spores - 1.8 x lQ^/mL on a continuous basis, no
chlorination.)
Time (hours
of operation)
ERDLATOR effluent ERDLATOR floe Filtered water
(number of organisms/mL) (10^ organisms/nr.L) (number of organisms/mL)
1.5
—
2.75
2.0
—
4.6
__b
2.5
—
5.6
__b
3.0
6000
6.7
17
4.0
4600
7
170
5.0
3200
7
2
6.0
3700
8
11
Si 70
Adapted from Pruett and Lindsten.
k Body feed net added.
2
diatomite precoat (0.10 Ib/ft ) and body feed (40 to 43 ppm) only and coagulation without
chlorination. The results were obtained with three different grades of diatomite, but this
grade variation appeared to be insignificant. The dosages of FeCi^ and limestone listed in
Table 12 were obtained by dividing the mass rate of addition of the chemical to the
ERDLATOR by the mass flow of the water through the F.RDLATOR.
Troubles with the MWPU were encountered on test days 46 and 47. The residual
turbidity in the filtered water was considerable and the observed removal of the coliform
bacteria was relatively poor. The presence of algae was noted on one of those days.
Excluding days 46 and 47, the arithmetic mean removal of turbidity was about 72% in the
ERDLATOR, and greater than 99% in the filters. The overall reduction in turbidity was
about 500-fold when taken across the entire treatment train. The corresponding values
ior the total coliform bacteria were about 86% for the ERDLATOR, more than 99% for
the filters, and 300-fold for the entire system.
No conditioning chemicals were added to the ERDLATOR on days 51 through 53, and
the raw water was seeded with the cysts of Naegleria gruberi, a nonpatbogenic cyst
simulant for E. histolytica. The filtered water was not analyzed for N. gruberi cysts in
one of the three runs, but no cysts were recovered in the remaining two runs for which the
Volume 8
Table 12. Average daily removal of turbidity and total coliform bacteria from Fotomac
River water in 1967 with the standard 3000-gph MWPU.a (pH = 7.7 to 8.2, alkalinity = 43
to 84 mg/L as CaCC>3, temperature = 59 to 79°F, filtration rate = 2.5 gal/min • ft2, no
chlorination, diatomite precoat = 0.10 lb/ft2, and body feed = 40 to 43 ppm.)
Day
Doses of conditioning
chemicals
Turbidity (NTU)
Total coliforms
(number / mL )
FeCl 3
(ppm)
cacoy
(ppm)
Raw ERDLATOR
water effluent
Filter
effluent
Raw
water
""kRDLATOR
effluent
Filter
effluent
l
25
80
47
1 4
0.2
575
62
1
2
28
80
50
1 1
0.1
1150
69
1
3
28
80
46
12
0
1000
104
2
6
43
80
54
34
0.2
1 175
164
1
7
41
83
59
\7
0
1925
165
5
8
43
86
52
16
0
10700
1070
67
46
22
60
51
NRC
4
600
NRC
43
47 b
73
167
46
1 1
2
325
44
58
48
40
178
47
7
0
390
33
10
11
44
102
59
19
0.1
6525
370
l
12
32
102
47
12
0
5500
350
1
13
30
107
49
8
0
5625
227
2
16
23
112
44
13
0.2
2275
181
l
17
21
1 10
48
10
0.1
2 425
184
3
18
21
84
52
10
0.2
1900
177
4
21
34
80
46
12
0
1000
83
0
a 7 1
Adopted from Lindsten and Schmitt.
b Algae present.
c NR = not reported.
input concentrations were 3200 and 20 cysts per gallon. The Potomac River water had a
turbidity of 20 and 6 NTU on those two days. The temperature was not reported. The
2
diatomite precoat was 0.10 Ib/ft and the body feed was 39 to 40 ppm. It was concluded
that the cysts of N_. gruberi could be effectively removed from the water by
sedimentation, and subsequent filtration with body feed through the precoated filters at a
2
filtration rate of 2.5 gal/(min • ft ).
72
Volume 8
The MWPU was also tested under closely controlled conditions in isolated, restricted
military locations where full-scale "live-agent" tests were possible —that is, where the
actual chemical or nuclear warfare agent was used rather than a simulant. The results
are summarized briefly as follows for the nuclear and chemical warfare agents.
NUCLEAR WARFARE AGENTS
72
The results of tests using nuclear warfare agents were reported by Lindsten et ah .
♦ The ERDLATOR in combination with the coagulant ferric chloride and the
limestone water conditioner removed 80 to 86% of 2-month-oid fission products from
water.
• The small amount of radioactive floe particles, which was carried over from the
coagulation step, was removed by the diatomaceous-earth filters.
• Conventional treatment in the MWPU followed by post-treatment with mixed-bed
ion-exchange resin removed an a
…[truncated]