the effect of orifice flow treatment on biosludge

The Effect of Orifice Flow Treatment on Biosludge Dewaterability

by

Krista Singh

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 Krista Singh 2015

ii

The Effect of Orifice Flow Treatment on Biosludge Dewaterability

Krista Singh

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2015

Abstract

This research assessed the potential for orifice flow treatment to improve biosludge

dewaterability by disintegrating the flocs, thereby releasing the interstitial water trapped within

them. Pulp and paper waste activated sludge, municipal waste activated sludge, and municipal

anaerobically digested sludge samples were orifice flow treated at strain rates up to 29,280

1060 s-1

, 34,540 s-1

, and 34,090 s-1

, respectively, and their particle size distribution, water

distribution, and dewaterability were assessed. Although orifice flow treatment disintegrated the

biosludge flocs, it did not significantly affect the interstitial water content. Overall, orifice flow

treatment worsened filterability. Orifice flow treatment did, however, increase the centrifuge

cake solids content of pulp and paper and municipal waste activated sludge by 10 and 15 %

respectively, showing the potential to improve biosludge centrifugability. Additionally, orifice

flow treatment was more effective in disintegrating pulp and paper waste activated sludge flocs

than sonication at the same energy output.

iii

Acknowledgments

I would like to thank my supervisors Professor Ramin Farnood and Professor D. Grant Allen, as

well as Professor Honghi Tran, for their guidance. I would also like to thank Professor Arun

Ramchandran for serving on my committee.

This research was funded by an Industry/Academia partnership through the Natural Sciences and

Engineering Research Council of Canada Collaborative Research and Development Grant

Program. I wish to thank the personnel at the Tembec Temiscaming and Tembec Kapuskasing

Pulp & Paper mills, especially Adrew Barquin, for providing our lab with sludge samples as well

as tours of the mills.

Thank you to all of my colleagues in Professor Farnood’s, Professor Allen’s, and Professor

Tran’s labs for their assistance. Special thanks to my fellow “sludgies” Sofia Bonilla, Parthiv

Amin, and Jordan Bouchard for their help with all things sludge-related; John Gibson for

building the orifice flow treatment apparatus; Torsten Meyer for his help with operating the

orifice flow treatment apparatus; Azad Kavoosi and Yaldah Azimi for driving me to Ashbridges

Bay wastewater treatment plant to obtain sludge samples; and Rosanna Kronfli and Parthiv Amin

for their help with editing this thesis.

I would also like to express my great appreciation to the Department of Chemical Engineering &

Applied Chemistry and the Chemical Engineering Graduate Student Association for making my

time at the University of Toronto truly enjoyable.

Last but not least, thank you to my family and friends, especially my parents Anand and Susan,

for their endless love and support throughout this journey, and to God for His many blessings.

iv

Table of Contents

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

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

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

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

Nomenclature .............................................................................................................................. xii

Chapter 1 ....................................................................................................................................... 1

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

1.1 Hypotheses ..................................................................................................................................... 2

1.2 Objectives ...................................................................................................................................... 3

Chapter 2 ....................................................................................................................................... 4

Literature Review ................................................................................................................... 4 2

2.1 Wastewater Treatment ................................................................................................................. 4

2.1.1 Activated Sludge Process ........................................................................................................ 4

2.2 Sludge Dewatering ........................................................................................................................ 5

2.2.1 Mechanical Dewatering ........................................................................................................... 6

2.3 Dewaterability ............................................................................................................................... 8

2.3.1 Rate of Dewatering .................................................................................................................. 8

2.3.2 Extent of Dewatering ............................................................................................................... 9

2.3.3 Motivation for Improvement ................................................................................................... 9

2.4 Challenges in Sludge Dewatering .............................................................................................. 10

2.5 Sludge Disintegration ................................................................................................................. 11

2.5.1 Sonication .............................................................................................................................. 12

2.5.2 Orifice Flow Treatment ......................................................................................................... 15

2.5.3 Sonication vs. Orifice Flow Treatment .................................................................................. 16

2.6 Assessment of Water Distribution in Sludge ............................................................................ 17

2.6.1 Drying Test ............................................................................................................................ 17

2.7 Assessment of Dewaterability .................................................................................................... 19

v

2.7.1 Rate of Dewatering ................................................................................................................ 19

2.7.2 Extent of Dewatering ............................................................................................................. 20

Experimental Methods ......................................................................................................... 22 3

3.1 Experimental Approach ............................................................................................................. 22

3.2 Sludge Samples ........................................................................................................................... 23

3.3 Sludge Treatment ....................................................................................................................... 24

3.3.1 Orifice Flow Treatment ......................................................................................................... 24

3.3.2 Sonication Apparatus ............................................................................................................. 26

3.4 Primary Sludge and Polymer Addition .................................................................................... 27

3.4.1 Primary Sludge Addition ....................................................................................................... 27

3.4.2 Polymer Solution Preparation ................................................................................................ 27

3.4.3 Polymer Addition .................................................................................................................. 27

3.5 Particle Size Distribution ........................................................................................................... 28

3.6 Water Distribution ..................................................................................................................... 28

3.7 Assessment of Dewaterability .................................................................................................... 29

3.7.1 Rate of Dewatering ................................................................................................................ 29

3.7.2 Extent of Dewatering ............................................................................................................. 29

3.8 Statistical Analysis ...................................................................................................................... 31

Chapter 3 ..................................................................................................................................... 32

Results and Discussion ......................................................................................................... 32 4

4.1 Sludge Storage............................................................................................................................. 32

4.2 Orifice Flow Treatment .............................................................................................................. 33

4.3 Particle Size Distribution ........................................................................................................... 34

4.3.1 Proportions of Settleable and Supracolloidal Particles .......................................................... 34

4.4 Water Distribution ..................................................................................................................... 39

4.5 Polymer Demand ........................................................................................................................ 43

4.6 Dewaterability Results ............................................................................................................... 45

4.6.1 Capillary Suction Time .......................................................................................................... 45

4.6.2 Specific Resistance to Filtration ............................................................................................ 47

4.6.3 Gravity Filter Cake Solids Content ........................................................................................ 49

4.6.4 Crown Press Cake Solids Content ......................................................................................... 51

vi

4.6.5 Pressure Filter Cake Solids Content ...................................................................................... 53

4.6.6 Centrifuge Cake Solids Content ............................................................................................ 55

4.6.7 Combined Gravity Filtrate and Crown Press Pressate Solids Content .................................. 58

4.6.8 Pressure Filtrate Solids Content ............................................................................................. 60

4.6.9 Centrate Solids Content ......................................................................................................... 61

4.7 Summary of Filterability Results .............................................................................................. 62

4.7.1 Pulp and Paper WAS ............................................................................................................. 62

4.7.2 Municipal WAS ..................................................................................................................... 63

4.7.3 Municipal ADS ...................................................................................................................... 63

4.7.4 Overall Effect ........................................................................................................................ 64

Chapter 4 ..................................................................................................................................... 66

Conclusions ........................................................................................................................... 66 5

Chapter 5 ..................................................................................................................................... 68

Recommendations ................................................................................................................. 68 6

References.............................................................................................................................. 70 7

Appendices ............................................................................................................................ 76 8

8.1 Appendix A: Determination of Sonication Parameters........................................................... 76

8.2 Appendix B: Calculation of Specific Resistance to Filtration ................................................. 79

8.3 Appendix C: Determination of Water Distribution................................................................. 82

vii

List of Tables

Table 2.1. The Effect of Sonication on Sludge Dewaterability .................................................... 13

Table 4.1. Biosludge Flowrate and Strain Rate through Orifice for Various Biosludges and

Orifice Radii.................................................................................................................................. 33

Table 4.2. Classification of Particles in Sludge based on Particle Size (Karr & Keinath, 1978) . 34

Table 4.3. The Effect of Orifice Flow Treatment on Pulp and Paper WAS, Municipal WAS, and

Municipal ADS Filterability ......................................................................................................... 62

Table 4.4. For a Given Biosludge and Filter Medium, was the Maximum Strain Rate Above or

Below the Critical Strain Rate?..................................................................................................... 65

viii

List of Figures

Figure 1.1. Mechanical Sludge Dewatering .................................................................................... 1

Figure 2.1. Activated Sludge Process ............................................................................................. 5

Figure 2.2. Typical Sludge Drying Curve ..................................................................................... 18

Figure 2.3. Crown Press (reprinted with permission) ................................................................... 21

Figure 3.1. Experimental Approach .............................................................................................. 22

Figure 3.2. Process Flow Diagram of Pulp and Paper Mill Central Wastewater Treatment Plant 23

Figure 3.3. Process Flow Diagram of Ashbridges Bay Wastewater Treatment Plant .................. 24

Figure 3.4. A) Photo and B) schematic diagram of Orifice Flow Treatment Apparatus .............. 25

Figure 4.1. Effect of time on median particle diameter of an untreated and a treated (E =

29,670 s-1) P&P WAS sample stored at 4°C .............................................................................. 32

Figure 4.2. Effect of treatment on particle size distribution. A) Orifice flow treated P&P WAS,

B) orifice flow treated municipal WAS, C) orifice flow treated municipal ADS (0.8 mm orifice

radius), and D) sonicated P&P WAS (460 kJ/ kg DS) ................................................................. 35

Figure 4.3. Effect of orifice flow treatment of P&P WAS at various strain rates on proportions of

settleable and supracolloidal particles in P&P WAS with and without polymer (0.815 kg/ tonne

DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)........................................... 37

Figure 4.4. Effect of orifice flow treatment (0.8 mm orifice radius) and sonication (460 kJ/ kg

DS) on proportion of supracolloidal particles (mean ± SD) in various biosludge samples .......... 38

ix


DS) on A) free, B) interstitial, and C) vicinal and hydration water contents (mean ± SD) of

various biosludge samples. ........................................................................................................... 40

Figure 4.6. Cake solids content as a function of water removed. A) P&P WAS, B) municipal

WAS, and C) municipal ADS. Open circle symbols represent critical solids contents. Lines a)

free water removed, b) free + interstitial water removed, c) free + interstitial + vicinal water

removed, d) free + interstitial + vicinal water + hydration water removed, e) current cake solids

content achieved, and f) minimum cake solids content for self-sustainable combustion ............. 42

Figure 4.7. Effect of treatment on polymer dose curve. A) Orifice flow treated P&P WAS, B)

orifice flow treated municipal WAS, C) orifice flow treated municipal ADS (0.8 mm orifice

radius), and D) sonicated P&P WAS (460 kJ/ kg DS) ................................................................. 44

Figure 4.8. Effect of orifice flow treatment of P&P WAS at various strain rates on CST (mean ±

SD) of P&P WAS with and without polymer (0.815 kg/ tonne DS) and/or primary sludge (7:3

primary sludge to WAS mass ratio) .............................................................................................. 46


DS) on CST (mean ± SD) of various biosludge samples .............................................................. 46

Figure 4.10. Effect of orifice flow treatment of P&P WAS at various strain rates on SRF (mean ±

SD) of P&P WAS with and without polymer (0.815 kg/ tonne DS) and/or primary sludge (7:3

primary sludge to WAS mass ratio) .............................................................................................. 47


DS) on SRF (mean ± SD) of various biosludge samples .............................................................. 48

Figure 4.12. Effect of orifice flow treatment of P&P WAS at various strain rates on gravity filter

cake solids content (mean ± SD) of P&P WAS with and without polymer (0.815 kg/ tonne DS)

and/or primary sludge (7:3 primary sludge to WAS mass ratio) .................................................. 49

x


DS) on gravity filter cake solids content (mean ± SD) of various biosludge samples ................. 50

Figure 4.14. Effect of orifice flow treatment of P&P WAS at various strain rates on Crown Press




DS) on Crown Press cake solids content (mean ± SD) of various biosludge samples ................. 52

Figure 4.16. Effect of orifice flow treatment of P&P WAS at various strain rates on pressure

filter cake solids content (mean ± SD) of P&P WAS with and without polymer (0.815 kg/ tonne

DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)........................................... 53


DS) on pressure filter cake solids content (mean ± SD) of various biosludge samples ............... 54

Figure 4.18. Effect of orifice flow treatment of P&P WAS at various strain rates on centrifuge




DS) on centrifuge cake solids content (mean ± SD) of various biosludge samples ..................... 56

Figure 4.20. Effect of orifice flow treatment of P&P WAS at various strain rates on combined

gravity filtrate and Crown Press pressate TSS (mean ± SD) of P&P WAS with and without

polymer (0.815 kg/ tonne DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio) . 58


DS) on combined gravity filtrate and Crown Press pressate TSS (mean ± SD) of various

biosludge samples ......................................................................................................................... 59

xi

Figure 4.22. Effect of orifice flow treatment of P&P WAS at various strain rates on pressure

filtrate TSS (mean ± SD) of P&P WAS with and without polymer (0.815 kg/ tonne DS) and/or

primary sludge (7:3 primary sludge to WAS mass ratio) ............................................................. 60


DS) on pressure filtrate TSS (mean ± SD) of various biosludge samples .................................... 61

Figure 8.1: Plot of tV vs. V for Pulp and Paper WAS Sample ...................................................... 80

Figure 8.2. Plot of Normalized Dry Water Content vs. Drying Flux with Segmental Linear

Regression for a Pulp and Paper WAS Sample ............................................................................ 83

Figure 8.3. Plot of Normalized Dry Moisture Content vs. Drying Flux for Normalized Dry

Moisture Contents less than the First Critical Moisture Content with Segmental Linear

Regression ..................................................................................................................................... 84

xii

Nomenclature

ADS Anaerobically digested sludge

BCTMP Bleached chemi-thermo-mechanical pulp

CST Capillary suction time

DS Dry solids

DSC Differential scanning calorimetry

DTA Differential thermal analysis

P&P Pulp and paper

pol Polymer

prim Primary sludge

PSD Particle size distribution

SRF Specific resistance to filtration

TGA Thermogravimetric analysis

TS Total solids (wt.%)

TSS Total suspended solids (g/L)

WAS Waste activated sludge

𝐴 Area (m2)

𝑏 Slope

𝐶𝐼 Confidence interval

𝐷𝑆 Dry solids content (kg DS/ m3)

𝐷50 Median particle diameter (μm)

𝐸 Strain rate (s-1

)

𝐸𝑚𝑎𝑥 Maximum strain rate (s-1

)

xiii

𝑒𝑑 Specific energy delivered (J/kg)

𝑒𝑠 Specific energy supplied (kJ/kg DS)

ℱ Friction heating per unit mass of fluid passing through system (J/kg)

𝑔 Acceleration of gravity (m/s2)

𝑚 Mass (kg)

𝑃 Pressure (Pa)

𝑃𝐷 Power delivered (W)

𝑃𝑆 Power supplied (W or kW)

𝑝 P-value

𝜌 Density (kg/m3)

𝑄 Volumetric flowrate (m3/s)

𝑅 Drying flux (kg water/ m2s)

𝑅𝑚 Filter medium resistance (m-1

)

𝑅 Specific resistance to filtration (m/kg)

𝑟 Orifice radius (m)

𝑟2 Coefficient of determination (linear regression)

𝑅2 Coefficient of determination (non-linear regression)

𝑡 Time (s)

𝜇 Filtrate dynamic viscosity (Pas)

𝑉 Volume (m3)

𝑣 Velocity (m/s)

𝑑𝑉

𝑑𝑡 Volume of filtrate passing through filter cake and filter medium per unit time

(m3/s)

𝜔 Dry mass of filter cake per unit volume of filtrate (kg/m3)

xiv

𝑑𝑊

𝑑𝑚 Work done on fluid per unit mass of fluid passing through system (J/kg)

𝑋 Normalized dry water content (wt.%)

𝑧 Elevation (m)

1

Chapter 1

Introduction 1

The pulp and paper industry, as well as other industries and municipal wastewater treatment

plants, commonly use the activated sludge process to treat their wastewater. A byproduct of the

activated sludge process is waste activated sludge (WAS), which consists of single particles and

flocs of solids, microorganisms, and extracellular polymeric substances (EPS) dispersed in water.

WAS, which is only about 0.5 to 1.5 wt.% solids, is usually mixed with primary sludge and

polymer, and mechanically separated into its solid and liquid phases (Figure 1.1)

(Tchobanoglous, Burton, & Stensel, 2003). The separated solid phase, referred to as the cake, is

generally disposed of by land application, incineration, and/or landfilling, while the separated

liquid phase, referred to as the filtrate, pressate, or centrate, is returned to the wastewater

treatment process. The separation of sludge into its solid and liquid phases is referred to as

dewatering. Dewatering decreases the quantity of sludge, and thus the costs associated with its

use and/or disposal. It also increases the heat of combustion of the sludge, decreasing the

quantity of auxiliary fuel required for, and thus the operating costs associated with incineration.

A minimum cake solids content of 40 wt.% is required for self-sustainable combustion

(Integrated Pollution Prevention and Control, 2001). In practice, mechanical dewatering achieves

a cake solids content of only 12 to 34 wt.% (Tchobanoglous et al., 2003). Thus, as 40% of

Canadian pulp and paper mill sludge is disposed of by incineration, there is motivation to

increase the cake solids achieved by mechanical dewatering (Elliott & Mahmood, 2005).

Figure 1.1. Mechanical Sludge Dewatering

2

A major challenge in sludge dewatering is removing the water trapped within the flocs and cells

and the water physically and chemically bound to the surface of the solids particles and cells.

Theoretically, the water trapped within the flocs and cells can be released by disintegrating the

flocs and cells, increasing the cake solids content that can be achieved by mechanical dewatering

(Vesilind, 1994). One method of sludge disintegration is orifice flow treatment, the pumping of a

liquid through a small orifice, which is thought to be a simpler, easier to scale-up, and lower

capital and operating cost alternative to sonication (Gogate et al., 2001). Although disintegrating

the flocs and cells may increase the cake solids content by releasing the water trapped within

them, it may adversely increase the filtrate solids content and decrease the rate of filtration, as

the smaller disintegrated flocs are more likely to pass through the filter medium and blind the

filter medium and cake.

1.1 Hypotheses

It is hypothesized that:

1. The orifice flow treatment of biosludge will:

a. Disintegrate the flocs;

b. Release the water trapped within the flocs;

c. Increase the cake solids content, thereby improving dewaterability, but decrease

the rate of dewatering and increase the filtrate, pressate, or centrate solids

content, thereby worsening dewaterability.

2. Mixing orifice flow treated biosludge with primary sludge and/or polymer will

counteract any decrease in the rate of dewatering and any increase in the filtrate,

pressate, or centrate solids content caused by orifice flow treatment, while preserving

any increase in cake solids content, resulting in an overall improvement in

dewaterability.

3

1.2 Objectives

The goal of this study was to assess the potential for orifice flow treatment to improve biosludge

dewaterability. More specifically, the objectives of this study were to determine how the orifice

flow treatment of biosludge affects the dewaterability of biosludge and mixtures of biosludge,

primary sludge, and/or polymer and how it affects the dewaterability of biosludge in comparison

to sonication.

4

Chapter 2

Literature Review 2

2.1 Wastewater Treatment

As with most industries and municipalities, the pulp and paper industry must treat the water they

use before returning it to the environment. The removal of contaminants from used water is

referred to as wastewater treatment and commonly consists of three main stages: preliminary

treatment, primary treatment, and secondary treatment. Preliminary treatment is the removal of

large objects and grit from wastewater by screening and gravitational or centrifugal

sedimentation, respectively. Primary treatment is the removal of floatable matter as well as

settleable solids from wastewater by gravitational sedimentation. The removed settleable solids

are referred to as primary sludge. Secondary treatment is the incorporation of non-settleable

suspended and dissolved solids into flocs or films by a biological process, followed by the

removal of the flocs or films from the wastewater by a physical process. The removed flocs or

films are referred to as secondary sludge or biosludge.

2.1.1 Activated Sludge Process

The most common process for secondary treatment is the activated sludge process (Figure 2.1).

In this aerobic, suspended-growth, and biological process, a suspension of microorganisms and

primary effluent are mixed and aerated in an aeration tank. The microorganisms feed on the

remaining organic matter in the wastewater, grow, and secrete extracellular polymeric substances

(EPS). The EPS bind the microorganisms and organic and inorganic solids in the wastewater into

settleable flocs (Madigan, Martinko, Stahl, & Clark, 2012). The contents of the aeration tank,

referred to as mixed liquor, are then sent to a secondary clarifier for the flocs to settle. Some of

the settled flocs, referred to as return activated sludge, are returned to the aeration tank to treat

incoming primary effluent, while the remaining settled flocs, referred to as waste activated

sludge (WAS), are removed from the process (Tchobanoglous et al., 2003).

5

Figure 2.1. Activated Sludge Process

2.2 Sludge Dewatering

Of the contaminants removed from wastewater during treatment, sludge is the largest in volume

and managing this sludge is a great challenge (Tchobanoglous et al., 2003). Primary sludge

consists of the settleable solids dispersed in water while WAS consists of single particles and

flocs of the unsettleable solids, microorganisms, and EPS dispersed in water. As primary sludge

and WAS are 91 to 95 wt.% water and about 98.8 to 99.2 wt.% water, respectively, the water in

sludge is usually separated from the solids and returned to the wastewater treatment process,

while the solids are used and/or disposed of by land application, incineration, aerobic and

anaerobic digestion, and/or landfilling (Tchobanoglous et al., 2003; US EPA, 1987). The process

of separating the water in sludge from the solids, producing a sludge cake and filtrate, pressate,

or centrate is referred to as dewatering. The dewatering of sludge significantly decreases its mass

and volume and increases its heating value, which is important if the sludge is to be incinerated,

thereby reducing the costs associated with its use and/or disposal (Tchobanoglous et al., 2003).

Sludge dewatering processes can be classified as being natural, mechanical, thermal, or

combinations thereof (Tchobanoglous et al., 2003). Natural dewatering uses drying beds and

lagoons to remove water by natural evaporation and gravity and/or induced drainage (United

States Environmental Protection Agency, 1987). Mechanical dewatering uses machines to

remove water by expression. Thermal dewatering uses dryers to remove water by evaporation.

6

2.2.1 Mechanical Dewatering

Mechanical dewatering is more common than natural and thermal dewatering, as it requires less

time and space than natural dewatering and less energy than thermal dewatering (Mahmoud,

Olivier, Vaxelaire, & Hoadley, 2010; K.-W. R. Tsang, 1989). Primary sludge and WAS are often

mixed and chemically conditioned prior to mechanical dewatering.

2.2.1.1 Sludge Mixing

Primary sludge and WAS are usually mixed prior to dewatering to produce a sludge that is more

dewaterable than WAS alone. During dewatering, the more numerous and larger solids in

primary sludge act as a filter aid, decreasing the compressibility and increasing the porosity of

the sludge cake by forming a rigid lattice structure, through which the water can easily drain

(Mowla, Tran, & Allen, 2013; Tchobanoglous et al., 2003).

Primary sludge and WAS are usually mixed in the ratio that they are generated, which has

typically been 70:30 primary sludge to WAS for pulp and paper mills (Elliott & Mahmood,

2005). However, with mills reducing their loss of fibres to wastewater, less primary sludge is

being generated and the ratio of primary sludge to WAS is decreasing (Elliott & Mahmood,

2005). This presents a challenge to mills because the dewaterability of the mixed sludge worsens

as the primary sludge to WAS ratio decreases (Mahmood & Elliott, 2006).

2.2.1.2 Chemical Conditioning

Chemicals are often added to sludge prior to dewatering to improve dewaterability by

coagulating and flocculating the solids in the sludge. The chemicals most commonly used for

sludge conditioning can be classified as inorganic chemicals, such as ferric chloride and lime, as

well as organic polymers (US EPA, 1987). Organic polymers are more commonly used than

inorganic chemicals, as they are required in lower dosages, and therefore do not add as much to

the mass and volume of the sludge, and they do not lower the heating value of the sludge, which

is important if the sludge is to be incinerated (US EPA, 1987). The type and dosage of chemical

conditioner required to achieve a desired level of dewaterability is important, with chemical

conditioner costs accounting for a major portion of dewatering costs (Mahmood & Elliott, 2006).

7

2.2.1.3 Mechanical Dewatering Devices

Common devices used for mechanical dewatering include belt-filter presses, screw presses, and

centrifuges.

A belt filter press consists of two porous belts and a series of rollers and can be divided into three

main zones: the gravity drainage zone, the low pressure zone, and the high pressure zone. In the

gravity drainage zone, sludge is fed onto the lower belt of the belt filter press. As the belt moves

forward, some of the water in the sludge drains by gravity. In the low pressure zone, the sludge is

sandwiched between the lower and upper belts, expressing some of the water from the sludge. In

the high pressure zone, the lower and upper belts with sludge in between them are passed over

and under a series of rollers with decreasing diameters that apply increasing amounts of pressure

to the sludge, further expressing water from the sludge. The sludge cake is discharged and the

pressate/filtrate is collected and returned to the wastewater treatment process (Komline-

Sanderson, 2014).

A screw press consists of a screw conveyor inside a perforated drum. Sludge is fed into the screw

press. The screw rotates slowly, moving the sludge forward. Near the inlet of the screw press, the

water in the sludge drains by gravity. As the sludge moves forward, it is compressed as a result

of the screw having an increasing diameter and/or decreasing pitch. As the sludge is compressed,

the water in the sludge is expelled through the perforations in the drum. At the end of the screw,

the sludge is forced through a restricted pathway, further compressing the sludge, and

discharged. The water expelled from the sludge is collected and returned to the wastewater

treatment process (Vincent Corporation, 2014).

8

The most common centrifuge used to dewater sludge is the solid-bowl centrifuge, which consists

of a screw conveyor inside a cylindrical-conical bowl (US EPA, 1987). The sludge is fed onto

the screw conveyor. The screw conveyor and bowl rotate in the same direction at different

speeds, creating a centrifugal force that pushes the solids in the sludge against the bowl wall (US

EPA, 1987). The conveyor moves the solids along the bowl wall to the solids discharge while the

liquid in the sludge or centrate, flows around and through the screw conveyor to the liquid

discharge (US EPA, 1987).

2.3 Dewaterability

The term dewaterability refers to the ability to separate the water from the solids in sludge. There

are two main aspects of dewaterability: the rate of dewatering and the extent of dewatering (D.

Lee & Wang, 2000; Peng, Ye, & Li, 2011).

2.3.1 Rate of Dewatering

The rate of dewatering is the rate at which the water can be separated from the solids in the

sludge and is determined by the ability of the water to move throughout the sludge (D. Lee &

Wang, 2000). The rate of dewatering is reflected in the sludge throughput, which should be

sufficiently high such that sludge does not need to be stored for more than 2 to 3 days before

dewatering, as it will deteriorate and become more difficult to dewater (Tchobanoglous et al.,

2003).

9

2.3.2 Extent of Dewatering

The extent of dewatering is the extent to which the water can be separated from the solids in the

sludge and is determined by the strength of the interactions between the water and solids (D. Lee

& Wang, 2000). The extent of dewatering is reflected in the solids content of the separated

water and solids. A complete separation in which the separated water is 0 wt.% solids and the

cake is 100 wt.% solids is rarely achieved (Seader, Henley, & Roper, 2011). Nonetheless, the

solids content of the separated water should be sufficiently low not to overload the wastewater

treatment process and degrade the quality of the final effluent when recycled back into the

process (Tchobanoglous et al., 2003). Similarly, the solids content of the cake should be

sufficiently high to meet any regulations for its use and/or disposal and to make its use and/or

disposal economical (US EPA, 1987).

Thus, a dewaterable sludge is a sludge that can be dewatered at a high rate and to a high extent

such that the desired sludge throughput and cake and filtrate/pressate/centrate solids contents can

be easily achieved. With the addition of primary sludge, polymer, and/or other dewatering aids,

the desired sludge throughput and filtrate/pressate/centrate solids content can generally be

achieved by mechanical dewatering (US EPA, 1987). The desired cake solids content, however,

is more difficult to achieve (US EPA, 1987). The cake solids content can be increased by

increasing the force applied to the sludge, but even then not all of the water in the sludge can be

mechanically removed, with most mechanical dewatering processes achieving a cake solids

content of only 12 to 34 wt.% (United States Environmental Protection Agency, 1987; Vesilind,

1994).

2.3.3 Motivation for Improvement

In 2002, 40% of Canadian pulp and paper mill sludge was disposed of by incineration,

converting the sludge primarily into carbon dioxide, water, and ash, and allowing for maximum

volume reduction (Elliott & Mahmood, 2005; Tchobanoglous et al., 2003). The sludge is often

incinerated in biomass boilers along with pulp and paper mill rejects, such as bark and wood

residues, and the generated energy is recovered (Monte, Fuente, Blanco, & Negro, 2009).

10

However, because of the low solids content of sludge, auxiliary fuel such as natural gas or oil

must be burned to sustain combustion. With the energy consumed from burning the auxiliary

fuel often being less than the energy generated from burning the sludge itself, auxiliary fuel is a

major operating cost of sludge incineration (Monte et al., 2009). The amount of auxiliary fuel

required to sustain combustion can be reduced by increasing the solids content of the sludge, and

at a solids content greater than or equal to 40 wt.%, auxiliary fuel is only required at the

beginning to heat the mixture and at the end when the volatile solids content of the mixture is

low (Integrated Pollution Prevention and Control, 2001; Tchobanoglous et al., 2003). Thus, there

is motivation to increase the extent of dewatering such that the cake solids content is at least 40

wt.%.

2.4 Challenges in Sludge Dewatering

Vesilind (1994) classified the water in sludge, based on its interactions with the solid particles

and cells in the sludge, as free and bound water, the latter of which was further classified into

interstitial, vicinal, and hydration water. Free water does not significantly interact with the solid

particles or cells, interstitial water is trapped within the flocs and cells, and vicinal and hydration

water are physically and chemically bound, respectively, to the surface of the solid particles and

cells. A combination of natural and mechanical dewatering can remove free water from sludge.

Bound water, however, is more difficult to remove. Mechanical dewatering can remove

interstitial water if enough energy is applied to compress and disintegrate the flocs and cells to

release the water within them. Nonetheless, only thermal dewatering can remove vicinal and

hydration water. Thus, at most, mechanical dewatering can remove all free and interstitial water

from sludge. Typically, however, mechanical dewatering removes all free water but only some

interstitial water (Vesilind, 1994). To remove more interstitial water from sludge by mechanical

dewatering, the flocs and cells in the sludge can be disintegrated prior to dewatering to release

the water trapped within them, thereby converting interstitial water into free water, which is

easier to remove (Erdincler & Vesilind, 2000; Ruiz-Hernando, Simón, Labanda, & Llorens,

2014).

11

2.5 Sludge Disintegration

Disintegrating biosludge flocs and cells may improve dewaterability by:

1. Releasing the water trapped within the flocs and cells, thereby converting interstitial

water into free water (Erdincler & Vesilind, 2000; Ruiz-Hernando et al., 2014; Vesilind,

1994).

2. Creating smaller flocs and particles that:

a. Upon the addition of primary sludge, form a more homogeneous mixture that is

less compressible and more porous (Thapa, Qi, Clayton, & Hoadley, 2009)

b. Upon the addition of polymer, form tighter flocs, thereby converting interstitial

water into free water (Huan, Yiying, Mahar, Zhiyu, & Yongfeng, 2009; Lo, Lai,

& Chen, 2001; Zhang, Wan, & Zhang, 2011)

Disintegrating biosludge flocs and cells may worsen dewaterability by:

1. Loosening the flocs, thereby creating more space for water to be trapped, and converting

free water into interstitial water (Chu, Chang, Liao, Jean, & Lee, 2001; Zhang et al.,

2011)

2. Creating smaller flocs and particles that:

a. Provide more surface area to which the water in the sludge can bind, thereby

converting free and interstitial water into vicinal and hydration water (Chu et al.,

2001; Feng, Lei, Deng, Yu, & Li, 2009; Ruiz-Hernando et al., 2014; United States

Environmental Protection Agency, 1987; Wang, Ji, & Lu, 2006; Zhang et al.,

2011)

b. Hinder the movement of water, especially by blinding the filter medium and cake

during filtration (Feng, Deng, et al., 2009; Karr & Keinath, 1978)

c. Increase the polymer demand (Dewil, Baeyens, & Goutvrind, 2006; Kopp,

Miiller, Dichtl, & Schwedes, 1997; United States Environmental Protection

Agency, 1987)

Consequently, there is controversy regarding the effect of sludge disintegration on

dewaterability.

12

2.5.1 Sonication

Sludge flocs and cells can be disintegrated by biological, chemical, mechanical, and thermal

methods and combinations thereof (Krogmann et al., 1999). Although they can be energy-

intensive, mechanical disintegration methods are advantageous in that unlike biological,

chemical, and thermal disintegration methods, they do not require biological or chemical aids,

which add to the amount of sludge, or high temperatures and pressures, which significantly

increase capital and operating costs (Weemaes & Verstraete, 1998).

One mechanical disintegration method is sonication, the application of sound with a frequency

greater than or equal to 20 kHz, known as ultrasound, to a substance. With ultrasound, the

particles in a liquid are mainly disintegrated by cavitation (Dewil et al., 2006). Ultrasound waves

have alternating regions of low and high pressure and as they propagate through a liquid, the

liquid expands and compresses, causing gas bubbles to form and subsequently implode (Dewil et

al., 2006). The implosion of the gas bubbles creates regions of extreme temperature (5000 K) and

pressure (500 atm), as well as shear forces that disintegrate particles in the liquid (Dewil et al.,

2006).

Although sonication has been found to be effective in disintegrating sludge, there is controversy

regarding its effect on sludge dewaterability. Table 2.1 summarizes the effect of sonication at

various specific energies and frequencies on the dewaterability of various sludges, as concluded

by various researchers. The specific sonication energy in kJ/ kg DS, 𝑒𝑠, was calculated using

Equation 1 (Dewil et al., 2006).

𝑒𝑠 =

𝑃𝑆𝑡

𝐷𝑆 ⋅ 𝑉

Equation 1

where 𝑃𝑆 is the supplied ultrasonic power in kW, 𝑡 is the sonication time in s, 𝐷𝑆 is the dry solids

content of the sludge sample in kg DS/ L, and 𝑉 is the sludge sample volume in L.

13

Table 2.1. The Effect of Sonication on Sludge Dewaterability

Reference Conclusion 𝒆𝑺 (kJ/kg

DS) Result Sludge Type

Frequency

(kHz)

Zhang, Wan, &

Zhang, 2011

Improved 70 - 560

Gravity settled sludge solids

content with and without

polymer* increased

Municipal WAS 25

Worsened 670 -

2,700

Gravity settled sludge solids

content with and without

polymer* decreased

Dewil et al.,

2006 Worsened

140 -

5,900

Gravity settled sludge volume

increased

Turkey

slaughterhouse

WAS

20 5,000 -

35,000

Filter cake solids content with

polymer decreased and polymer

demand increased Food processing

WAS

31,000 -

370,000 CST increased

Huan et al., 2009 Improved

220 Settling volume with polymer

decreased

Municipal WAS 25

650

Filter cake solids content with

FeCl3 increased and FeCl3

demand decreased

(Feng, Lei, et al.,

2009)

Improved 500-1,000 Settling velocity increased and

supernatant turbidity decreased

Municipal WAS 20

Worsened 5,000-

26,000

Settling velocity decreased and

supernatant turbidity increased

Feng et al., 2009

Improved

800 –

2,200

CST decreased and free water

content (drying test) increased

Municipal WAS 20

800 –

26,000 SRF decreased

800

Free water content

(centrifugation and filtration)

increased

Worsened

4,400 –

35,000

CST increased and free water

content (drying test) decreased

35,000 SRF increased

14

2,200 –

35, 000

Free water content

(centrifugation and filtration)

increased

(Ruiz-Hernando

et al., 2014)

Worsened 5,000 –

27,000

Free water content decreased

(DSC) Thickened

municipal WAS 20

Improved 27,000 Centrifuge cake solids content

increased

Wang, Ji, & Lu,

2006 Worsened

5,300 -

86,000

CST and SRF increased and free

water content (centrifugation)

decreased

Municipal WAS 20

Bougrier,

Albasi,

Delgenès, &

Carrère, 2006

Worsened 6,300 and

9,400 CST increased

Thickened

municipal WAS 20

Chu, Chang,

Liao, Jean, &

Lee, 2001

Improved 16,000 CST decreased

Food-processing

WAS 20

Worsened

32,000 -


16,000 –

140,000

Free water content (expression

test) decreased

Na, Kim, &

Khim, 2007

Worsened 17,000 -


Municipal ADS 28 Improved

55,000 –

450,000 CST decreased

17,000 -

670,000

Centrifuge cake solids content

increased

Erdincler &

Vesilind, 2000

Improved

32,000**

CST decreased and free water

content (centrifugation)

increased, centrifuge cake solids

content increased Simulated WAS 20

Worsened Free water content (DSC)

decreased

15

*Polymer was added before sonication

**In calculating the sonication energy, assumed that the sample volume was 500 mL based on the Branson 450 Ultrasonic Cell Disrupter that was

used and that the sample density was that of water.

Chu et al. (2001), Feng et al. (2009), and Zhang et al. (2011) found that there was a critical

sonication specific energy below and above which dewaterability improved and worsened,

respectively. Na et al. (2007) also found that there was a critical sonication specific energy, but

below and above which dewaterability worsened and improved, respectively. Erdincler &

Vesilind (2000) and Huan et al. (2009) and Bougrier et al. (2006), Dewil et al. (2006), and Wang

et al. (2006) found that the sonication of sludge either only improved or only worsened

dewaterability, respectively, although they may not have sonicated the sludge over a large

enough range of specific energies to observe a critical point. Generally, it appears that sonication

at low specific energy can improve dewaterability by slightly disintegrating the flocs, such that

some of the water trapped within them is released, but as few small particles as possible are

created, as they worsen dewaterability (Feng, Deng, et al., 2009; Zhang et al., 2011).

2.5.2 Orifice Flow Treatment

Another method of mechanical disintegration is orifice flow treatment, the pumping of a liquid

through a small orifice. In orifice flow treatment, one mechanism by which the particles in the

liquid may be disintegrated is cavitation. As the liquid enters the orifice, its velocity increases

and its pressure decreases, causing gas bubbles to form. Then, as the liquid exits the orifice, its

velocity decreases and its pressure increases, causing the gas bubbles to implode (Gogate et al.,

2001).

Another mechanism by which the particles in a liquid may be disintegrated in orifice flow

treatment is shear stress (Gibson et al., 2012). As the liquid flows through the orifice, the

particles in the liquid experience shear stress, are strained such that they elongate, and eventually

break apart (Sonntag and Russel, 1987; Fernandes 2012).

16

Gibson et al. (2012) investigated the orifice flow treatment of wastewater for improved UV

disinfection and found it to be an effective means of floc disintegration, with the degree of floc

disintegration correlating strongly with the strain rate in s-1

, 𝐸, as defined in Equation 2, but not

with the cavitation number, a measure of the tendency for hydrodynamic cavitation.

𝐸 =

𝑄

𝜋𝑟3

Equation 2

where 𝑄 is the flowrate through the orifice in m3/s and 𝑟 is the radius of the orifice in m

(Gibson, 2012).

This result suggests that the main mechanism of particle breakage in orifice flow is shear stress

rather than cavitation. Few studies have been conducted on the effect of orifice flow treatment on

sludge dewaterability. Analogous to the specific energy below which sonication improves

dewaterability, there may be a critical strain rate below which orifice flow treatment improves

dewaterability.

2.5.3 Sonication vs. Orifice Flow Treatment

Sonication is a more powerful method of sludge disintegration than orifice flow treatment,

achieving up to 100% cell disintegration compared to 75% cell disintegration (Weemaes &

Verstraete, 1998). Nonetheless, the equipment required for orifice flow treatment, consisting

primarily of a pump, pipe, and an orifice plate, is simpler, easier to scale-up and of lower capital

and operating costs than that required for sonication (Gogate et al., 2001). In terms of operating

costs, Gogate et al. (2001) found the energy efficiency (ie. power dissipated in liquid/ electric

power supplied to system) of 2 L and 50 L orifice flow treatment apparatuses to be 54% and

60%, respectively, whereas that of two different ultrasonic horns used to treat 50 mL samples, a

500 mL ultrasonic bath, and a 1.5 L ultrasonic flow cell, were only 3%, 17%, 39%, and 43%,

respectively. Thus, orifice flow treatment appears to be more suitable for industrial-scale

mechanical sludge disintegration than sonication.

17

2.6 Assessment of Water Distribution in Sludge

There are several methods for determining the water distribution in sludge and they can be

classified into two main types: mechanical and thermal. Mechanical methods for determining the

water distribution in sludge, such as filtration, expression, and centrifugation, involve directly or

indirectly measuring the amount of water that is removed when certain amounts of mechanical

force is applied to the sludge (K.-W. R. Tsang, 1989). The water that remains in the sludge is

considered to be some form of bound water. The main disadvantage of mechanical methods for

determining the water distribution in sludge is that its effect on mechanical dewatering cannot be

investigated since it was measured by mechanical dewatering (K.-W. R. Tsang, 1989).

Thermal methods for determining the water distribution in sludge, such as drying, dilatometry,

differential scanning calorimetry (DSC), differential thermal analysis (DTA), and

thermogravimetric analysis (TGA), involve directly or indirectly measuring the amount of water

in the sludge that freezes or evaporates at certain temperatures (K.-W. R. Tsang, 1989). The

water that does not freeze or evaporate is considered to be some form of bound water.

Although the drying test takes longer than other thermal methods, it uses a larger sample size

than DSC/DTA/TGA, which use sample sizes between 8 to 35 mg that may not be representative

of the entire sample, and allows for further differentiation of bound water into interstitial, vicinal,

and hydration water (Chen, Hung, & Chang, 1997; Lee & Lee, 1995).

2.6.1 Drying Test

The drying test is a thermogravimetric analysis method for determining the bound water content

in sludge and involves drying a sludge sample at a constant temperature and humidity and

weighing the sample with time, until the mass of the sample remains constant.

Plotting the drying flux of the sludge as a function of its dry or normalized moisture content

yields a drying curve, similar to that shown in Figure 2.2 (Deng et al., 2011; K. Tsang &

Vesilind, 1990)

18

Figure 2.2. Typical Sludge Drying Curve

In Figure 2.2, three distinct drying periods can be identified, namely, the constant-rate drying

period, the first falling-rate drying period, and the second falling-rate drying period (Seader et

al., 2011). During the constant-rate drying period, water evaporates at the surface of the sample

as water within the sample travels to the surface by liquid diffusion (Seader et al., 2011). The

drying flux is constant and depends only on external conditions (K.-W. R. Tsang, 1989). The

water that evaporates during this period is considered free water.

During the first falling-rate drying period, water still evaporates at the surface of the sample.

However, the rate at which water travels from within the sample to the surface decreases, and the

surface is no longer saturated with water. As such, the drying flux decreases, often linearly, with

decreasing moisture content (Seader et al., 2011). The water that evaporates during this period is

considered interstitial water.

19

During the second falling-rate drying period, the surface of the sample is now dry and water

evaporates from within the pores and the vapour diffuses to the surface (Seader et al., 2011). The

drying flux decreases even more with decreasing moisture content until it becomes zero and the

equilibrium moisture content is reached (Seader et al., 2011). The water that evaporates during

this period is considered vicinal water and the water that remains is considered hydration water.

2.7 Assessment of Dewaterability


2.7.1.1 Specific Resistance to Filtration

Specific resistance to filtration (SRF) is a common measure of the rate of filtration of sludge

derived from filtration theory. During the filtration of sludge, a filter cake forms on the filter

medium, and the filtrate must pass through both the filter cake and filter medium. The rate at

which the filtrate passes through the filter cake and filter medium, 𝑑𝑉

𝑑𝑡 can be described by

Equation 3, which is derived from Darcy’s law for the flow of a fluid through a porous medium

(Christensen & Dick, 1985b).

𝑑𝑉

𝑑𝑡=

𝑃𝐴

𝜇(

1

(𝑅𝜔𝑉

𝐴 ) + 𝑅𝑚

)

Equation 3

where 𝑉 is the filtrate volume in m3, 𝑡 is the filtration time in s, 𝑃 is the filtration pressure in Pa,

𝐴 is the filtration area in m2, 𝜇 is the filtrate dynamic viscosity in Pas, 𝑅 is the SRF of the filter

cake in m/kg (ie. the resistance of the filter cake to the flow of filtrate per unit dry mass per unit

area of cake), 𝜔 is the dry mass of the filter cake per unit volume of filtrate in kg/m3, and 𝑅𝑚 is

the resistance of the filter medium to the flow of filtrate, in m-1

(Christensen & Dick, 1985b).

If 𝑃, 𝐴, 𝜇, 𝑟, 𝜔, and 𝑅𝑚 are constant, integration of Equation 3 gives:

𝑡

𝑉=

𝜇𝑅𝜔

2𝑃𝑇𝐴2𝑉 +

𝜇𝑅𝑚

𝑃𝑇𝐴

Equation 4

20

Plotting 𝑡

𝑉 as a function of 𝑉 yields a straight line with slope, 𝑏, from which 𝑅 can be calculated

using Equation 5

𝑅 =

2𝑃𝑇𝐴2𝑏

𝜇𝜔

Equation 5

Thus, determining SRF involves vacuum or pressure filtering sludge and measuring the volume

of the filtrate with time. A low SRF is indicative of a high rate of filtration and vice versa.

2.7.1.2 Capillary Suction Time

As the SRF test can be time-consuming, an empirical capillary suction time (CST) test was

developed as an alternative measure of the rate of filtration. The simple and quick CST test

involves pouring sludge into a cylinder placed on filter paper sandwiched between two plastic

blocks, in which the top block contains two or three electrodes (APHA, AWWA, & WEF, 1999).

The water in the sludge moves through the filter paper by capillary action (APHA et al., 1999).

CST is the time in seconds, as recorded by an automatic timer, for the water to move the distance

between the electrodes. A low CST is indicative of a high rate of dewatering.


The extent of dewatering can be assessed by dewatering the sludge samples using laboratory-

scale equipment such as pressure filters and centrifuges, and measuring the solids content of the

cake and filtrate. One device that is specifically designed for laboratory-scale dewatering is the

Crown Press (Figure 2.3), which simulates industrial belt filter press dewatering. The gravity

drainage zone of the belt filter press is simulated by gravity filtering sludge, using the Gravity

Drainage Kit that accompanies the Crown Press, producing a gravity filter cake. The low and

high pressure zones of the belt filter press are simulated by sandwiching the gravity filter cake

between the two belts of the Crown Press and by pulling the belts over the crown of the Crown

Press, with increasing amounts of pressure.

21

Figure 2.3. Crown Press (reprinted with permission)1

1 Reprinted from D.J. Bouchard, Evaluating wood fines as a physical conditioner for dewatering biosludge, Page 28,

Copyright (2015), with permission from D.J. Bouchard

22

Experimental Methods 3

3.1 Experimental Approach

The objectives of this study were to determine how the orifice flow treatment of biosludge

affects the dewaterability of biosludge and mixtures of biosludge, primary sludge, and/or

polymer and how it affects the dewaterability of biosludge in comparison to sonication. The

experimental approach (Figure 3.1) involved obtaining sludge samples, treating the biosludge

samples by orifice flow or sonication, adding primary sludge and/or polymer to the biosludge

samples if required, and assessing the changes in the particle size distribution, water distribution,

rate of dewatering, and extent of dewatering of the samples after treatment.

Figure 3.1. Experimental Approach

23

3.2 Sludge Samples

Pulp and paper sludge samples were obtained from a Canadian pulp and paper mill that produces

bleached chemi-thermo-mechanical pulp (BCTMP), paperboard, cellulose, and chemical

products. The mill has a central wastewater treatment plant (Figure 3.2) and two additional

primary clarifiers for the BCTMP and paperboard processes. As the bolded streams in Figure 3.2

show, WAS and primary sludge samples were obtained from a secondary clarifier in the central

wastewater treatment plant and the paperboard primary clarifier, respectively. The samples were

couriered from the mill to the laboratory in about 3 days.

Figure 3.2. Process Flow Diagram of Pulp and Paper Mill Central Wastewater Treatment

Plant

For comparative purposes, municipal sludge samples were also collected from Ashbridges Bay

Wastewater Treatment Plant (Figure 3.3) in Toronto, ON, CA. Unlike the pulp and paper mill,

Ashbridges Bay Wastewater Treatment Plant thickens and anaerobically digests WAS before

dewatering. Thus, both WAS and anaerobically digested sludge samples were obtained. As the

bolded streams in Figure 3.3 show, the WAS and anaerobically digested sludge samples were

obtained from one of the secondary clarifiers and one of the anaerobic digestion tanks,

respectively. The samples were driven from Ashbridges Bay Wastewater Treatment Plant to the

laboratory in about 30 minutes.

24

Figure 3.3. Process Flow Diagram of Ashbridges Bay Wastewater Treatment Plant

Upon arrival at the laboratory, samples were stored at 4°C until required for experimentation.

Biosludge samples were used within about 3 days of arriving at the laboratory to minimize

changes in properties due to biological activity. Primary sludge was stored for longer periods of

time as it is less biologically active than biosludge. Prior to use, samples stored at 4°C were

warmed to room temperature using a water bath and mixed at 60 rpm for 30 minutes using a PB-

900 Programmable JarTester (Phipps & Bird Inc., Richmond, VA, USA).

3.3 Sludge Treatment

3.3.1 Orifice Flow Treatment

The orifice flow treatment apparatus (Figure 3.4) consisted of a 20 L feed tank, a positive

displacement progressive cavity pump (Continental Ultra Pump, Warrenton, MO, USA), a

bypass valve, a pressure gauge, and an orifice plate made from a sheet of aluminum, all

connected by 20 mm-inner diameter plastic tubing and polyvinyl chloride piping.

25

A

B

Figure 3.4. A) Photo and B) schematic diagram of Orifice Flow Treatment Apparatus

26

Biosludge was orifice flow treated 7 L at a time, as this was the minimum sample volume

required for the pumping system to reach steady state and for sufficient amounts of orifice flow

treated biosludge to be collected. First, biosludge was sieved to a size less than 1.6 mm to

remove large particles that would clog the orifice. The sieved biosludge was poured into the feed

tank and the pump was turned on with the bypass valve fully open. The bypass valve was then

fully closed, forcing the biosludge through the orifice of the orifice plate. Once the system

achieved steady state, the orifice flow treated biosludge was collected for analysis. The pressure

gauge reading was recorded, and the time required to pump 2 L of biosludge through the

apparatus was measured to calculate the flowrate. From the orifice radius and the flowrate, the

strain rate of biosludge through the orifice was calculated using Equation 2.

To vary the strain rate, three different orifice plates with 0.8 mm, 1.2 mm, and 2.4 mm radius

orifices were used. A 0.8 mm radius orifice was the smallest orifice that would not clog. Varying

the orifice size also caused the flowrate of biosludge to vary and both variations were accounted

for in the calculating the strain rate.

3.3.2 Sonication Apparatus

Sonication was conducted using a custom 20 kHz ultrasonic reactor (Advanced Sonic Processing

Systems, Oxford, CT, USA). The reactor consisted of a 5.4 cm radius by 25 cm tall open acrylic

cylinder mounted on a water-cooled magnetostrictive transducer driven by a generator with a

maximum ultrasonic power of 450 W.

To deliver the same amount of energy to the pulp and paper WAS (P&P WAS) that was

delivered by orifice flow treatment at the maximum strain rate, which was produced by the 0.8

mm radius orifice, the sludge was sonicated in 400 mL batches at 200 W for 28.5 s, which

translates to an energy input of 460 kJ/kg DS (see Appendix A: Determination of Sonication

Parameters).

27

3.4 Primary Sludge and Polymer Addition

3.4.1 Primary Sludge Addition

Pulp and paper primary sludge was added to pulp and paper WAS (P&P WAS) in a 7:3 mass

ratio, the same ratio in which primary sludge and WAS are mixed at the mill. Primary sludge

was added to WAS in an Erlenmeyer flask and mixed for 30 s at high speed using a magnetic

stirrer with a 1.5 inch stir bar. The sludge mixture was aged for at least 30 s before being

analyzed.

3.4.2 Polymer Solution Preparation

Organopol 5400 (BASF Corporation, Charlotte, NC, USA), a cationic, high molecular weight

polymer used by the mill, was added to the pulp and paper WAS samples. Flo Polymer GB 1000

(SNF Canada Ltd., Trois Rivieres, QC, CA), a cationic, high charge density, and ultra-high

molecular weight polymer used by Ashbridges Bay Wastewater Treatment Plant, was added to

the municipal WAS and anaerobically digested sludge samples.

Organopol 5400 and Flo Polymer GB 1000 were received in powder form. 0.5 wt.% aqueous

solutions of each polymer were prepared by adding 0.5 g of polymer to 100 mL of distilled water

in a 250 mL Erlenmeyer flask. Using a magnetic stirrer and a 1.5 inch stir bar, the solution was

mixed at high speed for 30 minutes and aged for at least 30 minutes before use. The polymer

solution was used within 3 days.

3.4.3 Polymer Addition

Using a magnetic stirrer and a 1.5 inch stir bar, sludge samples of at least 100 mL were mixed in

an Erlenmeyer flask at the minimum speed that caused a vortex to form. The polymer solution

was pipetted into the vortex and mixing was continued for 30 s. The polymer-dosed sludge was

aged for at least 30 s before being analyzed.

28

The optimum polymer dose for each type of biosludge was identified using polymer dose tests.

Multiple aliquots of a sludge sample were treated with varying amounts of polymer, and the

capillary suction time (CST) was measured (Section 3.7.1.1). The optimum polymer dose was

identified as the dose that minimized the CST, and this dose was used for all subsequent tests on

polymer-dosed sludge.

3.5 Particle Size Distribution

The particle size distribution (PSD) of the sludge samples was determined using a Malvern

Mastersizer S with a Large Volume Dispersion Unit (Malvern Instruments Ltd., Worcestershire,

UK). PSD was used to determine the effectiveness of orifice flow treatment and sonication in

disintegrating the sludge flocs. The instrument passes a laser through a dispersed sample and

captures the scattering pattern, from which the volume-based size distribution is derived. A

300RF lens that covers a particle size range of 0.05 to 880 μm was used. Sludge sample was

added to tap water in the dispersion unit while being stirred at low speed to disperse the particles

without disrupting them. Sample was added until the laser obscuration was within the ideal range

of 10 to 30%. The Malvern software was configured to use the Fraunhofer model to derive the

volume based size distribution from the scattering pattern.

3.6 Water Distribution

Drying tests were conducted to determine the water distribution in the sludge. 5 mL of sludge

were pipetted into a 53 mm-diameter aluminum dish and the dish was suspended from a balance

into a furnace (PSH Kilns & Furnaces, Oakville, ON, CA) at 30°C. Compressed air was passed

through the furnace at 2 L/min to control the humidity. The mass of the sample was

automatically recorded by a laptop every 5 minutes. After approximately 16 hours the sample

mass reached equilibrium, and the TS of the sample was measured to determine the equilibrium

water content. The sample mass-time data and the equilibrium water content were then used to

determine the water distribution in the sample (see Appendix C: Determination of Water

Distribution). Drying tests were conducted in duplicate.

29

3.7 Assessment of Dewaterability


The rate of dewatering was assessed by measuring the CST and determining the specific

resistance to filtration (SRF) of the sludge samples.

3.7.1.1 Capillary Suction Time

CST was measured in accordance with Standard Method 2710 G (APHA, AWWA, & WEF,

1999) using a Type 304M CST apparatus with 7 x 9 cm CST Paper (Triton Electronics Ltd.,

Essex, England) and 3 mL samples. CST was measured in replicates of five.

3.7.1.2 Specific Resistance to Filtration

SRF was determined by pressure filtering sludge samples and recording the mass of the filtrate

with time. 1 ⅞ inch diameter Corrosion Resistant Type 304 Stainless Steel Wire Cloth Discs

with 0.003 inch diameter openings (McMaster-Carr Supply Company, Elmhurst, IL, USA) were

used as the filter medium. A 100 mL sample was poured into a filtration vessel and the vessel

was sealed. 30 s into the filtration, a pressure of 4.91 x 104 Pa was applied to the vessel using a

pump. This is a typical pressure used in determining SRF (Kavanagh, 1980). The filtrate was

collected in a container placed on an analytical balance that recorded the mass of the filtrate to a

laptop at one second intervals. The sample was pressure filtered for 30 minutes. At the end of the

30 minutes, the volume and total suspended solids (TSS) (Section 3.7.2.1) of the filtrate were

measured. The time-filtrate mass data and the volume and TSS of the filtrate were then used to

calculate the SRF of the sample (see Appendix B: Calculation of Specific Resistance to

Filtration). SRF was measured in triplicate.


The extent of dewatering was assessed by dewatering sludge samples by gravity filtration,

Crown Pressing, pressure filtration, and centrifugation and measuring the total solids (TS) of the

cakes and the TSS of the filtrates, pressate, and centrate.

30

3.7.2.1 Total suspended solids

TSS was measured in accordance with Standard Method 2540 D (APHA, AWWA, & WEF,

1999) using Whatman Grade 934-AH Glass Microfiber Filters (GE Healthcare Life Sciences,

Piscataway, NJ, USA) and was measured in triplicate.

3.7.2.2 Total Solids

TS was measured in accordance with Standard Method 2540 B (APHA, AWWA, & WEF, 1999)

and was measured in triplicate.

3.7.2.3 Gravity Filtration and Crown Pressing

Gravity filtration and Crown Pressing was conducted using a Crown Press and the accompanying

Gravity Drainage Kit (Phipps & Bird, Inc., Richmond, VA, USA), both supplied with a standard

polyester 6 x 2 herringbone satin weave filter medium with a thread count of 64 x 24 and an air

permeability of 390 ft3/min (Clear Edge Filtration, Tulsa, OK, USA).

250 mL sludge samples were gravity filtered for 10 min, with the filtrate being collected in a

graduated cylinder. Next, the gravity filter cake was placed between the belts of the Crown Press

and, using the handle, tension was applied to the belts at a Crown Press gauge reading of 100 lbf,

150 lbf, and 200 lbf for 30 s each with rapid releases in between. As calculated by Amin (2014),

Crown Press gauge readings of 100 lbf, 150 lbf, and 200 lbf corresponded to applied pressures of

8.9 psi, 12.6 psi, and 16.3 psi, and lineal belt tensions of 59.2 lb/in, 83.5 lb/in and 107.8 lb/in,

respectively.

The Crown Press pressate was collected with the gravity filtrate and the Crown Press Cake was

removed from the belts. The volume and TSS of the combined gravity filtrate and Crown Press

pressate were measured, as well as the TS of the gravity filtration and Crown Press Cakes.

Simulated belt-press tests were conducted in triplicate.

31

3.7.2.4 Pressure Filtration

In measuring SRF, the sludge samples were pressure filtered. In addition to the volume and TSS

of the filtrate, the TS of the filter cake was measured and used as indicators of the extent of

dewatering.

3.7.2.5 Centrifugation

30 mL samples were centrifuged in 50 mL tubes at 5,000 G and 20°C for 10 minutes, using a

Beckman Coulter Avanti J-E centrifuge with a J-Lite JLA-16.250 Fixed Angle Rotor (Beckman

Coulter, Inc., CA, USA). The centrate was decanted and TS of the centrifuged cake was

measured. TSS of the centrate was not measured because it was below the detection limit of 0.01

g/L. Centrifugation tests were conducted in triplicate.

3.8 Statistical Analysis

Statistical analysis of data was performed using GraphPad Prism 6 (GraphPad Software Inc., San

Diego, CA, USA). All reported error and error bars represent the standard deviation of the mean.

Regression analysis of data was performed using the method of least squares. Standard unpaired

t-tests were performed on the data sets of the untreated and treated samples. The statistical

significance of results were determined at the 95% confidence level.

32

Chapter 3

Results and Discussion 4

4.1 Sludge Storage

Amin (2014) and Bouchard (2015) previously confirmed that the pH, CST, and TSS of the pulp

and paper waste activated and primary sludge samples are preserved in the 3 days that they are

couriered from the mill to the laboratory and for at least a month when stored at 4°C. In this

study, biosludge samples were orifice flow treated within 1 day of arriving at the laboratory and

analyzed within the following 2 days. When not being analyzed, the samples were stored at 4°C.

Sludge storage at 4°C for 2 days did not change the median particle diameter of the

untreated (𝑝 = .565, 𝑟2 = .399) and treated (𝐸 = 29,670 𝑠−1)(𝑝 = .171, 𝑟2 = .930) P&P

WAS samples (Figure 4.1). Hence, it was assumed that the effects of orifice flow treatment on

the biosludge samples were preserved for at least 2 days when stored at 4°C.

Figure 4.1. Effect of time on median particle diameter of an untreated and a treated

(𝑬 = 𝟐𝟗, 𝟔𝟕𝟎 𝒔−𝟏) P&P WAS sample stored at 4°C

0 1 20

20

40

60

Time after orifice flow treatment (d)

Me

dia

n p

art

icle

dia

me

ter

(mm

)

Untreated Treated

33

4.2 Orifice Flow Treatment

Table 4.1 shows the flowrate and strain rate through the orifice for various biosludges and orifice

radii. Using the orifice plate with the 0.8 mm radius orifice, the smallest orifice that would not

clog, a maximum strain rate of 29,280 1060 s-1

, 34,540 s-1

, and 34,090 s-1

was achieved for

P&P WAS, municipal WAS, and municipal ADS, respectively. Using a similar orifice flow

treatment apparatus with an orifice plate with a 0.8 mm radius, Gibson et al. (2012) achieved a

maximum strain rate of about 40,000 s-1

for municipal mixed liquor. As expected, the maximum

strain rates for biosludge were slightly less than that for mixed liquor, as biosludge is generally

more viscous than mixed liquor.

Table 4.1. Biosludge Flowrate and Strain Rate through Orifice for Various Biosludges and

Orifice Radii

Biosludge Type Orifice Radius

(m)

Flowrate (m3/s) Strain rate (s

-

1)

Pulp and paper waste activated

sludge

8.0 x 10-4

4.709 x 10-5 1.70 x

10-6

29,280 1060

1.2 x 10-3

7.407 x 10-5

13,990

2.4 x 10-3

9.916 x 10-5

2,283

Municipal waste activated sludge 8.0 x 10-4

5.556 x 10-5

34,540

Municipal anaerobically digested

sludge

8.0 x 10-4

5.483 x 10-5

34,090

34

4.3 Particle Size Distribution

Orifice flow treatment at the maximum strain rate shifted the particle size distribution (PSD) of

P&P WAS, municipal WAS, and municipal ADS towards smaller particle diameters, indicating

that orifice flow treatment at strain rates of about 30,000 to 35,000 s-1

effectively disintegrates

biosludge flocs (Figure 4.2). However, sonication at an energy input of 460 kJ/ kg DS, which

was calculated to deliver the same amount of energy to P&P WAS as orifice flow treatment at a

strain rate of 29,280 s-1

, only slightly shifted the PSD of P&P WAS towards smaller particle

diameters, suggesting that orifice flow treatment is more effective in disintegrating biosludge

flocs than sonication. This conclusion is supported by Gogate et al. (2001), who found orifice

flow treatment to be not only more energy efficient than sonication, but also more effective in

generating cavitation.

4.3.1 Proportions of Settleable and Supracolloidal Particles

Particle size is known to have a large effect on sludge dewaterability, with smaller particles

generally worsening dewaterability. Karr & Keinath (1978) classified the particles in sludge

based on their particle size, as shown in Table 4.2. They found that wastewater sludges mostly

contain settleable and supracolloidal particles, with supracolloidal particles having the largest

effect on SRF, as they blind the filter medium and cake, thereby increasing SRF.

Table 4.2. Classification of Particles in Sludge based on Particle Size (Karr & Keinath,

1978)

Particle Class Particle Size (μm)

Settleable ≥100

Supracolloidal 1 to 100

True colloidal 0.001 to 1

Dissolved ≤0.001

35

Figure 4.2. Effect of treatment on particle size distribution. A) Orifice flow treated P&P

WAS, B) orifice flow treated municipal WAS, C) orifice flow treated municipal ADS (0.8

mm orifice radius), and D) sonicated P&P WAS (460 kJ/ kg DS)

36

Untreated P&P WAS consisted of 28 ± 10 𝑣𝑜𝑙. % settleable particles and 72 ± 10 𝑣𝑜𝑙. %

supracolloidal particles. The orifice flow treatment of P&P WAS at increasing strain rate

decreased the proportion of settleable particles to a plateau of 13 𝑣𝑜𝑙. %

(95% 𝐶𝐼, 0 𝑡𝑜 25 𝑣𝑜𝑙. %), and to a similar extent, increased the proportion of supracolloidal

particles to a plateau of 87 𝑣𝑜𝑙. % (95% 𝐶𝐼, 74 𝑡𝑜 99 𝑣𝑜𝑙. %) (Figure 4.3). Thus, the orifice flow

treatment of P&P WAS at increasing strain rate predominantly disintegrated settleable particles

into supracolloidal particles until the proportions of both plateaued. Similarly, Yong (2007),

Yuan & Farnood (2010), and Gibson et al. (2012) found that at increasing energy input, the

sonication, shear treatment, and orifice flow treatment of primary effluent, WAS, and mixed

liquor, respectively, disintegrated large flocs into small flocs until the proportions of both

plateaued, suggesting that wastewater flocs have a core that is difficult to disintegrate

mechanically.

The optimum strain rate for the disintegration of P&P WAS was about 14,000 s-1

(Figure 4.3).

Above this strain rate, no appreciable increase in the proportion of supracolloidal particles was

observed. This optimum strain rate is greater than that of 10,000 s-1

obtained by Gibson et al.

(2012) for the disintegration of mixed liquor. This was expected as WAS is generally more

viscous than mixed liquor.

Although to a lesser extent, the orifice flow treatment of P&P WAS at increasing strain rate also

decreased and increased to plateaus the proportion of settleable and supracolloidal particles,

respectively, in P&P WAS mixed with polymer and P&P WAS mixed with primary sludge

(Figure 4.3). Orifice flow treatment of P&P WAS at increasing strain rate, did not however,

significantly affect the proportions of settleable and supracolloidal particles in P&P WAS mixed

with both polymer and primary sludge.

37

Figure 4.3. Effect of orifice flow treatment of P&P WAS at various strain rates on

proportions of settleable and supracolloidal particles in P&P WAS with and without

polymer (0.815 kg/ tonne DS) and/or primary sludge (7:3 primary sludge to WAS mass

ratio)

Orifice flow treatment at the maximum strain rate increased the proportion of supracolloidal

particles in P&P WAS, municipal WAS, and municipal ADS by 20 % (𝑝 = .04), 54 %, and 12

%, respectively (Figure 4.4). Possible explanations for which orifice flow treatment at the

maximum strain rate increased the proportion of supracolloidal particles in municipal WAS to a

greater extent than in P&P WAS and municipal ADS are:

1) Municipal WAS (𝐷50 = 83.19 𝜇𝑚) contained larger particles than P&P WAS (𝐷50 =

66.30 ± 15.30 𝜇𝑚) and municipal ADS (𝐷50 = 47.30 𝜇𝑚), and larger particles have been

found to disintegrate more readily than smaller ones when subjected to orifice flow treatment

(Gibson et al., 2012).

0 10000 20000 300000

20

40

60

80

100

Strain rate (s-1)

Pro

po

rtio

n o

f p

art

icle

s (v

ol.%

)

P&P WAS P&P WAS+pol

P&P WAS+prim P&P WAS+prim+pol

Supracolloidal particles Settleable particles

38

2) As shown in Table 4.1, in pumping the sludge through a 0.8 mm radius orifice, municipal

WAS experienced a higher strain rate than P&P WAS and municipal ADS, and strain rate

has been found to correlate positively with the extent of particle disintegration (Gibson et al.,

2012).

3) Municipal WAS may be of a composition such that it disintegrates more readily than P&P

WAS and municipal ADS.

As previously discussed, orifice flow treatment appears to be more effective in disintegrating

biosludge flocs than sonication. Orifice flow treatment at the maximum strain rate increased the

proportion of supracolloidal particles in P&P WAS by 20 % (𝑝 = .04), while sonication at the

same energy output increased the proportion of supracolloidal particles in P&P WAS by only 0.6

% (Figure 4.4).

Figure 4.4. Effect of orifice flow treatment (0.8 mm orifice radius) and sonication (460 kJ/

kg DS) on proportion of supracolloidal particles (mean ± SD) in various biosludge samples

Orfi

ce fl

ow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

20

40

60

80

100

120

Pro

po

rtio

n o

f s

up

rac

ollo

ida

l

pa

rtic

les

(v

ol.%

)

UntreatedTreated

39

4.4 Water Distribution

To determine the effect of orifice flow treatment on the water distribution in biosludge, drying

tests were conducted on the untreated and treated samples. Appendix C shows how the drying

test results were used to determine the water distribution of the biosludge samples. Orifice flow

treatment at the maximum strain rate and sonication at the same energy output did not

significantly affect the free, interstitial, vicinal, or hydration water contents of P&P WAS,

municipal WAS and municipal ADS (Figure 4.5).

Orifice flow treatment at the maximum strain rate and sonication at the same energy output

seemed to have decreased the free water content and generally increased the interstitial, vicinal,

and hydration water contents of P&P WAS, municipal WAS, and municipal ADS. These

decreases and increases, however, were not statistically significant. Nonetheless, these results

suggest that at higher energy inputs, the orifice flow treatment and sonication of biosludge would

convert more free water into interstitial, vicinal, and hydration water, than interstitial water into

free water, which is contrary to the hypothesis of this study. Thus, the effect of free water

becoming trapped within and binding to the smaller, more numerous disintegrated flocs appears

to outweigh that of the interstitial water being released from the flocs upon disintegration.

40


kg DS) on A) free, B) interstitial, and C) vicinal and hydration water contents (mean ± SD)

of various biosludge samples.

Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

20

40

60

80

100

No

rma

lize

d d

ry fre

e w

ate

r c

on

ten

t (w

t.%

)

A

Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

5

10

15

No

rma

lize

d d

ry in

ters

titia

l wa

ter

co

nte

nt (w

t.%

)

B

Orfi

ce fl

ow

trea

ted

P&P

WAS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

2

4

6

8

No

rma

lize

d d

ry v

icin

al a

nd

h

yd

ratio

n w

ate

r c

on

ten

t (w

t.%

)

C

Untreated Treated

41

The drying test results can also be used to calculate the maximum cake solids content that can be

achieved by mechanical dewatering for untreated P&P WAS, municipal WAS, and municipal

ADS (Tsang, 1989). Figure 4.6 shows the calculated cake solids content as a function of water

removed for untreated P&P WAS, municipal WAS, and municipal ADS. It can be seen that

removing the free and interstitial water, which constituted 98.6, 98.8, and 96.1 wt.% of the total

water, increased the cake solids content to 53, 39, and 33 wt.% for P&P WAS, municipal WAS,

and municipal ADS, respectively. These cake solids contents can be considered as the maximum

cake solids contents that can be achieved by the mechanical dewatering of these biosludges.

Thus, theoretically, mechanical dewatering can achieve the minimum cake solids content of 40

wt.% for self-sustained combustion for P&P WAS, but not for municipal WAS and municipal

ADS.

In practice, with the addition of primary sludge, polymer, and some other dewatering aids, the

pulp and paper mill and Ashbridges Bay Wastewater Treatment Plant mechanically dewater P&P

WAS and municipal ADS to solids contents of about 30 and 28 wt.%, respectively (Figure 3.2

and Figure 3.3). This level of dewatering corresponds to the complete removal of free water, and

the removal of 65 and 89 wt.% of the interstitial water for P&P WAS and municipal ADS,

respectively. If the remaining interstitial water, which constitutes only 2.3 and 1.2 wt.% of the

total water, is removed, cake solids contents of 53 and 33 wt.% could be achieved for P&P WAS

and municipal ADS, respectively. Although orifice flow treatment and sonication do not appear

to convert interstitial water into free water, in the case of P&P WAS, attempting to remove more

interstitial water by mechanical dewatering appears to be a worthy effort, as unlike municipal

ADS, its maximum achievable cake solids content for mechanical dewatering is well above the

minimum cake solids content of 40 wt.% for self-sustainable combustion.

42

Figure 4.6. Cake solids content as a function of water removed. A) P&P WAS, B) municipal

WAS, and C) municipal ADS. Open circle symbols represent critical solids contents. Lines

a) free water removed, b) free + interstitial water removed, c) free + interstitial + vicinal

water removed, d) free + interstitial + vicinal water + hydration water removed, e) current

cake solids content achieved, and f) minimum cake solids content for self-sustainable

combustion

0 50 1000

20

40

60

80

100

Water removed (wt.%)

Ca

ke

so

lid

s c

on

ten

t (w

t.%

)

A

a

e

f

b

c

d

0 50 1000

20

40

60

80

100


Ca

ke

so

lids

co

nte

nt (w

t.%

)

B

a

e

fb

c

d

0 50 1000

20

40

60

80

100


Ca

ke

so

lid

s c

on

ten

t (w

t.%

)

C

a

e

fb

cd

43

4.5 Polymer Demand

Polymer dose tests were conducted on untreated and treated biosludge samples to determine the

optimum polymer dose, i.e. the polymer dose required to minimize CST, as well as the effect of

orifice flow treatment on the optimum polymer dose and minimum CST. In general, by

increasing the polymer dose, CST first decreased and reached a minimum value before

increasing again (Figure 4.7). Furthermore, orifice flow treatment at the maximum strain rate and

sonication at the same energy output appeared to increase the optimum polymer dose and the

minimum CST. However, except for the increase in the minimum CST of the orifice flow treated

P&P WAS, the increases in optimum polymer dose and minimum CST were not statistically

significant. Nonetheless, these increases suggest that at higher energy input, orifice flow

treatment and sonication would worsen dewaterability by decreasing the rate of dewatering and

increasing the polymer demand. This conclusion is supported by Kopp (1997) and Dewil et al.

(2006) who found that the mechanical disintegration of WAS increased the polymer demand. In

the present study, the optimum polymer dose for the untreated samples was used for all

subsequent tests on untreated and treated polymer-dosed samples.

44

Figure 4.7. Effect of treatment on polymer dose curve. A) Orifice flow treated P&P WAS,

B) orifice flow treated municipal WAS, C) orifice flow treated municipal ADS (0.8 mm

orifice radius), and D) sonicated P&P WAS (460 kJ/ kg DS)

45

4.6 Dewaterability Results

The effect of orifice flow treatment on the rate of dewatering was assessed by measuring the

CST and determining the SRF. The effect of orifice flow treatment on the extent of dewatering

was assessed by dewatering sludge samples by gravity filtration, Crown Pressing, pressure

filtration, and centrifugation and measuring the total solids (TS) of the cakes and the TSS of the

filtrates, pressate, and centrate. The CST, SRF, cake solids content, and filtrate/pressate/centrate

solids content results are presented and discussed in this section.

4.6.1 Capillary Suction Time

Orifice flow treatment of P&P WAS at increasing strain rate linearly increased the CST of P&P

WAS (𝑝 < .001, 𝑟2 = .42), but did not significantly affect the CST of P&P WAS with polymer

and/or primary sludge (Figure 4.8). These increases in the CST of P&P WAS are indicative of a

decrease in the rate of filtration and are consistent with the increases in the proportion of

supracolloidal particles in P&P WAS that were observed at strain rates up to 29,280 1060 s-1

,

as supracolloidal particles have been found to decrease the rate of filtration by blinding the filter

medium and cake (Karr & Keinath, 1978).

Orifice flow treatment at the maximum strain rate increased the CST of P&P WAS, municipal

WAS, and municipal ADS by 55, 32, and 62 %, respectively, while sonication at the same

energy output increased the CST of P&P WAS by 60 % (𝑝 < .001) (Figure 4.9).

46

Figure 4.8. Effect of orifice flow treatment of P&P WAS at various strain rates on CST

(mean ± SD) of P&P WAS with and without polymer (0.815 kg/ tonne DS) and/or primary

sludge (7:3 primary sludge to WAS mass ratio)


kg DS) on CST (mean ± SD) of various biosludge samples

0 10000 20000 300000

5

10

15

20

25

Strain rate (s-1)

CS

T (s

)

WaterP&P WAS P&P WAS+pol


Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

50

100

150

200

CS

T (s

)

UntreatedTreated

47

4.6.2 Specific Resistance to Filtration

Appendix B shows how the pressure filtration results were used to calculate the SRF of the

biosludge samples. Orifice flow treatment of P&P WAS at increasing strain rate linearly

increased the SRF of P&P WAS (𝑝 < .001, 𝑟2 = .73), P&P WAS with polymer (𝑝 =

.001, 𝑟2 = .81), P&P WAS with primary sludge (𝑝 = .028, 𝑟2 = .43), and P&P WAS with

primary sludge and polymer (𝑝 < .001, 𝑟2 = .74) (Figure 4.10). These increases in SRF are

indicative of a decrease in the rate of filtration and are consistent with the increases in the

proportion of supracolloidal particles in P&P WAS with and without polymer and primary

sludge that were observed at strain rates up to 29,280 1060 s-1

. Unlike CST, the detrimental

effect of orifice flow treatment on SRF persisted even after the addition of primary sludge and/or

polymer. The orifice flow treatment of pulp and WAS may not have affected the CST of P&P

WAS with primary sludge and/or polymer because CST is less sensitive to changes in the

proportion of supracolloidal particles than SRF (Karr & Keinath, 1978).

Figure 4.10. Effect of orifice flow treatment of P&P WAS at various strain rates on SRF

(mean ± SD) of P&P WAS with and without polymer (0.815 kg/ tonne DS) and/or primary

sludge (7:3 primary sludge to WAS mass ratio)

0 10000 20000 30000109

1010

1011

1012

1013

Strain rate (s-1)

SR

F (m

/kg

)

P&P WAS+prim+polP&P WAS+prim

P&P WAS+polP&P WAS

48

Orifice flow treatment at the maximum strain rate increased the SRF of P&P WAS and

municipal ADS by 103 and 139 %, respectively, and decreased the SRF of municipal WAS by

94 %, while sonication at the same energy output increased the SRF of P&P WAS by 48 %

(𝑝 < .001) (Figure 4.11). As previously discussed in Section 4.3.1, orifice flow treatment at the

maximum strain rate increased the proportion of supracolloidal particles in municipal WAS to a

greater extent than in P&P WAS and municipal ADS. Thus, an explanation for the decrease in

the SRF of municipal WAS is that the orifice flow treatment of municipal WAS at the maximum

strain rate disintegrated the flocs to such a great extent that instead of blinding the filter medium

and cake, most passed through the filter medium (pore diameter = 76 μm), providing little

resistance to filtration.


kg DS) on SRF (mean ± SD) of various biosludge samples

Orfi

ce fl

ow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

2×1012

4×1012

6×1012

SR

F (m

/kg

)

UntreatedTreated

49

4.6.3 Gravity Filter Cake Solids Content

Orifice flow treatment of P&P WAS at increasing strain rate linearly decreased the gravity filter

cake solids content of P&P WAS (𝑝 = .020, 𝑟2 = .30), but did not significantly affect the

gravity filter cake solids content of P&P WAS with polymer and/or primary sludge (Figure

4.12). These decreases in the gravity filter cake solids content of P&P WAS are indicative of a

decrease in the extent of dewatering and are consistent with the slight decrease in the free water

content of P&P WAS that was observed at the maximum strain rate, as well as the increases in

CST and SRF that were observed at strain rates up to 29,280 1060 s-1

for P&P WAS, as at a

lower rate of dewatering, a lower cake solids content will be achieved in the same amount of

time. The detrimental effects of orifice flow treatment on the gravity filter cake solids content of

P&P WAS were almost completely masked by the beneficial effect of primary sludge and/or

polymer addition.

Figure 4.12. Effect of orifice flow treatment of P&P WAS at various strain rates on gravity

filter cake solids content (mean ± SD) of P&P WAS with and without polymer (0.815 kg/

tonne DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)

0 10000 20000 300000

2

4

6

8

Strain rate (s-1)

Gra

vity

filte

r c

ak

e s

olid

s

co

nte

nt (w

t.%

)

P&P WAS P&P WAS+pol


50

Orifice flow treatment at the maximum strain rate decreased the gravity filter cake solids content

of P&P WAS by 12 % (𝑝 = .019) and did not significantly affect that of municipal ADS, while

sonication at the same energy output did not significantly affect the gravity filter cake solids

content of P&P WAS (Figure 4.13). Although not significantly, orifice flow treatment at the

maximum strain rate slightly increased the gravity filter cake solids content of municipal ADS.

An explanation for this increase is that orifice flow treatment at the maximum strain rate

disintegrated municipal ADS flocs to such a great extent that most of them passed through the

filter medium (pore diameter > 76 μm), increasing the rate of gravity filtration, and thus the

gravity filter cake solids content, as at a higher rate of dewatering, a higher cake solids content

will be achieved in the same amount of time. Nonetheless, these increases in the rate of gravity

filtration and the gravity filter cake solids content were at the cost of a great increase in the

gravity filtrate solids content of municipal ADS, as to be discussed in Section 4.6.7. The effect

described above was more pronounced in the case of municipal WAS, such that the treated

sample did not form an appreciable amount of gravity filter cake for analysis.


kg DS) on gravity filter cake solids content (mean ± SD) of various biosludge samples

Orfi

ce fl

ow

trea

ted

P&P W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P

WAS

0

1

2

3

4

Gra

vity

filte

r c

ak

e s

olid

s

co

nte

nt (w

t.%

)

UntreatedTreated

51

4.6.4 Crown Press Cake Solids Content

Orifice flow treatment of P&P WAS at increasing strain rate decreased the Crown Press cake

solids content of P&P WAS without and with polymer to a plateau of 6.4 wt.%

(95% 𝐶𝐼, 5.3 𝑡𝑜 7.4 𝑤𝑡. %) and 7.7 wt.%, (95% 𝐶𝐼, 5.1 𝑡𝑜 10.4 𝑤𝑡. %) respectively, but did not

significantly affect the Crown Press Cake solids content of P&P WAS and primary sludge with

and without polymer (Figure 4.14). These decreases in the Crown Press cake solids content of

P&P WAS are indicative of a decrease in the extent of dewatering and are consistent with the

slight decrease in the free water content of P&P WAS that was observed at the maximum strain

rate, as well as the increases in CST and SRF that were observed at strain rates up to 29,280

1060 s-1

for P&P WAS, as at a lower rate of dewatering, a lower cake solids content will be

achieved in the same amount of time. The detrimental effects of orifice flow treatment on the

Crown Press cake solids content of P&P WAS were completely masked by the beneficial effect

of primary sludge addition.

Figure 4.14. Effect of orifice flow treatment of P&P WAS at various strain rates on Crown

Press cake solids content (mean ± SD) of P&P WAS with and without polymer (0.815 kg/

tonne DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)

0 10000 20000 300000

5

10

15

20

25

Strain rate (s-1)

Cro

wn

pre

ss

ca

ke

so

lids

co

nte

nt (w

t.%

)

P&P WAS P&P WAS+pol


52

Orifice flow treatment at the maximum strain rate and sonication at the same energy output

decreased the Crown Press cake solids content of P&P WAS by 22 % (𝑝 = .031) and 21 %

(𝑝 = .015), respectively (Figure 4.15). While orifice flow treatment at the maximum strain rate

decreased the Crown Press cake solids content of P&P WAS, it disintegrated the municipal WAS

and ADS flocs to such a great extent that most passed through the gravity filter, not forming

enough gravity filter cake for Crown Pressing.


kg DS) on Crown Press cake solids content (mean ± SD) of various biosludge samples

Orfi

ce fl

ow

trea

ted

P&P

WAS

Son

icat

ed

P&P W

AS

0

5

10

15

Cro

wn

Pre

ss

ca

ke

so

lid

s

co

nte

nt (w

t.%

)

UntreatedTreated

53

4.6.5 Pressure Filter Cake Solids Content

Orifice flow treatment of P&P WAS at increasing strain rate linearly decreased the pressure filter

cake solids content of P&P WAS (𝑝 < .001, 𝑟2 = .92), but did not significantly affect the

pressure filter cake solids content of P&P WAS with polymer and/or primary sludge (Figure

4.16). These decreases in the pressure filter cake solids content of P&P WAS are indicative of a

decrease in the extent of dewatering and are consistent with the slight decrease in the free water

content of P&P WAS that was observed at the maximum strain rate, as well as the increases in

CST and SRF that were observed at strain rates up to 29,280 1060 s-1

for P&P WAS, as at a

lower rate of dewatering, a lower cake solids content will be achieved in the same amount of

time. The detrimental effects of orifice flow treatment on the pressure filter cake solids content

of P&P WAS were completely masked by the beneficial effect of primary sludge and/or polymer

addition.


pressure filter cake solids content (mean ± SD) of P&P WAS with and without polymer

(0.815 kg/ tonne DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)

0 10000 20000 300000

10

20

30

40

Strain rate (s-1)

Pre

ss

ure

filte

r c

ak

e s

olid

s

co

nte

nt (w

t.%

)

P&P WAS P&P WAS+pol


54

Orifice flow treatment at the maximum strain rate decreased the pressure filter cake solids

content of P&P WAS by 68 % (𝑝 < .001) and did not significantly affect that of municipal

WAS and municipal ADS, while sonication at the same energy output decreased the pressure

filter cake solids content of P&P WAS by 25 % (𝑝 = .019) (Figure 4.17). Although not

significantly, orifice flow treatment at the maximum strain rate slightly increased the pressure

filter cake solids content of municipal WAS. An explanation for this increase is that orifice flow

treatment at the maximum strain rate disintegrated municipal WAS flocs to such a great extent

that most of them passed through the filter medium (pore diameter = 76 μm), increasing the rate

of pressure filtration (ie. the SRF), and thus the pressure filter cake solids content, as at a higher

rate of dewatering, a higher cake solids content will be achieved in the same amount of time.


kg DS) on pressure filter cake solids content (mean ± SD) of various biosludge samples

Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

5

10

15

20

Pre

ss

ure

filt

er

ca

ke

so

lids

co

nte

nt (w

t.%

)

UntreatedTreated

55

4.6.6 Centrifuge Cake Solids Content

Orifice flow treatment of P&P WAS at increasing strain rate linearly increased the centrifuge

cake solids content of P&P WAS (𝑝 < .001, 𝑟2 = .56), P&P WAS and primary sludge ( 𝑝 =

.030, 𝑟2 = .39), and P&P WAS and primary sludge with polymer (𝑝 = .006, 𝑟2 = .55), but did

not significantly affect that of P&P WAS with polymer (Figure 4.18). These increases in

centrifuge cake solids content are indicative of an increase in the extent of dewatering and can be

explained by the disintegration of flocs into smaller flocs and particles that were more

compactible and thus, formed a denser cake that contained less water (Erdincler & Vesilind,

2000). The beneficial effect of orifice flow treatment on centrifuge cake solids content was

generally maintained with the addition of primary sludge and/or polymer.


centrifuge cake solids content (mean ± SD) of P&P WAS with and without polymer (0.815

kg/ tonne DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)

0 10000 20000 300000

2

4

6

8

10

Strain rate (s-1)

Ce

ntr

ifu

ge

ca

ke

so

lids

co

nte

nt

(wt.%

)

P&P WAS P&P WAS+pol


56

Orifice flow treatment at the maximum strain rate increased the centrifuge cake solids content of

P&P WAS and municipal WAS by 10 % (𝑝 < .001) and 15 % (𝑝 = .016), respectively, and

decreased that of municipal ADS by 8 % (𝑝 = .008), while sonication at the same energy output

did not significantly affect the centrifuge cake solids content of P&P WAS (Figure 4.19). A

possible explanation for this decrease in the centrifuge cake solids content of municipal ADS is

the slight decrease in free water content that was observed, as well as a potential decrease in the

rate of dewatering by centrifugation caused by the smaller flocs and particles that hinder the

movement of water.


kg DS) on centrifuge cake solids content (mean ± SD) of various biosludge samples

Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

2

4

6

8

10

Ce

ntr

ifu

ge

ca

ke

so

lids

co

nte

nt (w

t.%

)

UntreatedTreated

57

Unlike municipal ADS, it appears that the increase in the compactibility of pulp and paper WAS

and municipal WAS outweighed the decreases in its free water content and rate of dewatering,

leading to an overall increase in centrifuge cake solids content. In fact, the rate of filtration of

pulp and paper and municipal WAS were substantially greater than that of municipal ADS. As

such, in centrifuging the pulp and paper and municipal WAS samples at 5,000 G for 10 minutes,

the rate of centrifugation may have been high enough to begin with that even at a lower rate of

centrifugation, the same cake solids content would have been achieved when dewatering samples

using the same amount of mechanical force for the same amount of time. Therefore, it appears

that at sufficiently high rates of centrifugation, orifice flow treatment increases the centrifuge

cake solids content by disintegrating the flocs into smaller flocs and particles that are more

compactible. The increases in the centrifuge cake solids content of P&P and municipal WAS,

which were accompanied by slight decreases in free water content, are supported by Erdincler &

Vesilind (2000) who found that sludge disintegration by alkali treatment, NaCl treatment,

sonication, and heat treatment, all decreased the free water content measured by DSC, but

increased the centrifuge cake solids content.

The orifice flow treatment of P&P WAS at the maximum strain rate of 29,280 1060 s-1

increased the centrifuge cake solids content by 10 ± 4 % or 0.67 ± 0.25 percentage points. The

sonication of P&P WAS at the same energy output as orifice flow treatment at the maximum

strain rate, which translates to an energy input of 460 kJ/kg DS, increased the solids content of

the cake obtained by centrifugation at 5,000 G for 10 minutes by 14 ± 6 %. Erdincler & Vesilind

(2000) and Na et al. (2007) found that the sonication of simulated WAS and municipal ADS at

32,000 kJ/kg DS and 17,000-670,000 kJ/kg DS increased the solids content of the cake obtained

by centrifugation at 2,800 G for 30 minutes and 3000 rpm for 60 minutes by 87% and 50-267%,

respectively. As centrifuge cake solids content appears to increase with increasing sonication

energy, an orifice flow treatment apparatus that can achieve a greater maximum strain than that

used in this study should be used to determine how great of an increase in centrifuge cake solids

content can be achieved by orifice flow treatment, how this increase will translate to industrial

centrifugation, and if this increase will be industrially significant.

58

4.6.7 Combined Gravity Filtrate and Crown Press Pressate Solids Content

Orifice flow treatment of P&P WAS at increasing strain rate linearly increased the combined

gravity filtrate and Crown Press pressate TSS of P&P WAS (𝑝 = .029, 𝑟2 = .26), but did not

significantly affect that of P&P WAS with polymer and/or primary sludge (Figure 4.20). These

increases in the combined gravity filtrate and Crown Press pressate TSS are consistent with the

decreases in the proportion of settleable particles and increases in the proportion of

supracolloidal particles that were observed at strain rates up to 29,280 1060 s-1

for P&P WAS,

as smaller supracolloidal particles (0 𝜇𝑚 < 𝑑 < 100 𝜇𝑚) are more likely to pass through the

filter medium (pore diameter > 76 μm) than larger settleable particles (𝑑 ≥ 100 𝜇𝑚).


combined gravity filtrate and Crown Press pressate TSS (mean ± SD) of P&P WAS with

and without polymer (0.815 kg/ tonne DS) and/or primary sludge (7:3 primary sludge to

WAS mass ratio)

0 10000 20000 300000

2

4

6

Strain rate (s-1)

Co

mb

ine

d g

rav

ity

filtr

ate

an

d

Cro

wn

Pre

ss

pre

ss

ate

TS

S (g

/L)

P&P WAS P&P WAS+pol


59

Orifice flow treatment at the maximum strain rate increased the combined gravity filtrate and

Crown Press pressate TSS of P&P WAS, municipal WAS, and municipal ADS by 54 % (𝑝 =

.015), 67 % (𝑝 = .016), and 396 % (𝑝 < .001), respectively, while sonication at the same

energy output increased the combined gravity filtrate and Crown Press pressate TSS of P&P

WAS by 115 % (𝑝 = .003) (Figure 4.21). An explanation for the greater increase in the

combined gravity filtrate and Crown Press pressate TSS of municipal ADS is that, as previously

discussed, orifice flow treatment at the maximum strain rate disintegrated municipal ADS flocs

to such a great extent that instead of blinding the filter medium and cake, most passed through

the filter medium (pore diameter > 76 μm).


kg DS) on combined gravity filtrate and Crown Press pressate TSS (mean ± SD) of various

biosludge samples

Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

5

10

15

20

25

Co

mb

ine

d g

rav

ity

filtr

ate

an

d

Cro

wn

Pre

ss

pre

ss

ate

TS

S (g

/L)

UntreatedTreated

60

4.6.8 Pressure Filtrate Solids Content

Orifice flow treatment of P&P WAS at increasing strain rate linearly increased the pressure

filtrate TSS of P&P WAS (𝑝 = .014, 𝑟2 = .36), but did not significantly affect that of P&P

WAS with polymer and/or primary sludge (Figure 4.22). These increases in the pressure filtrate

TSS are consistent with the decreases in the proportion of settleable particles and increases in the

proportion of supracolloidal particles that were observed at strain rates up to 29,280 1060 s-1

for P&P WAS, as smaller supracolloidal particles (0 𝜇𝑚 < 𝑑 < 100 𝜇𝑚) are more likely to pass

through the filter medium (pore diameter = 76 μm) than larger settleable particles (𝑑 ≥

100 𝜇𝑚).


pressure filtrate TSS (mean ± SD) of P&P WAS with and without polymer (0.815 kg/ tonne

DS) and/or primary sludge (7:3 primary sludge to WAS mass ratio)

0 10000 20000 30000

0

1

2

Strain rate (s-1)

Pre

ss

ure

filt

rate

TS

S (g

/L)

P&P WAS P&P WAS+pol


61

Orifice flow treatment at the maximum strain rate did not significantly affect the pressure filtrate

TSS of P&P WAS and municipal ADS but increased that of municipal WAS by 736 % (𝑝 <

.001), while sonication at the same energy output increased the pressure filtrate TSS of P&P

WAS by 356 % (𝑝 = .001) (Figure 4.23). An explanation for the greater increase in the pressure

filtrate TSS of municipal WAS is that, as previously discussed, orifice flow treatment at the

maximum strain rate disintegrated municipal WAS flocs to such a great extent that instead of

blinding the filter medium and cake, most passed through the filter medium (pore diameter = 76

μm).


kg DS) on pressure filtrate TSS (mean ± SD) of various biosludge samples

4.6.9 Centrate Solids Content

As previously mentioned in Section 3.7.2.5 the centrate TSS was not measured, as it was below

the detection limit of 0.01 g/L for both the untreated and treated samples. Further studies should

be conducted to determine the effect of orifice flow treatment on centrate solids content.

Orfi

ce flow

trea

ted

P&P W

AS

Orif

ce flow

treat

ed m

un. W

AS

Orif

ice

flow

treat

ed m

un. A

DS

Son

icat

ed

P&P W

AS

0

5

10

15

20

25

Pre

ss

ure

filt

rate

TS

S (g

/L) Untreated

Treated

62

4.7 Summary of Filterability Results

Table 4.3 summarizes effect of orifice flow treatment on pulp and paper WAS, municipal WAS,

and municipal ADS filterability.

Table 4.3. The Effect of Orifice Flow Treatment on Pulp and Paper WAS, Municipal WAS,

and Municipal ADS Filterability

Pulp and Paper

WAS

Municipal WAS Municipal ADS

CST Increase Increase Increase

SRF Increase Decrease Increase

Gravity filter cake solids

content

Decrease When treated, not

enough cake formed

for analysis

Slight increase (not

significant)

Crown Press cake solids

content

Decrease When treated, not

enough cake formed

for analysis

When treated, not

enough cake formed

for analysis

Pressure filter cake solids

content

Decrease Slight increase (not

significant)

Slight decrease (not

significant)

Combined gravity filtrate

and Crown Press pressate

solids content

Increase Increase Substantial increase

Pressure filtrate cake

solids content

Slight increase

(not significant)

Substantial increase Slight increase (not

significant)

4.7.1 Pulp and Paper WAS

Orifice flow treatment of P&P WAS at strain rates up to 29,280 1060 s-1

increased the CST and

SRF, decreased the gravity filter, Crown Press, and pressure filter cake solids contents, and

increased the gravity filtrate, Crown Press pressate, and pressure filtrate solids contents, thereby

decreasing the rate and extent of filtration and worsening filterability. The increases in CST and

SRF are consistent with the increases in the proportion of supracolloidal particles in P&P WAS

that were observed at strain rates up to 29,280 1060 s-1

. The decreases in gravity filter, Crown

Press, and pressure filter cake solids contents are consistent with the slight decrease in the free

water content of P&P WAS that was observed at the maximum strain rate and with the decreases

in the rate of dewatering that were observed at strain rates up to 29,280 1060 s-1

for P&P WAS.

The increases in gravity filtrate, Crown Press pressate, and pressure filtrate solids content are

63

consistent with the decreases in the proportion of settleable particles and increases in the

proportion of supracolloidal particles that were observed at strain rates up to 29,280 1060 s-1

.

4.7.2 Municipal WAS

Orifice flow treatment of municipal WAS at the maximum strain rate increased the CST and

decreased the SRF; caused the insufficient formation of gravity filter and Crown Press cake for

analysis, and increased the pressure filter cake solids content; and increased the combined

gravity filtrate and Crown Press pressate and pressure filtrate solids contents. An explanation for

these results is that the orifice flow treatment of municipal WAS at the maximum strain rate

disintegrated the flocs to such an extent that they blinded the CST filter (pore diameter = 8 µm),

but passed through the SRF/pressure filter (pore diameter = 76 µm), and gravity filter/Crown

Press belt (pore diameter > 76 µm). As a result of the disintegrated flocs blinding the CST filter,

the rate of dewatering by CST filtration decreased (ie. CST increased). In contrast, as a result of

the disintegrated flocs passing through the SRF/pressure filter, gravity filter, and Crown Press

belt, the rate of dewatering by pressure filtration (ie. SRF decreased), gravity filtration and

Crown Pressing increased, but the gravity filtrate, Crown Press pressate, and pressure filtrate

solids contents substantially increased. The increases in the rates of dewatering by gravity

filtration, Crown Pressing, and pressure filtration translated into increases in the gravity filter,

Crown Press, and pressure filter cake solids contents, as at a higher rate of dewatering, a higher

cake solids content will be achieved in the same amount of time.

4.7.3 Municipal ADS

Orifice flow treatment of municipal WAS at the maximum strain rate increased the CST and

SRF; increased the gravity filter cake solids content, caused the insufficient formation of Crown

Press cake for analysis, and decreased the pressure filter cake solids content; and increased the

combined gravity filtrate and Crown Press pressate and pressure filtrate solids contents. An

explanation for these results is that the orifice flow treatment of Municipal ADS at the maximum

strain rate disintegrated the flocs to such an extent that they blinded the CST filter and

SRF/pressure filter, but passed through the gravity filter and Crown Press belt. As a result of the

64

disintegrated flocs blinding the CST filter and SRF/pressure filter, the rate of dewatering by CST

filtration and pressure filtration decreased (ie. CST and SRF increased). The decrease in the rate

of dewatering by pressure filtration translated into a decrease in the pressure filter cake solids

content as at a lower rate of dewatering, a lower cake solids content will be achieved in the same

amount of time. In contrast, as result of the disintegrated flocs passing through the gravity filter

and Crown Press belt, the rate of dewatering by gravity filtration and Crown Pressing increased,

but the gravity filter filtrate and Crown Press pressate solids contents substantially increased. The

increases in the rates of dewatering by gravity filtration and Crown Pressing translated into

increases in the gravity filter, Crown Press, and pressure filter cake solids contents, as at a higher

rate of dewatering, a higher cake solids content will be achieved in the same amount of time.

4.7.4 Overall Effect

The pulp and paper WAS, municipal WAS, and municipal ADS filterability results suggest that

for a given biosludge and filter medium, there is a critical strain rate below which many of the

disintegrated solids blind the filter medium and cake, worsening all aspects of filterability, and

above which many of the disintegrated solids pass through the filter medium, increasing the rate

of filtration and filter cake solids content, but increasing the filtrate solids content even more.

Thus, at strain rates above the critical strain rate, orifice flow treatment is not a suitable method

for increasing the rate of dewatering and the cake solids content, as many of the solids will pass

through the filter and little solid-liquid separation will be achieved.

As summarized in Table 4.4, for pulp and paper WAS, the maximum strain rate was below the

critical strain rate for all filter media used in this study. For municipal WAS, the maximum strain

rate was above the critical strain rate for all filter media except the CST filter. For municipal

ADS, the maximum strain rate was below the critical strain rate for the CST filter and the

SRF/pressure filter, but above the critical strain rate for the gravity filter and the Crown Press

belt.

65

Table 4.4. For a Given Biosludge and Filter Medium, was the Maximum Strain Rate Above

or Below the Critical Strain Rate?

Filter Pore

Diameter (𝜇𝑚) P&P WAS

𝐸𝑚𝑎𝑥 = 29,280 𝑠−1 Municipal WAS 𝐸𝑚𝑎𝑥 = 34,540 𝑠−1

Municipal ADS 𝐸𝑚𝑎𝑥 = 34,090 𝑠−1

CST filter

8

(Sawalha &

Scholz, 2007)

Below Below Below

SRF filter/

pressure filter 76 Below Above Below

Gravity filter

and Crown

Press belt

>76 Below Above Above

The critical orifice flow treatment strain rate observed in this study does not agree with the

existence of a critical sonication energy below and above which dewaterability improves and

worsens, respectively, as observed by Chu et al. (2001), Feng et al. (2009), and Zhang et al.

(2011). Na et al. (2007), however, observed a critical sonication energy below and above which

dewaterability worsened and improved, respectively. However, it is interesting to note that none

of the studies on the effect of sonication on sludge dewaterability listed in Table 2.1 reported the

solids content of the filtrate, which is a key indicator of sludge dewaterability. In fact, none of

the studies reported all three of the key indicators of sludge dewaterability, that is, the rate of

dewatering, and solids contents of the cake and the filtrate.

66

Chapter 4

Conclusions 5

The objectives of this study were to determine how the orifice flow treatment of biosludge

affects the dewaterability of biosludge and mixtures of biosludge, primary sludge, and/or

polymer and how it affects the dewaterability of biosludge in comparison to sonication. Pulp and

paper WAS samples were orifice flow treated at various strain rates and for comparative

purposes, municipal WAS and ADS samples were orifice flow treated at their maximum strain

rates. Pulp and paper WAS samples were also sonicated at the same energy output as orifice flow

treatment at its maximum strain rate. The particle size distribution, water distribution, and rate

and extent of dewatering of the untreated and treated pulp and paper WAS, municipal WAS, and

municipal ADS samples were assessed, as well as the particle size distribution and rate and

extent of dewatering of mixtures of the untreated and treated pulp and paper WAS, primary

sludge and/or polymer. The rate of dewatering was assessed by measuring the CST and

determining the SRF of the samples and the extent of dewatering was assessed by dewatering

sludge samples by gravity filtration, Crown Pressing, pressure filtration, and centrifugation and

measuring the total solids (TS) of the cakes and the TSS of the filtrates, pressate, and centrate.

The main conclusions drawn from this study are as follows:

1. Orifice flow treatment of biosludge at increasing strain rate predominantly disintegrates

settleable particles into supracolloidal particles until the proportions of both plateau.

2. Orifice flow treatment of biosludge at strain rates up to about 35,000 s-1

did not significantly

affect the free, interstitial, vicinal, or hydration water contents.

67

3. Attempting to remove more interstitial water from biosludge by mechanical dewatering

appears to be a worthy effort for biosludges in which removing the free and interstitial water

will increase the cake solids content to at least 40 wt.%, the minimum cake solids content for

self-sustainable combustion. This is the case for pulp and paper WAS, but not for municipal

WAS and municipal ADS.

4. For a given biosludge and filter medium, there appears to be a critical orifice flow treatment

strain rate below which many of the disintegrated solids blind the filter medium and cake,

worsening all aspects of filterability, and above which many of the disintegrated solids pass

through the filter medium, increasing the rate of filtration and filter cake solids content, but

increasing the filtrate solids content even more. Thus, orifice flow treatment is not a suitable

method for improving sludge filterability.

5. At sufficiently high rates of dewatering, orifice flow treatment of biosludge increases the

centrifuge cake solids content, thereby improving centrifugability, by disintegrating the flocs

into smaller flocs and particles that are more compactible.

6. Mixing orifice flow treated biosludge with primary sludge and/or polymer does not provide

an additional improvement in dewaterability in comparison to mixing untreated biosludge

with primary sludge and/or polymer. Thus, the smaller disintegrated flocs and particles do

not seem to form a less compressible and more porous mixture with primary sludge, nor do

they seem to form denser flocs with polymer.

7. Orifice flow treatment disintegrates biosludge flocs to a greater extent than sonication at the

same energy output. Orifice flow treatment affects biosludge dewaterability in a similar

manner to sonication.

68

Chapter 5

Recommendations 6In this study, orifice flow treatment was found to effectively disintegrate biosludge flocs. As

such, it is recommended that the effect of orifice flow treatment on the anaerobic digestibility of

sludge and the dewaterability of the resulting anaerobically digested sludge be investigated.

Kopp et al. (1997) found that in disintegrating the flocs and cells, the stirred-ball milling and

high-pressure homogenization of sludge improved its anaerobic digestibility by making its

organic components more available for digestion, leading to greater biogas production and

sludge reduction. They also found that although the resulting anaerobically digested sludge had a

greater polymer demand, it contained denser flocs that were easier to dewater, leading to a

greater centrifuge cake solids content. As orifice flow treatment is another method of mechanical

sludge disintegration, it too has the potential to improve the anaerobic digestibility of sludge and

the dewaterability of the resulting anaerobically digested sludge. Nonetheless, as this study only

confirmed that orifice flow treatment disintegrates biosludge flocs, it should first be confirmed

that it also disintegrates the cells.

Orifice flow treatment was also found to disintegrate biosludge flocs to a greater extent than

sonication at the same energy output, suggesting that orifice flow treatment is more efficacious

in disintegrating biosludge flocs than sonication. As such, it is recommended that further studies

be conducted to compare orifice flow treatment to sonication and other methods of mechanical

sludge disintegration.

At sufficiently high rates of dewatering, orifice flow treatment of biosludge was found to

increase the centrifuge cake solids content by disintegrating the flocs into smaller flocs and

particles that are more compactible. As the extent of floc disintegration increases with increasing

strain rate, its is recommended that an orifice flow treatment apparatus that can achieve a greater

maximum strain than that used in this study be used to determine how great of an increase in

centrifuge cake solids content can be achieved by orifice flow treatment, if and how this increase

will translate to industrial centrifugation, and if this increase will be industrially significant.

69

Additionally, as the rate of dewatering by centrifugation and the centrate TSS were not measured

in this study, it is recommended that further studies be conducted to determine the effect of

orifice flow treatment on centrifugability.

70

References 7

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76

Appendices 8

8.1 Appendix A: Determination of Sonication Parameters

To compare the effect of orifice flow treatment on dewaterability to that of sonication, pulp and

paper WAS samples were sonicated such that the energy delivered to the samples was equal to

the energy delivered to the samples by orifice flow treatment at the maximum strain rate.

The energy delivered to the samples by the pump during orifice flow treatment at the maximum

strain rate, as produced by the 0.8 mm radius orifice, was calculated by conducting an energy

balance around the pump using Bernoulli’s equation (Equation 6) for steady flow of an

incompressible, homogenous fluid (de Nevers, 2005).

∆ (

𝑃

𝜌+ 𝑔𝑧 +

𝑣2

2) =

𝑑𝑊

𝑑𝑚− ℱ

Equation 6

where 𝑃 is the pressure of the fluid in Pa, 𝜌 is the density of the fluid in kg/m3, 𝑔 is the

acceleration of gravity in m/s2, 𝑧 is the elevation of the fluid in m, 𝑣 is the velocity of the fluid in

m/s, 𝑑𝑊

𝑑𝑚 is the work done on the fluid per unit mass of fluid passing through the system in J/kg,

and ℱ is the friction heating per unit mass of fluid passing through the system in J/kg.

Referring to Figure 3.1, applying Bernoulli’s Equation between the surface of sludge in the feed

tank and the pressure gauge gives:

𝑃2 − 𝑃1

𝜌+ 𝑔(𝑧2 − 𝑧1) +

(𝑣22 − 𝑣1

2)

2=

𝑑𝑊

𝑑𝑚− ℱ

Equation 7

where the subscripts 1 and 2 refer to the surface of the sludge and the pressure gauge,

respectively.

77

The pressure at the surface of the sludge in the feed tank is atmospheric and the pressure at the

pressure gauge was 90 psig (620,527.5 Pa) at the maximum strain rate. The elevation of the

surface of the sludge in the feed tank and the pressure gauge was 0.358 m and 0.572 m from the

base of the apparatus, respectively. The velocity at the surface of the sludge in the feed tank was

assumed to be zero and the velocity of the sludge at the pressure gauge was calculated using

Equation 8.

𝑣 =

𝑄

𝐴

Equation 8

where 𝑄 is the volumetric flowrate of the fluid and 𝐴 is the cross-sectional area of the pipe.

The volumetric flowrate of sludge flowing through the orifice flow treatment apparatus at the

maximum strain rate was measured to be on average 4.71 x 10-5

m3/s. The cross-sectional area of

the 20 mm inner diameter pipe is 3.14 x 10-4

m2. Substituting these values into Equation 8 gives:

𝑣 =4.71 × 10−5 𝑚3

𝑠3.14 × 10−4𝑚2

𝑣 = 0.15 𝑚

𝑠

Therefore, the velocity at the pressure gauge was 0.15 m/s.

The density of the pulp and paper WAS samples was assumed to be 998.2 kg/m3, the density of

water at 20 °C and 1 atm. The friction heating per unit mass of fluid passing through the system

was assumed to be zero since the pipe length was short. Substituting these values into Equation 7

and solving for 𝑑𝑊

𝑑𝑚 gives:

(620,527.5 𝑃𝑎 − 0 𝑃𝑎)

998.2 𝑘𝑔𝑚3

+ 9.8𝑚

𝑠2(0.572 𝑚 − 0.358 𝑚) +

(0.15 𝑚𝑠 )

2

− (0 𝑚𝑠 )

2

2=

𝑑𝑊

𝑑𝑚− 0

𝑑𝑊

𝑑𝑚= 624

𝐽

𝑘𝑔

78

Therefore, the work done on or energy delivered to the sludge by the pump per unit mass of

sludge passing through the orifice flow treatment apparatus at the maximum strain rate was 624

J/kg of sludge.

The sample volume and sonication power and time required to deliver 624 J/kg to the sludge was

determined as shown below.

It was decided that 400 mL sludge samples would be sonicated at 200 W. Using the same

ultrasonic reactor that was used in this study, Yong (2007) determined the power delivered in W

(𝑃𝐷) to 400 mL of distilled water as a function of the power supplied in W (𝑃𝑆) to the ultrasonic

reactor, as shown by Equation 9.

𝑃𝐷 = 0.2365(𝑃𝑠) − 38.547 Equation 9

Thus, at a power setting of 200 W, about 8.753 W would be delivered to 400 mL of sludge.

Then, the sonication time, 𝑡, for the specific energy, 𝑒𝐷, delivered to the sludge to be 624 J/kg of

sludge, was calculated using Equation 10.

𝑒𝐷 =

𝑃𝐷𝑡

𝑉𝜌

Equation 10

where 𝑉 and 𝜌 are the volume and density, respectively, of the sludge sample.

Substituting the values of 𝑒𝐷, 𝑃𝐷, and 𝑉 into Equation 10, assuming the density of pulp and paper

WAS to be 998.2 kg/m3, and solving for 𝑡 gives:

𝑡 =(624

𝐽𝑘𝑔

)(4 × 10−4 𝑚3)(998.2 𝑘𝑔𝑚3)

8.753 𝐽𝑠

𝑡 = 28.5 𝑠

Thus, the sonication time required to deliver 624 J/kg of energy to 400 mL of sludge at a

sonication power or 200 W is 28.5 s.

79

8.2 Appendix B: Calculation of Specific Resistance to Filtration

To determine the specific resistance to filtration (SRF), 100 mL sludge samples were pressure

filtered through 1⅞ inch diameter stainless steel wire cloth discs with 0.003 inch diameter

openings, at 4.91 x 104 Pa for 30 minutes. The mass of the filtrate was recorded at one second

intervals, and after 30 minutes elapsed, the volume and TSS of the filtrate were measured. With

this data, SRF, 𝑅, was calculated using Equation 5.

𝑅 =

2𝑃𝐴2𝑏

𝜇𝜔

Equation 5

where 𝑃 is the filtration pressure in Pa, 𝐴 is the filtration area in m2, 𝑏 is the slope of the line

obtained from plotting 𝑡

𝑉 as a function of 𝑉 in s/m

6, where 𝑡 is the filtration time and 𝑉 is the

filtrate volume, 𝜇 is the filtrate dynamic viscosity in Pas, and 𝜔 is the dry mass of the filter cake

per unit volume of filtrate in kg/m3.

The filtration pressure was 4.91 x 104 Pa, the filtration area was 1.781 x 10

-3 m

2, the area of the

1⅞ inch diameter stainless steel wire cloth disc, and the viscosity of the filtrate was assumed to

be 8.90 x 10-4

Pas, the viscosity of water at 20°C.

To determine 𝑏, the recorded time-filtrate mass data was converted into time-filtrate volume data

by assuming the density of the filtrate to be 998.2 kg/m3, the density of water at 20 °C and 1 atm.

Since pressure was not applied to the filtration vessel until 30 seconds into the filtration, the

time-filtrate data at 𝑡 = 30 was subtracted from all subsequent time-filtrate data. 𝑡

𝑉 was then

plotted as a function of 𝑉, as shown in Figure 8.1 for a pulp and paper WAS sample. The 𝑡

𝑉 vs. 𝑉

data may be non-linear at the beginning when the filter cake is just forming and/or at the end

when filtration has progressed into expression (Christensen & Dick, 1985b). Linear regression

analysis was performed on the linear portion of the data. The slope of the linear portion of the 𝑡

𝑉

vs. 𝑉 plot was 2.052 x 1011

s/m6 for this pulp and paper WAS sample.

80

Figure 8.1: Plot of 𝒕

𝑽 vs. 𝑽 for Pulp and Paper WAS Sample

The dry mass of the filter cake per unit volume of filtrate, 𝜔, was calculated using Equation 11.

𝜔 =

(𝑇𝑆𝑆𝑠𝑎𝑚𝑝𝑙𝑒𝑉𝑠𝑎𝑚𝑝𝑙𝑒 − 𝑇𝑆𝑆𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒𝑉𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒,𝑚)

𝑉𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒,𝑑

Equation 11

where 𝑉𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒,𝑚 is the measured volume of filtrate at the end of the test and 𝑉𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒,𝑑 is the

volume of filtrate at the end of the filtration phase of the test, as derived from the plot of t/V vs.

V as the volume at which linearity ends.

The expression in the numerator of Equation 11 was used for the dry mass of the filter cake

instead of the actual dry mass of the cake, as there would be less error due to incomplete

discharge of the cake. The dry mass of the cake was divided by the volume of filtrate at the end

of the filtration phase instead of the volume of filtrate at the end of the test, since the volume of

filtrate at the end of the test would also include the filtrate generated by expression, which is a

different process than filtration (Christensen & Dick, 1985a).

0 2×10-5 4×10-5 6×10-5 8×10-5 1×10-40.0

5.0×106

1.0×107

1.5×107

2.0×107

2.5×107

V (m3)

t/V

(s/

m3) t/V = 2.052 x1011V - 0.6701

R2 = 0.9997

81

For this pulp and paper WAS sample, 𝑇𝑆𝑆𝑠𝑎𝑚𝑝𝑙𝑒 was 24.00 g/L, 𝑉𝑠𝑎𝑚𝑝𝑙𝑒 was 0.1000 L,

𝑇𝑆𝑆𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 was 0.34 g/, and 𝑉𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒,𝑚 was 0.0890 L. From Figure 8.1, 𝑉𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒,𝑑 is 0.06997 L.

Substituting these values into Equation 11 gives:

𝜔 =(24.00

𝑘𝑔𝑚3) (1.000 × 10−4 𝑚3) − (0.3400

𝑘𝑔𝑚3)(8.90 × 10−5 𝑚3)

6.997 × 10−5 𝑚3

𝜔 = 33.9 𝑘𝑔

𝑚3

Now, substituting the values for 𝑃, 𝐴, 𝑏, 𝜇, and 𝜔 into Equation 5 gives:

𝑅 =2(49,100 𝑃𝑎)(1.781 × 10−3 𝑚2)2(2.052 × 1011

𝑠𝑚6)

(8.713 × 10−4 𝑘𝑔

𝑚 ∙ 𝑠)(33.9 𝑘𝑔𝑚3)

𝑅 = 2.16 × 1012 𝑚

𝑘𝑔

Thus, the SRF of this pulp and paper WAS sample was 2.16 x 1012

m/kg.

82

8.3 Appendix C: Determination of Water Distribution

To determine the water distribution in the biosludge samples, a drying test was conducted in

which 5 mL sludge samples were dried at 30°C and constant humidity until the mass of the

sample equilibrated. The mass of the sample was recorded every 5 minutes and once the mass

equilibrated, the TS of the sample was measured to determine its dry mass and its equilibrium

water content. With this data, the drying flux was plotted as a function of the normalized water

content.

First, the mass of water in the sample at time 𝑡, 𝑚𝑤𝑎𝑡𝑒𝑟,𝑡, was calculated using Equation 12.

𝑚𝑤𝑎𝑡𝑒𝑟,𝑡 = 𝑚𝑠𝑎𝑚𝑝𝑙𝑒,𝑡 − 𝑚𝑠𝑜𝑙𝑖𝑑𝑠 Equation 12

where 𝑚𝑠𝑎𝑚𝑝𝑙𝑒,𝑡 is the mass of the sample at time 𝑡 and 𝑚𝑠𝑜𝑙𝑖𝑑𝑠 is the mass of the solids in the

sample or the dry mass of the sample.

Next, the normalized dry water content, 𝑋, was calculated using Equation 13.

𝑋 = 100𝑚𝑤𝑎𝑡𝑒𝑟,𝑡

𝑚𝑤𝑎𝑡𝑒𝑟,𝑡=0

Equation 13

where 𝑚𝑤𝑎𝑡𝑒𝑟,𝑡=0 is the mass of water in the sample at 𝑡 = 0.

Then, the drying flux at time 𝑡 for the 5 minute interval that followed, 𝑅𝑡, was calculated using

Equation 14.

𝑅𝑡 = −

1

𝐴

𝑑𝑚𝑤𝑎𝑡𝑒𝑟

𝑑𝑡= −

1

𝐴(𝑚𝑤𝑎𝑡𝑒𝑟,𝑡+300 − 𝑚𝑤𝑎𝑡𝑒𝑟,𝑡

300 𝑠)

Equation 14

where 𝐴 is the exposed sample area.

The exposed sample area was taken to be 2.2 x 10-3

m2, the area of the 53 mm-diameter

aluminum dish in which the sample was dried, as the sample initially covered this area.

Inevitably, as drying proceeded, the exposed sample area changed due to shrinking, which

decreases the top exposed surface area of the sludge but also exposes the sides of the sample, and

83

cracking, which exposes the interior of the sample (Tao, Peng, & Lee, 2005). To minimize

shrinking and cracking, researchers often perform the drying test on mechanically dewatered

sludge (Tsang, 1989). In this study, however, the drying test was not performed on mechanically

dewatered sludge, as the untreated and treated sludge samples would mechanically dewater

differently and this might diminish or enhance the effect of treatment on the water distribution of

the sludge. As such, the effect of shrinking and cracking on the exposed sample area was not

accounted for. This leads to an inaccurate estimation of the drying flux, but was deemed

acceptable since the drying flux vs. normalized dry water content plot was similar to the typical

plot (Figure 2.2) and since major changes in the drying flux were still identifiable for

determining the critical water contents.

To determine the first critical water content at which the first falling-rate period began,

segmental linear regression was performed on the normalized dry water content-drying flux data

using GraphPad Prism 6 (GraphPad Software Inc., San Diego, CA, USA), as shown in Figure

8.2. Segmental linear regression fits two lines intersecting at 𝑋 = 𝑋0 to the data; 𝑋0 was taken

as the first critical water content, 𝑋𝐶,1.

Figure 8.2. Plot of Normalized Dry Water Content vs. Drying Flux with Segmental Linear

Regression for a Pulp and Paper WAS Sample

0 50 1000

2×10-5

4×10-5

6×10-5

Normalized dry water content (wt.%)

Dry

ing

flu

x (

kg

wa

ter/(

m2*

s))

XC,1= 6.9 wt.%

84

To determine the second critical water content at which the second falling-rate period began,

segmental linear regression was performed on the normalized dry water content-drying flux data

with a normalized dry water content less than the first critical water content, as shown in Figure

8.3. Again, 𝑋0 was taken as the second critical water content, 𝑋𝐶,2.

Figure 8.3. Plot of Normalized Dry Moisture Content vs. Drying Flux for Normalized Dry

Moisture Contents less than the First Critical Moisture Content with Segmental Linear

Regression

The third critical water content was taken to be the equilibrium water content, which was

determined by measuring the TS of the sample once its mass equilibrated.

0 2 4 6 80

1×10-5

2×10-5

3×10-5

4×10-5

Normalized dry water content (wt.%)

Dry

ing

flu

x (

kg

wa

ter/(

m2*

s))

XC,2= 1.3 wt.%

the effect of orifice flow treatment on biosludge

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