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Electro-dewatering Treatment of Pulp and Paper Mill
Biosludge: The Effects of Conditioners
by
Jaehoon Ya
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Jaehoon Ya 2017
ii
Electro-dewatering Treatment of Pulp and Paper Mill Biosludge:
The Effects of Conditioners
Jaehoon Ya
Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2017
Abstract
Biosludge has been a problem for pulp and paper mills due to its difficulty to dewater.
Electro-dewatering can significantly reduce the water content of biosludge, but the effect
of electro-dewatering on pulp and paper mill biosludge is not well understood. This study
examined the feasibility of using electro-dewatering on pulp and paper mill biosludge
and observed that over 40% dry solids content could be achieved. Chemical and
physical additives including synthetic polymers, cationic proteins, fly ash, lime mud and
wood fines were added to biosludge, and electro-dewatered at 20V using a batch-scale
electro-dewatering device. Overall, the addition of conditioners did not improve the
removal of water from biosludge. However, biosludge conditioned with a small dose of
weak cationic polymer (2% Organopol5400) reduced the energy consumption of electro-
dewatering by ~19%. Electro-dewatering of biosludge is also expected to consume less
energy compared to thermal drying for removing the same amount of water.
iii
Acknowledgments
I would like to express sincere thanks to my supervisors and mentors, Professor D.
Grant Allen and Professor Honghi Tran, for their valuable guidance throughout the
program. Without their consistent support and criticism, the completion of this study was
not possible. I also would like to thank our collaborator, Professor Dominic Frigon of
McGill University, for providing the electro-dewatering unit to our laboratory and his
support on data analysis.
I would like to thank all of the researchers and students in our research group. Special
thanks to Dr. Torsten Meyer, Dr. Sue Mao and Dr. Sofia Bonilla, for their help with
experiments and data analysis.
Lastly, I would like to thank my family members, especially my parents Hwasung Ya and
Young-Nam Kim for their support and unconditional love throughout the journey.
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Table of Contents Abstract .......................................................................................................................... ii
Acknowledgments ........................................................................................................ iii
List of Tables ................................................................................................................. vi
List of Figures .............................................................................................................. vii
Nomenclature ................................................................................................................. x
1. Introduction ............................................................................................................... 1
1.1 Objectives .............................................................................................................. 3
2. Literature Review ...................................................................................................... 4
2.1 Dewatering and Disposal Processes for P&P mills ................................................ 4
2.2 Basics of Electro-osmosis ...................................................................................... 6
2.3 Other Electro-kinetic Phenomena .......................................................................... 9
2.4 Previous Studies on Electro-dewatering of Sludge .............................................. 10
2.4.1 Energy Consumption during Electro-dewatering ........................................... 13
2.5 Conditioning of Biosludge to Increase Dewaterability .......................................... 14
2.5.1 Polymer-based Conditioner ........................................................................... 14
2.5.2 Physical Conditioners .................................................................................... 15
2.6 Significance of Objectives .................................................................................... 17
3. Materials and Methods ........................................................................................... 18
3.1 Experimental Approach ....................................................................................... 18
3.2 Materials .............................................................................................................. 19
3.2.1 Biosludge ...................................................................................................... 19
3.2.2 Synthetic Polymers ....................................................................................... 19
3.2.3 Physical Conditioners .................................................................................... 20
3.2.4 Proteins ......................................................................................................... 21
3.3 Test Methods ....................................................................................................... 22
3.3.1 Total Solids Measurement ............................................................................. 22
3.3.2 Thickening by Centrifuge ............................................................................... 22
3.3.3 Dewatering with Electro-dewatering Device .................................................. 22
3.3.4 Overall experimental sample flow during tests .............................................. 24
3.3.5 pH Measurement ........................................................................................... 25
3.3.6 Data Analysis ................................................................................................ 26
3.3.7 Conditioner Dose .......................................................................................... 26
v
4. Results and Discussion.......................................................................................... 27
4.1 Types of Water Removed during EDW ................................................................ 27
4.2 Temperature Elevation during EDW .................................................................... 29
4.3 Effects of Centrifugation on EDW ........................................................................ 33
4.4 Effect of Voltage .................................................................................................. 37
4.5 Cationic Polymer Addition .................................................................................... 42
4.6 Addition of Physical Conditioners ........................................................................ 48
4.6.1 Fly Ash and Lime Mud ................................................................................... 48
4.6.2 Silica ............................................................................................................. 54
4.6.3 Wood Fines ................................................................................................... 55
4.6.4 Discussion on the Effect of Physical Conditioners ......................................... 57
4.7 Protein Conditioning ............................................................................................ 58
4.8 Anionic Polymer Addition ..................................................................................... 59
4.9 Estimation of Energy Consumption ...................................................................... 62
5. Implications on Industrial Application .................................................................. 65
6. Conclusions ............................................................................................................ 67
7. Recommendations .................................................................................................. 68
8. References .............................................................................................................. 70
Appendices ................................................................................................................. 75
Appendix I: ................................................................................................................ 75
Appendix II: ............................................................................................................... 76
vi
List of Tables
Table 1. Various dewatering devices and the resultant percent DS content in sludge
cake (adapted from [2]) ................................................................................................... 4
Table 2. Various electro-dewatering studies on sewage sludge (adapted from [22]) .... 11
Table 3. The effects of physical conditioners on mechanical dewatering (adapted from
[42]) .............................................................................................................................. 16
Table 4. Polymers used in this study ............................................................................ 19
Table 5. Dry solids of the conditioners used in this study .............................................. 20
Table 6. Dosages reported for different conditioners used in this study ........................ 26
vii
List of Figures
Figure 1. Flow diagram of a typical wastewater treatment process at pulp and paper
mills ................................................................................................................................ 1
Figure 2. Illustration of an electrical double layer and charges around a colloidal sludge
particle in water (adapted from [9]), not to scale. ............................................................ 7
Figure 3. Illustration of a hydrated cation in the diffuse layer under the influence of an
electric field [14], not to scale. ......................................................................................... 8
Figure 4. Experimental approach taken in this study ..................................................... 18
Figure 5. Appearance of the physical conditioners used in this study ........................... 21
Figure 6. Schematic of electro-dewatering unit ............................................................. 23
Figure 7. Appearance of EDW device (right) and the attached controller ...................... 23
Figure 8. Images of sludge samples as they went through testing process .................. 24
Figure 9. Distribution of water after electro-dewatering tests under varying conditions. All
in triplicate with 3 different batches of sludge, EDW Time= 10 min. .............................. 28
Figure 10. Temperature profiles during EDW tests at 0, 20, 30, and 40V ..................... 29
Figure 11. Effect of initial sludge temperature on DS after electro-dewatering test,
Conditioned with 2% Zetag8165, EDW Time = 10 min. ................................................ 31
Figure 12. Comparison of 0V and 20V EDW tests at similar average temperature ....... 32
Figure 13. DS of centrifuged and electro-dewatered sludges at various centrifugal
conditions (shown on x-axis), Voltage = 20V, EDW Time = 10 min............................... 33
Figure 14. Average thickness and DS of sludge sample after electro-dewatering,
centrifugal conditions shown on x-axis, Sludge-1 batch, Voltage = 20V, EDW Time = 10
min. ............................................................................................................................... 35
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Figure 15. Average current during electro-dewatering, centrifuged at conditions shown
on x-axis, Sludge-1 batch, Voltage = 20V, EDW Time = 10 min. .................................. 36
Figure 16. Effect of increased voltage for the biosludge conditioned with 2% Zetag8165
cationic polymer ............................................................................................................ 38
Figure 17. Effect of increased voltage, Time = 10 min. ................................................. 40
Figure 18. DS and corresponding energy consumption rates at various voltages, Time =
10 min. .......................................................................................................................... 41
Figure 19. DS of sludges with or without Zetag8165 after electro-dewatering test,
Voltage = 20V, Time = 10 min. ...................................................................................... 43
Figure 20. Filtrate removal rates by Zetag8165 conditioning at 2%, Voltage = 20V ...... 44
Figure 21. Comparison of sludges conditioned with Zetag8165 or Organopol5400,
Voltage = 20V, Time = 10 min. ...................................................................................... 45
Figure 22. DS and the average thickness of sludge cakes (6 different batches of sludge),
showing cationic polymer data, dotted-lines representing upper/lower confidence
intervals (α = 95%) for the runs with no polymer, Voltage = 20V, EDW time = 10 min. . 47
Figure 23. Effects of fly ash or lime mud addition on biosludge, Pre-mixing, Voltage =
20V, Time = 10 min. ...................................................................................................... 49
Figure 24. Effects of fly ash and lime mud addition on biosludge, Post-mixing, Voltage =
20V, Time = 10 min. ...................................................................................................... 50
Figure 25. pH of sludges, in triplicate with samples prepared at 5% wt. in deionized
water ............................................................................................................................. 51
Figure 26. Electro-dewatering results comparing sludges conditioned to basic pH,
Voltage = 20V, Time = 10 min. ...................................................................................... 52
ix
Figure 27. DS and the average thickness of sludge cakes (6 different batches of sludge),
showing fly ash and lime mud data, dotted-lines representing upper/lower confidence
intervals (α = 95%) for the runs with no additive, Voltage = 20V, EDW time = 10 min. . 53
Figure 28. Effect of silica (SiO2) addition, Post-mixing, Voltage = 20V, Time = 10 min. 54
Figure 29. Effect of wood fine addition, Post-mixing, Voltage = 20V, Time = 10 min. ... 56
Figure 30. Effect of protein conditioning, Voltage = 20V, Time = 10 min. ...................... 58
Figure 31. Effect of anionic polymer conditioning, Voltage = 20V, Time = 10 min. ........ 60
Figure 32. Energy consumption and corresponding DS from electro-dewatering tests
with conditioners, Columns with the same colour and pattern indicate the same batch of
sludge, Voltage = 20V, Time = 10 min. ......................................................................... 62
Figure 33. Overview of the potential electro-dewatering and disposal process ............. 65
Figure 34. Repetitive runs of EDW tests at 30V for 10 min., 120g of sample each run . 75
x
Nomenclature
BOD Biochemical oxygen demand
BSA Bovine serum albumin
CST Capillary suction time
DS Dry solids
EDL Electrical double layer
EDW Electro-dewatering
EPS Extracellular polymeric substance
MDW Mechanical dewatering (such as belt press thickener or screw
press)
MMO Mixed metal oxide
P&P Pulp and paper
P/S Primary sludge / secondary sludge
SRF Specific resistance to filtration
WAS Waste activated sludge
1
1. Introduction
Pulp and paper (P & P) mills produces a large amount of wastewater effluent. Before
discharging the effluent into the environment, contaminants from the waste stream must
be removed in order to comply with environmental regulations. Common municipal or
industrial wastewater treatment plants use a waste activated system where the effluent
is aerated so that microorganisms can remove and degrade many of the contaminants
in the wastewater stream. However, a by-product of this process is a large amount of
sludge and can be categorized into two types: primary and secondary sludge, which, as
the name implies, are termed by their origin from the respective clarifiers (Figure 1).
Primary sludge is typically rich in both organic and inorganic matter while secondary
sludge, also known as biosludge or waste activated sludge (WAS), consists mostly of
organic matter, including microorganisms and extracellular polymeric substances (EPS).
The high concentration of organic matter in biosludge hampers the dewatering process
[1] and creates obstacle for P&P mills to efficiently treat sludge.
Figure 1. Flow diagram of a typical wastewater treatment process at pulp and paper mills
In Canada, a P&P mill produces an average of 40 oven dry tonnes (ODt/d) of sludge per
day, or approximately 15000 ODt/d of sludge annually [2]. In order to handle such large
amounts of sludge, P&P mills usually adapt a combination of mechanical dewatering
and disposal processes as outlined in Figure 1.
The treatment costs for the waste activated process can be as high as 60% of the total
operating costs of a water treatment plant [3] and are largely due to sludge
2
transportation, land filling, land application and/or incineration. Since the cost is primarily
due to the weight, volume and dry solids content (DS) of the sludge, a more efficient
dewatering process can lead to a reduction in weight and volume and an increase in
high dry solids content and thereby reduce disposal costs.
The challenge for P&P mills is that conventional dewatering methods are not effective in
dewatering biosludge due to the gel-like matrix and extracellular polymeric substances.
For this reason, biosludge is rarely dewatered alone on the mechanical dewatering
systems. To enhance the dewatering of sludge, biosludge is generally mixed with
primary sludge before dewatering. Moreover, a high primary to secondary sludge
mixture ratio is preferred. However, it has been noted that primary sludge production is
not favourable as P&P mills are seeking to become more efficient and thus reduce fibre
losses [2]. Furthermore, the production of biosludge is expected to increase [4].
Therefore, because of the reduction of primary sludge production and stringent
environmental regulations, the process of mixing primary and secondary sludges to
improve dewatering is not sustainable for P&P mills.
One possible dewatering technique that can significantly increase the dry solids content
of biosludge is the use of an electric field to enhance the dewatering rate. This technique
is commonly known as electro-dewatering (EDW) but is also referred to as electro-
osmotic dewatering or electric field-assisted dewatering. A large number of studies have
reported the effect of EDW on municipal sludge but only a few have described
dewatering on P&P mill biosludge. Therefore, this study focused on examining the
effects of EDW on P&P mill biosludge.
Prior to undergoing dewatering processes in P&P mills, sludge is usually conditioned
with chemicals or physical conditioners in order to enhance dewatering. One common
chemical flocculants are cationic synthetic polymers. Several studies attempted to
examine the effect of adding polymers to municipal sludge prior to electro-dewatering,
but the effect of adding polymers to P&P mill biosludge is still not clear. Other physical
conditioners often used to increase the filtration rate for mechanical dewatering in P&P
mills include fly ash, lime mud and wood fines. However, there is a lack of studies which
researched utilizing physical conditioners in combination with electro-dewatering on P&P
biosludge. The present study investigated electro-dewatering in conjunction with adding
3
cationic conditioners, fly ash, lime mud or wood fine, to biosludge in order to measure
their efficacies in enhancing water removal.
1.1 Objectives
The overall objective of this study was to investigate the effect of electro-dewatering on
P&P mill biosludge. Other specific objectives included adding chemical and physical
conditioners to the biosludge in order to understand their effects on the efficacy of
electro-dewatering. The specific objectives were to determine:
1. The effects of conditioner additions, such as charged proteins and synthetic
polymers, on electro-dewatering of P&P mill biosludge;
2. The effects on electro-dewatering when fly ash, lime mud and wood fines were
added to biosludge; and
3. Determine if the addition of conditioners had an effect on energy consumption
during electro-dewatering.
4
2. Literature Review
2.1 Dewatering and Disposal Processes for P&P mills
In pulp and paper (P & P) mills, sludges are dewatered and disposed of as part of the
wastewater treatment process. Common systems for the dewatering of sludges include
centrifuges, vacuum filters, belt filter presses, and screw presses. Among these
mechanical dewatering devices, belt filter presses and centrifuges are the most popular
due to cost efficiency [5]. These types of presses are categorized as mechanical
dewatering methods and, as its name suggests, pressure is applied to sludge so as to
separate water from solids. Once the sludge is dewatered, the resultant product is
referred to as sludge cake. These mechanical dewatering devices typically produce ~15-
30% dry solids content (DS) sludge cake and is dependent on the primary and
secondary sludge mixture ratio. For instance, when a screw press is used on mixture of
primary and secondary sludges, the generated sludge cake has ~31% DS [2]. The DS
content of sludge cake when using various mechanical dewatering devices in Canadian
P&P mills are shown in Table 1.
Table 1. Various dewatering devices and the resultant percent DS content in sludge cake (adapted from [2])
Dewatering Device
Average Sludge Cake DS (%)
Primary Sludge Combined Sludge (Primary & Secondary)
Belt Press 27 27
V-Press 33 27
Screw Press 41 31
Vacuum filter 17 N/A
Other 30 N/A
In P&P mills where both primary and secondary sludges are produced, secondary
sludge is generally mixed with primary sludge to improve the sludge dewaterability [6].
The average primary to secondary (P/S) ratio was reported to be 63/37 for 34 pulp mills
5
in Canada [3]. The data from Table 1 shows that the dryness of sludge cake is
decreased if dewatering is performed on the combined primary & secondary sludge
mixture as compared to primary sludge. It is known that a lower P/S ratio results in
reduced dewaterability of the sludge mixture [4] and a reduction in DS of the sludge
cake. A higher P/S ratio generally leads to an increase in DS because primary sludge
helps to enhance the overall dewaterability of the sludge mixture. However, due to
changes in the reduction of primary sludge production the P/S ratio is expected to
decrease. A study has noted that the optimization of fiber recovery system leads to a
reduction in primary sludge production because the primary sludge production depends
on efficiency of the fibre recovery system [4]. Moreover, secondary sludge production
will likely increase in the future due to two reasons: first, increased BOD loading and
second, governmental regulations demanding the removal of BOD/suspended solids in
the effluent and waste [4]. Therefore, the high P/S ratio for enhanced dewatering is not a
sustainable option and sludge dewatering will become even more challenging for P&P
mills to deal with.
Due to the trend of decreasing primary sludge production, it is desirable to perform
dewatering on the low P/S ratio mixture but still achieve a sufficiently high level of
dryness. However, colloidal materials and organic matter such as extracellular polymeric
substances (EPS) render biosludge extremely difficult to dewater and results in a low
DS [1]. For example, if only secondary sludge is dewatered, a belt filter press can
increase the DS up to 13-16% while a solid bowl centrifuge results in ~11% DS [7].
Therefore, the limitations of the conventional dewatering systems hinder mills from being
able to produce sludge cake with high DS and this leads to higher disposal costs.
In order to decrease sludge disposal costs, a study reports that sludge should contain
40% DS for a cost-effective reduction in volume [1]. The final disposal of sludge is
usually accomplished by one or a combination of land application, land filling, and
incineration [2]. For the disposal by land filling or land application, the water content in
sludge greatly increases the costs of transportation and also demands that more land be
available. If incineration is the final disposal method, the sludge entering a reboiler
should not have a moisture content above 40-50% otherwise burning is inefficient and
require more fuel [5, 6]. Otherwise, if the sludge DS is less than 40%, the use of
6
supplement fuel, such as oil, is required for the boiler operation. A dryness of 40%,
however, is difficult to achieve on either 100% biosludge or the low P/S ratio mixture by
the conventional mechanical dewatering devices. Thus, an alternative dewatering
solution with good dewatering capability is needed for P & P wastewater treatment.
2.2 Basics of Electro-osmosis
Electro-dewatering, commonly referred to as electro-osmotic dewatering, is primarily
based on the electro-kinetic phenomenon called electro-osmosis; therefore, electro-
dewatering is commonly referred to as electro-osmotic dewatering. Electro-osmosis is a
phenomenon that describes, under the influence of an electric field, the induced motion
of water around charged particles [8]. The electric field is created by providing electricity
to the anode and cathode electrodes of the electro-dewatering device thereby inducing
electro-osmosis inside the sludge. Because the zeta potential of sludge is typically
negative, the direction of electro-osmotic flow, and therefore the water, is from the
anode to the cathode.
The electro-osmosis phenomenon can be better described using the electrical double
layer (EDL) (Fig. 2). Colloidal particles immersed in water, such as biosludge particles,
are typically negatively charged. At the surface of a sludge particle in water, an electrical
double layer is formed due to positive ions attracted to the negative surface of the
particle. The first layer, also known as Stern layer, is formed by immobile cations
strongly bounded to the surface of the sludge particle. The secondary layer, called the
diffuse layer or Gouy-Chapman layer, is a region formed with both cations and anions.
These ions are more loosely attracted to the surface, allowing the ions to become
mobile. Figure 2 shows an illustration of the electrical double layer around the surface of
a negatively charged colloidal sludge particle.
7
Figure 2. Illustration of an electrical double layer and charges around a colloidal sludge particle in water (adapted from [9]), not to scale.
As shown in Figure 2, there is an imbalance of ions in the diffuse layer with more cations
in the region as compared to anions. If this colloidal particle is placed in an electric field,
the ions in the diffuse layer move towards one of the electrodes with cations moving
towards the cathode and anions migrating towards the anode [10]. Because more
cations are present in the diffuse layer of the negatively charged sludge particles, the
net flow of the charges is directed towards the cathode. This movement of positive ions
also results in the migration of water molecules because the ions are solvated with water
(hydration shell) by ion-dipole interaction as illustrated in Figure 3 [11, 12, 13]. The net
resulting flow of ions and water is known as electro-osmotic flow.
8
Figure 3. Illustration of a hydrated cation in the diffuse layer under the influence of an electric field [14], not to scale.
Several researchers have attempted to model the kinetics of the electro-osmotic flow.
One classical way to interpret the electro-osmotic flow is through the Helmholtz-
Smoluchowski equation, which describes the electrophoretic mobility in straight
cylindrical capillary tubes [15, 16]. The Helmholtz-Smoluchowski equation in its simplest
form is as follows:
𝑑𝑉
𝑑𝑡=
𝜀𝜁
𝜂∗ 𝐸 ∗ A
Where:
V = the water volume (m3) ,
t = time (s)
ε = dielectric permittivity of the medium (Fm-1)
ζ = zeta potential at the capillary wall (V)
η = viscosity of the liquid medium (kgm-1s-1)
E = electric field strength (V/m)
A = cross-sectional area of the capillary (m2)
When considering Ohm’s law, the electric field term, E, is related to the current and
conductivity of the electrolyte solution [16].
9
E = 𝐼
(𝐴 ∗ 𝑘)
E = Electric field strength (V/m)
k = Conductivity of the electrolyte solution (S/m)
A = Cross-sectional area (m2)
I = Current (A)
Thus, substituting above equation into the Helmholtz-Smoluchowski equation yields
𝑑𝑉
𝑑𝑡=
𝜀𝜁
𝜂𝑘∗ 𝐼
The above equation shows that the electro-osmotic flow is proportional to current, zeta
potential, and inversely proportional to the conductivity and viscosity of liquid. This
suggests that the listed parameters may influence the electro-osmotic flow which then
may have an impact on the dewatering and DS if electro-dewatering is applied to P & P
sludge. However, for complex mixtures such as sludge, more work is needed to
accurately predict the electro-osmotic flow. As noted by researchers [22], this is due to
the lack of measurements of constants including the fluid viscosity, the zeta potential,
and the permittivity. Furthermore, the above equation does not account for the effect of
pressurization, and thus it is not indicative of a pressurized EDW device.
2.3 Other Electro-kinetic Phenomena
In addition to electro-osmosis, electrolysis and electrophoresis are two other electro-
kinetic phenomena that may occur during the course of electro-dewatering. Electrolysis
refers to the decomposition of water by the influence of an electric field where redox
reactions lead to the production of hydrogen and oxygen gases. These gases are
explosive, however, necessitating a supplementary ventilation system to prevent an
explosion.
Electrophoresis refers to the movement of charged particles towards an electrode under
the influence of an electric field [17] and it is widely known for its application in gel
electrophoresis where different sizes of nucleic acids can be separated. A negatively
charged particle, such as a sludge particle, will move towards an anode when subjected
to an electric field. The electrophoretic effect on sludge is significant only when the
10
sludge has a high water content so that there is room for particles to move freely. If the
DS of sludge is moderately high, the sludge particles are restricted in terms of
movement and the electrophoretic movement of particles will be minimal. For the
experiment performed in this study, it was assumed that the electrophoretic effect is not
significant because the DS content of the sludge being electro-dewatered is usually high
(>9% DS). This assumption is evidenced by other studies similar to the present one [12,
17].
2.4 Previous Studies on Electro-dewatering of Sludge
Over the last two decades, a large number of researchers have investigated the effects
of electro-dewatering on various types of colloidal substances such as clay suspensions,
kaolin suspensions, industrial (e.g. drilling or food waste) and municipal sludges.
However, most studies related to electro-dewatering have been focused on municipal
sludges. Depending on the wastewater treatment process, the type of sludge
investigated have varied between primary, secondary, or anaerobically digested sludge.
The most common form of electro-dewatering device studied utilized a combination of
both mechanical pressure devices in conjunction with electro-dewatering. The
application of pressure ensures sufficient contact between the sludge and electrodes,
and additionally provides the advantage of mechanical dewatering, similar to
conventional dewatering systems. One example is a mechanical belt-filter press in
combination with electro-dewatering [18]. Although the type of mechanical devices used
in studies vary, the most common device, likely due to the simplicity of configuration, is a
vertical hydraulic piston mated to an electro-dewatering system [12, 19, 20, 21]. Table 2
shows various types of mechanical systems and EDW systems studied with municipal
sludges with the corresponding results of dryness gain.
11
Table 2. Various electro-dewatering studies on sewage sludge (adapted from [22])
Device Type
Pressure (kPa)
Operation Mode
Type of Sludge
Dryness Gain* (%)
Filter-press Pilot
25-400 20V Primary 4-7
Lab piston cell
122-1960 50V Activated 25-27
Lab piston cell
100-600 20V/cm Anaerobically Digested
4-7
Belt filter press
N/A 20V Activated 5-6
Lab piston cell
300 100A/m2 Activated 22
Belt filter press
N/A 20V Activated 4
Lab piston cell
250 30V Anaerobically Digested
29
Belt filter press
200-300 40A/m2 Activated 18-19
Diaphragm filter press
700-1600 N/A Activated 19-21
* The difference in the final DS between electro-dewatering and mechanical dewatering
As shown in Table 2, researchers reported a dryness gain ranging between 4-27% DS.
The dryness gain depends on many factors such as sludge type (e.g. primary, activated
or digested sludge), applied energy intensity, and the type of EDW devices. The
research indicates that electro-dewatering significantly increases DS when compared to
mechanical dewatering alone. Also, that a larger energy input by either increased
voltage or current results in higher dryness gain. It can be noticed that the dryness gain
is significantly greater for activated sludges, regardless of whether the gain was shown
on bench- or pilot-scale electro-dewatering systems. Such a large increase in the DS of
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pure activated sludge is unlikely by a mechanical dewatering device alone, and further
suggests that mechanical-EDW may improve P & P sludge dewatering.
A few companies have built commercial full-scale EDW devices for use on sludges
created in municipal and industrial settings. Similar to bench or pilot-scale systems,
most commercial EDW systems consist of a mechanical pressurization feature paired
with an EDW dewatering device. There are several well-known companies. Ovivo Inc.
(USA), under the brand name Cinetik, offers a linear EDW system that adapts a semi-
batch pressurization process. Ace Korea Inc. (Korea), under the brand name Elode,
uses a belt-filter press combined to an EDW system. Electrokinetic (United Kingdom),
offers a similar system to the previous and claims that their system can generate sludge
cake of 15-60% DS if used for sludge dewatering.
Other studies have reported using EDW but varying the experimental configurations
such as: device configuration, supplementary pre-treatment processes, and sludge
conditioning. For example, previous studies report experimental results on various
device configurations including horizontal EDW [9, 23, 24], vacuum filtration with EDW
[25], using adsorptive material combined with EDW [26], rotating anode EDW device
[27], anode flushing [28], or a continuous system for removing electrolysis products [29].
One study used a freeze-thaw process as a pre-treatment of sludge and coupled it with
EDW [30]. A number of studies researched varying methods to condition the sludge
such as adding surfactants [31] or magnetic micro-particles [32]. Another sludge
conditioning method, discussed in the later section of this thesis, included adding
charged polyelectrolytes or a physical conditioner.
Although studies on municipal sludges have shown promising results, studies
investigating electro-dewatering on P&P mill biosludge are scarce. Lucache et al. [6, 33]
examined the effect of electro-dewatering on cellulosic sludge from a P&P mill and
reported achieving ~45% DS, which is significantly higher than mechanical dewatering
as indicated above in Table 1. The Lucache et al. study suggests that applying EDW for
the dewatering of the P&P mill biosludge is promising and DS content achieved on
municipal sludges may be realizable in P & P sludges. However, the Lucache et al.
study is superficial as it does not provide specific details and also because it was
13
focused on cellulosic sludge. Given this scenario, it is worthwhile to investigate the effect
of electro-dewatering on P&P mill biosludge.
2.4.1 Energy Consumption during Electro-dewatering
In order to apply EDW in a P & P setting, it must be practical, achieve higher DS content,
and be cost effective so that the cost of energy consumption is not greater than the
reduction in transportation and disposal costs. Energy consumption is a crucial factor
because, being an electrical device, EDW requires a great deal of energy to operate. To
produce the sludge cake that is reasonably high in dryness, high voltage (or current)
may be required. Therefore, it is desirable to minimize energy consumption by
optimizing the EDW process. If the energy costs are below that of transportation and
disposal, then EDW is a highly promising technology which could help P & P mills deal
with a lower production of primary sludge and more stringent environmental regulations
requiring less contaminants to be released into the environment.
In the literature, the electro-dewatering treatment of sludge is usually compared to a
thermal drying process because both processes are similar in that they produce high DS
sludge and consume energy. Most studies that investigated the energy consumption of
EDW systems reported that the energy consumed is generally less than the energy
required for a thermal dryer. The enthalpy of evaporation of pure water is 0.62 kWh kg-1
(2200 kJ kg-1), and thus, the energy demand of the thermal drying process is in the
range of 0.62-1.20 kWh kg-1 of water removed (2200-4300 kJ kg-1) [34, 35].
The studies on both batch- and pilot-scale devices reported that the electro-dewatering
of sludges consumed relatively less amount of energy compared to thermal drying.
Mahmoud et al. reported a range of 0.10-0.24 kWh kg-1 of water removed (360-860
kJ/kg) on a laboratory scale EDW device [35]. Saveyn et al. examined a pilot-scale EDW
device on activated sludge and reported that the energy consumption was 0.22-0.28
kWh kg-1 (790-1000 kJ kg-1) for generating ~42% DS sludge cake [36]. Recently, Zhang
et al. studied the EDW performance of an industrial-scale device and reported that the
energy consumption rate was around 0.13 kWh kg-1 of water removed (470 kJ/kg) for
producing a biosludge cake with 40% DS [37]. These results by various researchers
14
suggest that EDW can be a more energy-efficient method for dewatering sludge as
compared to thermal drying.
However, using electro-dewatering on sludge to obtain a DS of 40-45% or higher may
not be cost-effective. Olivier et al. examined both operations of constant voltage and
constant current density and concluded that the instantaneous energy consumption
depends on the reached dryness of the sludge cake [38]. Their paper reported that
when generating a sludge cake of more than 45% DS, the EDW process consumed
more energy than the thermal drying process. This result is in line with the results by
Zhang et al. who noted that instantaneous energy consumption was greatly elevated
when they tried to increase the DS above 42% [37]. These reports suggest that as the
water content of the sludge becomes lower, a greater amount of energy is required to
further dewater the sludge. Regardless, these studies suggest that EDW may be an
effective solution for dewatering P & P mill sludge up to 40% DS.
2.5 Conditioning of Biosludge to Increase Dewaterability
2.5.1 Polymer-based Conditioner
Prior to dewatering, polyelectrolyte (charged synthetic polymers) conditioners are often
added to promote the flocculation of sludge so as to increase the rate of filtration. A P&P
mill survey reported that 95% of 48 mills in Canada use some form of chemical
coagulants or flocculants in the sludge dewatering process [2]. The use of chemical
polymers is costly for P&P mills. A 1982 survey found that polymer costs accounted for
11.7% of the total operating and maintenance costs of sludge dewatering and disposal
in a P&P mill [7]. Therefore, if EDW can reduce the polymer demand, the application of
EDW can be an attractive solution for the industry.
Various researchers have investigated the effect of cationic polyelectrolyte on EDW.
Most studies agree that the use of cationic polyelectrolyte is necessary to enhance
mechanical filtration and that they do not improve the water transport efficiency of the
sludge being electro-dewatered. Gingerish et al. [39] reported that polyelectrolyte dosing
on both aerobic and anaerobically digested sludges had no significant effect in
increasing total solids during EDW. Saveyn et al. [15] examined the effect of various
15
cationic polymers on activated sludge and concluded that the polyelectrolyte
conditioning did not increase the electro-osmotic water transport efficiency. He and
Mikkelsen [40] proposed that even when cationic polymers were added in excess, a
large part of the sludge remained negatively charged and that the increased mechanical
filtration is mainly due to the “bridging effect” of the polymer. Based on experiments with
both cationic and anionic polymers, Citeau et al. [41] also agreed that polyelectrolyte
addition had no impact on EDW performance, such as energy consumption. Tuan et al.
[30], in contradiction to the above studies, observed increases in the DS of the EDW
sludge after polymer conditioning, possibly due to increased filtration rates by polymers.
Iwata et al. [8] gave a different opinion in their review paper noting that the effect of
polymer dosing on sewage sludge has not yet been well clarified with respect to electro-
dewatering. Although these studies suggest that the use of polyelectrolyte can
potentially be minimized by electro-dewatering, the fundamental effect of adding
polymers to sludge is not well understood.
2.5.2 Physical Conditioners
Physical conditioners may assist sludge dewatering as “filter aids”. They do so by
helping to reduce the compressibility of sludge under pressure and maintain porosity by
providing a rigid structure which helps to retain paths for free water to flow within sludge
flocs [42]. The physical conditioners that may assist filtration include fly ash, lime mud
and wood fines, all of which are common wastes generated from P&P mills. Utilizing
these wastes with sludge dewatering may reduce the total amount of wastes and
increase the sustainability of the wastewater treatment process. The dewatering of
sludge with physical conditioners has been widely studied on mechanical dewatering
processes. Table 3 below shows some studies regarding the application of physical
conditioners for dewatering.
16
Table 3. The effects of physical conditioners on mechanical dewatering (adapted from [42])
Sludge Type
Additive Result
Waste Activated
Fly ash Improved filtration
Reduced SRF*
Refinery Oily
Wastewater
Fly ash,
Lime Reduced compressibility
Municipal Wastewater
Wood chip,
(conditioned with either ferric
chloride or alum)
Reduced filtration time
Increased filtrate
Aerobic digested
Wood chip Reduced adhesion on the filter
Reduced SRF*
*SRF = Specific Resistance to Filtration
The studies where fly ash, lime mud or wood fines were added showed improvement in
sludge dewatering by either enhancing filtration, reducing specific resistance to filtration
(SRF) or reducing compressibility. Based on these results, it is suggestive that the
addition of physical conditioners may be helpful for EDW by creating rigid and
incompressible structures which may improve dewatering of P&P sludge. Although
various groups of researchers have studied the effects of physical aids on mechanical
dewatering, very few studies have examined the effects of physical conditioners on
electro-dewatering performance.
One study on municipal sludge reported that EDW efficiency was increased 20-40% by
the addition of fly ash to the sludge [43]. The study suggested that the increased
efficiency was due to the addition of fly ash. However, there is still a lack of
understanding exactly how fly ash improves dewatering of sludge when coupled to EDW.
In addition, no previous studies have reported the use of lime mud or wood fines on
EDW. Therefore, it would be beneficial to study the effects of physical conditioners on
EDW.
17
2.6 Significance of Objectives This literature review has identified significant gaps in knowledge about applying EDW
on P&P mill biosludge. First, only a few studies have examined the utility of applying
EDW on the biosludge produced from P&P mills. Second, the effects of conditioners,
such as cationic/anionic synthetic polymers, charged proteins, fly ash, lime mud, and
wood fines are not clear as to their effects on the EDW treatment of biosludge. Lastly,
the energy consumption rates between different conditioning practices have not been
investigated previously. Therefore, the objectives of this thesis are to fill the mentioned
knowledge gaps, further expand the knowledge of EDW under varying conditions, and
measure the potential of its use for dewatering sludge produced by the P&P industry.
18
3. Materials and Methods
This section identifies materials and experimental procedures utilized to conduct the
EDW tests. Materials required for the EDW tests are shown first while subsequent
sections detail the procedures used.
3.1 Experimental Approach
The objectives of this thesis were to examine the effect of EDW on P & P biosludge and
the potential for using various physical and chemical conditioners. To achieve the
objectives, the experimental approach used is shown in Figure 4.
Figure 4. Experimental approach taken in this study
19
3.2 Materials
3.2.1 Biosludge
Our laboratory receives monthly shipments of secondary sludge (biosludge) from a
Canadian P & P mill which uses a sulphite pulping process. All of the experiments
presented in section 4 were performed with the sulphite mill biosludge. After receiving
the sludge samples from the mill, they were stored in a cold room at 4˚C. Before
performing an experiment, the biosludge was removed from the cold room and allowed
to reach room temperature.
3.2.2 Synthetic Polymers
Three types of synthetic polymers were used in this study: Zetag8165, Organopol5400
and Organopol5510, with all of them being supplied from BASF Inc. (Table 4). The
polymers differ in cationic strength and molecular weight. Zetag8165 is specifically
designed for use on biosludge and has a high charge density and molecular weight.
Organopol5400, however, is generally used for flocculating the primary and secondary
sludge mixture. An anionic polymer, Organopol5510, was also used to examine its effect
on EDW performance.
Table 4. Polymers used in this study
Synthetic Polymer Charge Charge Density* Molecular Weight*
Zetag8165 Cationic Medium-high Very high
Organopol5400 Cationic Low Not found
Organopol5510 Anionic Not found Not found
* Information provided by the supplier
A stock solution of 0.5% wt. concentration was prepared by adding pre-weighted
polymer beads into deionized water.
20
3.2.3 Physical Conditioners
Several physical agents were used to investigate their effects on EDW performance. Fly
ash, wood fine, and lime mud were provided by a pulp mill. Silica was also tested to
observe the effect of increasing the inorganic content on EDW performance. The DS of
the physical conditioners and their images are presented in Table 5 and Figure 5,
respectively.
Table 5. Dry solids of the conditioners used in this study
Conditioner Dry Solids (%)
Fly Ash 69.1
Wood Fine 94.6
Lime Mud 71.5
Silica (SiO2) 99.9
21
Figure 5. Appearance of the physical conditioners used in this study
3.2.4 Proteins
Two proteins with different charges were tested: Protamine and Bovine Serum Albumin
(BSA). The stock solution was prepared at 2% (2g 100mL-1) in deionized water, stirred
for 1 hour. Then, the stock solution was added to raw biosludge at a dosage of 10% (1g
dry protein / 10g dry sludge). The conditioned raw sludge was then incubated at room
temperature for 1 hour.
22
3.3 Test Methods
3.3.1 Total Solids Measurement
To measure the dryness of sludge samples, all in duplicate, total solids tests were
performed as per Standard Methods 2540B [44]. The weight of sample ranged 4-10g.
The precision of the analytical balance was ±1 mg.
3.3.2 Thickening by Centrifuge
A laboratory centrifuge (Beckman Coulter) was used to separate water (centrate) from
the sludge. Specifically, 475 ml of biosludge was poured into a polypropylene centrifuge
tubes and centrifuged at 3000G for 10 minutes, unless stated otherwise.
After centrifugation, the supernatant was removed by manually pouring out the centrate
from the conical containers and the thickened sludge removed with a spatula and
transferred into a separate container for further tests.
3.3.3 Dewatering with Electro-dewatering Device
The laboratory EDW device used in this study was originally built by Elmco Water
Technologies, now part of Ovivo Inc. The EDW was initially at the department of Civil
Engineering and Applied Mechanics at McGill University but was then transferred to the
University of Toronto for this project. The EDW device consists of two parts: first, the
EDW unit in which samples are dewatered and a separate cabinet which controls the
dewatering station. Figure 6 outlines the schematic of the EDW unit while Figure 7 is an
image of the EDW unit and the attached controller.
23
Figure 6. Schematic of electro-dewatering unit
Figure 7. Appearance of EDW device (right) and the attached controller
24
The EDW unit is equipped with anode and cathode electrodes whereby an electric field
is created between the two once power is supplied. The anode is composed of mixed-
metal-oxide (MMO) coated titanium designed to resist corrosion from electrolytic
reactions. The cathode is made of perforated stainless steel. Further, the EDW is
comprised of a hydraulic piston which mechanically pushes on the sludge and aids in
the dewatering process. As the electro-osmotic flow develops and the sludge is
dewatered, also aided by the piston, the water passes through the cathode plate and is
collected in a container residing on an electronic balance. The mass of the water is able
to be measured as the EDW test occurs.
The EDW unit is operated by a touch screen interface and the operating parameters,
such as voltage, current, and the time of each run can be manipulated as required. The
120V AC is converted into DC by a rectifier inside the electrical cabinet.
3.3.4 Overall experimental sample flow during tests
A B C
Figure 8. Images of sludge samples as they went through testing process
The raw biosludge (Figure 8A); the sludge after centrifugation (Figure 8B) and after
being dewatered using the EDW unit (Figure 8C).
25
3.3.4.1 Filter Medium
The filter mesh was 53-μm pore-size Spectra Mesh™ made of nylon and was used in all
EDW tests conducted in this study. A previous EDW study used the same filter material
as used here and found it had no effect on energy consumption or water removal [45].
3.3.4.2 Pressurizing Electro-dewatering Hydraulic Piston
The hydraulic piston of the EDW was pressurized to 20 Psi, for all experiments, using
the laboratories air supply. Rubber tubes were used to connect the air supply valve to
the EDW cabinet.
3.3.4.3 Electro-dewatering Test Protocol
Test protocol for the EDW experiments was as follows:
1. Power on the EDW device and ensure sufficient air is flowing to the system.
2. The cylinder stroke of the hydraulic piston is adjusted with an empty filter to
calibrate piston position (this step is required once after boot-up).
3. Sludge sample is placed on the filter and its mass measured using an electronic
scale.
4. The filter with sludge sample is placed inside the EDW unit and the front
protective cover of the EDW unit closed to ensure safety.
5. Input voltage, current, time, and pressure parameters as required through the
touch-screen interface on the cabinet.
6. Start the EDW test in “automatic mode”.
7. When test is completed, remove the dewatered sludge sample from the unit and
perform further analysis.
8. Remove the filtrate and filtrate container from the EDW unit and clean the EDW
device using a sufficient amount of water.
3.3.5 pH Measurement
pH was measured using an Orion 370 meter. Prior to measuring the pH, the meter was
calibrated by performing a 3-point-calibration at pH 4, 7 and 10. The pH of the thickened
26
sludge was measured by taking 5 g of sludge sample, mixing it with 95g of deionized
water, and taking the pH of the resulting slurry.
3.3.6 Data Analysis
Student’s t-test was performed to observe the difference of means between two groups
of data at a confidence level of = 0.5, or 95%, with two degrees of freedom. The error
bars shown in this thesis represent one standard deviation from the mean. Regression
by least squares method was used to compute the line of best fit. Confidence intervals
of the fitted line was drawn by Graphpad Prism 7 (USA) software at the confidence level
of 95%. Data was considered significantly different if the confidence level was = 0.5 or
lower.
3.3.7 Conditioner Dose
The chemical conditioners, such as synthetic polymers or proteins, added comprised ~1-
4% of the total mass of the sludge. However, physical conditioners including fly ash,
lime mud, wood fines and silica were added and made up ~15-55% of the sludge and
this affected the DS. Table 6 provides information as to how the percentage of the
individual conditioners was calculated.
Table 6. Dosages reported for different conditioners used in this study
Type of Conditioner Amounts Added
Chemical
(synthetic polymers, proteins)
g of conditioner
g of dry biosludge * 100%
Physical
(Fly ash, lime mud, wood fines and silica)
g of conditioner
g of dry mixture * 100%
where, dry mixture = dry biosludge + dry conditioner
27
4. Results and Discussion
In this section, results of the electro-dewatering (EDW) tests are presented along with
analyses on potential improvement in dewaterability. Dewaterability can be categorized
based on several parameters: dry solids content (DS), dewatering rate, and the extent of
solids capture. However, to simplify the assessment of dewaterability in this thesis, the
DS content of sludge is the main parameter for the dewaterability of sludge by EDW and
the addition of the conditioners. The results of the water removed, temperature and
voltages are presented first, followed by the results of the effect of chemical polymers
and physical conditioners. In order to account for the difference in the initial water
content between the treated sludge sample and blank (sludge without any additives), the
mass of water removed was divided by initial water mass. Lastly, the energy
consumption rates are compared between the studied conditioners.
4.1 Types of Water Removed during EDW
Removed water from biosludge during the EDW test can be categorized into two groups:
filtrate and evaporated water. Filtrate, the water collected in the filtrate container, is the
water removed due to electro-kinetic effects with the aid of mechanical pressure from
the hydraulic piston. Evaporated water (the water removed by the elevation of
temperature due to Joule heating) also accounts for a significant portion of the total
water removed during EDW [46]. The evaporated water escapes the EDW unit because
the system is open to atmosphere. Figure 9 shows the different types of water removed
during the EDW tests under varying conditions. The evaporated water fractions were
calculated based on mass balance with filtrate and the final DS of sludge.
28
Figure 9. Distribution of water after electro-dewatering tests under varying
conditions. All in triplicate with 3 different batches of sludge, EDW Time= 10 min.
The fractions of filtrate water under EDW were in the range of 54-67% while the
fractions of evaporated water were between 18-25%. The results show that a greater
amount of water was removed from sludge as filtrate while the evaporated water portion
was relatively lower compared to the filtrate. However, evaporated water still accounts
for a significant amount of water averaging about 35% of the total water removed. These
results of water removal agrees with the literature which report a similar range when
EDW was performed on municipal activated sludge [46]. Free water, which is the largest
part of sludge, is estimated to be in the range of 70-75% for municipal sludge [47].
Because the results in Figure 9 show that more than 80% water (filtrate & evaporated
water) was removed, it appears that electro-dewatering can remove most free water and
some of interstitial or surface water as well.
The distribution of the filtrate and evaporated varied by test conditions. For example,
sludge conditioned with Zetag8165 polymer decreased the portion of filtrate while
increasing the amount of remaining water and this was significantly different from the
29
sludge to which no polymer was added (P<0.05). A voltage application of 30V increased
the fraction of filtrate thereby lowering the fractions of both evaporated and remaining
water. Further analyses on the conditioning methods such as the effect of voltage or
polymer are presented in the later sections of this thesis.
4.2 Temperature Elevation during EDW
Due to the Joule heating effect, where temperature is elevated as a result of an electric
current flowing through a conductor, the temperature of the sludge sample was greatly
increased during the course of the EDW tests. This elevation of temperature may have
both lowered the viscosity of water in the sludge sample and led to an increase in the
amount of evaporated water [12]. Figure 10 shows the temperature profile as voltage
was increased from 0V to 40V. EDW was done for 10 minutes in all tests.
Figure 10. Temperature profiles during EDW tests at 0, 20, 30, and 40V
Figure 10 shows that applied voltage strongly influence the temperature of the sludge as
the greater the voltage, the greater the increase in temperature. This was because a
higher voltage led to a higher current and therefore an increase in Joule heating. The
increase in temperature by voltage is in agreement with the literature that examined the
30
EDW of municipal biosludge [23, 35]. The test at 0V, when there was no electric field
applied, the temperature of the bisludge did not change. At 40V, there was a sharp rise
and by four minutes the temperature reached ~95°C and remained at this level for the
duration of the test. This sharp rise in temperature with increasing voltage has
previously been reported in EDW of municipal sludge [46]. The plateau in the figure was
expected because increases in temperature is diminished by the increase in the rate of
heat loss. Greater evaporation of water with increasing temperature was also observed,
as expected. At 40V, steam was visually observed to emanate from the sludge, but this
was not the case for samples under 0V and 20V.
The temperature increases may have lowered the viscosity of water inside the sludge
cake and this may have had an effect on the dewaterability of the sludge as there was
an increase in the amount of filtrate. For the EDW of P&P mill biosludge, at 40V the
sludge temperature reached ~95⁰C and led to a reduction in the viscosity of water. For
instance, dynamic viscosity of free water reduces from 1.002 to 0.315 (10-3 Pascal-
second (Pa.S)) as temperature rises from 20⁰C to 90⁰C [48]. This suggests that the
electro-osmotic flow may have been increased by a factor of ~3 as per the Helmholtz-
Smoluchowski equation (discussed in Chapter 2) due to reduced water viscosity.
Therefore, it was necessary to investigate if the reduced viscosity was a major factor for
the observed increases in the filtrate. To further investigate the effect of temperature, a
set of experiments were performed with thickened sludge samples heated to varying
temperatures in heated water prior to performing EDW. Figure 11 shows the DS of the
temperature-modified sludge after EDW at 0V and 40V.
Figure 11 shows that the change in the initial temperature of sludge had a minimal effect
on increased DS of electro-dewatered sludge. The test runs at 0V yielded an average
DS of 12%, but this value was well below what was observed from the test runs at 40V
where the average DS was 48%. The large difference in DS between 0V and 40V runs
shows that the application of the electric field significantly improved dewatering
regardless of the initial temperature of the sludge.
31
Figure 11. Effect of initial sludge temperature on DS after electro-dewatering test,
Conditioned with 2% Zetag8165, EDW Time = 10 min.
The results of the effect of initial temperature were not consistent with the literature. In a
study comparing both non-cooled and cooled EDW systems on municipal biosludge,
Navab-Daneshmand et al. [46] reported that the dewatering rate was enhanced in non-
cooled EDW systems. Mahmoud et al. [35] reported that reduced viscosity caused by
Joule heating enhanced dewatering kinetics in municipal wastewater sludge. However,
Figure 11 shows contrary results in that the elevation of the initial temperature up to
45°C at 40V did not yield a noticeable difference in DS. This was because the rate of
increase in temperature was too high at 40V runs, as seen in Figure 10. Thus, the
temperature of the sludge cake mostly remained at ~95⁰C during the experiment,
resulting in no difference in the sludge DS. Additionally, due to experimental limitations,
the initial temperature of the sludge cake was only elevated to 45⁰C. Thus, the increase
in temperature may have been too small to observe any effect. For these reasons, the
increase in electro-osmotic flow, as predicted by the Helmholtz-Smoluchowski equation,
was not observed in Figure 11.
32
Further analysis of average temperatures and its effects on DS at a lower voltage
suggests that increasing temperature alone without EDW is not sufficient in elevating the
sludge DS to the same extent as EDW. The combination of pressure and high
temperature, without the aid of electro-osmosis, did not lead to the same level of DS as
EDW. Figure 12 shows the test results in sludges electro-dewatered at 0V and 20V, at a
similar average temperature of ~35⁰C.
A B
Figure 12. Comparison of 0V and 20V EDW tests at similar average temperature
Figure 12 A shows a temperature profile of the EDW tests at 0V and 20V against time,
while Figure 12 B shows average temperatures and DS and corresponds to Figure 12 A.
As shown in Figure 12 B, the DS of the electro-dewatered sludge at 20V was 27% while
the no EDW sludge DS at 0V was ~13%. Although the average temperatures in both
test cases were similar, the difference in DS was large. Therefore, these results suggest
that the increase in temperature, due to Joule heating, and the reduction in water
viscosity were not the main factors for increasing the sludge DS by EDW.
33
4.3 Effects of Centrifugation on EDW
Since the first step of the experimental procedure involved removing water from the
sludge by centrifugation, it is important to assess the effect of centrifugation on the
performance of the EDW test. A typical industrial centrifuge operates at 3000 RPM for
10 minutes [49]. Although the laboratory centrifuge may not exactly mimic the actual
performance of an industrial centrifuge, centrifugation at 3000G (g-force or gravitational
force) for 10 minutes was chosen to be similar to that used in industry. More effective
centrifugation of sludge is possible if the g-force or the length of time of centrifugation is
increased. Centrifugation by itself will increase the removal of water from sludge and
lead to an increase in the DS of the sludge.
Based on the reference condition as a starting point, the g-force of the centrifuge were
varied and their effects on EDW were investigated. The DS of EDW sludges at different
centrifugal conditions are shown in Figure 13. The tests were performed on two different
batches of biosludge.
Figure 13. DS of centrifuged and electro-dewatered sludges at various centrifugal
conditions (shown on x-axis), Voltage = 20V, EDW Time = 10 min.
34
As expected, the tests with Sludge-1 batch showed that the DS of the centrifuged sludge
cake (green plaid) increased as the centrifugal force or time increased. The DS of the
centrifuged sludge was significantly increased (P<0.05) from 8% to 11% as the
centrifugal condition was changed from 3000G/10minutes to 6000G/20minutes,
respectively. This increase in DS was due to more powerful separation between solid
and water during centrifugation. However, even with the increase in DS of centrifuged
sludge, the DS of EDW sludge did not increase proportionally. It was found that
centrifugation had no effect on the DS of electro-dewatered samples (blue) as there
was no difference for samples centrifuged at 3000G/10minutes and 6000G/10minutes
(Sludge-1), or between the sludges thickened at 3000G/10minutes and
5000G/10minutes (Sludge-2). Where the sludge samples were centrifuged at
6000G/20minutes, the DS of EDW sludge was significantly decreased (P<0.05) by ~5%
compared to the DS of the EDW sludge thickened at 6000G/10minutes. This reduction
in the DS can be explained by a deteriorating intensity of the electric field (V/m) during
the test. The increase in the DS by more powerful centrifugation, as in the case of
thickening at 6000G/20minutes, is likely due to a tendency to resist compression against
pressure due to the accumulation (piling up) of the sludge particles. The samples
thickened at 6000G for 20 minutes had a greater amount of sludge particles and this
likely led to higher resistance against pressure. This is because the same mass of the
sludge sample (100g) was consistently used for the EDW tests, but the samples varied
in their DS content. A thicker sludge cake will lower the intensity of the electric field
because the distance between the electrode plates is increased, thereby deteriorating
the EDW kinetics. These findings agree with the literature in that a reduction in the
thickness of sludge improves EDW kinetics and vice versa [38]. Figure 14 below
presents the average thickness of sludge cakes during the EDW tests with varying
centrifugal conditions. The thickness values were averaged between 10 seconds and 10
minutes of the experiment in order to negate the initial compression by the hydraulic
piston which occurs from 0 to 10 seconds.
35
Figure 14. Average thickness and DS of sludge sample after electro-dewatering,
centrifugal conditions shown on x-axis, Sludge-1 batch, Voltage = 20V, EDW Time = 10 min.
The results in Figure 14 reveal that the sludge centrifuged at 6000G/20minutes had a
thickness of 4.44 mm and lower DS. The relationship between sludge thickness and DS
also suggests that thinner sludge cake does not necessarily mean an increase in the DS
content of sludge. Between the sludges centrifuged at 3000G/10minutes and
6000G/10minutes, there was a significant difference (P<0.05) of ~0.65 mm in the
thickness but not in DS. Therefore, EDW kinetics was not affected at low thickness but
with increasing sludge thickness EDW performance deteriorated.
Furthermore, the thickness did change the current in the sludge. Figure 15 shows the
average current at different centrifugal conditions. Because the tested voltage was
constant at 20V, the current in the sludge fluctuated during the EDW test so that the
current values shown is the average between 10 seconds and 10 minutes of the test.
36
Figure 15. Average current during electro-dewatering, centrifuged at conditions
shown on x-axis, Sludge-1 batch, Voltage = 20V, EDW Time = 10 min.
Figure 15 shows that the average current was 4.5A for the centrifuged sludge at
3000G/10minutes, and the currents were lower for the sludges centrifuged at
6000G/10minutes and 6000G/20minutes. The trend of decreasing current is explained
by the change in the resistance of the sludge cakes. For the thicker sludge cake, the
resistance against current is expected to be larger. Therefore, in order to maintain
constant voltage as per Ohm’s law (V=I*R) and neglecting the effect of capacitance, the
current proportionally decreased during the EDW test, indicating that the intensity of the
electrical field was reduced.
In summary, these results show that the EDW performance was affected by the
compressibility of the sludge cake because the intensity of the electric field was altered
by the thickness of the sludge cake. In order to increase the DS of biosludge as much as
possible, the compressibility of sludge should be adjusted such that optimal thickness of
sludge is maintained during EDW. The thickness of sludge can be an important factor in
industrial applications because the EDW processes is dependent on the level of
37
thickness of sludge cake. Figure 14 shows that the optimal thickness for the EDW tests
in this study was around 3.2 mm where the final sludge DS was high. It was found that a
thickness higher than 3.2 mm lowers the DS of the sludge cake.
The change in thickness of the sludge cake also depends on the amount of water
removed during the EDW test. This is because as more water is withdrawn from the
sludge cake, the sludge cake is expected to be thinner. Therefore, it can be concluded
that the final DS of the sludge cake after EDW is dependent on thickness, which then is
related to both the compressibility and also the amount of water removed from the
sludge.
4.4 Effect of Voltage
One of the main operating parameters of EDW is voltage. In this section, the results
from the EDW tests performed at various voltages are presented. Increases in the final
filtrate mass, as shown in Figure 16, are observed as the applied voltage was increased.
The amount of filtrate represents water removal by both pressurization and the electro-
kinetic effects. However, since the applied pressure was kept the same at 20 Psi and
only voltage was changed, the increase in filtrate amount was due to the change in the
applied voltage.
38
Figure 16. Effect of increased voltage for the biosludge conditioned with 2% Zetag8165 cationic polymer, Initial mass of sludge = 120g
The increase in voltage elevated both EDW rates and the extent of water removal.
Faster EDW rates are observed as the voltage increases as seen by the steeper curves
in Figure 16. For instance, the 40V and 80V runs have steeper curves than the test run
at 20V. However, when comparing the runs between 40V and 80V, the rates of increase
in filtrate are similar, showing that the increase in voltage does not proportionally
increase the dewatering rate. As more water was removed from the sludge sample, the
rate eventually diminished over time and a plateau was reached. This trend of the
reduction in the filtrate was expected because water became extremely difficult to
remove with less amount of free and interstitial water remaining in the sludge sample.
The result of the high water removal rate with increasing voltage is in agreement with
other studies on municipal biosludge [19]. As expected by the model based on the
Helmholtz-Smoluchowski equation, which is presented in Chapter 2 of this thesis, the
increase in voltage creates stronger electric field, thereby leading to a faster electro-
osmotic flow rate.
39
The increase in voltage also affects the extent of water removed by EDW. The plateau,
which is the final filtrate mass, signifies the extent of EDW at a specific voltage. The
plateaus reached for both 40V and 80V runs show that the filtrate increase by EDW
stops after approximately 10 minutes; however, for the run at 20V, the plateau was not
reached because the rate of increase in filtrate is slower. Similar to the EDW rates, the
extent of water removed is increased with higher voltage. However, the rate of water
removed is not linear with the increase in the applied voltage. The filtrate collected at
80V is 82g, which is slightly more than 75g obtained from the 40V run. Although the
voltage was doubled from 40V to 80V, the amount of filtrate collected did not
correspondingly double. These results also suggest that there is a limit for EDW. The
application of voltage more than 80V would lead to only a minor increase in dewatering
of the sludge, and hence, would not be energy efficient.
The amount of filtrate collected is reflected in the final DS of biosludge. Figure 17 shows
the DS of EDW sludge and the corresponding energy consumption rates in kWh kg-1 of
water removed. Because of the limited availability of sludge samples and the large
consumption in order to obtain a single data point, the test results shown in Figure 17
were replicated at one voltage and is reported in the appendix section of this thesis. The
data showed consistent results with a standard deviation of 1.72% and is in line with the
literature [38].
40
Figure 17. Effect of increased voltage, Time = 10 min.
As expected from the previous results where filtrate rates increased with increasing
voltage, Figure 17 shows that DS is also noticeably increased as the voltage is
increased from 0 to 80V. This agrees with findings on municipal sludges which report
that a higher voltage increases the sludge DS [18, 21, 36]. However, the increase in
voltage did not proportionally increase the DS of the sludge cake. While the application
of 20V produced a sludge cake with ~37% DS, quadrupling the voltage to 80V led to a
DS of ~60%. The change in voltage affects energy consumption. At 80V, the amount of
energy consumed was 0.52 kWh kg-1 of water removed (~1900 kJ kg-1). Figure 18
shows the relationship between the DS and energy consumption rates.
41
Figure 18. DS and corresponding energy consumption rates at various voltages,
Time = 10 min.
Figure 18 shows a trend that energy consumption increases linearly with the increase in
the sludge DS. The trend is in line with the literature which notes that higher energy
input is required to elevate the DS of EDW sludge [38]. The energy consumption rate
would be expected to rise exponentially beyond 60% DS, as reported in the literature
[38], possibly due to the last fraction of water being very difficult to remove from the
sludge.
Energy consumption is the key parameter for determining the operational costs of an
EDW device. Because the increase in energy consumption also elevates the operational
costs, the EDW device should be optimized to meet the target DS while minimizing the
energy input. The DS requirement for dewatered sludge, however, may be different
among applications due to factors such as the type of wastewater treatment/disposal
processes, type of sludge, or operational budget. In order for the EDW technology to be
economically viable, energy consumption rates should be minimized while producing
reasonably high DS. In the literature, dewatering biosludge to 40% DS seems to be
cost-effective [1, 50]. The present experimental results suggest that 20V is the ideal
42
voltage to obtain a sludge DS of ~40%. Therefore, the rest of data presented in this
thesis was obtained with a voltage of 20V.
The applied voltage of 20V allows one to test for the rate of EDW. As can be seen in
Figure 16, two parameters are involved in the analysis of dewaterability: the rate and the
extent of electro-dewatering. If 20V is applied, a plateau is not reached and therefore
only the rate of EDW is assessed. This further eliminates the need to assess the extent
of EDW at a higher voltage. For the results comparing the effect of conditioners, the
experiments were performed at 20V for 10 minutes. Thus, the DS of EDW sludge
represents the EDW rate in 10 minutes.
4.5 Cationic Polymer Addition
Polymer conditioning is commonly used in P&P mills to flocculate sludge during the
wastewater treatment process, but it is rather costly. One of the potential benefits of
using EDW may be the reduction of polymer demand and therefore the costs associated
with dewatering. Several researchers who studied municipal sludges have reported that
polymer conditioning had no significant effect on EDW performance [15, 39, 41]. To
examine the effect of cationic polymers on P & P mill biosludge, raw sludge sample was
mixed with a polymer solution and EDW conducted at 20V for 10 minutes.
Zetag8165 (BASF), with its high cationic charge density, had shown the best
performance as measured by capillary suction time (CST) test on biosludge. Based on
the CST test, it was selected to examine its effect on filtrate removal and DS after EDW.
Figure 19 shows the results of a blank sludge sample (no conditioner added) and one
treated with 2% Zetag8165 and EDW performance. In this thesis, blank refers to a
sludge sample without any additives.
43
Figure 19. DS of sludges with or without Zetag8165 after electro-dewatering test, Voltage = 20V, Time = 10 min.
Figure 19 shows that conditioning sludge with 2% Zetag8165 actually decreased the DS
of sludge cake as compared to the blank. Adding the polymer showed an adverse effect
and significantly less reduction (P<0.05) in dryness as compared to the blank. In order to
account for the difference in the variance of initial water between the treated sample and
blank, the mass of the total water removed was divided by the mass of the initial water
content, yielding the fraction of water removal. Similar to the DS result, the fraction of
water removed was lower 0.83 to 0.76 when the polymer was added.
Based on the information provided by the vendor, Zetag8165 polymer is known to carry
medium to high cationic charges which promotes flocculation by neutralizing the
negative charges in biosludge. The results of the EDW tests indicate that neutralizing
the charge by adding cationic polymer to sludge may adversely affect EDW performance.
44
Cationic polymers such as Zetag8165 work as flocculants by two main mechanisms: first,
they reduce the repulsion between negatively charged particles by charge neutralization,
and second, the linkage of sludge flocs together with the long chains of the polymer
molecules, known as a “bridging effect”. Both mechanisms promote the flocculation of
the sludge particles, thereby enhancing mechanical dewaterability. This effect of
flocculation by the polymer is also reflected in the increased rate of filtrate at the early
stage of EDW as shown in Figure 20. The filtrate collected from the sludge conditioned
with 2% Zetag8165 is higher up to 1 minute of EDW, possibly due to the effect of
cationic polymer enhancing the mechanical dewaterability under pressure. After
approximately 1 minute of the run, the sludge treated with no polymer dominated in
terms of the filtrate collected until 10 minutes of the test. The results provides insight that
the addition of 2% Zetag8165 deteriorates EDW performance.
Figure 20. Filtrate removal rates by Zetag8165 conditioning at 2%, Voltage = 20V
To further investigate if there is an effect in the DS and filtrate dewatering rate when
performing electro-dewatering, another cationic polymer which carries a lower cationic
charge than Zetag8165 was tested. Therefore, Organopol5400 (BASF), was tested at
various concentrations. Figure 21 shows sludges conditioned with either Zetag8165 or
Organopol5400 with concentrations varying from 0 to 4%.
45
Figure 21. Comparison of sludges conditioned with Zetag8165 or Organopol5400, Voltage = 20V, Time = 10 min.
Figure 21 shows that adding Organopol5400 resulted in no significant difference in the
DS of sludge for all runs from 0% to 4% concentrations. At 4%, Organopol5400 yielded
38.9% DS while at the same concentration Zetag8165 yielded ~28.7% DS. Extensive
charge neutralization, which is occurring at 4% Zetag8165, resulted in a drop of ~10% in
DS as compared to Organopol5400. These results indicate that Organopol5400, which
carries less amounts of cationic charge than Zetag8165, is better at electro-dewatering
and that the EDW performance is strongly degraded by charge neutralization. In
conclusion, the studied cationic polymers are not effective in increasing the DS and the
rate of EDW.
However, these findings contradict what have been previously reported in the literature.
Most researchers have claimed that polymer conditioning has no effect on the EDW of
municipal sludges regardless of the conditioning history [15, 41], and some reported an
improvement in the DS and dewatering rates [17]. Based on the results in the literature,
it is suspected that the results of reduction in the DS by cationic polymer may be due to
supplementary effects such as the change in the compressibility of sludge (thickness) as
46
seen in the previous sections. Several hypotheses can be made in regards to the
decrease in the DS by Zetag8165 conditioning.
1. Charge neutralization by Zetag8165 reduces zeta potential (ζ) of the sludge
medium, decreasing the electro-osmotic flow as suggested by the Helmholtz-
Smoluchowski equation.
2. Charge neutralization in combination with the bridging effect promotes the
flocculation of sludge. The accumulated sludge particles of the thickened
samples have the effect of reducing compressibility, thereby causing the sludge
cake to resist pressure during the EDW test. Thus, the change in the
compressibility may decrease the intensity of the electric field by increasing the
overall thickness of sludge cake, ultimately hindering the electro-kinetic effect.
3. A combination of hypotheses 1 and 2
If the second hypothesis holds true, the adverse effect by charge neutralization cannot
be easily concluded because the reduction in DS could be due to the change in the
compressibility of sludge regardless of the polymer conditioning. This hypothesis, in turn,
is in agreement with the findings from the literature that polymer conditioning has no
significant effect on EDW efficiency. To evaluate if the second hypothesis is valid, an
analysis can be performed by comparing the DS at the same level of thickness between
the sludges conditioned with or without polymer. Figure 22 shows the DS of sludge
cakes and corresponding average thickness values recorded during the EDW tests. The
trend curve in the figure indicates the linear regression of the data for the sludge
samples with no polymer (blanks), showing a collective trend based on 23 observations
obtained from 6 different batches of sludge thickened at various centrifugal conditions.
For example, more powerful centrifugation increases the DS of thickened sludge,
leading to larger sludge thickness during the EDW test. For the linear fit of the trend, the
upper and lower bounds of the confidence interval at the confidence level of 95% are
shown for the test runs with no polymer. The data points for polymer conditionings,
either Zetag8165 or Organopol5400, show the DS and the corresponding thickness
values at the dosages between 0 and 4%. More specifically, a small black box in the
graph shows the data points for the sludges conditioned at 4% Zetag8165 where intense
charge neutralization would be expected to be occurring.
47
Figure 22. DS and the average thickness of sludge cakes (6 different batches of sludge), showing cationic polymer data, dotted-lines representing upper/lower
confidence intervals (α = 95%) for the runs with no polymer, Voltage = 20V, EDW time = 10 min.
Figure 22 shows that at a thickness of 4 mm, the sludges conditioned with 4%
Zetag8165 performs poorly by skewing the DS values outside the lower confidence
interval of the linear regression fit. For example, the DS values at 4% Zetag8165 were
29.3%, 28.7%, 27.9% and 27.6% with the average being 28.4%. However, the expected
DS by as calculated by linear fit should be ~33.4%. These results suggest that the
polymer addition decreases the DS even further than the DS expected from blank runs.
Although the graph has a limitation that multiple batches of sludge were used to plot the
trend, a conclusion can be made in regards to the effect of the change in compressibility
by the polymer: the skewed results at the high dose of Zetag8165 suggest the rejection
of the 2nd hypothesis. The weakened intensity of the electric field by an increase in the
sludge cake thickness is not the main factor for the reduction in the DS of the sludge
conditioned with the cationic polymer. This analysis on the thickness implies that the
48
charge neutralization by cationic polymers deteriorates the EDW kinetics regardless of
the thickness of sludge cakes.
4.6 Addition of Physical Conditioners
To examine any beneficial effect of physical conditioners on EDW, various physical
conditioners were added to biosludge followed by performing EDW on them. The mixing
of the conditioners can be performed at two different points: either before centrifugation
or after. For simplicity, the terms “pre-mixing” (before) and “post-mixing” (after) are used
to refer to the way samples were conditioned.
Three different wastes from P & P mills were tested followed by EDW treatment: fly ash,
lime mud, and wood fines. To further examine the effect of increasing inorganic content
to sludge, the addition of silica was also investigated. This section presents the EDW
results categorized by the different conditioners tested.
4.6.1 Fly Ash and Lime Mud
Two common wastes from a pulp mill, fly ash and lime mud, were mixed with biosludge
to examine any potential effect on EDW. As a preliminary study, the pre-mixing of the
conditioner was performed on sludge, and the EDW results are presented in Figure 23.
The fly ash or lime mud was added to the raw sludge at dosages of between 0 and 50%.
For example, as stated in the method section, the dosage of physical conditioner at 50%
refers to 0.5g of dry conditioner per 1g of dry mixture.
49
Figure 23. Effects of fly ash or lime mud addition on biosludge, Pre-mixing, Voltage = 20V, Time = 10 min.
Figure 23 shows that both fly ash and lime mud conditioning at the dosage of 17-50%
decrease the DS of the mixture. These results indicate that the EDW performance
deteriorates when fly ash or lime mud is added. They cannot be used for EDW on P&P
mills sludge. For lime mud, the rates of decrease in the DS moderates at a dosage of
30%, followed by a slight increase in DS at 50%. The slight increase at the higher
dosage is suspected to be because of the increase in the amounts of solids added.
Although Figure 23 is an accurate reflection of the experiments conducted, the data
does not take into account the loss of conditioners during the removal of centrate. A
more accurate way to measure the effects fly ash and lime mud is to directly add the
solids to a thickened sludge: post-mixing. For this, another set of experiments were
performed where fly ash or lime mud was post-mixed. Figure 24 shows the effect of fly
ash and lime mud at different dosages between 25% and 50%.
50
Figure 24. Effects of fly ash and lime mud addition on biosludge, Post-mixing, Voltage = 20V, Time = 10 min.
The results were similar to what was observed in the pre-mixed samples. For both
conditioners, there is a decrease in the DS content at 25% dosage followed by a slight
increase at 50%. It is suspected that in the latter case, the increase in the DS of sludge
cake at the high dosage is due to the addition of solids since both fly ash and lime mud
used in this study had the high DS of 69-71%. However, the drop in the DS at the 25%
dosage is possibly due to the conditioners hampering the dewatering of sludge. There
could be a number of reasons why the DS is lowered, but one could be that adding fly
ash or lime mud increased the pH of the sludge.
Both fly ash and lime mud, when dissolved in water (or watery sludge), alters the pH of
the mixture and this could have affected the EDW performance resulting in lower DS as
compared to blank samples. Researchers have investigated and claimed that pH is one
of the major factors influencing EDW performance [41, 46]. Navab-Daneshmand et al.
[46] reported that increasing pH reduces the EDW rate on municipal biosludge, which
could have occurred in the present study. To measure the effect of pH change by fly ash
51
or lime mud, the pH of a mixture was measured at a concentration of 25% (the largest
drop in DS was observed at this dosage). Another set of experiments was performed by
conditioning sludge with sodium hydroxide, increasing pH to the same level of both fly
ash and lime mud conditioning. This addition of caustic is aimed so as to compare data
between sludges and to examine the effect of the pH increase. Figure 25 shows the pH
values measured for the different conditioning methods.
Figure 25. pH of sludges, in triplicate with samples prepared at 5% wt. in deionized water
At a dosage of 25%, the pH of blank samples were measured to be ~6.8, and adding
either fly ash or lime mud increased the pH of the sludge sample to ~7.6-7.8 pH. By the
addition of NaOH to raw biosludge, the pH of thickened sludge is elevated to 7.77 and is
in similar to the sludges conditioned with fly ash or lime mud. Subsequently, EDW tests
were performed on the same sludges shown in Figure 25. The corresponding DS and
the water removal rates for those sludges are presented in Figure 26.
52
Figure 26. Electro-dewatering results comparing sludges conditioned to basic pH, Voltage = 20V, Time = 10 min.
The results from Figure 26 reveal that the pH elevation through the addition of either fly
ash or lime mud has a minimal effect on decreasing the DS of the sludge after EDW.
The sludge with the increased pH by NaOH shows no significant difference in DS as
compared to the untreated sludge where both generated DS of ~40%. This DS value
was, however, much higher than what was observed for both fly ash and lime mud
conditioned sludge where it was ~31%. Therefore, this EDW experiment shows that the
pH increase is not the major factor which causes a reduction in the percent DS
generated when fly ash or lime mud are added to sludge.
As seen previously, an increase in the thickness of mixtures does degrade EDW kinetics.
Adding physical conditioners such as fly ash or lime mud may reduce the compressibility
of the mixture and the thickness of the sludge sample during the EDW test may have
increased thereby degrading the EDW kinetics. To test this, DS and thickness of fly ash
or lime mud conditioned sludge were compared. Figure 27 shows the DS and
corresponding thickness values for the fly ash and lime mud conditioned sludge and a
blank with no conditioners.
53
Figure 27. DS and the average thickness of sludge cakes (6 different batches of sludge), showing fly ash and lime mud data, dotted-lines representing
upper/lower confidence intervals (α = 95%) for the runs with no additive, Voltage = 20V, EDW time = 10 min.
Figure 27 shows that for both fly ash and lime mud there was a decrease in the DS of
sludge cake and was even lower than the expected DS at the same level of thickness.
Thus, this results proves that reduced compressibility by either fly ash or lime mud is not
the main reason for the reduction in the DS. Further discussion on the degraded EDW
rate by the fly ash or lime mud addition is presented in later sections of this thesis.
54
4.6.2 Silica
A previous study on municipal digested sludge reported that increasing the inorganic
content by adding fly ash improved EDW efficiency [43]. However, here it was observed
that fly ash conditioning did not increase the DS of sludge even at the high dosage of
50%. To investigate whether increasing the inorganic content affects EDW, pure
inorganic chemical can be added to the sludge. To observe the effect of inorganic
chemical, silica (SiO2), with a DS of ~99%, was chosen as a conditioner to be added to
the biosludge followed by EDW performed on the mixture. Figure 28 shows the results
of the silica addition at a dosage of 55%.
Figure 28. Effect of silica (SiO2) addition, Post-mixing, Voltage = 20V, Time = 10 min.
Adding silica dramatically increased the DS of the mixture by ~14% as compared to the
one with no silica. In the silica sludge, the DS was 52.6% after EDW. While this increase
is large, the results should be evaluated cautiously because the increase may not have
been due to the beneficial effect of silica but rather was simply due to the addition of
pure solids. In the latter case, adding silica is not justified because the addition would
55
only increase the volume of the mixture being treated. Prior to EDW, the blank and the
treated sample had a DS of 9.6% and 19%, respectively. Therefore, the fraction of water
removed was examined to take into account the difference in the initial water mass
between the blank and silica sludge samples. Figure 28 shows that the fraction of water
removed actually decreased, from 0.82 to 0.78, in the silica added sludge. In fact, less
water was removed by the addition indicating that silica slightly deteriorates EDW
performance. Moreover, the difference between the post-EDW DS is accountable to the
~10% difference in the blank and silica added sludge which was already present prior to
conducting EDW. Therefore, it can be concluded that increasing the inorganic content of
P & P biosludge does not help improve EDW. If incineration is taken as the final disposal
method for sludge, the burning efficiency in a boiler would be decreased since the silica
would only increases the amount of ash produced.
4.6.3 Wood Fines
Wood fines are another type of carbon-based waste that can be utilized in P & P mills. In
some P&P mills, wood fines have been used as a physical conditioner to enhance
dewaterability of sludge. However, based on a previous study, mass balance revealed
that wood fines absorb more water than help releasing it during mechanical dewatering
[51]. Even though the study reports this adverse effect, it is interesting to examine the
effect of wood fines for EDW treatment. It is theorized that wood fines would provide
structural rigidity to the sludge sample. Based on the previous results of thickness, this
would create a thicker sludge cake, thereby reducing the electric field intensity. In order
to test what would occur, wood fines at the dosage of 55% were mixed with sludge and
this mixture was followed by EDW. Figure 29 shows the results of the wood fine addition.
56
Figure 29. Effect of wood fine addition, Post-mixing, Voltage = 20V, Time = 10 min.
Similar to other previously examined conditioners, the results of wood fines show
adverse effects in both DS and the fraction of water removed. By the addition, the
fraction of water removed is greatly decreased from 0.82 to 0.43 while the DS of the
mixture is also reduced from 37% to 28%. These results support the argument that the
addition of physical conditioners is ineffective at removing more water from sludge.
57
4.6.4 Discussion on the Effect of Physical Conditioners
In summary, the physical conditioners fly ash, lime mud, silica or wood fines have
adverse effects on electro-dewatering when both DS and water removal fraction are
examined. Although adding silica increases the DS of the mixture, the fraction of water
removal is diminished, indicating adding silica does not enhance EDW. An attempt to
examine the possible effect of pH change did not verify the assumption that the pH
increase was the main factor for hindering EDW performance. The analysis on thickness
shown in Figure 27 revealed that the increase in the thickness of the mixture and
reduced compressibility is also not a significant factor for decreasing the EDW rate when
the conditioners are added.
The possible explanation why the DS and water removal rates are deteriorated may be
one or a combination of water absorption and adsorption by the physical conditioners.
The physical conditioners had relatively higher DS, in the range of 69-99%, compared to
the thickened sludge which had around 10% DS. Because the conditioners are drier
than the sludge, it is suspected that the water in the sludge is transferred to the
conditioner solids. Thus, water absorption or adsorption is suspected to be occurring
onto the conditioner solids when the mixture is made. Adding physical conditioners
saturated with water can potentially eliminate the absorption by solids. However, this
method is not practical as it would only increase the volume of conditioners which then
increases the total volume of mixture that requires dewatering and disposal.
Based on the results in this study, adding physical conditioners is ineffective for the
EDW of biosludge and is due primarily to two reasons: first, the addition of physical
conditioners greatly increases the total volume of the mixture to be handled at the final
disposal stage; and second, the added solids do not help release more water by
providing a structural advantage during EDW. Therefore, it can be concluded that the
addition of the studied physical conditioners actually hampers EDW treatment of
biosludge.
58
4.7 Protein Conditioning
To further examine the effect of charge neutralization by cationic additives, biosludge
was conditioned with proteins which have different amount of cationic charges.
According to a previous study, cationic charged proteins increase the mechanical
dewaterability of sludge [52]. In the present study, protamine and bovine serum albumin
(BSA) were used to investigate how they affected the dewaterability of P & P biosludge.
Protamine carries a large amount of cationic charges which can neutralize the negative
charges of sludge flocs. In a CST test, this protein improved filtration of sludge. BSA is
another protein that carries relatively lesser amounts of cationic charges than protamine
[52]. Based on the previous results of cationic polymers on EDW, it was expected that
the EDW performance would deteriorate with the addition of a highly cationic protein
such as protamine, possibly due to charge neutralization. Figure 30 presents the effect
of cationic proteins from the EDW tests at the dosage of 10%. The dosage was selected
because it yielded the lowest CST in the previous study.
Figure 30. Effect of protein conditioning, Voltage = 20V, Time = 10 min.
59
As expected, the addition of protamine decreases the DS to ~23% while the blank
sample with no protein produces a sludge of ~38% DS. The addition of BSA, however,
yields ~36% DS with no significant difference as compared to the untreated sample.
Water removal fraction is also decreased for the sludge conditioned with protamine, but
there was no effect by the BSA addition. Therefore, the results show that protamine
strongly degrades EDW performance and decreases the DS of the sludge cake.
Similar to the results of a strong cationic polymer such as Zetag8165, protamine with its
high cationic charge is thought to heavily neutralize the negative charges present in
sludge. The results of cationic proteins addition supports the finding that EDW
performance is strongly affected when the sludges charge is neutralized. In the case of
the BSA addition, the protein carries a lesser amount of cationic charges than protamine
and therefore does not change the charge profile of sludge as drastically. Thus, the
treatment with BSA does not significantly deteriorate EDW performance.
So far, the results of cationic additives, such as polymers and proteins, suggest that the
conditioning of sludge with the cationic additives has negative effect on EDW by
reducing both the DS and water removal. These results suggest that the already
negative charge present in the sludge may help increase the rate of EDW. Thus,
another experiment was performed by using an anionic additive to observe the effect of
elevated negative charges and is shown below.
4.8 Anionic Polymer Addition
To further examine the effect of polymer conditioning, sludge was mixed with an anionic
polymer, Organopol5510 (BASF), so that it became more negatively charged.
Organopol5510 polymer has been used by pulp mills, in combination with bentonite as
the settling agent in a clarifier. The results of the sludge treated with Orgaonopol5510 at
2% dosage is shown in Figure 31.
60
Figure 31. Effect of anionic polymer conditioning, Voltage = 20V, Time = 10 min.
The results in Figure 31 show that Organopol5510 does not help increase the DS for
EDW. Instead, the DS of the EDW sludge with 2% Organopol5510 was significantly
lower (P<0.05) than the untreated sample with the difference being 3.3%. In contrast to
the DS result, the fractions of water removed was not significantly different between the
blank and treated samples. These results imply the difference in initial water content that
more water present in the polymer conditioned sample causes the reduction in the DS.
The DS of the thickened sludges after centrifugation were 9.1% and 8.2% for blank and
the treated sample, respectively. The reduction in the sludge DS after centrifugation is
expected because adding an anionic polymer increases the amount of negative charges
in sludge. The negative charges then degrade the mechanical dewaterability by
increasing repulsion between sludge flocs, thereby resulting in lower DS after
centrifugation. Thus, the difference in the final DS of sludge after EDW may be
explained by the difference in the initial water content of the samples.
No effect of adding the anionic polymer can be explained by the limitations that further
increase in DS is difficult as the sludge dryness is increased. Therefore, the increase in
61
negative charges do not have a significant effect on increasing the EDW rate. The EDW
sludge without any polymer already was close to ~35-40% DS. At this point, water
removal is difficult because there is a small amount of water remaining in the sludge
sample, hampering the flow of water during EDW. Thus, even with an increase in
negatives charges through the addition of an anionic polymer, the EDW rate does not
increase.
It can be concluded that the studied anionic polymer at the tested dose does not
increase the rate of electro-dewatering. Because increasing negative charges in sludge
may only degrade mechanical dewaterability during the thickening process, adding
anionic polymer alone is not effective for the dewatering of biosludge using EDW
treatment.
62
4.9 Estimation of Energy Consumption
Energy consumption is one of the important aspects for the operation of an industrial-
scale EDW device. Therefore, it would be beneficial to quantify the energy consumption
rates for the studied conditioners. The energy consumption rates were calculated based
on the mass of water removed. The results are presented in Figure 32 and categorized
by the conditioners studied. Figure 32 also presents the DS of electro-dewatered sludge
to compare the EDW rates and corresponding energy values.
Figure 32. Energy consumption and corresponding DS from electro-dewatering tests with conditioners, Columns with the same colour and pattern indicate the
same batch of sludge, Voltage = 20V, Time = 10 min.
The data shows that the addition of a strong cationic additive, as in 4% Zetag8165 or 10%
protamine, greatly reduces the energy consumption rates as compared to other
conditioning methods. For the runs with 4% Zetag8165 and 10% protamine, the energy
consumption rates are decreased by 47% and 36%, respectively, as compared to blank
runs. However, the DS of EDW sludge is also decreased. If the dosage of cationic
polymer is decreased, the DS and energy consumption rate increase. For example, the
conditioning of sludge with 2% Zetag8165 results in higher DS and energy consumption
rates than the runs with 4% Zetag8165. These results suggest that the charge
neutralization by strong cationic additives, either polymer or protein, decreases the
energy consumption rates of EDW and also the DS of sludge.
63
Among the tested cationic additives, conditioning with Organopol5400, which is a
relatively weaker cationic agent than Zetag8165, showed the best performance when
both the DS and the energy consumption rates are considered. For example, the
addition of 2% Organpol5400 significantly reduced the energy consumption rate by
~19%, but there is no significant difference in the DS when compared to the untreated
samples. These results provide the insight that appropriate use of cationic polymers,
possibly at low dosage, may help reduce the energy consumption rate of EDW while still
maintaining the dewatering performance.
A possible explanation for the reduction in energy consumption by adding a cationic
polymer is given by Smollen & Kafaar [18]. They proposed that the amount of electrical
current is strongly dependant on the remaining negative charges in sludge, and because
cationic polyelectrolytes can neutralize the negative surface charge of sludge, the
electrical current become much lower which, in turn, decreases the energy consumption
rates. The results in this study also agree with the explanation in that only sludges
conditioned with cationic additives exhibited lower energy consumption rates as
compared to the blank samples.
It was found that the use of physical conditioners, such as silica, lime mud, or fly ash,
does not help reduce the energy consumption rate of EDW. For fly ash, the energy
consumption rate is dramatically increased, reaching the highest value of 0.57 kWh kg-1
(~2000 kJ kg-1) among the conditioners. When compared to the untreated samples,
which was 0.21 kWh kg-1 (760 kJ kg-1), the addition of fly ash shows the worst
performance as it doubled the cost of energy consumption compared to the blank.
Ohm’s law suggests a possible reason for the increased energy consumption when fly
ash or lime mud was added. The addition of fly ash or lime mud could reduce the
electrical resistance of the sludge sample and increases current at the constant applied
voltage, thereby elevating energy consumption. The results also imply that the increase
in the intensity of current, as seen in the fly ash addition, does not effectively help
increase the DS of the sludge cake.
The EDW treatment, for its ability to generate a very high DS sludge cake, is usually
compared to a thermal drying process. The energy consumption for an industrial scale
drying process is estimated to be in the range of 0.62-1.2 kWh kg-1 of water removed
64
(2200-4300 kJ kg-1) [38]. The results in this study show the energy consumption rates of
EDW ranging between 0.17–0.21 kWh kg-1 of water removed (610-760 kJ kg-1) for the
untreated samples and possibly lower for the samples conditioned with cationic
conditioners. The literature also reports a similar range of energy consumption for the
EDW of municipal biosludge: 0.10-0.28 kWh kg-1 of water removed (360-1000 kJ kg-1)
[35, 36, 37]. Therefore, at the studied DS of sludge, the EDW tests on P&P mill
biosludge consumed much less energy than the thermal drying processes.
The energy costs for EDW can be roughly estimated. As of August 2017, the average
price of residential electricity in Ontario is in the range of 9.5-13.2 cents kWh-1. Based on
the energy consumption rates from the test results in this study, the price of electricity
for EDW is then approximately 1.6-2.8 cents kg-1 of water removed. In the case where
the biosludge DS is increased from 10% to 40%, the electricity costs of EDW (raw
biosludge and no additives) is estimated to be in the range of $0.77-1.32 million/year for
a P & P mill producing 17 ODt/d of biosludge [2].
65
5. Implications on Industrial Application
The findings from this study may suggest the potential applicability of the EDW
treatment for the P&P industry. Several notes on industrial application can be made as
follows.
With electro-dewatering, more than 40% DS on biosludge is achievable depending on
the amount of energy input, sludge conditioning, sludge loading rates and etc. As
high DS is preferred for decreasing the total volume of sludge for disposal, such high
dryness of sludge can significantly reduce the disposal costs associated with sludge
transportation, incineration, land filling or land application.
If the incineration of sludge is selected as the final disposal method, the high dryness
of sludge (>40% DS) can potentially reduce the requirement of the auxiliary fuel used
in a boiler. This could potentially lead to cost savings.
A reduction in polymer demand may be possible with EDW. The addition of chemical
polymers was not effective for increasing the EDW rate; thus, cationic polymers may
only be used for enhancing mechanical dewatering of sludge (e.g. thickening
processes). However, the costs for EDW, such as the cost of electricity consumption
should be carefully analyzed and quantified in order to decide if the benefit of polymer
reduction outweighs the operational costs.
The potential sludge treatment process using the EDW process in combination with
incineration is illustrated in Figure 33.
Figure 33. Overview of the potential electro-dewatering and disposal process
66
Low dosage of a cationic polymer can help reduce energy consumption, thereby
reducing the operation costs for EDW.
The addition of physical conditioners such as fly ash, lime mud or wood fines under
the studied conditions is not effective for increasing the rate of EDW. As the addition
of physical conditioners increases the total volume of sludge, it is recommended that
only sludge without any physical additive be treated using EDW.
The temperature of sludge for EDW can rise to ~95°C and possibly higher with a low
amount of water remaining in sludge, therefore, safety issues for the operation of the
EDW device need to be addressed.
67
6. Conclusions
The main objective of this thesis was to investigate the effect of electro-dewatering on P
& P mill biosludge by examining the DS content and the amount of water removed
during EDW. A secondary objective was to investigate if conditioners, both chemical and
physical, show any beneficial effect on the EDW treatment of biosludge. Several
conclusions can be drawn from the present work:
1. With EDW, the DS of secondary sludge from a sulphite P&P mill can be
increased to more than 40% DS without any conditioner addition.
2. Increasing applied voltage (electric potential) increases the final DS of the EDW
sludge.
3. The thickness of the sludge cake plays a significant role. Increasing sludge
thickness reduces the intensity of the electric field, thereby decreasing the EDW
rate.
4. Cationic additives at a high dose deteriorate the EDW rate, resulting in a lower
DS of sludge. It is probably a result of charge neutralization by cationic polymers.
5. Adding anionic polymer alone at the tested dose does not help improve the EDW
rate.
6. Physical conditioners such as lime mud, fly ash and wood fines do not help
increase the EDW rate of biosludge. The adverse effect of these additives is
likely due to the difficulty in removing the water that they physically absorb from
the biosludge.
7. Small amounts of cationic agents (polymer or protein) can help reduce energy
consumption for EDW. Among all conditioners tested in this study,
Organopol5400 at 2% dose performed the best by yielding a 19% reduction in
energy consumption while the DS of EDW sludge was the same as a blank
sludge.
68
7. Recommendations
Multiple recommendations can be made on areas not covered in this study.
1. Effect of EDW on kraft mill sludge - Only biosludge from a pulp mill that uses
sulphite pulping process was investigated in this study. Due to the wide
popularity of kraft pulping process in the P & P industry, it would be beneficial to
study the effect of EDW on kraft mill sludges, and possibly perform a study
comparing sulphite, kraft and municipal sludges.
2. Potential synergetic effect between the conditioners - For example, a physical
conditioner may be studied together with the addition of cationic polymers to
observe any beneficial effect.
3. Effect of mixing primary and secondary sludges at different ratios – This is
because mixing of two sludges is the common practice at P&P mills.
4. The thickening method used in this study does not fully represent an industrial
thickener. Therefore, using different thickening methods, such as a gravity
thickener or crown press, or another way to replicate an industrial thickener can
be conducted prior to the EDW test to yield more practical results.
5. Developing a continuous EDW process to simulate the performance of an
industrial scale dewatering system.
6. Exploring ways to utilize the excess heat generated during the EDW process
7. Because the EDW treatment generates a sludge cake with high dryness, some
solid particles are strongly attached to the filter medium after dewatering, creating
a blinding problem. This blinding of the filter medium could be problematic for an
efficient treatment. Thus, future studies can be focused on developing a method
or materials to minimize the blinding problem.
8. As water travels from anode to cathode during the course of EDW, drier sludge
cake is observed near the anode compared to the cathode. Thus, it would be
beneficial to study the gradient of water along the sludge cake.
69
9. Previous studies suggest that an interrupted EDW process by the intermittent
application of an electric field can reduce energy consumption [53, 54]. Therefore,
it could be beneficial to study the effect of an interrupted current on the EDW of
P&P mill sludges.
10. Adding synthetic polymers to biosludge can change the viscosity of liquid in the
sludge. However, the effect of viscosity change by conditioners was not
examined in this study. Thus, future studies may focus on investigating how the
change in viscosity, through the addition of conditioners, affects EDW
performance.
70
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75
Appendices
Appendix I:
Repetition of Electro-dewatering tests at 30V
Figure 34. Repetitive runs of EDW tests at 30V for 10 min., 120g of sample each run
76
Appendix II:
Estimation of the mass of evaporated water
The mass of evaporated water was estimated because the EDW system used in this
study was open to atmosphere and the evaporated water was not collected. If the
sample mass of sludge, the dry solids contents of initial (before EDW) and final (after
EDW) samples were known, then the mass of evaporated water was estimated as
follows:
𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 = 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 ∗ (1 − 𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙)
Where, m denotes mass in gram and DS denotes dry solids in fraction (e.g. DS = 0.10)
Similarly,
𝑚𝑠𝑜𝑙𝑖𝑑𝑠 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 = 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 ∗ (𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙)
Here, it was assumed that no solids was lost during EDW; thus, the mass of solids was
assumed constant. Then,
𝐷𝑆𝑓𝑖𝑛𝑎𝑙 =𝑚𝑠𝑜𝑙𝑖𝑑𝑠
𝑚𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒
+ 𝑚𝑠𝑜𝑙𝑖𝑑𝑠
𝑚𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟 =𝑚𝑠𝑜𝑙𝑖𝑑𝑠
(𝐷𝑆𝑓𝑖𝑛𝑎𝑙)− 𝑚𝑠𝑜𝑙𝑖𝑑𝑠
= 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 ∗ (𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙
𝐷𝑆𝑓𝑖𝑛𝑎𝑙) − 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 ∗ (𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙)
Thus,
𝑚𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟 = 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 ∗ [𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙
𝐷𝑆𝑓𝑖𝑛𝑎𝑙− 𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙]
The mass of water removed by EDW can be expressed as follows:
𝑚𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝐸𝐷𝑊 = 𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑎𝑡𝑒𝑟𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒
− 𝑚𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔𝑤𝑎𝑡𝑒𝑟
77
𝑚𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝐸𝐷𝑊
= 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒
∗ (1 − 𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙) − 𝑚 𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒
∗ [𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙
𝐷𝑆𝑓𝑖𝑛𝑎𝑙− 𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙]
Therefore,
𝑚𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝐸𝐷𝑊
= 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒
∗ (1 −𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙
𝐷𝑆𝑓𝑖𝑛𝑎𝑙)
If the amount of filtrate water was known, the mass of evaporated water was calculated.
𝑚𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝐸𝐷𝑊 = 𝑚𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 𝑤𝑎𝑡𝑒𝑟
+ 𝑚𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑒𝑑 𝑤𝑎𝑡𝑒𝑟
𝑚𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑒𝑑 𝑤𝑎𝑡𝑒𝑟
= 𝑚𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝐸𝐷𝑊
− 𝑚𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 𝑤𝑎𝑡𝑒𝑟
Finally,
𝑚𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑒𝑑 𝑤𝑎𝑡𝑒𝑟
= 𝑚𝑠𝑙𝑢𝑑𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒
∗ (1 −𝐷𝑆𝑖𝑛𝑖𝑡𝑖𝑎𝑙
𝐷𝑆𝑓𝑖𝑛𝑎𝑙) − 𝑚𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒
𝑤𝑎𝑡𝑒𝑟