DUAL-CHEMICAL CONDITIONING FOR DEWATERING MUNICIPAL
WASTEWATER SLUDGES
A Thesis
Subrnitted to the Faculty of Graduate Studies and Research
In Partial Fulfillment of the Requirements
for the Degree of
Master of Applied Science
in Environmental Systems Engineering
University of Regina
by
Shivakumar Krishnamurthy
Regina, Saskatchewan
March, 2001
Copyright 2001: S. Krishnamurthy
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The Regina Wastewater Treatment P'lant presently uses Percol 757, a cationic
polymer, to condition its anerobically digezsted wastewater sludge. This study was
initiated to examine the feasibility of using f&rric chloride in conjunction with either of
two polyrners - Percol 755 or Percol 757 - to condition the sludge. Specific resistance to
filtration ( S E ) , capillary suction time (CST), and time-to-filter (TTF) were the three
parameters used to evduate conditioning pemforrnance and dewaterability. The roIe of
charge neutralization in the conditioning process was examined by zeta potential
measurements. Optimum dosages of femc chloride, Percol 755 and Percol 757 were
determined to be 80.6, 2.9 and 2.7 kg/T of- dry solids respectively. Optimum dosing
resulted in complete charge neutralization.. It was concluded that zeta potential
measurements could not be reliably used to predict dewaterability. Sludge conditioning
experiments done using different combinations of optimal dosages of femc chIoride and
polymers indicated that al1 combinâtions of femc chloride and polymer conditioning
resulted in better dewaterability than conditioming with femc chloride or polymer alone.
Dual-chemical conditioning was cheaper than femc chloride conditioning, but more
expensive than polymer conditioning. It was shown that dual-chernical conditioning using
a combination of 50% of optimum dosage of Wrric chloride and 50% of optimum dosage
of Percol 757 had the lowest CST, SRF and TTF values. It was noticed that a percentage
of the optimum dosage of polymer could b e replaced by a similar percentage of the
optimum dosage of ferric chloride without adwersely affecting the dewatering efficiency.
Both CST and TTF were found to have excelle-nt correlation with SRF data.
1 express my greatest esteem and gratitude to my supervisor Dr. T. Viraraghavan,
Professor, Faculty of Engineering, University of Regina, for his guidance throughout my
graduate study. 1 am thankful to +he cornmittee members for their valuable time and
constructive criticisms of my work. 1 am gratefd to Dr. S. Sankaran, Faculty of
Administration, University of Regina, for his encouragement and support.
1 thank the staff of the Regina Wastewater Treatment Plant in general, and Mr. A.
Fries (Biochemist) and Mr. Mike Wild (Supervisor), in particular, for their assistance
during my visits to the Plant for sampling purposes.
This study was partially supported by a research g a n t to my supervisor, Dr. T.
Viraraghavan, by Natural Sciences and Engineering Research Council of Canada. I thank
the Faculty of Graduate Studies and Research, University of Regina, for financial support
in the form of a Graduate Teaching Assistantship during Winter '99. 1 also thank the
Faculty of Engineering, University of Regina, for financial support in the form of
teaching assistantships during the course of this program.
1 am extremely thankful to Mr. Ashref Darbi, doctoral candidate, for his assistance
with the experiments. 1 thank fellow graduate students - Subbu, Kripa, Anoop, Yan, Fu,
Bayo, Tirupathy, Arasu and Hadi - for ail their help.
1 dedicate this work to my parents, my sister and my brother-in-law. Their love,
support and constant encouragement was largely instrumental in the successful
completion of my graduate study.
TABLE OF CONTENTS
. . ABSTRACT ....................................................................................................................... 11
... ACKNOWLEDGEMENTS ............................................................................................ 111
. . * LIST OF TABLES ................... ... ............... ................... .................. vil1
LIST OF FIGURES ................... .................................................... ................. ix
NOTATION ........... .......................... ........................................................................ x
1.0 INTRODUCTION ................ .... .......................................................................... 1
...................................................................................................................... 1.1 General 1
......................................... ......................... 1.2 Regina wastewater treatrnent plant ... 1
.............................................................................................. 1 -3 Objectives of the study 4
1.4 Scope of the study ..................................................................................................... 5
........................... 2.0 LITERATURE REVIEW ............................... .......................... 6
2.1 Conditioning ............................................................................................................ 6
2.1 . 1 Factors affecting conditioning ......................................................................... 7
...................................................................... 2.1.2 Inorganic chernical conditioning 10
........................................................................ 2.1.3 Organic chernical conditioning 11
............................................................................. 2.1.4 Dual-chernical conditioning 12
.............................................................................................................. 2.2 Dewatering 13
............................................................................ 2.2.1 Factors affecting dewatering 14
...................................................................................... 2.3 Dewaterability parameters 15
............................................................ 2.3.1 Specific resistance to filtration (SRF) 17
.................................................................................. 2.3.1.1 Solids concentration 19
................................................................................. 2.3.1.2 Solids characteristics 20
..................................................................................................... 2.3.1.3 Pressure 21
............................................................................. 2.3.1.4 Effective filtration area 22
.................................................................................................... 2.3.1.5 Viscosity 23
2.3.1.6 Determination of dry mass of cake per unit volume of filtrate ................. 24
.............................................................................................. 2.3.1.7 Other factors 25
............................................................................................. 2.3.1.8 Units of SRF 26
2.3.L.9 Applications of SRF as a dewaterability parameter ................... .... ..... 26
2-32! Capillary suction time (CST) ........................................................................... 29
2.3.2.1 Properties of the paper ...................... .. ................................................... 29
........................................................................ 2.3.2.2 Surface tension ............ ,. 30
.............................................................................................. 2.3.2.3 Temperature 30
.......................................................................... 2.3.2.4 Suspended solids content 31
................................................................................ 2.3.2.4 Solids characteristics 31
.................................. 2.3.2.5 Applications of CST as a dewaterability parameter 32
......................................................................................... 2.3.3 Time-to-filter (TTF) 34
............................................................. 2.3.3.1 Factors affecting the TTF results 35
.................................. 2.3 3.2 Applications of TTF as a dewaterability parameter 36
......................................................................................... 2.3 -4 Zeta Potential (ZP) 37
...................................................................................... 2.3.4.1 Theory behind ZP 37
......................................................... 2.3.4.2 Applications of ZP ..................... .. 39
3.0 MATEFUALS AND METHODS ....................... .. ............................................ 41
......................................................... 3.1 Sludge sample collection ........................ .. 41
.................................................................... 3.2 Preparation of conditioner solutions 42
3.2.1 Preparation of femc chionde solutions ............................................................ 42
..................................................................... 3 .2.2 Preparation of polymer solutions 42
........................................... ............................... 3.3 Conditioning sludge samples ..... 43
.......................................................................... 3.3.1 Single-chernical conditioning 44 3 CI ............................................................................. 1 . 3 . 2 Dual-chemical conditioning 41
3.4 Specific resistance to filtration ................................................................................ 44
.................................................................................................. 3.4.1 Test procedure 46
................... 3.4.2 Determination of weight of dry solids per unit volume of filtrate 50
............................................................................................. 3.5 Capiliary suction time 50
3.6 Time-to-filter ........................................................................................................... 52
3.7 Zeta potentid .......................................................................................................... 53
.......................................................................................... 3.7.1 Sample preparation 53
3.7.2 Test procedure .................................................................................................. 56
.......................................................... 3.7.2.1 Specific conductance measurement 56
3.7.2.1 Particle tracking ........................................,.. 57
4.0 RESULTS A N D DISCUSSION ................................ .......... ................... 60
................................................... 4.1 Interpretation of dewaterability parameter values 60
........................................... 4.2 Determining individual optimum conditioner dosages 62 . . ............................................................. ...... 4.2.1 Femc chlonde condi tioning .... 62
....................................................................................... 4.2.2 Polyrner conditioning 67
.................................... 4.3 Cornparison of performance of Percol 755 and Percol 757 73
..................... 4.4 Performances of SRF, CST and TTF as dewaterability parameters .. 73
4.4.1 Specific resistance to filtration ......................................................................... 73
4.4.2 Capillary suction tirne ..................................................................................... 75
.................................................................................................... 4.4.3 Tirne-to-filter 77
4.5 Role of charge neutralization in sludge conditioning ......................................... 79
....................................................................... ....... 4.6 Dual-c hemical conditioning .. 82
.................................................... 4.7 Costs of single- and dual-chemical conditioning 84
5.0 CONCLUSIONS AND RECOMMENDATIONS ................................................. 86
5.1 Conclusions ............................................................................................................. 86
................................................................................................... 5.2 Recommendations 88
.......................................................................................... 5.3 Need for fùture research 88
APPENDICES ................................ ............................................................................ 104
Appendix A: Percent polymer activity calculation ........... .. .......................... 105
........................................................ Appendix B: SRF. CST. TTF and ZP results 108
.............................. Appendix C : SRF Test Readings ............................ ............... 124
Appendix D: 'End of cake formation' filtrate volumes data and dopes of "tN vs.
V" plots ...................................................................................................................... 179
Appendix E: Details of cost calculations for single- and dual-chernical . . . condihoning ............................................................................................................... 181
vii
LIST OF TABLES
Table Title Page
Table 3.1. Example of SRF test data ................................................................................ 47
.... . Table 3.2. Specific conductance and max recornmended voltage (Zeta-Meter . 1977) 59
Table 4.1. Results of dual-chemical conditioning ........................................................... 83
Table 4.2. Dual chernical conditioning filtrate characteristics .......................................... 85
Table 4.3. Single- and dual-chemical conditioner costs per ton of dry solids .................. 85
... Vll l
LIST OF FIGURES
Figure Title Page
Figure 1.1. Schematic of the wastewater treatment process .............................................. 3
Figure 3.1. Filtration apparatus used for SRF and TTF tests .......................................... 45
.................................................................... Figure 3 .2 . Example of 'ti N 1 versus Vif plot 49
Figure 3.3. Determination of slope of linear portion of ' t i N i versus VIf plot 49 ..................
Figure 3 .4 . Capillary suction time apparatus .................................................................... 51
Figure 3 .5 . View of Zeta-Meter (Mode1 ZM-77) .............................................................. 54
Figure 4.1. Variation of SRF with increasing femc chloride dosages 63 ..............................
Figure 4.2. Variation of CST with increasing femc chloride dosages .............................. 63
Figure 4.3. Variation of TTF with increasing femc chloride dosages .............................. 64
................................. Figure 4.4. Sludge pH and solids variation due to FC conditioning 66
Figure 4.5. Filtrate pH and solids variation due to FC conditioning ................................ 66
Figure 4.6. Variation of SRF with increasing polymer dosages ....................................... 68
Figure 4.7. Variation of CST with increasing polyrner dosages ....................................... 68
Figure 4.8. Variation of TTF with increasing polymer dosages ....................................... 69
Figure 4.9. Sludge pH and solids variation due to Percol 755 conditioning .............. ..... 71
.................. Figure 4.10. Filtrate pH and solids variation due to Percol 755 conditioning 71
Figure 4.1 1 . Sludge pH and solids variation due to Percol 757 conditioning ................... 72
.................. Figure 4.12. Filtrate pH and solids variation due to Percol 757 conditioning 72
............. Figure 4.13. Effect of FC dose on filtrate volumes at the end of cake formation 76
.......... Figure 4.14. Effect of polymer dose on filtrate volumes at end of cake formation 76
Figure 4.15. Correlation of SRI? and CST test results .................................................. 78
Figure 4.16. Correlation of SRF and TTF test resdts .............................. ... ...................... 78
.............................................................. Figure 4.17. Effect of FC dose on zeta potential 80
...................................................... Figure 4.18. Effect of polymer dose on zeta potential 80
NOTATION
C
c c
Cs
CSA
CST
DC
EM
FC
G
I
in, Hg
ISR
K
lb/sq.in.
lb/T
min
ML
ares of filter paper
altemating current
dope of 'W verszrs V plot7'
radial clearance of piston or width of the annulus.
suspended solids concentration of sludge cake
suspended solids concentration of sludge
capillary suction apparatus
capillary suction time
direct current
electrophoretic mobility
ferric chloride
mean velocity gradient
strearning current
pressure in inches of mercury
initial settling rate
dirnensiodess polyrner dose constant
pounds per square inch
pounds of conditioning chernical per ton of dry solids
minute
mega liter
pressure in millimeters of mercury
pressure of filtration
rpm
RWWTP
S
SCD
SFF
SRI?
SS
t
Tm
TTD
TT.F
v
petammeter
specifnc resistance to filtration
radius of the piston
revolu~tions per minute
Regina wastewater treatment plant
coefficient of compressibility
stroke of the piston
streaming curent detector
specifüc filtrate flow rate
specifiic resistance to filtration
suspemded solids
mixing time
time
terameters
time-ta-drain
time-to-filter
volunee
ratio ot f weight of dry cake solids per volume of filuate
fiequemcy of pump in cycles per second
zeta patentid
dieleclaic constant
zeta pcntential
filterability constant
xii
filtrate viscosity
micro meter
micro siemens
a..
Xll l
1.0 INTRODUCTION
1.1 General
Municipal wastewater sludge treatment and disposa1 are some of the most
expensive operations carrïed out by wastewater treatrnent facilities worldwide. It is
estimated that sludge management constitutes between 30 and 40% of the capital cost of a
treatrnent plant, and about 50% of the operating cost (Vesilind, 1979). in most cases,
conditioring chemicals are the largest single-cost cornponent of the sludge management
process (Chitikela and Dentel, 1998). This being the case, any attempt to reduce
wastewater treatment cost shouId focus on economizing the sludge management aspects
in general, and reducing the amount spent on conditioning chemicals in particular.
Considering the high conditioner costs, it makes sense to continuously monitor and
evaluate the performance of conditioning chemicals used. There is also a need to examine
ways to optimize the conditioning process by using different conditioners, or by using
two conditioners in tandem.
1.2 Regina Wastewater Treatment Plant
The Regina Wastewater Treatment Plant (RWWTP) treats approximately 70 ML
of domestic wastewater on a daily basis. The wastewater is collected at the McCarthy
Boulevard pumping station and purnped to the RWWTP. Afier screening, the wastewater
enters the primary treatment plant and passes through two aerated rectangular grit
charnbers, where heavier inorganic particles settle out. The wastewater then passes into
three rectanguiar sedimentation tanks where much of the suspended matter settles out.
The primary effluent undergoes biological secondary treatment through an aerated
facultative lagoon system. The lagoon efnuent flows by gravity into the tertiary treatrnent
plant housing two round "Gravery' darifiers. Alum and an anionic polymer are added here
for removal of phosphoms.
The sludge handling facility of the primary treatment plant consists of two-srage
high rate anaerobic digesters. There are two primq digesters each with an approximate
retention time of one month and one secondary digester with a retention time of fifteen
days. The sludge and scurn (2-3% solids) fiom the primary sedimentation tanks are
puxnped to both pnmary digesters. In the secondary digester, the solids are allowed to
settle d o m and the liquid supernatant is returned to the front end of the plant for
treatment.
The sludge fiom the secondary digester (8-1 0% solids) is conditioned with Percol
757, a cationic polymer. Potassium permanganate is added for odor control and the
conditioned sludge is sent to one of two "Passevanty' vacuum belt filter presses for
dewatering. A sludge cake of approximately 30% of solids is usually achieved, and the
dewatered sludge (approximately 750 dry tonnes per year) is stockpiled on site. A
schematic of the wastewater treatment process is s h o w in Figure 1.1.
Like a majority of wastewater treatment facilities around the world, the RWWTP
uses polymer to condition its anerobicdly digested sludge. Until not long ago, treatrnent
plants worldwide mainly used femc chloride dong with lime for sludge conditioning.
However, the requirement of large quantities of ferric chloride and lime for conditioning,
with the resultant increase in the sludge mass requiring disposal, has resulted in treatrnent
facilities opting for polyrner conditioning, despite higher costs. Using a relatively
inexpensive conditioner like ferric chlonde for part of the conditioning needs would
significantly reduce the arnount being spent on conditioning chemicals.
However, it was necessary to examine whether a combination of ferric chloride
and polymer for conditioning would perform as well as, if not better than, polymer alone.
Hence, it was necessary to determine the performance and econornic feasibility of using a
combination of conditioning chernicals to dewater the RWWTP shdge.
1.3 Objectives of the study
The basic objective of the study was to evaluate the performance of dual-chernical
conditioning (polyrner + femc chloride) for dewatering the RWWTP sludge. The other
objectives of this study included the following:
1) an exarnination of the performance of capillary suction time (CST), specific resistance
to filtration (S RF), and time-to-filter (TTF) as dewaterzbility parameters; and
2) an economic evaluation of dual-chernical conditioning in cornparison with the
polyrner conditioning presently being practiced at the RWWTP.
1.4 Scope of the study
The scope of the study included the foliowing tasks:
calculation of the individual optimum dosages of femc chlofide, and the two
polymers vercol 755 and Percol 757), that would be required to condition the
RWWTP sludge;
cornparison of the performance of two cationic polymers - Percol755 and Percol757;
examination of the mathematical correlation of SRF and CST;
a thorough review of the state-of-the-art of the four pararneters - S E , CST? and TTF
- and cornparing and contrasting their performances as dewaterability pararneters;
possible use of zeta potential (ZP) values as a dewaterability parameter, and
investigation of the role of charge neutralization in optimum conditioning for
dewatering;
evaluation of the performance of dual-chemical conditioning of the RWWTP sludge.
This involved conditioning sludge sarnples with combinations of different
percentages of the optimum dosages of femc chloride and Percol755/Percol 757; and
cornparison of the conditioning costs of single- and dual-chemical conditioning.
2.0 LITERATURE RE-W
The selection of a conditioning chernical to aid sludge dewatering, and the
determination of conditioner optimum dosages can be complicated. The type of
conditioners to be used would depend on, among others, the physical properties of the
sludge solids, and the specific dewatering process used. The selection of a conditioning
chernical depends on the proper evaluation of its effèctiveness in enhancing
dewaterability. The conditioner selection process, and the calculation of optimal dosages
of conditioners to be used, is usually accomplished with jar tests alone, or in conjunction
with dewaterability parameters such as CST, SRF, and TTF. To better understand the
dewaterability parameters, it is important to know the basic principles involved in the
processes of conditioning and dewatering. The following sections provide a review of
information available in literature on sludge conditioning, dewatering, and dewaterability
parameters.
2.1 Conditioning
Conditioning involves the chernical andor physical treatment of sludge to
enhance removal of water and improve solids capture. The primary objective of
conditioning is to increase the particle size by combining the small particles into larger
particles. Sludge solids, by virtue of being typically negatively charged, repel each other
rather than attract. Conditioning is employed to neutralize the effects of this electrostatic
repulsion, and to enable particles to collide and increase in size. It is a two-step process
consisting of coagulation and flocculation. Coagulation involves destabilization of the
sludge particle by decreasing the magnitude of the repulsive electrostatic interactions
between the particles. Flocculation involves promoting the agglomeration of the
suspended matter by gentle rnixing (U.S.EPA, 1987).
The basic mechanisms by which conditioning processes transform the amorphous
gell-like sludge mass into a porous material which allows release of water are not known.
An analysis of equations of flow through porous media would seem to indicate that the
goal of conditioning must be to increase porosity, particle diameter, particle shape factor,
and particle rnass density, and to reduce the coefficient of compressibility (Dick, 1980).
The three most cornmon conditioning systems use inorganic chemicals, organic
chemicals, or thermal treatrnent. Relatively less fiequently used conditioning methods
include combined organic and inorganic chernical conditioning, elutriation, use of
bulking materials, acidification, Ereeze-thaw, solvent extraction, irradiation, and
potassium permanganate (Water Pollution ControI Federation, 1987).
Literature on selection of coagulants and cornputing coagulant requirements are
available (Genter, 1946; Water Pollution Control Federation, 1987; Dente1 et al., 1988;
Dentel, 199 1 ; Dente1 et al., 1993; Dente1 et al., 1995).
2.1.1 Factors affecting conditioning
Sludge conditioning performance Is affected by variations in three basic
parameters: 1) dosage of chemicals, 2) mixing time, and 3) mixing intensity (Christensen
et al., 1995).
Novak and Haugan (1980) reported attempts to develop a better understanding of
rnechanisms by which cationic polymers improve the dewaterability of activated sludge.
The influence of polymer dosing and mixing on sludge dewaterability and floc stability
was evafuated. It was reported that the optimal dose of cationic polymer depended upon
the sludge solids concentration, mixing speed, and mixing tirne.
Werle et aI. (1984) determined the effects of polymer dose, mixing time, md
rnixing energy on the relative filterability of water and wastewater sludges. Tt was shown
that jar testing devices using low mixing energies tended to under-predict polymer dose
requirements in cases where sludges, especially primary sludges, were to be subjected to
hgh-stress dewatering. The product of the mean velocity gradient (G) and the mixing
time ( t ) , was found to be a critical factor in detennining optimal polymer dose. Increase in
mixing energy input (Gt) was found to result in an increase of optimum polyrner
requirements while conditioning alum, activated and primary shdges.
m i l e Werle et al. (1984) used only one polymer in their experiments, Novak et
al. (1988) used several polyrners to determine if polymer selection was dependent on
mixing intensity. They also concluded that polyrner requirements increased as Gî
increased for both alum and activated sludge. Polyrner selection was shown to be more
important at high mixixg energy inputs than at low energy inputs.
Cole and Singer (1985) reported that charge neutralization was not a prerequisite
for effective conditioning. Effective sludge conditioning was shown to be achievable with
either high or low charge density cationic polymers.
Novak and Bandak (1989) evaluated the effects of shear (G) and mixing time (f)
on the performance of sludge conditioners. In addition to improving dewaterability,
chemical conditioning improved the resistance of sludges to shear. Mixing requirements
for optimal conditioning of sludge c m be represented by the following equation (Novak
and Bandak, 1989):
@ t = K (2- 1)
where: G = shear (tirne"), r was the mixing time, and K a dimensionless polyrner dose
constant. The G exponent, x, was found to vaxy between 2.8 for unconditioned sludges to
values of 1 or less.
Novak and Lynch (1990) felt that it was possible to sirnulate the shear in
dewatering processes, thereby facilitating b e seiection of conditioning chemicals and
doses using laboratory mixing devices. Experimental findings suggested that shear
occurring in a filter cake during mechanical dewatering could increase polyrner dosage
requirements.
Langer and Klute (1993) found that imcreasing the mixing energy input and thus
promoting turbulence for a period of about orne second after polyrner addition resulted in
an improvement of the dewaterability of the conditioned sludge. Dewaterability was not
found to deteriorate with excess rnixing. Nt \vas shown that poor mixing could be
compensated by a higher polymer dose. Al: so, the lower the polymer dose, the more
important was rapid mixing.
Christensen et al. (1993) studied the rnechanisms responsible for overdosing (very
high conditioner dosages) in polyrner and iriorganic sludge conditioning. Experimental
findings showed that overdosing was associated only with polyrner conditioning.
2.1.2 Inorganic chernical conditioning
Inorganic chemicais predominantly used include femc chioride (FeC13), in
conjunction with lime (Cao). Ferrous sulfate, ferrous chloride, and aluminum sulfate
have also been used. Researchers have investigated the role of lime in sludge
conditioning (Webb, 1974; Sontheimer, 1967). Webb (1 974) evaluated a number of
conditioners used with and without lime and explained the effect of lime addition on
femc chloride conditioned sludge in terms of a calcium link between negatively charged
ferric hydroxide precipitate and negatively charged sludge particles. Christensen and
Shdc (1979) feIt that lime was mainly added to enhance filterability and raise pH. They
investigated the effectiveness of ferrous and femc salts with and without lime and
demonsîrated that al1 the iron salts provided better conciitioning with lime. Ferric sdts
were also found to provide better conditioning than ferrous salts with or without lime.
Christensen and Wavro (1 98 1) reported that iron-lime conditioning of sludge was
an inherently more stable process that organic polyelectrolyte conditioning. The
instability of the poIyelectroIyte conditioning was reported to be due to the signifi cant
variability in the surface chernistry of polyrners. Variable concentration of sludge solids at
a given plant, dong with the specific nature of the polymer-sludge solids interaction, and
the possibility of process deterioration due to overdosing, was also thought to contribute
to the instability. Iron-lime conditioning, on the other hand, was found to be inherently
more stable because changes in the siirface chemistry andor the concentration of the
sludge solids did not significantly affect process results. Other benefits of iron and lime
conditioning include disinfection, odor control, and an ability to immobilize heavy
rnetals.
Zall et al. (1987) conditioned an oily sludge with lime and fly ash to reduce the
compressibility for dewatering by a laboratory filter press. These additives were found to
greatly improve dewatering compared to polyrner alone.
Novak et al. (1999) applied liquid pressure to charactenze the rate and extent of
activated sludge dewatering in the 'expression' phase. Polymer and an iron-lime
combination were tried as conditioners. Iron-lime conditioning was found to alter sludge
compressibility and improve floc structure, and was reported to have advantages over
polymer conditioning alone.
Typical conditioning dosages of ferric chloride and lime for anerobically digested
municipal wastewater prirnary sludges are 60-100 I b n (30-50 kgMg) and 200-260 lb/T
(100-130 kgMg) of dry solids (U.S.EPA, 1987). This is the major disadvantage of the
inorganic chemical conditioning process because it increases the sludge mass
considerably, thereby increasing sludge disposa1 costs. In addition, inorganic conditioning
reduces the fuel value of sludge for incineration. Scaling problems and the generation of
chemical solids (iron and calcium precipitates) are the other disadvantages (Christensen
and Wavro, 1987).
2.1.3 Organic chemical conditioning
Organic polyelectrolytes have been increasingly used for sludge conditioning over
the last three decades. Organic polyelectrolytes or polymers are large, water-soluble
organic molecules developed fiom smaller building blocks (monomers) repeated in a long
chain.
Gale and Baskerville (1970a) compared the effects of 16 polyelectrolytes on four
wastewater sludges with aluminum chlorohydrate as a control. The important variables in
the practical application of polyelectrolytes in sludge dewatering were identified as
method of preparing solutions, method of mixing the solution with the sludge, storage of
solutions, and the type of water used.
Typical polyrner conditioning dosages of anerobically digested municipal
wastewater primary sludges for dewatenng by belt filter presses are 2-10 IblT (1-5
kg/Mg) (US .EPA, 1 98 7).
The various advantages of polymers over inorganic conditioners are as follows
(Water Pollution Control Federation, 1987):
1) reduced conditioner costs (in spite o f the higher unit costs of polyrner, the
conditioning process is cheaper because of the much reduced polymer dosages
typically required);
2) smaller volumes and weights of dewatered solids;
3) elimination of lime handling and storage problems; and
4) influence on downstream processes (unlike iron and lime, polyrner conditioning does
not reduce the fuel value of sludges).
2.1.4 Duàl-chemical conditioning
The instances of dual-chemical conditioning discussed in this section refer to a
combination of organic and inorganic chemicals, or of two organic conditioners (two
polymers).
Studies on the dual-chernical conditioning of sludges have not been numerous-
Chitikela and Dente1 (1998) hvestigated the use of an inorganic chemical conditioner
(ferric chloride) or a cationic surfactant (hexadecyltrimethyl ammonium bromide) in
combination with a cationic polymer (Percol 757). They found that the cost-effectiveness
of the dual-chernicd conditioning depended on the polymer in use and the chemical costs
relevant to the particular site. However, they also reported that results fiom bench-scale
and pilot-scale studies showed that using femc chloride and a mannich polymer would
provide annual savings of US $275,000 over the use of an emulsion polymer alone.
Roberts and Olsson (1 975) found that polyelectrolyte requirements for activated
sludge conditioning could be reduced by either elutriation of colloidal particles in the
sludge with water, or by partial neutralization of the colloidal particles with positively
charged alurninurn hydroxide. Preconditioning of the activated sludge with a 0.5 g/L dose
of alum was found to reduce the poIyelectrolyte requirement to 40% of its original dose.
Senthilnathan and Sigler (1993) reported that dual polyrner conditioning of an
aerobically digesîed activated sludge h m a pharmaceutical industry enhanced the
dewatering by 20 to 30% and increased the solids content of the dewatered sludge cake. A
low molecular weight, kigh cationic charge polymer, in combination with a high
molecular weight, high cationic charge polymer was used in their study.
2.2 Delvatering
The primary objective of dewatenng is removal of water fiom sludge, thereby
reducing the sludge volume lefi for disposal. Techniques for dewatering wastewater
sludges may broadly be classified under two categories: 1) mechanical dewatering
systems, and 2) air drying processes, where moisture in the sludge is removed by natural
evaporation and gravity or induced drainage (U.S .EPA, 1 987).
Air drying processes include sand beds, fieeze assisted sand bed dewatering,
vacuum assisted beds, wedgewire beds, sludge lagoons and paved beds. Other innovative
processes include solar sludge drying beds, utilization of additives (sawdust and wood
chips), and growing reeds or bulrushes to dewater the sludge (USEPA, 1 987).
Mechanical dewatering systems are generally used where land is a premium
andior open air drying might be offensive. Typical mechanical dewatering systems
Ulclude belt filter presses, centrifuges, filter presses, and vacuum filters. Other dewatenng
systems that have come up in recent years include Expressor Press, Som-A-System,
Centripress, HIW Screw Prcss, and Sun Sludge System (U.S.EPA, 1987).
There are severai publications that discuss the various mechanical dewatenng
systems in detail (U.S. EPA, 1979; Vesilind, 1979; Dick and Ball, 1980; U.S. EPA, 1987;
Viessman and Hamrner, 1 988).
2.2.1 Factors affecting dewatering
Dick (1 980) surnrnarized the factors influencing dewaterability as follows:
1) fluid properties like viscosity, ionic strength, density, bound water;
2) sludge particle properties like size and shape distribution, surface potentisl, surface
area, and density; and
3) sludge properties like suspended solids (SS) concentration, pemeability, yield
strength, and electrokinetic properties.
Mashimoto and Hiraoka (1990) studied the characteristics of sludges for 33 factors
afkcting dewaterability of sludge by a belt press filter.
A lot of effort has been taken in atternpting to establish the role of water in the
dewatering process. Vesilind (1991) descrïbed the water in sludge as being made up of
fiee (or bulk) water, interstitial water, vicinal water, and water of hydration. It was
reported that mechanicd sludge dewatering could not remove any more than bulk and
interstitial water fiom the sludge. However, this observation was disputed by Dick and
Drainville (1995), who also felt that any measurement of bound water would be affected
by the method of measurement.
Based on experimental data, Smollen (1 986c, 1988, 1990) categorized municipaf
sludge moisture into fiee, irnmobilized, bound, and chemically bound moisture.
Information on different methods of measurement of bound water has been published
(Lee and Hsu, 1995; Colin and Gazbar, 1995, Chu and Lee, 1999).
2.3 Dewaterability parameters
Several tests that are used to evaluate the performance of different conditioners
also provide valuable information that can be used to evaluate the dewaterability of
sludges.
A simple jar test is comrnonly used for a quick initial screening of poljmers. It
involves pouring 500 mL of sludge sarnples into each of six jars in the jar test apparatus.
Different dosages of polymer solutions are then dispensed into the jars using a syringe.
The polymer and sludge are then mixed at approxirnately 200 rprn for about two minutes.
The speed is then reduced to 30 rpm for 5 minutes to allow for flocculation. The sludge
sample is then inspected for formation of strong flocs and the clarity of free water above
the settled sludge. These would be indicative of the polyrner's ability to condition the
sludge. The test results are based on visual observation of floc formation. The main
disadvantage of a jar test is that it does not provide quantitative information. In addition,
while tbis test provides information only on the degree of flocculation achieved, it does
not indicate actual dewaterability (Dentel et al, 1993). Moreover, the jar test actually
characterizes only the thickening behavior, and not dewaterability. Hence, this test rnay
only be a reasonable means of qualitatively screening conditioning aids prior to more
elaborate tests, and not an indicator of dewaterability (Dente1 et al., 1988).
Attempts have been made to use rheology as a tool to predict dewaterability of
wastewater sludges. Campbell and Crescuolo (1 982, 1989) used the shear stress versus
the shear rate curve of conditioned sludge for optimization of chemical conditioning.
Abu-Orf and Dente1 (1999) attempted to identifi specific rheological characteristics of
sludges that rnay be used to assess conditioning and dewatering processes. They
compared rheological characteristics with dewaterability in laboratory evaluations
(indicated by CST, streaming curent, viscosity, and solids measurements). Experimental
results indicated no correlation between the proposed rheological parameters and
conditioning indices.
The tests mainly used by wastewater treatment plants to evaluate dewaterability
include specific resistance to filtration (SRF), capillary suction time (CST), tirne-to-filter
(TTF) and fiIter yield (by filter leaf testing). Al1 these tests provide quantitative values
that can be used to assess the dewaterability of sludges.
2.3.1 Specific resistance to filtration (SRF)
Specific resistance is nurnencally equd to the pressure required to produce unit
rate of flow through a cake having unit weight of dried solids per unit area, when the
viscosity of the liquid is unity (Gale, 1977).
The SRF test has its roots in a filtration theory proposed by Ruth et al. (1 933) and
Carman (1933, 1934 a,b, 1938). Carman supplied detailed experimental data to support
the filtration equation he developed. The term "average specific resistance" was also
introduced at around this point of time. However, it was not until 1956 that basic
conditions for sludge filtration were developed and experimental techniques involved in
the method of calculating specific resistance described (Coackley and Jones, 1956; Jones,
1956). n i e caiculation of specific resistance, fkom that point of time until much later,
involved the following procedure.
Sludge was filtered through a moistened filter p a p a placed at the bottom of a
Buchner h e l . A suitabIe time (say two minutes) was dlowed for cake formation after
the application of the required vacuum pressure. The volume of filtrate coming off the
sample was then noted down every minute for ten minutes and then at two minute
intervals for another ten minutes (Coackley and Jones, 1956). -4 graph of VV versus V
was plotted, where t is the time in seconds for V mL of filtrate to accumulate. The slope
of the graph was measured and the specific resistance cdculated from the following
equation:
where: specific resistance, c d g
pressure of filtration, lb/sq.in.
area of filter paper, cm2
siope of 'W versus V plot", sec/mL2
filtrate viscosity, poise
ratio of weight of dry cake solids per volume of liquid in the sludge
before filtration, g/mL
A detailed description of the theoretical background of filtration, experimental
procedures involved and the apparatus required for carrying out the SRI test c m be found
in Gale (1 977).
The overall accuracy of the SRF value would depend on the accuracy of
individual determinations of pressure (P), effective filtration area (A), dope of "tN
versus V plot" (b), viscosity (p), and ratio of weight of dry cake solids per unit volume of
liquid in the sludge (w) (Christensen and Dick, 1985a,b). The SRF varies with pressure,
area, solids concentration and liquid viscosity (Vesilind, 1979). The following sections
summarize the effects of these and other variables on the SRF value.
2-3.1 -1 Solids concentration
Theoretically, SRF is sulpposed to be independent of sludge solids concentration.
However, experimental findings; by many researchers do not bear this out. Coackiey and
Jones (1956) carried out several tests to examine the effects of solids content on the SRF
value. They investigated the eEects of dilution of a thickened digested sludge by tap
water, supernatant liquor, and aiistilled water. In al1 instances, they reported that SRF
decreased with a decrease in solids content.
Christensen and Dick (1585 a,b) exarnined the effects of slurry suspended solids
concentration on SRF values. TLtiey found that the SRF of a floccdent slurry was a very
strong function of suspended solids concentration at low concentrations, and relatively
independent of concentration at Ehigher values. They observed that this finding could have
implications on the translatability of the SRF data as a measure of dewaterability.
Smollen (1986b) conducted SRF experiments on six categories of municipal
sludge, with solids concentratimns ranging from 0.3% to 6.7% fiorn eleven municipal
wastewater treatment plants throughout South Afnca. Solids concentrations were fo und
to affect SRF values. SRF v d u e s of anerobically digested sludge increased with a
decrease in solids concentration.. In activated sludge slurries, SRF values were found to
decrease with a decrease in solihs concentration.
However, Kavanagh (1 980) conducted filtration experiments using sludges having
solids concentrations ranging from 5 to 50 kg/m3, and found that the results obtained
supported the theoretical concept of the SRF not being influenced by the solids content.
They explained that previous investigators (Coackley and Jones, 1956) failed to include
the filter cake solids in the calculation of 'wr, and that was the reason their experimental
findings did not tally with the theoretical predictions.
2.3.1.2 Solids characteristics
Trubnick and Mueller (1958) stated that the size, shape, and density of sludge
solids affected filterability by virtue of the role they played in compaction and in
requirements of coagulation chemicals.
Karr and Keinath (1978a,b) observed two major limitations of the SRF test. They
reported that SRF test was dependent on the solids concentration of sludge when the
concentration of supracolloidal solids was sufficient to cause blinding. This observation
was contrary to the SRF theory. The SRF does not adequately quanti@ the dewatering
characteristics of a sludge that contains supracolloidal fines. These fines could blind
either the passageways through the sludge or the filter medium during filtration. This
would also increase the measured resistance, whereas according to theory, resistance
should increase only because of solids deposition as cake during filtration. To quanti@
this degree of blinding, they developed a Blinding Index. which they felt would irnprove
the interpretation of the SRF data. Another limitation of the SRF test is that it does not
account for al1 the sludge properties, such as pick-up and release charactenstics, scrolling
properties, and particle resistance to shear.
Smollen (19S6b) also reported that blinding interfered with the SRF by creating
either cake or filter medium resistance. Sludge characterized by high proportions of
colloids and fines in the supernatent were found to create cake blinding during filtration.
Huang and Diggelman (1986) reported that particles in the range 0.45 to 2.5 pn
most influenced dewaterability and sludge filter cake solids concentration of a waste
activated sludge.
2.3.1.3 Pressure
According to Carman's study of the mechanisin of filtration (1934a), SRF was a
constant for ideal cakes, and a variable depending on P, pressure of filtration, for
compressible cakes, Carman's theory (1 933, 1 934a,b) also suggested that the following
empirical equation expressed the variation of SRF with pressure.
r, = rPS (2.3)
where: P = pressure,
s = coefficient of compressibility, and
r=SRFwhenP=l-
Coackley and Jones (1956) identified a range of values for the coefficient of
compressibility (s) of different sludges. This value denoted the degree of compressibility
of sludges, Le., the greater the value of's', the more compressible was the sludge.
Kavanagh (1980) exarnined the influence of pressure, in the range of 5-1 00 kPa,
on SW values. They reported that SRF depended critically on the operating pressure
used, and suggested an operating pressure of 49.1 kPa (368 mm Hg) be adopted as a
standard value.
Smollen (1 986b) found that anerobically digested sludge had the lowest
coefficient of compressibility (s=0.5) while activated sludge the highest (~1 .4 ) . Due to
the high compressibility displayed by biological sludges, an increase in pressure applied
was not found to result in a proportionate improvement in filtration or reduction in SRF.
It was reported that the method of differential pressure application (whether the SRF
rneasurement was made under pressure or suction) appeared to be an important factor in
creating cake or filter medium resistance.
2.3.1.4 Effective filtration area
Swanwick and Davidson (1961) assessed the degree of accuracy of the SRF
experiment and camed out experiments to detemine Sie effective area of filtration. In the
SRF equation, the parameter 'A' which denotes the effective area of filtration, is squared,
and any discrepancy in its measurement would adversely affect the accuracy of the SRF.
As the perforations in a typical Buchner funne1 arnount to only about 9% of the total
perforated area, Swanwick and Davidson (196 1) thought that the effective filtering axa
might be Iess than the total perforated area. However, when they tried to increase the
effective filtering area by placing a disc of woven wire under the filter paper, the found
that it did not increase the rate of filtration. So they concluded that the area of filtration,
A, may be taken as the area of the filter paper. However, placing discs of woven wire
under the filter paper was suggested as an additional support for the filter paper during
prolonged filtration experirnents.
Kavanagh (1980) presented the results of m experimental study aimed at critically
assessing the validity of Carman's filtration theory, and conduded that the basic filtration
equation developed by Carman could be used quantitatively to describe the filtration of
biological sludges. Kavanagh also asserted that the use of filter paper area as a measure of
the effective filtration area of the Buchner funriel was justified, md that its use did not
ïntroduce any significant error into the determination of Sm.
Christensen and Dick (1985b) surnmarïzed the findings of different researchers
regarding the effective filtration area in SRF measurernents. They affirmed that
uncertainty about the effective filtration area in a Buchner funne1 or other laboratory
filtration device continued to prevail. It was felt that an effort should be made to
minimize the problem by pIacing a plastic window screening under the filter media.
2.3.1.5 Viscosity
Trubnick and Mueller (1958) reported that filtrate viscosities did not Vary
significantly and hence were only of academic interest in the filtration of sewage sludges.
However, Christensen and Dick (1985b) found sludge filtrate viscosities to be 3-24%
higher than viscosities of water at the same temperatures, and felt that errors in the
calculation of SRF values would be minimized if the filtrate viscosities were used in the
SRF equation.
Changes in liquid viscosity are most likely caused by variations in ambient
temperature. Viscosity measurements showed that filtrate viscosities were within 1-2% of
pure water at the same temperature. Expenmental findings revealed that a linear
relationship exists beiween 'b' and 'p' indicating that the SRF of the sludge was constant
over the temperature (viscosity) range examined (Kavanagh, 1980).
2.3.1.6 Determination of dry mass of cake per unit volume of filtrate
Christensen and Dick (1985b) reported the lack of a standard method for the
calculation of 'w', dry mass of cake per unit volume of filtrate. For a long tirne,
researchers have used any of the following procedures for determining w:
1) divide the mass of the dried cake by the final volume of filtrate;
2) divide the product of the sluny concentration and sample volume by the final volume
of filtrate; or
3 ) rneasure the sluny and cake solids concentrations and use them in the following
equation
w = C,C, / [l OO(C,-Cs)] (2-4)
where: Cs = sluny suspended solids concentration, and
Cc = cake suspended solids concentration
Christensen and Dick (1985b) argued that al1 the above methods make the error of
including cake drying in the calculation of 'w'. The. felt that the calculation of 'w' should
be based on the filtrate volume measurement at the end of the cake formation period. This
c m be done by dividing the total rnass of cake solids by the volume of filtrate generated
at the end of the cake formation period. The total mass of cake solids can be determined
directiy by weighing or indirectly by the product of slurry solids concentration and sample
volume. The filtrate volume associated with the end of cake formation is that volume
where 'tN versus V' plot breaks away from a straight line.
2.3.1.7 Other factors
Coackley and Jones (1956) observed that the timing of the initial filtrate volume
readings shoutd allow for the formation of a cake with a resistance equal to or greater
than that of the filter medium. They felt that two minutes would be a suitable time to
allow for the formation of such a cake.
Christensen and Dick (1985b) also examined the effects of 'initial tirne and
filtrate volume readings'. They reported that the timing of the initial filtrate volume
readings did not theoretically affect the slope of the 't/V versus V' plot used to calculate
SRF. This timing of the initial reading is the time allowed for a thin cake formation that
would reduce the significance of the resistance of the filter media. It was observed that
the time allowed to elapse before the first reading did not affect the slope of the plot, as
long as the:
1) pressure differential was applied irnmediately afier the sarnple is introduced to the test
apparatus, and
2) time and filtrate volume readings are synchronous.
Thus, the time to initial filtrate volume readings was not critical as long as the
pressure application is instant upon pouring of sarnple in the Buchner funnel.
Zingler (1970) described the limitations of the Buchner funnel test, and drew
attention to the significance of the quantity of water stored in the filtration apparatus
during d?e filtration test. He concluded that Buchner h e l filtration would accurately
indicate performances in operzting plants only where the technical dewatering processes
used were in agreement with the filtration phenornena, Le., where vacuum belts, plane-
and disc filters were used for dewatering.
Agerbæk and Keiding (1993) showed that SRF varied with pH and conductance.
A significant electrokinetic effect was found to reduce SRF with increasing pH and
decreasing conductance-
Dick and Bal1 (1980) mentioned that while the SRF test provided rapid and
reproducible results, it did not simulate several phenomena encountered in cake filtration,
particularly as c k e d out in a vacuum filter.
2.3.1.8 Units of SRF
Cornrnonly used units for SRF are s ~ / ~ , but cm/g or m/kg are the preferred units
(Gale, 1977). Christensen (1983) reported on the various units that have been used by
researchers to report values of SRF. It was felt that consistent reporting of SRF values in
terameters per kilogram (Tm/kg) would irnprove communication on the dewaterability of
water and wastewater sludges. SRF values of industrial sludges, which are significantly
higher or lower, c m be reported in petameters per kilogram (Pmkg) or gigarneters per
kilogram (Grnkg).
2.3.1.9 Applications of SRF as a dewaterability parameter
In cornparison with other dewaterability parameters, there is a virtual unanirnity in
the acceptance of SRF as the most fundamental measure of dewaterability based on
established theories. There have been several instances of researchers using SRF as a
pararneter to assess conditioner and dewatering performance.
The yield of different types of dewatering systerns was reported to be related to
SRF values. Baskerville et al. (1 97 1) showed that, in filter presses, the time required to
achieve a cake was directly proportional to the SRF. It was shown that the yield of a
vacuum filter was inversely proportional to the square of the SRF (Gale and Baskerville,
l97Ob).
Swanwick et al. (1964) conducted SRF tests to determine the effects of
rnesophilic anerobic digestion on the dewatering characteristics of sludges fkom eight
sewage works. Experimental findings showed that SRF values could be correlated to the
rate of drainage of the sludges. A relation between SRI? and the overall dewaterability on
beds was indicated.
Gale and Baskerville (1970a) used SE2F values as a parameter in comparing the
effects of 16 polyelectrolytes on four different sewage sludges. Gale and Baskenille
(1 970b) observed that reducing SRF values increased the filter yield. However, a reduced
SRF value would not necessarily affect the final solids content in al1 circumstances. If the
SRF reduction was obtained by using flocculents that produced cakes of open structure
and low solids content, then the cake will be wetter.
Gale (197la) reported that SRF of the cake forrned on a filter paper in a Buchner
funnel would not necessarily be the sarne as that formed on a filter medium of very open
structure. In such a filter medium, fine particles that were retained by the filter paper
would p a s through the open-structure medium and the coarse retained particles would
form a cake of lower specific resistance than when al1 particles were retained.
Baskerville et al. (1971) showed that the time required to produce a filter press
cake was directly related to the SRF of the sludge feed, over a wide range of filtrabilities,
for a thick and thin raw sewage sludge. It was reported that filter press performance could
be estimated from SRF measurements in combination with values for maximum cake
solids expected from a filter press.
Srnoilen (1986a,b) used SRF results to compare the relative dewatering properties
of various municipal sludges from f 1 m~micipal wastewater treatment plants in South
Afica. She concluded that that the test could be used as a measure of relative
dewaterability provided it was conducted in a consistent marner.
Kempa and Fukas-Plonka (1 982) expressed a need to uni@ sludge testing methods
as it was felt that differences in test procedures between laboratories influenced the SRF
results. They stated that, dong with SRF results, various other properties such as type of
&el, filtration surface, temperature, viscosity, type of filter medium, pressure value and
duration of measurement should be determined and descnbed expiicitly. This information
should then be used in conjunction with the SRF values for estimating the filtration
properties of the sludge.
Ideally, a filterability pararneter should be independent of test conditions. SRF
was shown to be highly dependent on the applied pressure when the compressibiIity of
the sludge was hi&. A new pararneter, specific filtrate flow rate (SFF) with units of
kg2/(m's2) was suggested as a possible alternative to SRF. It was shown that, for presures
in the range normally applied in full-scale dewatering, SRF was a better filterability
pararneter only for low compressibility materials as TiO' and bentonite. For highly
compressible materials such as biological wastewater solids, silica, and attapulgite, SFF
proved to be a better filterability parameter (Sorensen et al., 1996).
2.3.2 Capillary suction time (CST)
Baskerville and Gale (U968) developed a simple instrument called the capillary
suction time (CST) apparatus, that was easy to operate, produced a rapid indication of
filterability, and showed good reeproducibility of results.
The CST test is performed by placing a sludge sarnple in an upright metallic
cylinder, or sludge reservoir, xresting on a chrornatography grade paper. The capillary
suction of the paper extracts ~e Iiquid fiom the sludge, wetting the paper. The time
required for the filtrate to flow 1 cm radidly is recorded as the capillary suction time or
CST (Dente1 et al., 1993).
Baskerville and Gale (1968) reported that the following factors affectecl the CST:
1) properties of the paper;
2) suface tension;
3) temperature; and
4) SS content of the sludge
Karr and Keinath (197Sa) reported the influence of fiagile settleable solids on
CST values. The following secttions surnmarize the effects of various factors on the CST
values.
2.3.2.1 Properties of the papes
The two properties of :importance are a) capillary suction pressure, and b) the
filtrate-absorbing-capacity per mnit area.
The capi1Iary suction pressure determines the pressure difference available for
filtration purposes. For the CST to be independent of the depth of the sludge in the test
cell, the capillary suction pressure needs to be large compared with the hydrostatic head
of the sludge. The filtrate-absorbing-capacity per unit area of the paper determines the
amount of cake built up in the test ce11 when a given area of paper has been wetted
(Baskerville and Gaie, 1968).
2.3.2.2 Surface tension
The surface tension of the filtrate will affect the capiliary suction pressure and
hence the CST. Baskerville and Gale (1968) found that, with tap water, the CST was
influenced only by the resistance of the paper to the flow. Concentrations of an anionic
detergent below 50 mg/L were not found to affect the CST. Above 50 rn& the CST was
fond to increase significantly. Therefore, for sludge filtrates greater that 50 mgK, the
CST would increase significantly with an increase in filtrate concentrations.
2.3.2.3 Temperature
Baskerville and Gale (1968) determined the CST of two sewage sludges of
vvying filterabilities, at varying temperatures. For both sludges, the CST was found to
decrease with an increase in temperature, signiSing greater dewaterability at higher
temperatures.
Kavanagh (1980) also noted that, but for the increased costs involved, the rate of
filtration of biological sludges could be increased substantialiy by preheating the sludge
to elevated temperatures.
2.3.2.4 Suspended solids content
The thickness of a cake, and the resistance it offers to flow through it, affects the
volume of filtrate. Likewise, the CST also would be af5ected by the solids content. It was
found that CST of slowly filtering sludges increased significantly with increasing solids
concentration, when compared to the rapidly filtering sludges. This variation in CST with
solids concentration emphasizes the need to specie the solids content along with
experimental results (Baskewille and Gale, 1 968).
2.3.2.4 Solids characteristics
Although the CST test is affected by concentration of fines in sludge, it is less
sensitive than SRF to this solids fraction. This is because the rate of flow of filtrate in the
CST is small and hence fewer fines are carried into the critical pathways. However, the
CST test is more sensitive to changes in the concentration of fragile settleable solids
(solids that are too fragile to be removed by filtering through a 100 pm mesh, yet large
enough to settle under quiescent conditions). The reason for this is, as oniy a small total
volume of filtrate is withdrawn during the CST test, very little compaction of the sludge
solids occurs. Therefore, CST results are more sensitive to initial packing and specific
characteristics of the particles (Karr and Keinath, 1978a).
2.3.2.5 Applications of CST as a dewaterabiility parameter
The CST test is a purely empirical test and is not based on a theoretical analysis of
sludge dewaterability (Vesilind, 1 988). However, numerous researchers have reported
excellent correlations between CST and the more fundamental SRF values.
Kavanagh (1980) plotted CST values against the product of solids concentration
and SRF values and reported excellent correlation. U.S.EPA (1987) used data from
studies with iron-lime sludge conditioning by Christensen and Stulc (1979) to establish a
strong correlation between SRF and CST values. Several researchers have reported
excellent correlations between CST and SRF values in their studies (Sarikaya and Al-
Marshoud, 1993; Hwa and Jeyaseelan, 1997). However, Smollen (1 986) reported the lack
of a quantitative relationship between SRF and CST.
The evidence of a mathematical reIation between SRF and CST values indicates a
possibility of using the simpler CST test as a substitute for the more cumbersome SW
test.
Unno et al. (1983) attempted to provide a mathematical basis for the CST test.
They developed a complex relationship between the SRF and a linearly measured
capillary suction time (LCST).
Vesilind (1988) mathematically analyzed the CST apparatus and suggested a
mode1 that includes both equiprnent variables and variables related to slridge
characteristics. It was reported that previous attempts to correlate SRF and CST have
failed because CST, as opposed to SRF, was not a fundamental measure of
dewaterability. He suggested that a filterability constant, X, which is also a fundamental
measure of dewaterability, should be correlated with SRF-
Tiller et al. (1990) felt that Vesilind's mode1 would not provide a theoretical basis
for calculating the average specific resistance of the cake, as it did not include the
conductivity of the chromatography paper. They developed a new capillary suction
apparatus (CSA) with a rectangular reservoir and proposed a method for calculating the
average specific resistance. The method was based on fitting experimental data to an
equation relating the distance travelled by the Iiquid front to the time taken.
Chen et al. (1996) evahatec! the feasibiIity of using CST to characterize
dewaterability of excess activated sludges. The CST was reported to be a good index for
the product of solids concentration and average specific resistance. However, use of CST
values to evahate the bound water in the sludge would be unreliable.
Christensen et al. (1993) used both CST and SRF to characterize dewaterability of
conditioned sludge. An apparatus resistance associated with the CST measurements,
which was significant compared to sludge resistance, was noted. Therefore, a
modification of the CST test was proposed to take into account the impact of CST
rneasurement associated with the apparatus resistance, filtrate viscosity, and the sludge
solids concentration.
Huisman and van Kesteren (1 998) applied large-strain consolidation theory to the
CST apparatus. It was reported that dry matter content could be predicted with respect to
time using a coefficient of consolidation determined by CST test. The effect of polymer
conditioning on dewatering can then be rneasured quantitatively as an increase of the
coefficient of consolidation. However, as the coefficient of consolidation was to be
determined fiom the results of a series of ineasurements, the procedure was found to lack
the simplicity of the CS?' test.
Christensen et al. (1 993) evaluated sludge conditioning performance using four
parameters - CST, SRF, time-to-drain (TTD), and initial settling rate (ISR). Experimental
findings showed that, unlike the other three parameters, CST was not affected by
variations in the mixing regime. The lack of sensitivity to variations in mixing regime
was attributed to an inherent apparatus resistance, and it was felt that CST was not very
appropriate as a measure of sludge conditioning.
There have been several other instances where CST had been used as a pararneter
to evduate conditioner pefiam~ance and dewaterability (Gale and Baskenille, 1970a;
Christensen and Stulc, 1979; Novak and Haugan, 1980; Werle et al., 1984; Novak et al.,
1988; Novak and Bandak, 1989; Christensen et al., 1993; Novak and Lynch, 1990; Cole
and Singer, 1985; Langer and Klute, 1993).
2.3.3 Time-to-filter (TTF)
The time-to-filter test (TTF) was developed by Dentel et al. (1988) as an attempt
to simpli@ the cumbersome procedures involved in conducting the more fundamental
SRF test. Using the same laboratory procedure as the SRF test, the TTF test involves
calculation of the time required for the filtration of a sludge or a conditioned sludge
sample rather than the specific resistances (Dentel, 199 1).
The TTF test consists of placing a sludge sample in a Buchner funnel lined with a
filter paper, applying vacuum, and measuring the time required for a fixed volume of
filtrate (usually 50% of the sample volume) to collect in the graduated cylinder. The
amount of time taken to collect the fixed volume of filtrate is the TTF. Optimum sludge
conditioning is indicated by the minimum TTF value (DenteI et al.. 1993).
The major advantage of the TTF test over the SRF test is that it eliminates the
need to determine the variables (solids content, applied pressure, filter area, sludge
temperature and viscosity) usualiy needed for the SRF test. The main disadvantage of the
TTF test is that, unlike the SRF test, it does not address the effect of such operational
variables as appiied pressure. In addition, the TTF is nearly linearly dependent on sludge
solids concentration, and hence, possibilities of cornparhg the TTF results for sludges of
different solids concentrations are limited (Dentel, 199 1).
2.3.3.1 Factors affecting the TTF results
The factors that influence the TTF results are 1) solids concentration, 2) sample
volume, 3 ) filter paper, 4) vacuum pressure, and 5) sludge temperature. Al1 tests should
be conducted under similar test conditions to minimize errors introduced by variations in
sample volume, filter paper, vacuum pressure, and sludge temperature (Dente1 et al.,
r 993).
Sludge solids concentration has a significant effect on the TTF results. The effect
of solids concentration on the test results c m be avoidcd by adhering to proper sample
preparation procedures to ensure homogeneity between sludge samples. Comparisons of
TTF data between different sarnples should not be atternpted unless the solids
concentrations of the sludges are comparable. TTF results of sludges tested on different
days should also not be compared (Dentel et al., 1993).
2.3.3.2 Applications of TTF as a dewaterability parameter
Dente1 et al. (1988) investigated several methods that could be used to evaluate
polymer performance in coagulation, sedimentation, filtration, and sludge conditioning. A
correlation between TTF and SRF values for a given sludge was confirmeci and TTF was
recornmended, as it was a much sirnpler method. In a water treatment plant in Edgewood,
MD, good correlation was observed when comparing the TTF test results at various
polymer dosages with the dewatering performance while centrifuging alun sludges with
the same polymer dosages.
Novak and Haugan (1980) used TTF and CST values to examine the influence of
polyrner dosing and rnixing on activated sludge dewaterability and floc stability. Results
indicated that TTF values could be relied upon to accurately predict dewatering
performance provided the mixing time and intensity closely matched the actual
dewatering process.
Novak and Lynch (1990) used a modified TTF test procedure to evahate the
effects of potymer conditioning on shear during cake filtration. The filtrate from filtration
tests was refiltered back through the sludge cake and the refiltration time was noted with
each cycle. A 200 mL waste activated sludge sarnple was used and the time to refilter
50 mL of filtrate was measured.
Christensen et al. (1995) used a test procedure similar to the TTF test to evaluate
the time-to-drain (TTD) a fixed volume of filtrate through the support medium in the
cylinder used for SRF testing. Experimental findings showed that TTD, along with SRF
and initial settling rate (ISR), was more sensitive to variations in mixing conditions than
the CST.
2.3.4 Zeta Potential (ZP)
Zeta potential (ZP) is the electric potential which exists at the shear plane around
a solid particle. The shear plane is the interface behveen the boundary layer of water
which remains with the solid and the water farther from the particle which moves in
relation to the particle. ZP measurements have ofien been used to evaluate particle
stability and effective flocculation, because of its quantification of the electrical potential
that causes inter particle repulsion. A near zero ZP indicates minimal electrical repulsion,
making it possible for attractive van der Waals forces to effect particle agglomeration
(Dentel, 199 1).
2.3.4.1. Theory behind ZP
Two phases in contact are always marked by a separation of electric charges at an
interface between them. There is an excess charge of one sign near to or on the surface of
one layer, and the balancing charge is distributed through the adjoining surface regions of
the other phase. If phase 1 is positively charged at the surface, its electrostatic potential
will be positive with respect to the buik of phase II. If phase II is a Iiquid containing
dissolved ions, the potential decreases as one moves into phase II until it becomes
constant in the bulk liquid at distances greater than approximately 5-200 nm fiom the
surface of phase 1. The region where the Iiquid has a positive electrostatic potential will
accumulate an excess of negative ions which gradually Iowers the electrostatic potential
to zero in the b u k electrolyte. The arrangement of charges on the surface of phase I and
opposing charges in the liquid phase II is referred to as the electrical double Zayer at the
interface. When either of the phases are caused to move tangentially past the second
phase, elecrrokinetic eflects take place, and depending on the way the motion is induced.
they are classified as 1) electrophoresis, 2) streaming potential, 3 ) electroosmosis. and 4)
sedirnentation potential (Hunter, 198 1). Any of these four principles can be applied in the
calculation of ZP.
Another electrokinetic parameter that is being increasingly used for monitoring
coagulant dosage is streaming current, measured by streaming current Cetectors (SCD).
The advantages of the SCD are rapidity of measurernent, and the possibility of continuous
control of optimum conditioning dosages. The disadvantages are the cost of the device,
the apparent offset of a SCD zero reading from zero ZP. and the lack of correlation
betwcen charge and optimal flocculation under some conditions (Dentel, 199 1)
The SCD was deveioped and patented by Gerdes (1966), who also established a
relationship between streaming curent and ZP:
where: I = streaming current, w = frequency of pump in cycles per second, s = stroke of
the piston, r = radius of the piston, 6 = ZP, E = dielectric constant, and c = radial clearance
of piston or width of the annulus.
Information on the theoretical principles and practical applications of SCDs in
sludge conditioning is available (Gerrdes, 1966; Dente1 and Kingery, 1989; Dentel and
Abu-06 1993 ; Dente1 et ai., 1989qb; Abu-Orf and Dentel, 1997; Abu-Orf and Dentel,
1998; Dentei and Abu-Orf, 1995; and Dentel, 1995).
2.3.4.2 Applications of ZP
ZP is a cornrnonly measured electrokinetic parameter to evaluate effective
coagulation. Measurernent of ZP in tht= water and wastewater treatment industry is usualty
through the calculation of the electrophoretic mobiIity (EM) of the particles. EM, which
signifies the velocity of a particle per mi t electric field, gives a measure of the net charge
on the solid particle.
Although the American Society for Testing and Materials (ASTM) provides a
procedure for determining ZP, the determination is fairly instrument specific. The
cornrnon Zeta Meter (Zeta Meter Inc-, Long Island City, NY) relies on timing particles
visually using a microscope equipped with an ocular micrometer (Dentel, 199 1).
Dente1 (1991) reported findings fiom literature, which indicated that a few plants
have used ZP measurements on a regular basis for coagulant dose detemination.
However, ZP values have been noted to be inappropriate for treatment plant application
because their determination involved a very cornplicated technique.
Many researchers have used Z P and EM measurements, typically to evaluate the
role of charge neutralization in the conditioning process (Roberts and Olsson, 1973;
Novak and Haugan, 1980; Novak et a l . , 7988).
Cole and Singer (1985) statedi that predictions of optimum conditioning dosages
should not be made using EM rneasurernents, primady because of the lack of a standard
sarnple preparation procedure. It !vas reported that charge neutralization was not a
prerequisite for effective sludge conditioning. They also summarized different sarnple
preparation methods for EM measurernent.
Riddick (1968) felt that reducing the opacity of the sample by simple dilution
would greatly alter the liquid phase and materially change the ZP. Hence, dilution of
sample should not be done to reduce opacity for ce11 electrophoresis.
3.0 MATEMALS AND METHODS
3.1 Sludge sample collection
In diis study, anerobically digested sludge samples were collected fiom the
RWWTP. The sampling was done fiom outiets of pipes leading fiom the secondary
digester to the belt filter press. The collected sludge sample must be adequately
representative of the anerobically digested sludge that is to be condirioned. To ensure this,
sludge was allowed to flow for two to three minutes before 20 L sarnples were collected.
At the laboratory, the sludge was passed through a 4.75 mm sieve to remove gross solids.
The solids content of the sludge was then tested and always found to be between 8 and
9% (the digested sludge at the plant is typicdly between 840% solids).
Aging of sludge can have a significant effect on the dewaterability of the sludge.
Gale (1977) reported that the ef5ect of aging of sludge c m be significant and hence testing
should be completed as soon as possible after sampling. Vesilind (1979) stated that
prolonged storage of sludge could increase the CST fourfold. According to the U.S.EPA
(1987) aging has been known to double the specific resistance in one hour. Therefore, it
becomes imperative that the experiments are conducted within a short time afier
obtaining the sludge sarnples.
In this study, sludge sarnples were preserved at 4OC and tested within 24 hours of
sarnpling.
3.2 Preparation of conditioner solutions
Detailed methodology for conditioner solutions preparation can be found in
Dente1 et al. (1993). The procedures involved are briefly described here.
3.2.1 Preparation of ferric chloride solutions
Ferric chloride (formula weight 162.21) was obtained fiom Fisher Scientific,
Edmonton, Canada, and a 5% (50 g/L) ferric chionde solution was prepared and stored
before collection of sludge samples.
3.2.2 Preparation of polymer solutions
Cationic polymers, Percol 755 and Percol 757. were supplied free of cost by
Allied Colloids (Canada) Inc., Brampton. Ontario.
As polymer products are nat composed of 100% actual or active polymer (Dente1
et al., 1993), the active polyrner content of the polymers had to be determined. The
procedures for the determination of active polyrner content can be found elsewhere
(Dentel et al., 1993). The active polyrner content of Percol 755 and Percol 757 were
determined to be 97.2% and 97.8% respectively. At the RWWTP, polymer is added to the
digested sludge as 7 g L (0.7%) solution. So, it was decided to use a 7 g L solution in Our
studies. The weight of polymer needed ro prepare 1 L of a 7 g L (0.7%) solution was
calculated from the percent activity of the polymers as follows:
Percol755: Percent activity = 97.2% (see Appendix A for calculation)
Weight of ~ I Y polymer added to 1 L of water for 7 g/L solution =
- - 7.2 g
Percol757: Percent activity = 97.8% (see Appendix A for calculation)
Weight of dry polymer added to 1 L of water for 7 g/L solution
- A 7.16 g
The required amount of polymer, as calculated above, was then weighed out. In a
2 L beaker, one liter of dilution water was taken and placed under a Phipps and Bird
bench stirrer paddle. The stirrer was t m e d on to its maximum speed, and the polymer
was slowly spnnkled into the beaker. It was ensured that the polymer grains did not form
lumps. Mixing was done until the polymer fomed uito a homogeneous viscous mixture.
The polymer was then allowed to age ovemight. The preparation of polymer solution was
usually done the evening before sarnple collection.
3.3 Conditioning sludge samples
The sludge sample fiom the plant was thoroughly mixed by transfemng it
repeatedly fiom one container to another. 500 mL of the sludge sample was then taken in
a 1 L glass beaker for conditioning.
3.3.1 Single-chemical conditioning
The specified dose of conditioner was dispensed into the sludge sarnpIe. Ferric
chloride solution was dispensed with a pipette, whik polymer solutions were dispensed
with syringes. The contents of the beaker were sirnultaneously mixed with a Braun Mode1
MR 300 handheld household blender for about 20 seconds, which was approximately the
time taken to uniformly dispense the conditioner solution into the sludge sarnple. Using
the jar test apparatus, rapid mixing was then done for 20 seconds at 100 rpm. The
conditioned sludge was then flocculated gently at 30 rpm for two minutes.
3.3.2 Dual-chernical conditioning
For dual-chernical conditioning, the sarne procedure as mentioned above was
used. The required dosage of femc chloride solution was first added and mixed with the
hand held blender for about 20 seconds. The polymer dosage was then added and rapidly
mixed for 20 seconds. Using the jar test apparatus, the sludge was then mixed at 100 rpm
for 20 seconds, followed by gentle flocculation at 30 rpm for two minutes.
3.4 Specific resistance to filtration
The filtration apparatus used for the SRF test is shown in Figure 3.1. It consisted
of a 100 mL graduated cylinder with a built in adapter and fntted glass side m. A 55
mm diameter Buchner h e l was fixed to the top of the adapter with a nibber stopper. A
tube connected the side ann to a vacuum purnp, through a vacuum reservoir.
5 or 9 c m aarneter
250 ml Graduated Cylinder wi th
AR^^^^^ ~ u b i n ~ / -: \ 1 - -
Vacuum
Z ource with Gauge
Vacuum Flask
Figure 3.1. Filtration apparatus used for SRF ancl TTF tests
45
The graduated cylinder and adapter set up was custom made by La Salle
Scientific, Montreal. A 55 mm diarneter Whatrnan No. 1 filter paper was used. The ak
motor vacuum pump was manufactured by General E!ectric, USA.
3.4.1 Test procedure
Before perfonning the filtration test, the pH, temperature, and solids content of
the sludge sample were determined. The filtration uias carried out in the following steps:
1) The filter paper was sealed to the Buchner h e l base by moistening the filter paper
before placing it in the furinel. Vacuum was applied for a few seconds to drain out the
moisture in the filter paper. The water in the cylinder was drained out before
proceeding further.
2) Exactly 100 mL of sludge sarnple was gently poured into the funne1 and a vacuum of
15 in. Hg was applied at zero time. A stopwatch was started simultaneously.
3) The filtrate volume collected in the cylinder was noted every minute for the first 10
minutes. For the next 20 minutes, readings were taken every two minutes, and as the
filtration proceeded, the time (t) taken for collection of filtrate volumes (V) was noted
every 10 minutes for the first hour and every haif an hour until the vacuum broke.
4) Once the vacuum broke, the sludge cake was gently taken out, the filter paper peeled
out from the bottom, and the total cake solids determined.
5) The solids content of the filtrate, and the pH of the filtrate were noted.
The time (t) taken for the collection of volume (V), of filtrate was noted as shown
in Table 3.1.
Table 3.1. Example of SRlF test data
Experimental Data Derived Data Ratio t (minutes) V hm t~ (seconds
O n nt, 1 0.25
Derived data, t, and VI, was calcdated in the following manner:
a) the first 120 seconds of the filtration was ignored, to allow for a thin cake formation
that would reduce the significance of the resistance offered by the filter paper.
b) the filtrate collected during the first 120 seconds was discounted frorn the folIowing
filtrate readings. In Table 3.1, it can be seen that 1.5 mL of filtrate had collected in the
first 120 seconds. So, the derived volume, VI = V- 1 S. Similarly, the derived tirne,
t i = t - 120.
A graph of ' t iNl versus VIt was plotted as shown in Figure 3.2. Only the linear
portion of the ' t iNI versus VI' plot was used to determine 'b', the dope of the plot. The
slope b is determined to be 6.5 s/m~', as shown in Figure 3.3. The SRF of the sludge was
then calculated as follows:
where: r = SRF, Trnlkg,
P = applied vacuum, (@n2)
A = area of the filter paper (cm2)
b = dope of the YIN 1 versus VI ' plot (seclrn~')
p = viscosity of filtrate, taken as that of water (g/cm.sec)
w = weight of dry solids per unit volume of filtrate (g/mL)
Figure 3.2. Example o f 'tlN1 versus Vit plot
--
Figure 3.3. Determination of slope o f linear portion of VIN1 versus VI' plot
3.4.2 Determination of weight of dry solids per unit volume of filtrate
The weight of dry solids per unit volume of filtrate, W. was calculated as follows:
1) the point at which the linearity of the ' t iN i versus VIy plot did not hold was the
filtrate volume at the end of the cake formation period (Christensen and Dick, 198%).
From Figure 3.2, it can be noted that the linearity did not hold at a filtrate volume of
67.5 mL.
2) the feed solids concentration of the sludge sample was already noted at the beginning
of the filtration. The weight of dry solids per unit volume of filtrate was then
calculated as:
Cake solids concentration x sampIe volume - - W filtrate volume at the end of the filtration phase
Conducting SRF tests for sludges, especially poorly conditioned sludges, c m be a
time consurning &air. Therefore, bearing in mind the need to complete experiments as
quickly as possible after sample collection, the SRF values were taken as an average of
only two test results. However, the results were monitored for unacceptably large
differences in SRF values for the same conditioner dose.
3.5 Capillary suction time
The CST (Mode1 1) apparatus, shown in Figure 3.4, and the chromatography grade
papers used in this study were purchased from Venture Innovations, Inc., Lafayette,
Louisiana.
Sludge
Sludge I
Automatic Counter (seconds) -
BIwk holding probec
Base
Reference marks on block holding probes
1 Probes resting on - filter paper
Figure 3.4. Capillary suction tinie apparatus
The CST apparatus consists of a tirner case, a test head with two plastic test
blocks, and a stainless steel cyiinder with 1 cm diameter at one end and 1.8 cm diameter
at the other end.
The filter paper was placed between the upper and lower plastic test blocks. The
stainless steel cylinder was placed inside the cavity in the test head. To ensure even
contact with the filter paper, the cylinder was rotated applying a slight downward
pressure. The timer case was twned on and 7-8 mL of the sludge sample was withdrawn
using a large bore pipet and slowly dispensed inside the metal cylinder.
The filtrate fiom the sludge was absorbed radially outward on the filter paper. As
the moisture flowed about 0.8 cm past the cylinder edge, it triggered off the first two
contacts, and an electric signal started the timer. As the moisture travelled a further 1 cm
and touched the third contact, the timer stopped. The CST, in seconds, was read directly
fiorn the timer display.
Al1 experiments in our study were conducted using the 1.8 cm diameter end. A
minimum of three measurements was taken per sample, and the average of the readings
was taken as the CST.
The filtration set up used for the TTF test was the sarne as the apparatus used for
the SRF test. The experimentd set up is shown in Figure 3.1.
A 55 mm diameter Whatman No. 1 filter paper was moistened and placed inside
the Buchner b e l . The vacuum pump was brkfly tumed on and stopped, and water fiom
the filter paper that had dropped into the cylinder was drained off thoroughly.
The vacuum pump was turned on, and 100 rnL of sludge was gently poured into
the Buchner h e l . A stopwatch was started simultaneously. TTF was noted as the time
taken for 50 mL of filtrate to accumulate in the cylinder.
TTF values were taken as the average of two tests. However, if the results showed
an unacceptably large difference in TTF values for the same conditioner dose, the tests
were repeated.
3.7 Zeta potential
Determination of ZP in this study was based on caiculation of the electrophoretic
mobility (EM) of the charged particles by timing their rate of movement in a DC voltage
field. The ZP measurements were made using Zeta-Meter Model-77, rnanufactured by
Zeta-Meter, Inc., New York, N.Y. A view of the instrument is shown in Figure 3.5.
3.7.1 Sample preparation
Cole and Singer (1985) reported the lack of a standard sarnple preparation
procedure for EM measurements and conducted tests on sarnples prepared using the three
following methods:
1) the solid and liquid fractions of the sludge were separated by filtration using a
Buchner h e l , and the filtrate was analyzed for EM;
Figure 3.5. View of Zeta-Meter (Mode1 ZM-77)
2) 5-6 drops of conditioned sludge sample were resuspended into 40 mL of filtrate,
obtained by the previous method, using a blender. The EM of the particles in the
suspension was then measured; and
3) the sludge sample was subjected to high speed centrifbging at 3000 rpm for 15 min,
after which 250 mL of the resultant centrate was dosed with polymer and blended For
5 minutes. The EM of the particles in the centrate was then measured.
Riddick (1968) reported that ZP of concentrated systems could be accurately
determined by separating the colloid from its continuous phase, then reconstituting the
system by adding a single small drop of the original slurry to the clarified bulk liquid.
This procedure ensured that, both the bulk liquid and the colloid with its adsorbed ions,
remained substantially unchanged. It also reduced opacity to an extent that readily
permitted electrophoresis.
Cole and Singer's (1 985) second method described above suggested the use of 5-6
drops of sludge. However, upon resuspending 5-6 drops of sludge into the filtrate, it was
found that the opacity of the sample was too high to enable proper visualization of
particles. On the other hand, when only two or three drops of sludge were tised, the
visualization of particles was much better.
Therefore, for this study, two or three drops of sludge were resuspended in
approximately 150 mL of filtrate obtained frorn the filtration experiments conducted on
that particular sludge. Just before canying out the ZP measurements, the filtrate and the
drops of sludge were thoroughly mixed by vigorously shaking the container for about a
minute.
3.7.2 Test procedure
A Mode1 ZM-77 Zeta-Meter consists of a power unit and a rnicroscopic module.
The power unit provides a precision regulated DC output voltage. The rnicroscopic
module carries the microscope and two illuminators that enable particle tracking. The
module also supports the electrophoresis ce11 below the microscope.
A brief description of the principles involved in ZP measurement and the test
procedure are given below. Zeta-Meter (1977) provides a more detailed explanation of
the experimental procedure.
The Zeta-Meter essentially measures the mobility of charged particles in a DC
voltage field. The best results will be obtained by using the maximum allowable voltage.
The maximum allowable voltage depends on particle velocity and thermal overturn.
Thermal overturn is related to specific conductance, which is a measure of the ability of
the sample to convey electric current. Therefore, the specific conductance of the sample
would determine the maximum voltage to be applied.
3.7.2.1 Specific conductance measurernent
The following procedure was used to determine the specific conductance of the sample:
1) The sample was filled in the cell, the electrodes were installed and the leads attached.
2) Each cell was pre-calibrated to a specific "K" factor, which determines the DC
voltage to be applied. The DC voltage was set to 65 volts, which matches the "K"
factor of the cell.
3) With the MICROAMPS switch on 1000X, the polarity switch was depressed. The
switch was turned clockwise until the MICROAMMETER read between 10 and 100.
4) The specific conductance of the sample was equal to the microammeter reading times
the MICROAMPS multiplier.
Once the specific conductance of the sample was known, the recommended
maximum voltage to be applied was read fkom the Table 3.2.
3-7.2.1 Particle tracking
Once the specific conductance of the sample was measured and the recommended
maximum applied voltage known, the next step in ZP determination was timing the
velocity of charged particles. Tracking the particles involved the following steps:
1) The sampIe was filled in the cell, the electrodes were installed and the leads attached;
2) The frontal illurninator was W e d on and the positioning line was located;
3 ) The ocular micrometer was aligned with the positioning line using the ce11 calibration;
4) The side illurninator was then switched on. Without any applied voltage, the particles
seen should not move.
5) The recommended voltage, as read from Table 3.2, was then applied and the
movernent of the particles was noticed along the tracking line. Negative particles
move to the lefi, and positively charged particles move to the right.
6 ) Immediately afier the voltage was applied and the particles start moving, a particle
near the tracking line was selected and its movement along the tracking line is timed.
7) A minimum of 10 particles was tracked, and the average tracking time was
deterrnined. Once the average tracking time had been detemined, the ZP was
calculated using the methodology specified in Zeta-Meter (1 977).
The accuracy of zeta potential measurements is known to be dependent on the
number of particles tracked in their linear path, i-e., greater the nurnber of particles
tracked, the more accurate zeta potential values are expected to be.
However, in this study, it was not always possible to track several particles. In
many instances, it was possible to track a maximum of only £ive particles before thermal
overturn caused the particles to deviate fiom their linear path. In such cases, the zeta
potential readings may not be very accurate.
Table 3.2. Specific conductance and mau. recommended vdtage (Zeta-Meter, 1977)
Specific conductance (@cm) 1 Recommended maximum applied voltage ] Less than 300
700 300 200
4.0 RESULTS AND DISCUSSION
Jar tests, which rely on visual observation, can be used for an initial screening of
conditioning chemicals. However, when there is a need to evaluate conditioner
performance or dewaterability in detail, parameters such as SRF, CST or TTF provide a
much more reliable option.
In this study, the determination of optimum dosages of single- and dual-chernical
conditioning, and evaluation of their dewaterability potential was conducted based on
dewaterability parameters. Hence, it was necessary to define the criteria applied in
interpreting these pararneter values.
4.1 Interpretation of dewatera bility parameter values
W l e operating with pararneter values, there is a need to equate numerical values
obtained by experimental methods to the degree of dewaterability they signfi. For
exarnple, it is obvious that a sludge with a CST value of 20 s would definitely be less
easily dewaterable than a sludge with a CST value of 19 S. But that is not reason enough
to choose, as an optimum, the dose that produces a CST of 20 S. simply because fùrther
addition of conditioner did not result in a cornparably signifiant lowering of the CST
value. We have to bear in mind that the reduction in CST to 19 s might resuit in a much
more dewaterabie sludge, in which case the economics involved would justiQ the
increased conditioner dosage.
There needs to be a basis by which optimum conditioner dosages are determined.
Some information on dewaterability parameter values and the degree of dewaterability
that they s ignie is available. For example, a CST of 20 s is a generally accepted measure
of good dewaterability (Vesilind, 1988; Abu-Orf and Dentel. 1999).
There is a general range of SRF values that indicate the degree of dewaterability
of sludges. Gale (1971 b) reported that raw wastewater sludges have SRF values of
1 0- 100 Tm/kg; well-conditioned sludges ordinarily have SRF values of O. 1 - 1 Tmkg. A
SRF value of 1 Tm/kg was suggested as a limit for economical sludge filtration.
While it is known that zero ZP values indicate complete charge neutralization, it
has also been shown (Cole and Singer, 1985) that charge neutralization is not always
required to achieve an optimal dosing of a sludge.
Optimal dosing does not necessarily mean conditioning the sludge to produce
maximum dewaterability. In this study? an optimal dosage was determined based on two
criteria, bath of which had to be satisfied by a dosage in order to be chosen optimum:
1) if the decrease in two consecutive parameter values, for sarnples conditioned with
increasing dosages of conditioners, was less than 5% of the value for the raw sludge,
the lower dosage would be taken as the optimum.
2) a CST value of less that 20 s and a SRF value of less that 1 T d g was indicative of
good dewaterability. An optimum dosage should produce CST and SRF values not
more that 20 s and 1 Trnkg, respectively.
An evaluation of optimum dosages based on TTF values alone are possible only
by using criterion 1, as there were no studies that related TTF values to ease of
dewaterability.
4.2 Determining individual optimum conditioner dosages
To decide on the individual conditioner dosages for dual-chemical conditioning, it
was necessary to first determine the individual optimum dosages for the three
conditioning chemicals - ferric chloride (FC), Percol755 and Percol757.
4.2.1 Ferric chloride conditioning
500 mL sludge samples were conditioned with 5% FC solutions to obtain dosages
ranging from 2000 mg/L to 9000 mglL. The pH, temperature. and solids content of the
raw and conditioned sludges were measured. The SW, CST and TTF values of the raw
sludge were determined to be 29.9 Tmkg, 201 -8 s, and 274.3 min, respectively. The SRF,
CST, and TTF values of the conditioned sIudge samples were measured.
Figures 4.1, 4.2 and 4.3 show the results o f SRF, CST, and TTF tests,
respectively. The complete results (dong with pH, temperature and solids concentration
data) are presented in Appendix B. Complete SRF test readings are given in Appendix C.
As per the S W test results in Figure 4.1, a FC dose of 69.1 kg/T produced a SRF
value of slightly below 1 Tmkg (0.99 Trnkg). A subsequent dose of 80.6 kglT produces
a SRF of 0.8 Trnkg. Therefore, if we consider SRF values alone, then the FC dose of
69.1 kg/T had to be the optimum, as it satisfied both critena.
According to the CST data in Figure 4.2, the 69.1 kg/T FC dose produced a CST
of 20.9 s, whereas the subsequent dose produced a CST of 19.3 S. Obviously, only the
latter dose of 80.6 kg/T satisfied both the criteria, and hence had to be the optimum dose.
40 60 30
Ferric chioride dose (kgT dry solids)
Figure 4.1. Variation of SRF with increasing ferric chloride dosages
- O 10 20 30 40 50 60 70 80 90 100 110
Ferric chloride dose (kg/T dry solids)
Figure 4.2. Variation of CST with increasing ferric chloride dosages
O IO 20 30 40 50 60 70 80 90 100 110
Ferric chloride dose (kg/T dry solids)
Figure 4.3. Variation of TTF with increasing ferric chloride dosages
Based on TTF test results shown in Figure 4.3, the optimum dosage had to be
69.1 kg/T, which produced a TTF of 11.9 min, whereas the subsequent 80.6 kg/T dose
reduced the TTF only by a further 0.2 min.
The 80.6 kg/T dose, which resulted in SRF and CST values of 0.8 T d g and
f 9.3 s, respectively, was the lowest dose that satisfied the two criteria for selection of an
optimum dosage. Hence, 80.6 kg/T was chosen as the optimum FC dose.
The pH of the sludge was expected to decrease with femc chloride conditioning.
Also, increase in the volume of FC conditioner solution used to condition the sludge
would decrease the solids content of the sludge sarnples. Figure 4.4 shows the anticipated
reduction in pH and solids content with increasing FC conditioner dosages. Figure 4.5
shows the reduction in pH of filtrate and increase in filtrate solids with increasing FC
dosages.
Subsequent to the dewatering process, the supernatant collected is usually
returned to the head of the plant, where it is mixed with the raw wastewater. Increasing
solids content in the filtrate, as noticed with FC conditioning, could appear to be a
downside of FC conditioning. However the increase, due to the filtrate solids, in the
overall solids content of influent raw wastewater and filtrate mixture, is generally too
minimal to pose a serious concern. For example, the amount of sludge conditioned per
year is approximately 10 ML, which would yield a filtrate volume of approximately
0.35 ML, whereas the total arnount of wastewater treated m u a l l y is 25,000 ML. As can
be seen, the volume of filtrate is almost negligible in cornparison with the raw wastewater
volume. So, the increased pH and solids content of the filtrate due to FC conditioning
would not have a significant bearing on the influent wastewater quality.
--- - Sludge sol ids (%)
50 100
FC dose (kg/T dry solids)
Figure 4.4. Sludge pH and solids variation due to FC conditioning
- Filtrate solids (%)
8 - - + - - Filtrate pH *.
20 40 60 80 100
FC dose (kg/T dry solids) --.-_--II -
Figure 4.5. Filtrate pH and solids variation due to FC conditioning
4.2.2 Polymer conditioning
Two cationic polyrners - Percol 755 and Percol 757 - were used to condition the
RWWTP sludge. 500 mL sludge samples were conditioned with 0.7% polyrner solutions
to obtain dosages ranging fkom 126 mg/L to 280 mgL.
The SRF of the raw sludge was found to be in the range of 24.3-25.2 Trn/kg, the
CST, 15 1.3-167.7 s, and the TTF, 246-258 min, respectively. Figures 4.6, 4.7 and 4.8
show the results of SRF, CST, and TTF tests, respectively. The cornplete results (dong
with pH, temperature and solids concentration data) are presented in Appendix B. In
addition. the complete SRF test readings are given in Appendix C.
From the SRF results presented in Figure 4.6, it can be seen that the lowest
dosages that produced SRF values below 1 Tm/kg were 2.9 kg/T (SRF = 0.8 Tmlkg) for
Percol 755, and 2.7 k f l (SRF = 0.9 Tm/kg) for Percol757.
As per the CST results in Figure 4.7, the lowest polyrner doses that produced CST
values < 20 s were 2.9 kg/T (CST = 14.1 s) for Percol755, and 2.7 kg/T (CST = 19.2 s).
Based on TTF test results from Figure 4.8, it was evident that using criterion 1 in
deciding optimum dosages might be erroneous. This was because, for Percol 755, an
increase in conditioner dose fkorn as low as 2 kg/T to 2.3 kg/T produced a decrease in
TTF of less that 5% (TTF reduces from 38.9 min to 26.3 min). Based on corresponding
CST and SRF data, these dosages were definitely not optimal.
1 2 3
Polymer dose (kg/T dry solids)
Figure 4.6. Variation of SRF with increasing polymer dosages
1 2 3
Polymer dose ( k f l dry soli&)
Figure 4.7. Variation of CST with increasing polymer dosages
68
1 2 3
Polymer dmse (kgfi dry solids) - -
Figure 4.8. Variation of TTF with increasing polymer dosages
The 2.9 kg/T Percol 755 dose, which produced a CST of 14.1 sec, and a SKF
value of 0.8 Tmkg was the lowest dose that satisfied both the criteria for an optimal
dosage selection. Sirnilarly- a Percol 757 dose of 2.7 kg.T (CST = 19.2 sec and
SRF = 0.9 Trnkg) also satisfied both the criteria. Therefore, these values were chosen as
the optimal polymer dosages.
The change in sludge pH due to polyrner conditioning was expected to be minimal
in comparison with FC conditioning- Figure 4.9 shows the variation in sludge pH and
soIids content with PercoI 755 dosage. Sirnilarly Figure 4.10 shows the effect of Percol
755 dose on the filtrate pH and solids content. Unlike with FC conditioning, Percol 755
conditioning did not increase the filtrate solids content.
Figures 4.1 1 and 4.12 show the corresponding variations in pH and solids content
of the sludge and filtrate due to conditioning with Percol 757. Once again, there was
minimal variation in pH and solids content of the sludge and filtrate, in comparison with
FC conditioning.
Sludge solids (%)
1 2 3 4
Percol 755 dose (kg/T dry solids)
Figure 4.9. Sludge pH and solids variation due to Percol755 conditioning
- Filtrate solids (%)
i - - + - - Filtrate pH L
1 2 3 4
Percol 755 dose (kg/T dry solids) -
Figure 4.10. Filtrate pH and solids variation due to Percol755 conditioning
Sludge solids (%)
- - + - - Siudge pH
1 - 7 3
Percol 757 dose (kg/T dry solids)
Figure 4.11. Sludge pH and solids variation due to Percol 757 conditioning
7-
-t Filtrate solids (%) :
i - - + - - Filtrate pH
1 2 3
Percol 757 dose (kg/T dry solids)
Figure 1.12. Filtrate pH and solids variation due to Percol757 conditioning
4.3 Cornparison of performance of Percol755 and Percol757
Figures 4.6,4.7 and 4.8 give the results of SRF, CST, and TTF tests, respectively,
with an increase in the Percol755 and Percol757 doses.
An optimal dosage of 2.9 kg/T of Percol 755 was found to reduce the raw sludge
CST by 91.6%, the SW by 97%, and the TTF by 96.5%. An optimal dosage of 2.7 kgîT
of Percol757 reduced the raw sludge CST by 91.6%, the SRF by 97.3%, and the TTF by
96.7%. Also, the variation in pH and solids content of the sludge and filtraie due to Percol
755 and Percol 757 conditioning (Figures 4.9, 4.10, 4.1 1 and 4.12) were very sirnilar. No
significantly superior performance of one polyrner over the other was observed.
4.4 Performances of SFW, CST and TTF as dewaterability parameters
4.4.1 Specific resistance to filtration
In this study, a SRF value of less than 1 Tm/kg was assurned indicative of good
dewaterability. Based on this criterion, the SRF test results indicated an optimal FC dose
of 69.1 kg/T, a Percol 754 dose of 2.9 kg/T, and a Percol 757 dose of 2.7 kg/T,
respectively.
As described in section 3.4.2, the calculation of 'w' involved the determination of
the filtrate volume at the end of the cake formation period. Christensen and Dick (1985b)
reported that the standard model for slurry filtration was based on physically changing the
sludge into a cake and that the model did not include dewatering after the slurry was
depleted.
Figures 4.13 and 4.14 show the reduction of filtrate volume at the end of cake
formation with increasing conditioner dosages. The filtrate volumes at optimal dosages of
FC, Percol 755, and Percol 757 were approximateIy 55 mL, 47 mL, and 46 mL
respectively. The filtrate volume at the end of cake formation for raw sludge was found to
be behveen 62 and 67 mL-
The filtrate volume at the end of cake formation period was detemined using the
'tN versus V graph' plotted with values from the filtration tests (see Figure 3.2). A
sumrnary of the different 'end of cake formation' filtrate voIumes and slopes for the
various SRF tests is presented in Appendix D. The complete readings of the SRF tests
conducted with the different conditioners are presented in Appendix C.
Filtrate volume at the end of cake formation is the point at which the linearity
does not hold. However, there was not always a clear point where the linearity did not
hold. Many times the break in linearity was very gradual, making the determination of
filtrate volume very difficult. In such instances, there was always a scope for marginal
error in determining the end of filtrate cake formation.
Figures 4.13 and 4.14 show the variation of 'filtrate volumes at end of cake
formation' with FC and polyrner dosing, respectively. From the figures, it is evident that
the filtrate volumes at the end of cake formation showed an overall downward trend with
conditioner addition.
The above observation can be explained by the known fact that the addition of
conditioner causes the sludge particles to form flocs. With increasing dosage, the number
of flocs become more andlor individual flocs corne together ta form larger flocs. During
floc formation, water in the sludge is trapped in the flocs. Vesilind (1994) called this
water 'interstitial water' and Smollen (1988) calied this 'immobilized moisture'. Water
held in the flocs c m become free (or bulk) water if the floc is destroyed.
In Figure 3.2, the filtration phase is the one until 67.5 mL of filtrate is
accumulated in the cylinder. Beyond this phase is what is called as the expression phase.
The filtration and expression phases are described in more detail in Novak et al. (1999).
The observed reduction in filtrate volumes with increasing conditioner dosage mems that
the water trapped in the flocs is not completely released in the filtration phase. In the
expression phase? upon further application of pressure, the flocs break and the water
trapped inside is released. This was confirmed in this study by the fact that, in al1
filtration experiments, at the tirne of vacuum break, the final filtrate volumes were
between 70 and 75 mL approximately.
4.4.2 Capillary suction time
A CST value less than 20 s was assumed to be indicative of good dewaterability.
Based on this criterion, the CST test results indicated an optimal FC dose of 80.6 kg/T, a
Percol 754 dose of 2.9 kg/T, and a Percol 757 dose of 2.7 kg/T. respectively. While the
optimal FC dose as per SRF values was 69.1 kg/T, the optimum polymer conditioning
doses estimated by CST matched the SRF values for optimum doses.
40 60 80
FC dose (kfl dry solids)
Figure 4.13. Effect of FC dose on filtrate volumes at the end of cake formation
1 2 3 4
Polymer dose (kg/T dry solids)
Figure 4.14. Effect of polymer dose on filtrate volumes at end of cake formation
SRF is a dewaterability pararneter that is based on a theoretical mode1 of filtration
(Smollen, 1986a), and has a very strong fundamental basis. On the other hand, CST is a
purely ernpirical phenomenon and is not based on a theoretical analysis of sludge
dewaterability (Vesilind, 1988). In addition, while the SRF test lacks simplicity, and is a
diff~cult, time-consuming test to conduct on a routine basis, the CST is an extremely
simple test. Therefore, if a correlation between CST and SRF values can be obtained, it
would be much simpler to nin a series of CST tests for diffèrent conditioner dosages, and
approximate the SW values from the correlation. The accuracy of this correlation c m be
improved if the CST is plotted against the product of SRF and the sludge solids
concentration (Vesilind, 1 979).
Figure 4.15 shows a plot of CST results against the product of SRF and the
corresponding sludge concentration values. An excellent correlation (~'=0.99) was
obtained in this study between the CST and SRF values.
There is presently no information relating a specific range of TTF values to the
degree of dewaterability they signi@. In the absence of any known standard relating TTF
values to dewaterability, it was not easy to accurately estimate optimal dosages using TTF
test results alone. However, the overall trend of the TTF results matched the SRF test
results. Figure 4.16 shows the correlation between TTF and SRF values. Optimum
dosages, as determined by CST and SRF values, yielded TTF values between 9 and
12 min.
Percol755 Percol757 y = 1.42~ - 18.1 y = 1 . 6 3 ~ - 23.8
Ferric chloride y = 1.3% - 21.9
Ferric chloride
i PercoI 755 A Percol757
--
100 150
CST (secs)
Figure 4.15. Correlation of SRF and CST test results
1 O0 150
TTF (min) - -___ ___ _ - - _ - -__- - -
Figure 4.16. Correlation of SRF and TTF test results
4.5 Role of charge neutralization in sludge conditionimg
The ZP of anerobically digested RWWTP slindge was found to be between
-26 mV and -18 mV. Figures 4.17 and 4.18 show -the increase in ZP with sludge
conditioning.
It can be seen fiom the figures that the ZP value a t the optimum dosages was zero
mV. This indicates that optimum dosing with d l three conditioners completely
neutralized the negative charge of the sludge particles.
In a11 instances, the ZP values continued to r e m a h zero even afier further addition
of conditioner beyond the optimum dosages. It wouId. be expected that the ZP value
would increase to a positive value with hrther addition of conditioners. The reason that
this did not happen in this study could be because ZP mezisurements at very low near-zero
values were extremely difficult to measure. At near zero ZP values, the colloidal particles
in the suspension travel at a very slow rate. The pamticles have to move dong the
horizontal tracking Iine to be tracked accurately. Haowever, due to the very slow
movement of the particles, the particles did not stay a l o n s the horizontal line long enough
for thern to be tracked before the occurrence of thermal overturn. During thermal
overturn, particles begin to travel in a helical path rather than in a straight line
(Zeta-Meter, 1977). In such cases, the ZP was noted a s zero, although they could very
well have been marginally positive.
Ferric chloride dose (kg/T dry solids) ---
Figure 4.17. Effect of FC dose on zeta potential
-30 ' Polyrner dose (kgT dry solids)
Figure 4.18. Effect of polymer dose on zeta potential
Charge neutralization and particle bridging are two likely mechanisms involved in
effective sludge conditioning (Chitikela and Dentel, 1998). Whife charge neutralization
completely occurred at optimal conditioner dosages, m e r addition of conditioners
brought about a marginal decrease in SRF, CST and TTF values (see Figures 4.1,4.2,4.3,
4.6, 4.7 and 4.8) during FC and Percol 757 conditioning. However, the corresponding ZP
values did not register any change fiom the zero mV reading at the optimal dosage. The
marginal decrease in SRF, CST and TTF values indicates better dewaterability. But, as
the corresponding ZP values continue to remain zero with conditioner addition, charge
neutrdization could not have contributed to this increased dewaterabiIity. Hence the
increase in dewaterability was most certainiy as a resuIt of particle bridging. However, it
was extremely difficult to assess the individual roles of particle bridging and charge
neutralization in conditioning at optimal dosages. Chitikela and Dentel (1998) also
reported difficulties in identifiing the roles played by the individual mechanisms.
In this study, complete charge neutralization was always achieved at the optimum
dosages. However, it has been shown (Cole and Singer, 1985) that dosing to achieve
charge neutralization was not a prerequisite for effective sludge conditioning. Also, the
fact that there is no standard sarnple preparation procedure for ZP measurements
indicated that predictionç of optimum dosage coiild not be reliably made by ZP
measurements alone. However, used in conjunction with parameters such as SRF and
CST, ZP measurements provide a good basis for understanding the contribution of charge
neutralization to the conditioning process.
4.6 Dual-chernical conditioning
The optimum dosages of FC, Percof 755 and Percol 757 were determined to be
80.6 kg/T, 2.9 kg/T, and 2.7 k g , respectively. Dual-chemical conditioning was carried
out by adding the following proportions of f e m c chloride and polymer to 500 mL sludge
sarnples:
I ) 25% of optimum dosage of FC + 75% of optimum dosage of Percol757
2) 50% of optimum dosage of FC + 50% of optimum dosage of Percol 757
3) 75% of optimum dosage of FC + 25% of optimum dosage of Percol757
Sludge sarnples were also conditioned with similar proportions of FC and Percol
755. The results of dual-chernical conditioning are given in Table 4.1.
Dual-chernical conditioning was found to result in ZP values of zero mV. Sludge
sarnples conditioned with a combination of 5 0 % of the optimal dosage of FC and 50% of
the optimal dosage of Percol 757 were found to result in the least SRF, CST 2nd TTF
values. AI1 six combinations of FC and polymer provided SRF, CST and TTF values that
were significantly lower than the corresponding values observed at optimal conditioning
with these conditioners individually. This indicated that a percentage of the optimum
dosage of polymer used for conditioning could be replaced by a similar percentage of the
optimum dosage of FC. This substitution of a proportion of polymer with FC was also
found to marginally improve dewaterability. This finding is similar to that reported by
Chitikela and Dente1 (1998).
Table 4.1. Results of dual-chernical conditioning
Conditioner detaih TTF
(min)
Polyrner CST
(sec)
Table 4.2 presents the pH and solids content of the filtrate of dual-chemical
conditioned sludge sarnples. It can be seen that the pH and solids content of the filtrate
increased with the addition of increasing volumes of FC. However, as explained in
section 4.2.1, the effect of increase in the fihate soiids would be minimal, when
comparing the lower filtrate volume with the much higher influent wastewater volume.
4.7 Costs of single- and dual-chernical conditioning
Based on the expenmental findings, the single- and dual-chemical conditioner
costs were estimated. Detaiis of cost estimation are presented in Appendix E. The
conditioner costs per ton of dry solids are given in Table 4.3.
Dual-chemical conditioning costs \vert- significantly higher than polymer
conditioning alone. Percol 757, presently being used at the RWWTP, is the cheapest
option avaiIable for conditioning the sludge. However, dual-chemical conditioning
enhanced dewaterability in cornparison with single-chemical conditioning. Enhanced
dewatering may result in significant savings in disposai costs. Presently the dewatered
cake is stockpiled at the R W T P . If there is a Euture need to landfill the dewatered cake,
the higher cake solids content resulting fiom enhanced dewaterhg with dual-chemical
conditioning might prove to be a very invaluable option.
Table 4.2. Dual chernical conditioning filtrate characteristics
Table 4.3. Single- and dual-chemical conditioner costs per ton of dry solids
Conditioncr combination FC dose
- - - - - - - - - -
Conditioner dose (kfl)
Conditioner cost
per ton of dry solids
($1
-
PH
7.1 6.6 5.4 7.2 6.5 5.4
1
FC 18.3 36.6 54.9 18.3 36.6 54.9
- ~olidsContent (%)
0.36 0.5 1 0.72 0.34 0.52 0.70
Percol755 2.0 1.3 0.7 O O O
PercoI757 O O O
2.0 1.3 0.7
5.0 CONCLUSIONS AND RECOMMENDATIONS
The objective of this study was to evduate the feasibility of dual-chemical
conditioning of the anerobically digested wastewater sludge at the RWWTP. Three
parameters - CST, SRF and TTF - were used to evaluate conditioner performance and
dewaterability. An additional parameter, ZP, which measures the overall charge on
particles in suspension, was used to study the role of charge neutralization in the
conditioning process.
5.1 Conclusions
FC, Percol 755 and Percol 757 conditioning were a11 found to significantly reduce
the SRF, CST, and TTF values of the RWWTP sludge. The individual optimum dosages
of FC, Percol 755 and Percol 757 were detemined to be 80.6, 2.9, and 2.7 kglT,
respectively. SRF was found to have an excellent correlation with CST and TTF test
results. CST and TTF are much sirnpler tests to conduct when compared to the tirne
consuming and complicated SRF test. Because of the excellent correlation shown
between these results, it is possible to conduct a series of CST or ?TF tests, and
approxirnate SRF values fiom them.
Charge neutralization was found to occur at the optimum dosages of al1
conditioners. However, the precise role of particle bridging and charge neutralization in
the sludge conditioning mechanism was still not very clear. ZP measurements, when used
in conjunction with CST or SRF values, provide excellent information about the
contribution of charge neutralization to the conditioning process. However, îhey should
not be used as the sole basis for predicting optimum dosages. UnIike CST, SRF and TTF
tests, ZP measurements cannot be used to predict dewaterability with any accuracy.
Dud-chemical conditioning using different proportions of the optimum dosages
of FC and polymer indicated the following:
1) A proportion of polyrner used for conditioning can be replaced by a similar proportion
of FC, without adverseIy affecthg the dewatering eficiency.
2) Sludge samples conditioned with a combination of 50% of the optima1 dosage of FC
and 50% of the optimal dosage of Percol 757 were found to have the least Sm, CST
and TTF values.
3) Dual-chernical conditioning costs were significantly higher when compared with
conditioning using FC or polymer alone.
4) Dual-chemical conditioning marginaIly increased the dewaterability of the sludge. If
there is a future need to Iandfill the RWWTP sludge cake, dual-chemical conditioning
might prove to be an economically more feasible option because of its enhanced
dewatering po tential.
FC conditioning of the RWWTP sIudge was found to result in increased filtrate
pH and solids content, when compared with polyrner conditioning alone. Subsequent to
the dewatenng process, the filtrate collected is usually retumed to the head of the plant,
where it is mixed with the raw influent wastewater. However the increase, due to the
filtrate solids, in the overdl soIids content of influent raw wastewater and filtrate mixture,
is generdly too minimal to pose a serious concern.
5.2 Recommendations
Experimentd fmdings have shown the enhanced dewatering capacity of dual-
chemical conditioning. Ferric chlonde conditioning of sludges serves to controI odor, and
prevents scale formation on pipes canying the sludges. Estimating the savings involved in
these advantages would be extremely difficult. Conducting a pilot-scale study at the
RWWTP would be a better means of evaluating the efficacy of dual-chernical
conditioning. Because of its excellent ccrrelation with SRF, the more fundamental
dewatering parameter: CST testing c m be used on a regular basis at the RWWTP to
monitor conditioning performance.
5.3 Need for future research
The following are identified as areas that require further study:
1) Mixing tirne and intensity have been known to greatly influence the conditioning
mechanism. This study incorporated only a particular mixing regime to condition the
sludge sample. Different combinations of mixing times and intensities could be
examined to develop the optimum operating conditions.
2) In this study, femc chloride conditioning was carried out by adding varying arnounts
of a 5% fer+ chloride solution to make up different conditioning dosages. A 2000
mgL conditioner dosage required the addition of 20 mL of the 5% solution to the 500
mL sludge sample. As much as 70 mL of solution had to be added to the sludge
sarnple for an optimal dose of 7000 mglL or 80.6 kg/T. Addition of this conditioner
volume increases the sludge volume by 14%. To prevent the increase in sludge
volume, it would be interesting to evaluate the conditimning performance of using a
constant volume of conditioners of different strengths to prepare varying conditioner
dosages.
3) Further studies are necessary to cleariy explain the indiwidual roles played by particle
brïdging and charge neutralization in the sludge conditiobning mechanism.
4) There is a need to evolve a standard sample prieparation procedwe for ZP
measurements. The various sample preparation methodsz for ZP measurements have to
be studied exhaustively and one method has to be chosem as the standard.
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APPENDICES
APPENDIX A: Percent polymer activity calculation
Percent polymer solids determination for Percol755
Description 1
Weight of polymer used = A
Weight of beaker + magnetic stirrer bar = B 11 63.6809 1 61.4205
Weight of beaker + dried polymer + magnetic stirrer bar = C 11 68.5530 1 66.2443
)/o Polyrner solids 1 % Non-polymer solids 2.25 3.45
-
(
(
- -
(C-B)xlOO % polymer solids or polyrner activity, P =
A
% non-polymer solids, P' = % Total solids - % polyrner solids
Average % polyrner solids or polymer activity, P = 97.2
Average % non-polymer solids, P' - 2.8
Percent polymer solids determination for Percol757
Description 1/8arnple # L Sarnple # 2
Weight of polyrner used = A 4.8644 4.9837
Weight of beaker + magnetic stirrer bar = B 56-7834 61.40 1 1
Weight of beaker + dried polymer + magnetic stirrer bar = C 6 1 .5396 66.2800
YO PoIymer solids Il 97-78 l 97-90 % Non-polymer solids 2.22 2.10
% polyrner solids or polymer activity, P = (C-B)xT00 A
% non-polymer solids, P' = % Total solids - % polymer solids
Average % polymer solids or polyrner activity, P = 97.8
Average % non-polymer solids, P' 2.2
APPENDIX B: SRF, CST, TTF and ZP results
Date : 14- 1 5/June/1999
Data Sheet #1
Conditioner : Ferric chloride (FC)
Conditioner Concentration : 50,000 ing/L
Sludge Volume : 500 mL
Specific resistance to filtration (SRF) test
Sludge Characteristics Conditioner (FC) SRF (Tmlkg)
#2
29.8
8.7
2.7
1 .O
0-8
0.8
0,6
Average
29.9
8.7
2.4
1 .O
0,8
0.7
0.6
Date : 1 4- 1 SlJunel1999
Data Sheet #3
Tinic-to-Filter (TTF) Test
1 Conditioner : Ferric chloride (FC) I 1 Conditioner Concentration : 50,000 m d L I 1 Sludge Volume : 500 rnL I Conditioner (FC) .-
Vol. (mL) Dose (mgIL) Dose (kf l )
O O O .i-Ir%T Sludge Characteristics TTF (min)
-
#2 Average
274.5 274.3
Temp. (OC)
21.5
% Solids
8.68 PH 7.2
Date : 22-23/June/l999
Data Sheet #6
Capillary Suction Time (CST) Test
1 Conditioner : Pm01755
1 Conditioner Concentration : 7,000 mg/L (0.7%)
I Sludge Volume : 500 mL
1 Sludge Characteristics
% Solids pH Temp. CC 8.70 7.4 23.0
Conditioner WC)
Vol. (mL) 1 Dose (mgIL) 1 Dose (kg/T)
CST (sec) I I
Date : 22-23/Juile11999
Data Sheet #7
Time-to-Filter (TTF) Test
Conditioner : Percol 755
Conditioner Concentration : 7,000 mg/L (0.7%)
Sludge Volume : 500 mL
Sludge Characteristics II Conditioner (FC) I/ -- . . L
% Solids PH Temp. (OC) Vol. (mL) Dose (mg/L) Dose (kg/T)
8.70 7.4 23 .O 0.0 O 0.0 258.0
8.57 7.4 23 .O 9.0 126
8.50 7.1 23.0 12.5 175 2,O
8.56 7.2 23.5 14.0 196 2.3
TTF (min)
#2 1 Average
-- - - . . . . -. . .
Date : 12- 1 3/July/ 1 999
Data Sheet #13
Specific Resistance to Filtration (SRF) Test
Conditioner : FC + Percol 75YPercol 757
Conditioner Concentration : FC = 50,000 mg/L ; Poly111er = 7,000 ing/L
Sludge Volume : 500 IL
Sludgc Characteristics Conditioner (FC + Percol755 1 Percol757) SRF (min)
Solids pH Temp. FC 1 Percol755 / Perco1757 #1 1 #2 1 Avg. , YO (OC) Vol. (rnL)I~ose (rng/L)1 Dose (kg/T) Vol. ( r n ~ ) I ~ o s e (rng/L)1 Dose ( k u ) @II
Percol7SS 5 I Percol7.57 J
Date : 12- 13lJulyl1999
Data Sheet #15
Time-to-filter (TTF) Test
Conditioner : FC + Percol755lPercol 757
Conditioner Concentration : FC = 50,000 mgIL ; Polymer = 7,000 mg/L
Sludge Volume : 500 mL
APPENDIX C: Sm Test Readings
Run 1 : O kg/T FC dose
Run 2 : O kg/T FC dose
Run 1 : 23 kg/T FC dose
Run 2 : 23 kg/T FC dose
=un 1 : 46.1 kg/T FC dose
II Vacuum Break
Run 2 : 46.1 kg/T FC dose
Run 1 : 69.1 kg/T FC dose
1 ! ~xperimentd~ata - --
1 Derived Data 1 1 Ratio
1 Vacuum Break
Run 2 : 69.1 kg/T FC dose
Vacuum Break
Run 1 : 80.6 kg/T FC dose
Run 2 : 80.6 kg.T FC dose
Run 1 : 92.2 kg/T FC dose
Run 2 : 92.2 kg/T FC dose
Experimental Data 1 Derived Data Ratio Time, t (min) Filtrate, V (mL) Tirne. ti (sec) Filtrate. VI (mL) tiNi (sec/rnL)
O 0.00 1 12.25 O O
Rua 1 : 103.7 kg/T FC dose
Run 2 : 103.7 kg/T FC dose
Vacuum Break 1 1 - 1
Run 1 : O kgFî Percol755 dose
Run 1 : 1.5 kg/T Percol755 dose
Run 2 : 1.5 kg/T Percol755 dose
Run 1 : 2.0 kg/T Percol755 dose
Run 2 : 2.0 kg/T PercoI 755 dose
I! Experimental Data 1 Derived Data 1 Ratio
1 Vacuum Break
Run 1 : 2.3 kg/T PercoI 755 dose
Run 1 : 2.6 kg/T PercoI 755 dose
Run 2 : 2.6 kg/T Percol755 dose
Run 1 : 2.9 kg/T Percol755 dose
Run 2 : 2.9 kg/T Percol755 dose
Run 1 : 3.2 kg/T Percol755 dose
Run 2 : 3.2 kg/T Percol755 dose
Run 1 : O kg/T fercol757 dose
ExperimentaI Data Derived Data Ratio Time, t (min) Filtrate. V (mL) Time. t (sec) Filtrate, VI (mL) ti/Vi (sec/mL)
O 0.00 2 1 .O0 O O
Run 2 : O kg/T Percol757 dose
Experimental Data Derived Datz Ratio Time. t (min) 1 Filtrate. V (mL) Time, ti (sec) 1 Filtrate, VI (mL) t l N l (sec/mL)
Vacuum Break
Run 1 : 1.4 kg/T Percol757 dose
Run 2 : 1.4 kg/T Percol757 dose
Run 1 : 1.9 kg/T Percol757 dose
Run 2 : 1.9 kg/T Percol757 dose
Run 1 : 2.1 kg/T Percol757 dose
Run 2 : 2.1 kg/T PercoI 757 dose
Run 1 : 2.4 kg/T Percol757 dose
Experimental Data Derived Data Ratio Time. t (min) 1 Filtrate. V (mL) Tirne. ti (sec) 1 Filtrate, V 1 (mL) tJVi (sec/mL)
Vacuum Break
Run 2 : 2.4 kg/T Percol757 dose
Run 1 : 2.7 kg/T Percol757 dose
Vacuum Break i v L
Experirnental Data 1 Time. t (min) 1 Filtrate. V (mL) 1
Derivcd Data T h e . ti (sec) 1 Filtrate, VI (mL)
Ratio tlNl (sec/mL)
Run 2 : 2.7 kg/T Percol757 dose
Run 1 : 3.1 kg/T Percol757 dose
Run 2 : 3.1 kg/T Percol757 dose
Run 1 : 18.3 FC + 2 kg/T Percol755 dose
Run 2 : 18.3 FC + 2 kg/T Percol755 dose
Run 1 : 36.6 FC + 1.3 kg/T Percol755 dose
Run 2 : 36.6 FC + 1.3 kg/T Percol755 dose
Run 1 : 54.9 FC + 0.7 kg/T Percol755 dose
1 Vacuum Break
Run 2 : 54.9 FC + 0.7 kg/T Percol755 dose
Run 1 : 18.3 FC + 2 kg/T Percol757 dose
Run 2 : 18.3 FC + 2 kg/T Percol757 dose
Run 1 : 36.6 FC + 1.3 kg/T Percol757 dose
Run 2 : 36.6 FC + 1.3 kg/T Percol757 dose
Run 1 : 54.9 FC + 0.7 kg/T Percol757 dose
Run 2 : 54.9 FC + 0.7 kg/T Percol757 dose
I! Experimental Data !! Derived Data 1 1 Ratio 1 Time. t (min) 1 Filtrate. V (mL) 1 Time. t i (sec) 1 Filtrate. V, (mL) ( tiNi (seclrnL) r
Vacuum Break
APPENDIX D: 'End of cake formation' filtrate volumes data and slmpes of 't/V
versus V' plots
'End of cake formation' filtrate volumes and slopes of ' t N versus V' plots
1 Conditioner dose WjT) L Slo e sec/ml2 Filtrate vo FC 1 Percol755 1 PercoI757 .
FC conditioning 1 -
0-0 I - I -
Run 1 Run 2
Percol755 conditioning - I 0.0 I ..
Percol757 conditionine - - 0.0 - - 1.4 - - 1.9
1 Dual-chernical conditioning 1
* refers to fiGate volume at the end of cake formation
APPENDIX E: Details of cost calcuIations for single- and dual-chernical
conditioning
Unit costs per kg of conditioners used for calculations:
Femc chloride : $ 0.67
Percol755 : $ 5.25 * (100/97.2)' = $ 5.40
Percol757 : $ 5.18 * (1 00/97.8)' = $ 5.30
' increase in cost by taking percent polymer activity into account
1) Conditioner: 100% FC
FC costs - - 80.6 kg/T * $ 0.67fkg - - $ 54.00
2) Conditioner: 100% Percol755
Percol 755 costs - - 2.9 k g K * $ 5.4ikg - - $ 15.66
3) Conditioner: 100% Percol757
PercoI 757 costs - - 2.7 k f l * $ 5.3ikg - - $ 14.3 1
4) Conditioner combination: 25% FC + 75% Percol755
FC costs - - 28.3 kg/T * $0.67/kg - - $ 12.26
Percol 755 costs - - 3.0 kg/T * $ 5.40/kg - - $ 10.80
Conditioner costs per T of dry solids
5) Conditisner combination: 50% FC + 50% Percol755
FC costs - - 36.6 kg/T * $ 0.67kg
Percol755 costs - - 1 -3 kg/T * $ 5.40/kg
Conditioner costs per T of dry solids
6) Conditioner combination: 75% FC + 25% Percol755
FC costs - - 54.9 kg/T * $0.67/kg
Percol 755 costs - - 0.7 kg/T * $ 5.40kg
Conditioner costs per T of dry solids
7) Conditioner combination: 25% FC + 75% Percol757
FC costs - - 18.3 kg/T * $ 0.67kg
Percol757 costs - - 2.0 kg/T * $ 5.301kg
Conditioner costs per T of dry solids
8) Conditioner combination: 50% FC + 50% PercoI757
FC costs - - 36.6 kg/T * $ 0.67kg
Percol 757 costs - - 1.3 kg/T * $5.30/kg
Conditioner costs per T of dry solids
9) Conditioner combination: 75% FC + 25% Percol757
FC costs - - 54.9 kg/T * $ 0.67kg - - $ 36.78
Percol757 costs - - 0.7 kg/T * $ 5.30/kg - - $ 3-71
Conditioner costs per T of dry solids - - $40.49
