Drying Characteristics of Biosludge from Pulp and Paper Mills

Geanna Roswitha Hovey

A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science

Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Geanna Roswitha Hovey 2016

ii

The Drying Characteristics of Biosludge from Pulp and Paper

Mills

Geanna Roswitha Hovey

Masters of Applied Science

Chemical Engineering and Applied Chemistry

University of Toronto

2016

Abstract

Biosludge disposal has been a problem for pulp and paper mills due to its high moisture content

and poor dewatering/drying characteristics. Most biosludge is landfilled, although some has been

used as soil amendment. Some mills mix biosludge with primary sludge, dewater the mixture to

15-30% solids and burn it with hog fuel in biomass boilers. For biosludge to burn effectively, its

solids content must be increased to at least 30-35%. However, during drying biosludge becomes

sticky, agglomerating and adhering to the dryer wall, decreasing dryer efficiency. This study

examines the drying characteristics and sticky behaviour of pulp and paper mill biosludge. The

cohesive strength of biosludge was found to be stronger than the adhesive strength, reaching a

maximum at 20% solids and 13% solids respectively. The moisture and organic appear to influence

sticky behaviour. The addition of wood fines and fly ash did not affect the drying rate, but reduced

stickiness.

Acknowledgments

“Home is behind, the world ahead, and there are many paths to tread, through shadows to the edge of night, until the stars are all alight.”

J.R.R. Tolkien

This has been an interesting and exhausting journey. The accomplishments of which I am proud

of and would not have been possible without the expertise and guidance of my supervisor and

mentor, Professor Honghi Tran. His contagious enthusiasm and meticulous nature were excellent

motivators. The skills I have developed during my time here, are ones I will continue to build on

over my life time.

I would like to thank Sue Mao for her insight into my work and consistent willingness to assist

me. I am grateful to have had the opportunity to collaborate and exchange ideas with Professor

Grant Allen, Professor Markus Bussmann, and all of the graduate students in our research group.

Thank you for your expertise and suggestions towards my research problems. I would also like to

thank our industrial partners, their feedback was always encouraging. I would especially like to

thank Tembec for permitting myself and others the opportunity to the visits to the mill site. It

was an amazing and eye opening experience to see all of the working components of a pulp and

paper mill first hand.

However, this journey could never have begun if it were not first and foremost for my loving

parents, Rita and Gary Hovey. Who encouraged and inspired me to achieve, to grow, to learn,

and to never give up. Who gave me ground to walk on when things were tough, and who helped

to shine light into the dark places. Finally, I would like to thank my dearest Craig, to whom I

give my most sincere thanks for his endless patience, unwavering support and love.

Table of Contents

Acknowledgments.......................................................................................................................... iii

Table of Contents ........................................................................................................................... iv

List of Tables ................................................................................................................................ vii

List of Figures .............................................................................................................................. viii

Introduction .................................................................................................................................1

1.1 Objectives ............................................................................................................................3

Literature Review ........................................................................................................................4

2.1 Biosludge .............................................................................................................................4

2.1.1 Sludge Structure .......................................................................................................4

2.1.2 Dewatering and Disposal Process ............................................................................5

2.2 Motivation for Drying ..........................................................................................................6

2.3 The Drying Process ..............................................................................................................7

2.3.1 Drying Phases ..........................................................................................................7

2.3.2 Heat and Mass Transfer ...........................................................................................9

2.4 Challenges with Drying Biosludge ....................................................................................10

2.4.1 Behaviour of Sludge during Drying .......................................................................10

2.4.2 Sticky Behaviour of Sludge ...................................................................................11

2.4.3 Other Challenges ....................................................................................................14

2.5 Conditioning ......................................................................................................................15

2.5.1 Polymer ..................................................................................................................15

2.5.2 Solid Additives.......................................................................................................15

2.6 Current Drying Technologies ............................................................................................16

Materials and Methods ..............................................................................................................17

3.1 Materials ............................................................................................................................17

3.1.1 Biosludge ...............................................................................................................17

3.1.2 Wood fines .............................................................................................................17

3.1.3 Biomass Boiler Fly ash ..........................................................................................18

3.1.4 Polymer ..................................................................................................................18

3.2 Test Methods ......................................................................................................................19

3.2.1 Dewatering Protocol ..............................................................................................19

3.2.2 Drying Test Protocol ..............................................................................................20

3.2.3 Irreversible Drying Test Protocol ..........................................................................21

3.2.4 Stickiness Test Apparatus ......................................................................................21

3.2.5 Addition of Solids Test Protocol............................................................................22

Results and Discussions ............................................................................................................23

4.1 Composition .......................................................................................................................23

4.2 TGA and DSC Analysis .....................................................................................................24

4.3 Fourier Transform Infrared (FTIR) Spectroscopy .............................................................26

4.4 Environmental Scanning Electron Microscope .................................................................28

4.5 Drying Kinetics ..................................................................................................................32

4.5.1 Drying Curves ........................................................................................................32

4.5.2 Model .....................................................................................................................38

4.5.3 Internal Temperature ..............................................................................................40

4.5.4 Effect of Polymer ...................................................................................................43

4.5.5 Effect of Additives on Drying................................................................................44

4.6 Sludge Behaviour ...............................................................................................................48

4.6.1 Effect of Fly ash and Wood fines on Shrinkage and Cracking ..............................50

4.6.2 Water absorption properties of dried sludge ..........................................................52

4.6.3 Adhesive and Cohesive Forces ..............................................................................53

Conclusions ...............................................................................................................................62

Recommendations .....................................................................................................................63

References .................................................................................................................................64

List of Tables

Table 1: Composition of biosludge from a sulphite mill and a kraft mill..................................... 23

Table 2: Data for TGA and furnace comparison test .................................................................... 25

Table 3: Observations of sulphite mill biosludge FTIR spectra ................................................... 27

Table 4: Arrhenius parameters for the drying of sulphite and kraft mill biosludge from 110-220°

C .................................................................................................................................................... 35

Table 5: Critical moisture contents of the tested biosludge. Obtained from Figure 21. ............... 37

Table 6: Common drying models proposed by various authors ................................................... 38

Table 7: Model constants and results of the goodness of fit statistical analysis ........................... 39

Table 8: Summary of maximum adhesive and cohesive stickiness for all tested samples ........... 61

List of Figures

Figure 1: Water distribution within sludge particle [18] ................................................................ 5

Figure 2: Typical wastewater sludge drying curve (adapted from [34]) ........................................ 7

Figure 3: Sample of assorted wood fines ...................................................................................... 17

Figure 4: Sample of biomass boiler fly ash .................................................................................. 18

Figure 5: Bench Scale Laboratory Crown Press [70] ................................................................... 19

Figure 6: Laboratory scale thermogravimetric furnace ................................................................ 20

Figure 7: Stickiness test apparatus adapted from [43]; (a) Adhesion Test, (b) Cohesion Test ..... 21

Figure 8: Kraft and Sulphite mill biosludge TGA and DSC curves ............................................. 24

Figure 9: Comparison of Drying Curves: Furnace and TGA with different sample sizes ........... 25

Figure 10: FTIR scans for sulphite mill biosludge dried at different temperatures ...................... 26

Figure 11: ESEM images of bone-dry biosludge .......................................................................... 28

Figure 12: ESEM images of biosludge at 75% dry solids ............................................................ 29

Figure 13: ESEM image of biosludge at 75% dry solids and then dried in the ESEM ................ 29

Figure 14: ESEM image of raw sulphite mill biosludge: (a) wet, (b) dried ................................. 30

Figure 15: ESEM image of dewater biosludge, approximately 13% dry solids ........................... 31

Figure 16: Drying curve for sulphite and kraft mill, and municipal biosludge, T = 220° C ........ 32

Figure 17: Effect of air flow rate on sulphite mill biosludge drying curve, T = 220° C ............. 33

Figure 18: Effect of temperature on sulphite mill biosludge drying curve ................................... 34

Figure 19: Arrhenius plot of sulphite and kraft mill biosludge .................................................... 35

Figure 20: General drying rate curve of sulphite sludge, T = 220° C ........................................... 36

Figure 21: Krischer curves for sulphite mill, kraft mill, and municipal biosludge, T = 220°C .... 37

Figure 22: Relationship between moisture content and internal temperature of biosludge .......... 40

Figure 23: Location of thermocouple in cylindrical sample ......................................................... 41

Figure 24: Sample temperature during drying a different locations ............................................. 41

Figure 25: Internal temperature when dried at 220° C and 160° C .............................................. 42

Figure 26: Effect of polymer on biosludge drying ........................................................................ 43

Figure 27: Effect of biomass boiler fly ash on biosludge drying at 140° C.................................. 44

Figure 28: Effect of wood fines on biosludge drying at 140° C ................................................... 45

Figure 29: Effect of 20%wt fly ash in biosludge drying at different temperatures ...................... 46

Figure 30: Smoke emitted from sample of sulphite biosludge mixed with 20% fly ash, dried at T

= 220° C ........................................................................................................................................ 47

Figure 31: Crack development and shrinkage during first 40 min of drying at 220° C ............... 48

Figure 32: Effect of adding fly ash and wood fines on shrinkage during drying, T = 220° C ..... 50

Figure 33: Comparison of crack formation between pure biosludge and biosludge mixed with fly

ash and wood fines ........................................................................................................................ 51

Figure 34: Comparing Initial %MC vs Final %MC after soaking ................................................ 52

Figure 35: Adhesive and cohesive strength of sulphite mill biosludge ........................................ 53

Figure 36: A comparison of adhesive stickiness between sulphite and kraft mill biosludge ....... 55

Figure 37: A comparison of cohesive stickiness between sulphite and kraft mill biosludge ....... 55

Figure 38: The effect of temperature on the adhesion of sulphite mill biosludge ........................ 56

Figure 39: The effect of temperature on the cohesion of sulphite mill biosludge ........................ 57

Figure 40: The effect of fly ash on the adhesion of sulphite mill biosludge ................................ 58

Figure 41: The effect of fly ash on the cohesion of sulphite mill biosludge ................................ 58

Figure 42: The effect of wood fines on the adhesion of sulphite mill biosludge .......................... 59

Figure 43: The effect of wood fines on the cohesion of sulphite mill biosludge .......................... 59

Figure 44: A comparison of adhesive stickiness between pure biosludge and mixed sludge ...... 60

Figure 45: A comparison of cohesive stickiness between pure biosludge and mixed sludge ...... 60

1

Introduction

The pulp and paper industry is essential to the Canadian and global economy, contributing 20

billion dollars to the domestic GDP [1] and 500 billion dollars globally [2]. With the global

annual growth forecasted at 2.5%, there will be an increase in the global sludge production, from

3 million tons/yr to between 4 and 5 million tons/yr [2]. Currently, Canada produces

approximately 1.5 million tons of sludge/yr [3], with 350,000 ton/yr from Ontario pulp and paper

mills [4].

The sludge originates from primary and secondary wastewater treatment processes. There are

two types: primary sludge and secondary sludge (biosludge). Traditionally, the activated sludge

process is used at pulp and paper mills for wastewater treatment. Since this method uses

microorganisms, the microbial population and activity must be maintained. To do this some of

the biosludge is recycled back into the process and the remainder must be disposed of. This is

called waste activated sludge.

Handling and disposal of biosludge has been a persistent problem for many pulp and paper mills,

due primarily to its high moisture content, 98% water [5, 6] and poor dewatering/drying

characteristics. Some mills mix biosludge with primary sludge, which is more fibrous, to

improve the dewatering properties of biosludge. The sludge mixture is then dewatered to 15-30%

solids and then disposed of, either in a landfill (41%), incinerated (54%), or land applied (5%)

[3]. However, disposal is becoming more difficult because of rapidly diminishing landfill space,

rising environmental concerns, and poor economics associated with reducing the water content to

be suitable for landfilling or combustion in a biomass boiler. The costs of sludge disposal have

been reported to be as high as 60% of the total wastewater treatment plant operating costs [7].

Furthermore, in order to comply with increasingly stringent effluent regulations, the wastewater

treatment facilities must become more efficient, increasing the amount of sludge generated and

worsening the current problem. These regulations are forcing the industry to move away from

conventional disposal methods and towards more sustainable strategies.

2

The cost effective and sustainable disposal of sludge is one of the major challenges facing many

pulp and paper mills [8]. As a result, there is an effort from pulp and paper companies in

exploring strategies for improving water removal from biosludge in order to, reduce costs

associated with transportation, maximize existing landfill space, and improve the combustion

properties of biosludge. For example, in North America there has been a gradual shift from

landfilling sludge to burning a portion in existing biomass boilers [9]. However, in order for

biosludge to burn effectively, it must be dewatered or dried to at least 30-35% solids. Typically

the sludge is dewatered mechanically. Unfortunately, due to physical limitations, mechanical

dewatering can only increase the solids content to about 13%. Thermal drying can remove the

residual water to achieve an acceptable dry solids content to facilitate sludge combustion [10].

Mill experience shows that at some intermediate moisture content during drying, biosludge

becomes sticky, either agglomerating into a large lump or adhering to heat transfer surfaces. This

reduces drying efficiency and may lead to corrosion or equipment failure. Although drying is

practiced in the industry, the fundamentals behind pulp and paper mill biosludge drying are not

well understood. Most existing literature pertains exclusively to municipal wastewater sludge,

with tests limited to primary or mixed sludge, and discusses only dewatering and drying

technologies rather than the changes in the physical properties of the sludge. Sludge stickiness

has only been discussed briefly with little insight [11, 12]. It is not known if the stickiness is due

to a thermal effect that causes the organics in the sludge to polymerize, or simply a concentration

effect due to the reduction of water causing greater particle-particle interaction.

Research on the drying properties and the sticky behaviour will provide essential information for

improving current disposal methods, which is critical for successful sludge drying. The results

from this study will provide the industry with a reference point for their biosludge, and may help

to develop strategies for controlling and mitigating the sticky behaviour.

3

1.1 Objectives

The objectives of this thesis are to:

1. Characterize the drying behaviour of pulp and paper mill biosludge from sulphite and

kraft mills

2. Observe the sticky phenomenon and quantify the stickiness at different moisture contents

3. Determine if the sticky behaviour is caused by a thermal effect or a concentration effect

4. Evaluate the effects of fly ash or wood fine addition on drying and stickiness

The following sections present an overview of relevant literature on the generation and physical

characteristics of biosludge and the drying theory related to sludge drying. Note that the majority

of related literature pertains to municipal sludge. The experimental methods used are then

presented and followed by the experimental results and discussions.

4

Literature Review

This section provides background on what biosludge is, where it is produced, its structure, and

the current dewatering and disposal methods. Note that the majority of information from this

section pertains to municipal sludge and has been used to discuss pulp and paper mill biosludge.

2.1 Biosludge

The wastewater purification process at pulp and paper mills generates two type of sludge;

primary sludge and secondary sludge. Primary sludge is produced in the primary clarifier and is

mostly composed of cellulose and lignin fibres. Secondary sludge or biosludge, is a colloidal

suspension of small solid particles in water [6] produced in the secondary clarifier. It is

composed of micro-organisms, unsettled fibres, and undigested organics. A portion of biosludge

is recycled back into the treatment process to maintain microbial concentrations; the remainder is

disposed of, and is called waste activated sludge. Many of the challenges regarding sludge

disposal result from the poor dewatering characteristics of biosludge. Biosludge has a low solids

content, about 2 wt% [6]. To improve dewatering, primary sludge is often is mixed with

biosludge to increase the solids content in a 3:1 ratio [13, 14]. However, many mills would rather

retain the primary sludge fibres for revenue generating products. To address this challenge the

use of thermal drying to facilitate biosludge disposal is the focus of this thesis.

2.1.1 Sludge Structure

Colloidal materials, small particles in suspension, and extracellular polymeric substances (EPS)

tightly bind water molecules within a bio-polymeric network and also to the particle surface,

contributing to the difficulty of dewatering [6, 15, 16]. The way water is bound in colloidal

materials differs from what occurs in conventional particulate suspensions [15]. It is unknown

which properties cause stronger bonding to water molecules, but it is expected that particle size

and distribution, chemical composition, compressibility, and fibre length play a role. Research on

municipal biosludge found the particles to be uniformly distributed within the polysaccharide

hydrogel, ranging from 1.2-600 µm [16]. However, there is a lack of detailed knowledge of the

sludge floc structure, and a uniform distribution of microorganisms is often assumed [15].

5

The polysaccharide hydrogel lowers dewatering efficiency [17] and consists of bound and

unbound water [6, 18, 19, 20]. The water distribution is further characterized in Figure 1 as; free,

interstitial, surface, and bound water. Free water is not contained within the particles and can be

separated by gravity filtration. Interstitial moisture is retained by adhesive and cohesive forces

within the sludge flocs and the capillaries of the sludge cake, and can only be removed by

breaking the cell wall. Surface water or vicinal water [6], is physically bound on the particle

surface by adsorption and adhesive forces and cannot be removed by mechanical dewatering

methods. Bound water, or hydration water [6], is chemically bound to the particles and can only

be removed thermally. In general the unbound water consists of the free, interstitial, and surface

water, whereas the bound water includes only the chemically bound water [18].

Figure 1: Water distribution within sludge particle [18]

2.1.2 Dewatering and Disposal Process

The first step in dewatering is to thicken the sludge by adding a cationic polymer to flocculate

the suspended particles. Then gravity filtration removes most of the free water and increases the

solids content to approximately 8-10%. The remaining water is contained within a hydrogel,

where most is trapped within the solids or bound to their surface. The sludge is then compressed

using a screw press, belt filter press, or centrifuge to increase the solids content to 25-45% for

mixed sludges [18, 21, 22]. Pure biosludge cannot be mechanically pressed. The dewatering and

disposal costs vary per mill, and account for approximately 40-45% of the total wastewater

treatment costs [23]. The dewatered sludge is then disposed of in a landfill or an incinerator.

6

2.2 Motivation for Drying

Research on municipal sludge found the solids to be hydrophilic, which contributes to the

retention of water [24]. This property places a practical physical limit on conventional

mechanical dewatering methods, as they cannot reduce the moisture content sufficiently to

sustain combustion (approximately 30-35 wt% solids [13]). As a result, blending with wood

waste is often required. Research on municipal sludge [17, 25, 26] found that the binding

strength between water and the sludge solids is weaker at higher water contents, where

approximately 20% of the water can be readily removed. After this point the binding strength

increases, increasing the energy required to dewater. Studies by Chu et al. [27] on dewatering

activated municipal sludge reported that the energy required to remove the residual water was

appromximately1kJ/kg ds for water contents greater than 97%, but the energy exceeded 1 MJ/kg

at water contents below 33%. This implies that sludge with lower residual water cannot be

dewatered because conventional dewatering techniques are unable to provide sufficient force.

Furthermore, the microorganisms within the biosludge contain water, which can only be

removed via cell destruction. Thermal drying is a potential solution, for it can remove the

residual water by breaking the gel-like matrix, decrease the water affinity of the sludge solids,

and degrade the cell walls [24, 28].

7

2.3 The Drying Process

Drying of a material is a process in which the water in the material is thermally removed. The

driving force behind the drying process is due to a concentration gradient between the wet

sample and the surrounding dry air. During drying, heat and mass transfer processes occur

simultaneously: the evaporation of the surface water and the movement of the internal water to

the surface of the solids [29, 30]. The diffusivity of water is a function of temperature and the

water content within a sample. External conditions such as temperature, air humidity and surface

area govern surface evaporation, whereas the internal moisture transfer is governed by the

physical structure of the material, the moisture content, and the temperature [19, 31].

2.3.1 Drying Phases

Previous work found that municipal sludge has a constant drying rate period and two falling rate

periods, where the interstitial water was removed during the first falling rate and the surface

water was removed during the second [19, 24, 32, 33]. Figure 2 shows a typical sludge drying

curve where a linear decrease in moisture content is followed by a non-linear decrease until

reaching the equilibrium moisture content. The moisture distribution was proposed from work by

Vesilind et al. [24] and is often used a reference throughout this field of study. Experimental

work by Bennamoun et al. found that that drying kinetics are influenced by the origin of the

sludge and operating conditions [32].

Figure 2: Typical wastewater sludge drying curve (adapted from [34])

8

In the constant drying rate period the sludge is completely saturated. If the sludge is

mechanically dewatered prior to drying, the constant rate period is unobserved or observed

briefly [28]. The constant drying rate is shown by line (AB) in Figure 2. The material surface

remains wet until the critical moisture content is reached. This indicates the start of the first

falling rate; line (BC) Figure 2. It is characterized by unsaturated surface drying, a decrease in

drying rate, and dry spots occurring on the sample surface [18, 30, 31, 33]. This phase continues

until the entire surface film is evaporated.

After further drying, a second critical moisture content is reached, indicating the start of the

second falling rate; line (CD) Figure 2. The residual surface water and the chemically bound

water are removed [18, 32]. The surface is completely dry, thus the evaporation interface has

moved into the solid [31]. This period is characterized by an increase in the material temperature;

from the wet bulb temperature to the temperature drying of the drying medium [29]. The critical

moisture content for sludge is typically between 0.40-0.80 kg water/ kg dry solid [30], and equals

the bound water content [18, 24, 34].

9

2.3.2 Heat and Mass Transfer

Heat and mass transfer are essential for drying. Heat transfer occurs through radiation,

convection, and conduction to increase the temperature of the wet solids and to evaporate the

water. Mass transfer occurs when water migrates from the interior, where it is wet, to the surface,

where it is drier, via diffusion and subsequently evaporation. These two processes occur

simultaneously, and the drying rate is governed by the rate of these two processes. The heat and

mass transfer in porous media is a complex phenomenon, and the fundamental transfer methods

such as capillary action, absorption, desorption, and shrinkage are still not well understood [35].

The drying rate is dependent on the contact area between the drying medium and the material;

the temperature and the humidity of the drying air; the speed and direction of the drying air; the

mixing of sludge; retention time, and the method of contacting the sludge with heating factor

[36]. Typically, there are three heat transfer resistances: the contact resistance at the hot surface,

the penetration resistance of the particle, and the penetration resistance of the bulk. Whereas, the

moisture mass transfer must overcome two resistances: the movement of the internal moisture,

which is a function of the moisture content and the internal structure of the solid; and the

movement of water vapour from the material surface, which is dependent on external conditions

[35, 37]. The moisture can be transported within a material by several mechanisms; capillary

forces, diffusion, vaporization-condensation, and mass-transfer potential [29]. The agglomeration

of the sludge solids, as a result of the sticky phase, significantly reduces the heat and mass

transfer potential due to the decrease in contact area between the wet sludge and the drying

surface. It was found that about 2595 kJ (0.72 kW), of energy is required to evaporate 1 kg water

[33], and that the energy required depends on the drying stage.

10

2.4 Challenges with Drying Biosludge

Pulp and paper mills produce varying amount of sludge depending on the raw materials, process,

and final products. As a result the composition of sludge varies between mills and even between

months or days at the same mill [13].

2.4.1 Behaviour of Sludge during Drying

Sludge drying is complex and involves extensive shrinkage, cracking, and crust formation; which

can alter the heat and mass transfer mechanisms [32, 38, 39]. Initially, the sludge is in a wet

phase, behaving as a viscoelastic solid with free-flowing behaviour [40]. The sludge then passes

through a sticky phase (solids content between 55% and 70%) [18], developing a skin layer on

the surface and exhibits shear thinning and non-Newtonian behaviour. This causes paste-like

behaviour and clumping inside the drier, requiring strong shearing for proper mixing. To avoid

the sticky phase it is common practice in indirect drying systems to mix dried sludge with raw

sludge to approximately 70% solids [12]. With further drying, the sludge becomes granular [18,

41], crumbling easily and mixing more freely [33, 36]. These phases are often detected by

measuring torque, where the sticky phase measures the highest values [36]. The sticky phase has

been observed by the pulp and paper industry, but no attempts to research or understand the

sludge behaviour have been pursued.

11

2.4.2 Sticky Behaviour of Sludge

Stickiness is a colloquial term referring to both particle-wall interactions (adhesion) and particle-

particle interactions (cohesion) [28, 42]. Stickiness causes agglomeration inside the dryers,

which decreases drying efficiency, causing severe issues in dryers [11, 43, 44, 45]. The majority

of research on stickiness pertains to foods and powders.

When the sludge is sticky it adheres to the dryer walls and agglomerates into large clumps,

fouling heat transfer surfaces and preventing thorough mixing. These clumps are difficult to

break apart and dry, causing a decrease in drying efficiency, which may lead to equipment

failure by altering the hydrodynamics of the dryer [28, 32, 42]. The significance of this

behaviour was discussed by Hirosue and Shinohara, who found that the heat transfer coefficients

decreased by 60% for sticky materials versus non-sticky materials [46].

From research on the drying of municipal sewage sludge, the sticky region has been defined: as

the moisture content range where a material exhibits strong sticky behaviour; the range is

dependent on the nature and source of the sludge. Several ranges are provided from different

studies: 30-45% [18], 45-65% [12], 50-62% [42], and 35-60% [47]. Li et al. [28] tested the

adhesion and cohesion characteristics of centrifugally dewatered mixture of primary and

secondary settled municipal sludges, which were conditioned with polyaluminum chloride

(PACl), during drying using an experimental apparatus adapted from the Jenike shear test. They

found that the sludge has maximum adhesive and cohesive strength at moisture contents of 45%-

70% and 30-60%, respectively, and that when the adhesive shearing stress is at a maximum, the

cohesive stress continues to rise. This suggests that the resistance between the sludge and the

contact surface is less than the internal resistance of the sludge particles. These tests were also

performed at temperatures of 120° C and 200° C and found that temperature does not play a

significant role in sewage sludge stickiness. The organic matter, such as the proteins and

polysaccharides, and their interactions with moisture are believed to be the key factors

contributing to stickiness in sludges [43, 48, 49]. It is suspected that organic matter will also

have a similar effect on pulp and paper mill biosludge stickiness.

12

2.4.2.1 Stickiness

Stickiness is a common problem affecting many industries. Much of the research on stickiness

pertains to foods and powders. In the food industry for example, most of the literature on the

cause of stickiness, related mechanisms, and testing methods are product specific, and no single

mechanism or test method is successful enough to generalize and characterize food stickiness.

This may also be the case for sludge stickiness.

Many material properties can cause stickiness such as: water, temperature, viscosity, and surface

tension. Water, in particular, is a ubiquitous plasticizer and can exist on the particle surface as an

adsorbed mono or multilayer or as capillary condensation. This reduces surface roughness,

allowing for closer particle-particle interactions and increasing attractive forces [50]. Particle

size can also influence the cohesion and adhesion of a material. For example micro or nano-sized

particles are greatly influenced by electrostatic molecular forces of attraction as a result of their

high surface area to volume ratio, where Van der Waals forces become significant for particles

below 10 microns and the inter-particle distance becomes sufficiently small [51].

2.4.2.1.1 Stickiness Mechanisms

Several mechanisms contribute to stickiness: inter-particle attraction, such as intermolecular and

electrostatic forces, liquid bridges, wetting, thermodynamic adsorption, tack, and rheology. Inter-

particle mechanisms are organized into four groups: intermolecular and electrostatic forces,

liquid bridges, solid bridges, and mechanical interlocking [51]. Van der Waals forces are the

primary intermolecular mechanism acting on particles. Typically, liquid bridges influence

particle behaviour in powdery materials, and most likely do not play a large part in sludge

stickiness. Whereas, solid bridges form due to sintering, melting, crystallization, dissolution, and

drying, and are held together by mechanical interlocking and primary chemical bonds [11, 50].

Wettability refers to whether a liquid will spread onto a surface as a continuous film or retract

into droplets [50]. The theory behind the thermodynamic adsorption mechanism was developed

by several researchers [52, 53, 54] who found that it is caused by electrodynamic intermolecular

forces acting at the liquid-liquid, liquid-solid, and solid-solid interfaces. Tack refers to the energy

required to separate two objects which are not permanently bound [55], and is often associated

13

with surface adhesion of sticky material [50]. Rheology relates to the deformation and flow of

material, especially non-Newtonian liquid and the plastic flow of solids.

The factors that influence adhesion and cohesion may be similar, but high adhesion does not

necessarily contribute to high cohesion, and vice versa. The strength and behaviour of cohesion

and adhesion depend on the nature of the material, individual particle characteristics, and

environmental factors, such as moisture and heat.

2.4.2.1.2 Theories on Adhesion and Cohesion

Adhesion is an interfacial property and a measure of the attractive forces between dissimilar

materials. Several theories [11, 56, 57] have been proposed to describe adhesion. Unfortunately,

there is little agreement on which theory is most relevant for describing the bonding between a

material and a surface. These theories include: the mechanical interlock theory, the adsorption

theory, the chemisorption theory, the electrostatic theory, the diffusion theory, and the weak

boundary layer theory. The most applicable theory describing sludge behaviour in a contact dryer

would be the adsorption theory and the mechanical interlock theory [28]. Adsorption theory

states that adhesion is a result of molecular contact between two materials, causing the

development of surface forces such as Van der Waals forces. The mechanical interlock theory

describes the penetration of sludge into the microscopic crevices on the dryer surface. This

causes mechanical adhesion to the surface and forms strong surface bonds.

Cohesion is more complex and difficult to describe as a result of the inconsistent and variable

composition of the sludge. It is a measure of the attractive force between particles, resulting in

agglomeration. Cohesive effects are a result of the interactions between adhesive polymers,

mineral materials, and crystal matrices [28]. Furthermore, chemical bonds, crosslinking,

intermolecular interactions, and mechanical forces cause organic materials to stick together [58].

Additionally, small mineral particles in the presence of moisture contribute to the plasticity of

the sludge and promote water affinity of the sludge particles; similar results were found in

research on the adhesion and cohesion of soils [59, 60, 61]. These attractive forces contribute to

the stiffness and rigidity of dried soils, which is also seen in sludge. Li et al suggest that the

metal salts contained in the sludge, crystalize as the moisture content decreases, contributing to

the firmness and hardness of dried sludge [28].

14

2.4.2.2 Reducing Stickiness in Industry

Additives may reduce or avoid stickiness. For example, in the spray drying of pineapple, orange,

and tomato juices large amounts of additives to improve drying such as maltodextrin or sodium

chloride are used. For orange juice, it has been suggested that the dryer walls be cooled to below

the sticky point temperature of the dried product [42]. Other methods to overcome stickiness in

spray drying in the food industry are to introduce a small amount cold air from the bottom of the

chamber. Additionally, controlling the dryer wall temperature, scraping the drying surfaces [11,

50], and breaking up agglomerates may help improve drying capacity. Conventionally, the

method of avoiding sticky behaviour is trial and error experimentation to determine the drying

conditions which bypass or limit the sticky characteristics of the material [11].

2.4.3 Other Challenges

Aside from stickiness, additional issues exist with thermal treatment. During drying the sludge

emits an odour, which can be a nuisance for nearby residences. Additionally, corrosion can occur

on drying equipment as a result of corrosive compounds becoming more concentrated. However,

stickiness is the most significant challenge, the difficulty in solid-liquid separation arises because

conventional designs optimize only the unit operations and do not consider the nature of the

thickened product and its handling issues [62].

15

2.5 Conditioning

2.5.1 Polymer

Sludges are often conditioned with long chain polymers to flocculate the particles. This is

generally not required for pure primary sludge, but needed for pure biosludge or for a mixture of

primary and secondary sludge. There are several types of flocculating polymers available on the

market, the type depends on the sludge, the ratio between primary and secondary, and the

dewatering techniques used. This is often determined by mill personnel in conjunction with the

polymer supplier performing trial and error testing.

The polymers act on particle charges to form larger aggregates, increasing the amount of water

which can be removed by gravity filtration. The amount of polymer required varies between 2-15

kg polymer/tonne dry solids, and is generally not the optimal amount due to the daily variability

in sludge composition, poor mixing, and lack of dosing control [23].

2.5.2 Solid Additives

Sand is mixed with samples of spent black liquor to facilitate drying for laboratory testing at

kraft pulp mills. Thus, it is suspected that additives may also accelerate biosludge drying. Fly ash

and wood fines, common solid waste materials at pulp and paper mills, were added to the

dewatered biosludge to determine if the drying rate can be increased. Fly ash has been shown to

be and effective filter aide for sludge dewatering as it improves drainage in the sludge cake and

increases the solids content [63, 64, 65]. Wood fines and bark have also been added to mixed

sludge (primary and biosludge) to increase the solids content and improve combustion in

biomass boilers [21, 66]. Presently no studies evaluate the impact of fly ash and wood fines on

the drying of pulp mill biosludge.

16

2.6 Current Drying Technologies

Several commercial dryers claim to be able to dry sludge. However, there is a lack of evidence

showing exactly how successful these systems are at mitigating or avoiding the sticky phase of

the sludge, indicating that this challenge has yet to be overcome by dryer manufacturers.

Nonetheless, dryers are used at some mills to increase the solids content as part of the disposal

process. The dryers are classified as direct dryers, which include rotary drum, flash, moving belt

or centri-dryers; and indirect dryers, which include, paddle, thin film, rotary disc, or rotary tray

dryers; or combined dryers such as a fluidized bed [18].

In indirect dryers, heat is transferred via conduction between the hot dryer surface and the

sludge. Typically, thermal oil or saturated steam is used as a heating fluid [32]. Mechanical

agitation is required to maintain a good contact between the heated dryer wall and the sludge. In

contrast, in direct dryers, the heat is transferred via convection, where hot drying medium,

typically air, dries the material by direct contact [32]. Direct dryers have a simpler design and

operate a much higher temperatures, allowing them to generate higher drying rates, and can dry

to 55% and nearly 95% solids [18]. However, the vapours generated during drying must be

separated from the drying medium. This is not the case for indirect dryers, making them easier to

manage, but also more complex [67].

The cost of sludge dewatering and drying is dependent on the process selected. Using waste heat

from sludge combustion or other waste heat sources can lower drying costs. The use of flue gas

to dry sludge prior to incineration can save up to 60% in cost compared with direct incineration

in large sludge facilities [68]. There is also potential for energy recovery biosludge combustion,

but this would only be feasible if the moisture content of the dewatered sludge can be reduced

efficiently. Unfortunately mechanical dewatering techniques, which are presently the most

energy efficient method to removing this moisture, cannot reduce the moisture content enough to

make biosludge combustion economically sustainable.

17

Materials and Methods

The initial step in this project was to first characterize the drying behaviour and properties of

pulp and paper mill biosludge. Since most of the existing research pertains exclusively to

municipal sludge it was used as a point of reference and for comparisons. The sticky phase was

studied and methods to improve biosludge drying and to mitigate sludge stickiness were

explored. One notable challenge with sludge is that there may be variations in composition

throughout the year, which may lead to variations between the samples.

3.1 Materials

3.1.1 Biosludge

The biosludge was collected and delivered to our laboratory from a sulphite pulp and paper mill

in sealed 20 L pails on a bi-weekly basis. The pails were stored at 4° C. The characteristics of the

biosludge may vary between pails, thus each test used sludge from a single pail when possible in

order to reduce variability. Kraft mill biosludge was shipped once in three sealed 20 L pails.

Municipal biosludge was collected once in order to compare with the pulp mill biosludge.

3.1.2 Wood fines

The wood fines used were assorted fines dried in an oven over night at 105° C, Figure 3. Prior to

oven drying the wood fines had a moisture content of approximately 8%.

Figure 3: Sample of assorted wood fines

1 cm

18

3.1.3 Biomass Boiler Fly ash

The biomass boiler fly ash used was dried in an oven over night at 105° C, Figure 4. Prior to

oven drying the fly ash had a moisture content of approximately 45%. The fly ash particle size

ranged from 150-600 µm and contained traces of unburned biomass.

Figure 4: Sample of biomass boiler fly ash

3.1.4 Polymer

Zetag 8165 (BASF), a high cationic polymer, was used in the dewatering stage to flocculate the

biosludge solids and facilitate gravity filtration. A 250 mL stock solution was prepared by

mixing the polymer crystals with distilled water to a concentration of 0.5 wt%. The stock

solution was prepared at least 24 hours prior to use. It had a shelf life of seven days. The amount

of polymer required for dewatering sludge was dependent on the total solids of the pail. To

determine the amount of polymer required a total solids test was performed, in duplicates, for

each pail according to Standard Methods 2540 G [69].

1 cm

19

3.2 Test Methods

3.2.1 Dewatering Protocol

A bench scale laboratory crown press (Phipps & Bird Inc.) was used to dewater the biosludge,

shown in Figure 5. The filter belt material is HF7-7040 white polyester with a 64x24 count in a

6x2 H’bone weave pattern (Clear Edge Filtration); this material was also used in the gravity

filtration step. After each pressing of the sludge the filter was rinsed to remove any residual

solids and dried.

The biosludge and polymer slurry was poured onto the gravity filter and filtered for 2-3 min; if

no polymer was added to flocculate the sludge, the filtering time necessary to remove enough of

the filtrated would be 10 min. The sample was then removed from the gravity filter and situated

between the two filter belts at the top of the crown. The sludge was then pressed between the two

filter belts at 9 kPa for 30 s followed by a quick release, then pressed at 87 kPa for 30 s with

another quick release, then pressed again at 112 kPa for another 30 s. The sludge cake has a

solids content of about 13% (wet basis).

Figure 5: Bench Scale Laboratory Crown Press [70]

20

3.2.2 Drying Test Protocol

A thermogravimetric furnace, Figure 6, was used to dry the samples. This apparatus can

simultaneously measure and record the sample weight during drying; allowing for accurate

determination of the moisture content and drying curves. The apparatus is comprised of a

balance mounted to the wall above the furnace. A weighing dish is hung from the balance

directly above the opening of the furnace. The sludge cake is place onto the weighing dish, then

the furnace is lifted upwards by stepping on the lever till it locks into place and the sample is

sitting in the centre of the furnace. A viewing window is located at the side of the furnace.

Various temperatures from 110-220° C were applied throughout this project. Air was supplied

from the bottom at a flow rate of 1 SLPM.

Figure 6: Laboratory scale thermogravimetric furnace

21

3.2.3 Irreversible Drying Test Protocol

The purpose of this test was to observe if the drying of biosludge is irreversible, and does not

revert back to its raw sludge state when submerged in water for 24 hr. This test will also

determine how much water is absorbed by the solids when they are re-hydrated. The results will

provide insight on the storage properties of dried and partially dried biosludge.

Sludge samples were dried at 220° C and 110° C to 75%, 50%, 25%, and 0% moisture. Triplicate

experiments were performed at each temperature and for each moisture content. The samples

were removed from the furnace once the target moisture content was reached and pulverized to >

6 µm to homogenize the moisture content. The solids were added to water to a concentration of 2

wt% solids, typical of raw biosludge. The mixture was agitated for 2 min and left to soak for 24

hr (+/- 3hr). Afterwards, the solids were separated from the water via vacuum filtration at 0.5 bar

and dried overnight in an oven at 103°C.

3.2.4 Stickiness Test Apparatus

A stickiness test apparatus was adapted from Peeters et al. [43] to measure the adhesive and

cohesive stickiness of the biosludge at different moisture contents by applying sufficient force to

cause slip, see Figure 7.

Figure 7: Stickiness test apparatus adapted from [43]; (a) Adhesion Test, (b) Cohesion Test

22

Dewatered sludge was placed into a hollow steel cylinder situated on a stainless steel surface

(adhesion) or a plate with a 5 cm hole (cohesion). The hollow cylinder had a mass of 214 g, an

internal diameter of 5 cm, and a height of 10 cm. A heavier solid cylinder with a mass of 1.71 kg

and an external diameter of 4.7 cm was placed on the sludge for 1 min. This was done to

consolidate the sludge. The solid cylinder was removed and a continuous stream of water is

added to the bucket, which is connected to the hollow cylinder by a steel cable. When the

cylinder and the sludge slip, the flow of water into the bucket is immediately stopped and the

bucket is weighed. To test the reproducibility of the apparatus the tests were repeated a minimum

of ten times. To do this the hollow cylinder and sludge are placed back to the starting position

and the sludge is broken up using a metal spatula prior to repositioning the solid cylinder. The

stress applied to cause slip (τ, Pa) was calculated:

� = � × �� Where:

M = the mass required for slipping (kg)

g = acceleration due to gravity (9.81 m/s2)

A = the contact area between the sludge and the steel surface (19.6 x 10-2 m2)

τ = the stress required for slipping (Pa)

3.2.5 Addition of Solids Test Protocol

Fly ash and wood fines in concentration of 10 wt%, 20 wt% and 30 wt% were added to 10 g of

dewatered biosludge to observe the effect of additives on drying behaviour (see 3.2.2) and

stickiness (see 3.2.4). The moisture content of the solids was adjusted by adding the required

mass of water to bone-dry solids, which were dried overnight in an oven at 103°C.

23

Results and Discussions

4.1 Composition

The sludge used was from the activated sludge system of the wastewater treatment system from a

sulphite mill and a kraft mill. It is composed of suspended solids in water. The ash content was

determined from the weight loss during ignition, where the sample weight after ignition at 800°

C represents the inorganic content. The higher the inorganic content, the less suitable for

incineration. The composition of sulphite mill and kraft mill biosludge are shown in Table 1. On

a dry basis, the sulphite mill biosludge had a higher volatile content, higher heating value, and

lower ash content than the kraft mill biosludge.

Table 1: Composition of biosludge from a sulphite mill and a kraft mill

Sulphite Mill Kraft Mill

Proximate (wt%), Dry Basis

Volatiles 71.8 56.7

Ash 14.0 25.0

HHV (Btu/lb) 8817 7540

Total Suspended Solids 1.7 3.3

Ultimate (wt%), Dry Basis

Carbon 44.7 39.8

Hydrogen 5.5 4.6

Nitrogen 6.0 3.2

Sulphur 2.1 2.4

Oxygen 31.3 27.0

24

4.2 TGA and DSC Analysis

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to

determine if thermal transitions occur within the sludge, such as melting or glass transition,

which may contribute to the sticky behaviour. The samples were heated in air at a rate of 20°

C/min from 10° C to 900° C.

The TGA and DSC curve for the kraft and sulphite mill biosludge are shown in Figure 8. There

is an initial warming for both samples as indicated by plateaus in the heat flow and weight %. At

250° C the heat flow rate and weight loss rate increase for both samples as volatiles are emitted

and burned. This is followed by continuing decrease in weight and a slight decrease and plateau

in heat flow, indicating that the rate of heat absorbed equals the rate of heat released. This is a

result of continued volatile burning occurring simultaneously with the heating of the solid

particles. At 460° C for sulphite mill biosludge sludge and 535° C for kraft mill biosludge, the

samples show a dramatic increase in heat flow and a sharp reduction in sample weight as the

solids burn (char burning). The heat flow during combustion is 1324 J/g for sulphite mill

biosludge and 1242 J/g for kraft mill biosludge. This difference is most likely a result of the

higher heating value of the sulphite mill biosludge, which contains higher organic matter and

lower ash contents, shown in Table 1. After combustion, the heat flow decreases for both

samples and no change in sample weight is observed. No phase transitions were observed.

Figure 8: Kraft and Sulphite mill biosludge TGA and DSC curves

25

Since the TGA is accurate and reliable it was used to verify the accuracy of the larger furnace

used for the drying experiments (3.2.2), where Sample 1 was dried in the larger furnace and

Sample 5 was dried in the TGA. The drying curves from the TGA and the furnace, set at 220° C,

follow the same trend and appear to be comparable (Figure 9). A shift along the x-axis was

noticed between Sample 1 and Sample 5, where Sample 5 shifted closer to the origin. It was

suspected that the shift was a result of the difference in sample size; approximately 13 g for the

furnace and 0.050 g for the TGA. Three additional sludge samples each with different sizes were

dried in the larger furnace (data in Table 2) and their drying curves were compared against the

initial drying curves from the TGA and the furnace, shown in Figure 9.

Table 2: Data for TGA and furnace comparison test

Sample Initial Mass (g) Surface Area

(cm2)

Surface Area/Mass

(cm2/g)

Drying Equipment

1 12.14 29.15 2.40 Furnace

2 3.78 13.19 3.49 Furnace

3 1.01 4.67 4.62 Furnace

4 0.42 2.97 7.07 Furnace

5 0.05 N/A N/A TGA

Figure 9: Comparison of Drying Curves: Furnace and TGA with different sample sizes

The noise in Sample 3 and Sample 4 can be attributed to the variability of the balance

measurement as a result of the small size of the sample. The data shows that as the size of the

sample decreases the drying curves also decrease, shifting towards the origin along the x-axis.

Thus the furnace can accurately measure drying curves because they are comparable to the TGA.

26

4.3 Fourier Transform Infrared (FTIR) Spectroscopy

An FTIR spectrometer was used to further examine the chemical composition and organic

functional groups of the sludge. Samples of sulphite mill biosludge were dried at 110° C, 170° C,

and 220° C and tested to observe any differences in compounds. The IR spectra were collected in

the range of 4000 – 650cm-1, the results are shown in Figure 10 and observations made in Table

3. Changes in peak height were observed between the samples suggesting that changes occurred

to the compounds within the sludge as the sludge is dried at higher temperatures. Decreases in

peak height indicate degradation of organic matter as the sludge is dried at higher temperatures.

Decreases in peak height indicate degradation of organic matter as the sludge is dried at higher

temperatures. This may provide insight into drying temperatures that can dry the sludge

efficiently, yet minimize calorific loss from the degradation of organics.

650

110° C

170° C

220° C

Figure 10: FTIR scans for sulphite mill biosludge dried at different temperatures

27

Table 3: Observations of sulphite mill biosludge FTIR spectra

Wavelength (cm-1) Functional Group Change in peak shape at

different temperature

3400-3200 - H-bonded OH groups of alcohols,

phenols, organic acids [71]

- H-bonded N-H groups [71]

- Broader at higher temperatures

2972

2980 - symmetrical and asymmetrical -C-H

vibrations- [71]

- Sharp peaks - Decrease at higher

temperatures

1734 - C=O stretching in ketones, carbonyl, and

ester groups [72]

- Only at 110° C and 170° C

1660-1630 - Carbonyl - Decrease at higher temperature

1515 - Possibly from lignin or lignocellulose

materials [72]

- Only at 220° C - May have been masked by

other bands at lower

temperature

1480-1380 - CH3 and the OH group in phenols [71, 72] - Broader as higher temperature

1251 - carboxylic acids and amide III - Only at 110° C and 170° C

1250-900 - C-O stretching of carbohydrates and

alcohol functional groups, indicative of

cellulose or polysaccharides [71, 73]

- Broader at higher temperatures

- Less peaks at higher temperatures

900-660 - Aromatic CH deformation - OH bending

- Broader at higher temperatures

These experiments provide qualitative insight into biosludge composition and are specific to this

particular sulphite mill biosludge. It should be noted that determining the chemical compounds

via FTIR spectra is challenging and not accurate due to the similarities in functional groups of

compounds of polysaccharides originating from microorganisms and wood fibres [74]. However,

it is likely that pulp and paper mill biosludge would contain amounts of lignin, cellulose,

hemicellulose, mineral ash, lipids, and proteins.

28

4.4 Environmental Scanning Electron Microscope

One of the challenges with testing biosludge was that the majority of equipment typically used to

characterize materials could only operate with completely dried samples. The biosludge could

not be tested at various moisture contents to observe the evolution of the sticky phase. However,

the environmental scanning electron microscope (ESEM) is a type of scanning electron

microscope (SEM) which can be used to image wet specimens. This technique allowed for

images to be taken of the biosludge at different moisture contents.

Figure 11 shows two ESEM images of a bone dry sample of biosludge. By visual inspection the

particles are very rigid and angular. Small porous areas are also noticeable.

Figure 11: ESEM images of bone-dry biosludge

29

Figure 12 shows a biosludge sample which was partially dried to 75% dry solids. The biosludge

appears to be less rigid, and is clumped together more than the bone-dry sample.

Figure 12: ESEM images of biosludge at 75% dry solids

Figure 13 is an image of the same sample from Figure 12, but it was dried in the ESEM to

observe changes after drying. Other than appearing darker, there is no significant difference

between the two images. This suggests that significant physical changes do not occur when the

material is nearly dry.

Figure 13: ESEM image of biosludge at 75% dry solids and then dried in the ESEM

30

Figure 14 is an ESEM image of raw biosludge, where image (a) is of wet raw biosludge and

image (b) is of completely dried raw biosludge. The sludge was dried inside the ESEM to get a

clearer image since the ESEM cannot image through water. The exact moisture content of the

image (b) is not known. No significant differences were observed, but there are some cracks and

larger separations in the dried sample. In both images the sludge appears as one large

interconnected mass and there does not appear to be any individual particles.

Figure 14: ESEM image of raw sulphite mill biosludge: (a) wet, (b) dried

31

Figure 15 are images of dewatered biosludge, approximately 13% dry solids. The sludge has soft

edges and larger particles, appearing composed of smaller particles. This may be a result of the

polymer that was added during the dewatering process. Undigested wood fibres are also visible.

Figure 15: ESEM image of dewater biosludge, approximately 13% dry solids

32

4.5 Drying Kinetics

Experiments were conducted to examine the drying characteristics of biosludge from sulphite

and kraft pulp mills. Similar drying testes were also done on a sample of municipal biosludge

obtained from Toronto’s Ashbridges Bay municipal wastewater treatment plant to provide

insight into whether the existing research on municipal sludge drying can be used as a reference

for pulp and paper mill biosludge. Dewatered biosludge was used for each of the tests. The initial

solids content for the sulphite and kraft mill biosludge was 13%, and 10% for the municipal

biosludge. Unless specified the drying temperature was 220° C with an air flow rate of 1 SLPM.

4.5.1 Drying Curves

Figure 16 shows the drying curve for sulphite and kraft mill biosludge, and municipal biosludge

at 220° C. The trends are typical of a material during drying [30, 31, 75]. The graphs were

obtained from experimental results, and the moisture content was normalized as is represented by

a dimensionless moisture ratio, to account for variations in initial weight, as follows:

������������� = ��� ��������������������� � ������������ Where:

X(t) – Moisture content at time ‘t’ [g water/g dry solids]

Xinitial – Initial moisture content [g water/g dry solids]

Xequilibrium – Equilibrium (final) moisture content [g water/g dry solids]

Figure 16: Drying curve for sulphite and kraft mill, and municipal biosludge, T = 220° C

33

The results show that the dewatered sulphite and kraft mill biosludge are nearly identical, and

all sludge types follow a similar trend. Both pulp and paper mill biosludges had better

dewatering capabilities than the municipal biosludge. This may be due to the different origins

of the sludge, which would have an impact on composition. Additionally, the dewatered

municipal biosludge was less viscous and smeared more easily than the sulphite biosludge,

this may be related to particle roughness or composition.

Figure 17 shows the effect of air flow rate on the drying of sulphite mill biosludge at 220° C.

The biosludge drying rate increases as the air flow rate increases, as expected. This is

because the faster velocity is more effective at removing the vapourized water from the

sample, thus maintaining a concentration gradient. The difference between these curves is

slight. This is most likely due to the high drying temperature, which reduced the effect of air

flow rate. If the drying experiments were carried out at lower temperatures, the air flow rate

would have had a larger effect of the drying and the difference between the curves would

have been more noticeable.

Figure 17: Effect of air flow rate on sulphite mill biosludge drying curve, T = 220° C

34

Figure 18 shows the effect of drying temperature on sulphite mill biosludge. The trends are

as expected, demonstrating that higher temperatures increase the drying rate increases and

shorten the drying time.

Figure 18: Effect of temperature on sulphite mill biosludge drying curve

The Arrhenius plot, shows the effect of temperature on the drying rate of sulphite and kraft mill

biosludge. Drying can be considered a first order reaction, thus:

� = �!"#� Where:

X(t) – Moisture content at time ‘t’ [g water/g dry solids]

Xo – Initial moisture content [g water/g dry solids]

t – Time [min]

k – Rate constant [(g water/g dry solids-min)/min]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300

Mo

istu

re R

ati

o

Time (min)

35

The rate constant (k) for each temperature can be determined by linearizing the above equation

and plotting the natural logarithm of the drying rate against time and determining the slope,

shown in Table 4.

Table 4: Arrhenius parameters for the drying of sulphite and kraft mill biosludge from

110-220° C

Sulphite Mill Kraft Mill

Temperature

(° C)

Temperature

(K)

1/T(K) k ln(k) k ln(k)

110 383 0.00261 -0.0135 -4.31 -0.0115 -4.47

140 413 0.00231 -0.0145 -4.23 -0.0153 -4.18

160 433 0.00221 -0.0208 -3.87 -0.0198 -3.92

180 453 0.00231 -0.0296 -3.52 -0.0270 -3.61

200 473 0.00211 -0.0337 -3.39 -0.0422 -3.17

220 493 0.00203 -0.0428 -3.15 -0.0520 -2.96

From this the Arrhenius plot can be developed, as shown in Figure 19. Drying temperature

clearly has a significant effect on the drying rate. The slope of this curve indicates the activation

energy, which is the minimum energy required for the process to proceed. The activation energy

was determined to be 2133.50 kJ/mol for the sulphite mill biosludge and 2694.30 kJ/mol for the

kraft mill biosludge.

y = -2694.3x + 2.4294

R² = 0.9597

y = -2133.5x + 1.1213

R² = 0.9374

-4.75

-4.5

-4.25

-4

-3.75

-3.5

-3.25

-3

-2.75

0.002 0.0021 0.0022 0.0023 0.0024 0.0025 0.0026 0.0027

ln(-

k)

1/T(K)

Kraft

Sulphite

Figure 19: Arrhenius plot of sulphite and kraft mill biosludge

36

Figure 20 shows the drying rate of sulphite mill biosludge dried at 220° C. It was determined by

calculating the moisture content on a g water/g DS basis from the experimental data. The rate

was then determined as the difference between two moisture contents in a 1 min time interval.

The drying rate reached a maximum of 0.18 g water/g dry solids-min at 9 min. During this time

the material under goes heating as most of the thermal energy is absorbed by the water on the

surface. Once enough thermal energy has been absorbed, the water on the sample surface will

vapourize and the mass transfer rate will increase, where the initial vapourization briefly occurs

at the sample surface. Once the maximum drying rate is reached there is a sudden decrease in

drying rate as the moisture content decreases. This is because the outer layer of the material has

dried, forming a crust. The moisture must migrate from the sample interior in order to vapourize.

Since the distance the water must travel increases throughout the drying process, the rate is

controlled by the diffusion of water towards the sample surface. Eventually, the drying rate

reaches zero at 75 min, because the moisture concentration is approximately zero and the

concentration gradient between the sample and the hot air no longer exists. This is the

equilibrium moisture content.

Figure 20: General drying rate curve of sulphite sludge, T = 220° C

Note, that unlike some materials, the constant mass transfer rate for biosludge occurs briefly.

This is most likely due to the dewatering process prior to drying.

37

The most useful method for observing the drying phases is a time independent graph of the

drying rate versus the moisture content, called a Krischer curve [76]. The Krischer curves for

municipal, sulphite mill, and kraft mill biosludge dried at 220° C are shown in Figure 21. The

drying rate is represented by the change in moisture content [X g water/g dry solid] per minute.

The sulphite sludge drying rate is lower than for the kraft mill sludge. This may be due to the

higher organic content in the sludge, which has been thought to lower the drying potential [77].

Figure 21: Krischer curves for sulphite mill, kraft mill, and municipal biosludge, T = 220°C

As expected [75, 78], drying occurs primarily in the falling rate period and two critical moisture

contents can be observed, as indicated by the changes in slope and shown in Table 5.

Table 5: Critical moisture contents of the tested biosludge. Obtained from Figure 21.

Critical Moisture

Content

Sulphite Mill

[g water/g dry solid]

Kraft Mill

[g water/g dry solid]

Municipal

[g water/g dry solid]

First 5.50 6.12 6.42

Second 0.78 0.80 0.89

The second critical moisture content of the pulp and paper mill biosludges agree with reported

values, 0.40-0.80 g water/g dry solid [30]. However, the experimental value for municipal sludge

is outside of this range, falling into the gelatin, and gel category (> 80 g water/ g dry solid). This

may be because the municipal sludge did not have the same dewatering capacity as the pulp and

paper mill sludges (10% solids versus 13% solids respectively). Thus, it may have been more

gel-like, which is typical of sludges with 93-90% moisture content. Note, that the critical

moisture content is not solely a material property, but also depends on the drying conditions

(temperature, drying rate, and humidity); thus it must be determined by experimentation [30].

38

4.5.2 Model

Thin layer drying was taken as the model reference for biosludge drying in these experiments

because this method has been frequently used to model drying kinetics. Thin layer drying is the

process of drying in one layer of sample particles or slices. There are several thin film drying

models proposed by previous researchers were used to determine an appropriate model for

sludge drying, shown in Table 6. The data was normalized using the moisture ratio (MR).

Table 6: Common drying models proposed by various authors

Model Name Model Equation

Newton [79, 80] MR = exp(-kt)

Henderson & Pabis [81, 82, 83] MR = a exp(-kt)

Page [84, 85, 86] MR = exp(-ktn

)

Two Term [87, 88] MR = a exp(k1t) + b exp(-k

2t)

Wang & Singh [89] MR = 1 + at + bt2

Logarithmic [90] MR = a exp(-kt) + b

Diffusion Approximation [91, 92] MR = a exp(-kt) + (1-a) expt(-kbt)

Midlli [93] MR = A exp(-kt

n

) + Bt

Verma [94] MR = A exp(-kt) + (1-A) exp(-Gt)

Several statistical parameters were used to evaluate the model: the reduced chi-square (Χ2), root

mean square error (RMSE), mean relative percent deviation (%P), and the coefficient of

determination (R2). These were calculated as follows:

Χ% = ∑ ����'(���������,� ���(��*�+��*,��%,�-. / � 0 ��12 = 31/5���(��*�+��*,� ����'(���������,��%

,

�-.6./%

8 = 100/ 5:���'(���������,� ���(��*�+��*,�:���'(���������,�,

�-.

Where:

MRexperimental – the moisture ratio determined by experimental observation

MRpredicted – the moisture ratio predicted by the model

N – the number of data points

n – the number of model constants

39

The smaller the values of Χ2, RMSE, and P, and the larger the R2, indicate the best model.

Nonlinear regression was used to determine the model constants, this was performed using the

Solver function in Excel. The results in Table 7 show that the Page model best describes the

drying of the tested biosludge, with the Midlli model a close second. However, none of the tested

models could accurately predict the second falling rate period, indicating the difficulty in

capturing the effects of changing heat and mass transfer mechanisms during drying.

Additionally, most existing models do not consider the development of sludge stickiness and its

impact on drying [75, 95, 96, 97, 98].

Table 7: Model constants and results of the goodness of fit statistical analysis

Model Name Model

Constants

Χ2 RMSE %P R2

Newton k = 0.0342 0.00106 0.0325 85.7 0.9898

Henderson & Pabis k = 0.0381

a = 1.1279

0.0005 0.0229 68.6 0.9915

Page k = 0.0094

n = 1.3611 3.7x10

-5

0.0060 58.9 0.9994

Two Term k1 = 0.03814

k2 = 0.99461

a = 1.12787

b = 0

0.00053 0.0229 68.6 0.9915

Wang & Singh a = 0

b = 5x10-9

0.84152 0.9136 31661 0.2709

Logarithmic k = 0.03814

a = 1.12788

b = 0

0.00053 0.0229 68.6 0.9915

Diffusion Approximation k = 0.04613

a = 0

b = 0.74174

0.00107 0.0325 85.7 0.9899

Midlli k = 0.00518

a = 0.96076

b = 1.6x10-5

n = 1.51625

8x10-5

0.0089 71.3 0.9986

Verma k = 0.03421

a = 0.04767

g = 0.03421

0.00107 0.0325 85.7 0.9898

40

4.5.3 Internal Temperature

A type K thermocouple was inserted into the centre of the sludge cake to record the sample

temperature during drying. Figure 22 shows the relationship between the sample moisture

content and the internal temperature of sulphite mill biosludge during drying.

Figure 22: Relationship between moisture content and internal temperature of biosludge

Three distinct regions occur in both curves, indicated by corresponding changes in the slope. As

the sludge sample enters the drying chamber, the first region occurs for approximately 5 min and

the drying rate is relatively constant. A slight change in the moisture content is observed as the

internal temperature increases rapidly. All of the heat supplied is absorbed by the sample to heat

it to the wet-bulb temperature, which is approximately that of the surrounding water, and the

drying rate reaches a maximum. Once the wet-bulb temperature has been reached, the drying rate

decreases, indicating the start of the first falling rate period. The sample temperature remains

constant at the wet-bulb temperature as a result of the latent heat of vapourization. Then the

second falling rate period occurs as the drying rate decreases, indicated by a change in the slope

of the moisture content curve. The sample temperature increases exponentially to the furnace

temperature. This is because the amount of remaining water is so small that it can no longer

absorb the heat from the dryer, thus the heat is absorbed by the surrounding solids. Research on

municipal sludge found that it also proceeds through three phases and is independent of sludge

type, with the wet bulb temperature ranging from 50-85° C [12].

0

20

40

60

80

100

120

140

160

180

200

220

240

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110 120

Inte

rna

l T

emp

era

ture

(°

C)

Mo

istu

re C

on

ten

t (%

wet

)

Time (min)

Moisture Content

Internal Temperature

41

4.5.3.1 Temperature Profile

A temperature profile of the sample during drying was prepared by recording the temperature at

different locations in the sample, shown in Figure 23.

Figure 23: Location of thermocouple in cylindrical sample

Figure 24 shows the temperature profiles at each location in the sample. As the sample dries the

temperature of the surface increases faster than at the centre. The centre temperature cannot

increase until the water has evaporated from the previous layers and the temperature of each

layer has increased to the furnace temperature, resulting in a plateau. This also explains the

sudden increase in temperature towards the temperature of the furnace, which is visible both at

the quarter and centre points of the sample but not at the surface.

Figure 24: Sample temperature during drying a different locations

0

20

40

60

80

100

120

140

160

180

200

220

240

0 10 20 30 40 50 60 70 80 90

Inte

rnal

Tem

per

atu

re (

°C

)

Time (min)

Surface

Quarter

Centre

Surface

Quarter

Centre

42

Figure 25 shows the internal temperature of the biosludge at the centre of the sample when it was

dried at 220° C and 160° C. The drying temperature at the centre ranges from 80-90° C at both a

furnace temperature of 220° C and 160° C. Suggesting that the centre temperature is independent

of dryer temperature and will only increase when the water from previous layers has evaporated

and the temperature of the solids has increased. This agrees with findings on the internal

temperature of municipal sludge during drying, which found the centre temperature to range

from 50-85° C [12]. Perhaps to reduce energy costs, the dryers only need to operate around 80-

100° C. However, the dryer temperature has a significant impact on the drying time, the lower

the temperature, the longer the drying time.

Figure 25: Internal temperature when dried at 220° C and 160° C

The trends in Figure 25 show similar temperature profile to results from previous research on

municipal sludge [12, 99].

0

20

40

60

80

100

120

140

160

180

200

220

240

0 20 40 60 80 100 120 140

Inte

rnal

Tem

per

atu

re (

°C

)

Time (min)

220° C

160° C

43

4.5.4 Effect of Polymer

Polymer is added to the biosludge in the dewatering stage to flocculate the solids prior to gravity

filtering, thus it is important to observe if polymer alters the drying process.

4.5.4.1 Drying Process

Biosludge samples were prepared with and without polymer and dried at 220° C. Figure 26

shows their drying curves with respect to the mass of water lost during the drying process. As

suspected the polymer does not have a significant effect on drying. The difference in initial water

between the samples is the most apparent difference. This is because more water can be removed

in the dewatering stage if polymer is used, leading to higher solids in the sludge cake prior to

drying.

Figure 26: Effect of polymer on biosludge drying

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50

Mois

ture

Rati

o

Time (min)

No Polymer

Polymer

44

4.5.5 Effect of Additives on Drying

Fly-ash and wood fines, common solids waste products at pulp mills, were added to the

dewatered sludge mixture to test whether their addition can improve biosludge drying

characteristics. These materials were oven dried and added to 10 g of biosludge in amounts of 10

wt%, 20 wt%, and 30 wt%. These tests were conducted at 140° C, since higher temperatures

would cause volatile burning of the wood fines.

Figure 27 shows the effect of fly ash on the drying of biosludge. From this, it can be concluded

that the addition of fly ash has no effect on the drying of biosludge. This could be due to the low

temperature (140° C) used in this study, which did not allow the fly ash particles to reach high

enough temperatures to alter the drying rate of biosludge.

Figure 27: Effect of biomass boiler fly ash on biosludge drying at 140° C

45

Figure 28 shows the effect of wood fines on the drying of biosludge. The addition of wood fines

adversely impacts the drying of biosludge. This might be a result of the bone dry wood fines

absorbing moisture from the biosludge and that the moisture transfer out of the wood fines is

slower than the moisture transfer out of the biosludge. This agrees with findings on municipal

sludge which suggest that increasing the organic content of the sludge adversely affect the drying

potential [77].

Figure 28: Effect of wood fines on biosludge drying at 140° C

46

4.5.5.1 Fly ash at different temperatures

Based on Figure 27, it is suspected that the reason the fly ash did not impact the drying