THERMAL TREATMENT OF PULP AND PAPER MILL

BIOSLUDGE AND DIGESTATE TO ENHANCE THEIR

ANAEROBIC DIGESTIBILITY

by

Lei Chen

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Department of Chemical Engineering & Applied Chemistry

University of Toronto

© Copyright by Lei Chen 2015

ii

Thermal Treatment of Pulp and Paper Mill Biosludge and

Digestate to Enhance Their Anaerobic Digestibility

Lei Chen

Master of Applied Science

Department of Chemical Engineering & Applied Chemistry

University of Toronto

2015

Abstract

Anaerobic digestion of pulp and paper mill biosludge has the potential to reduce sludge

disposal costs and generate energy through biogas production. Thermal treatment can

couple with anaerobic digestion to enhance the sludge digestibility. Conventionally,

thermal pretreatment of biosludge is used by the industry. In this study, three possible

thermal treatment-assisted anaerobic digestion configurations were compared:

1. Thermal pretreatment of biosludge;

2. Digestate thermal treatment;

3. Digestate thermal treatment and only recycling the hydrolysate.

The thermal pretreatment (1) of biosludge at the theoretical optimal conditions (170o C

for 1 h) did not extend the ultimate biogas yield. The digestate thermal treatment (2)

increased the rate and extent of the biogas production; a synergistic effect on biogas

production was observed by co-digesting biosludge and treated digestate. Only recycling

the hydrolysate (3) is recommended if the thermal treatment is conducted at high

intensities such as the one at 210o C for 30 min.

iii

Acknowledgments

I would like to thank my supervisor Prof. Ramin Farnood, and Dr. Torsten Meyer for

their guidance and support throughout the research project. I would also like to thank the

members of Department of Chemical Engineering and Applied Chemistry, BioZone, Prof.

Farnood’s research group, and Prof. Allen’s research group including Mr. Xian Huang,

Mrs. Sofia Bonilla, Dr. Chaoyang Feng, Mrs. Isabela Medina, Mr. Karim Saleh and Mrs.

Debby Repka.

I also appreciate the funding from the Natural Sciences and Engineering Research

Council of Canada (NSERC).

Finally, thanks to my parents for the understanding and support.

iv

Table of Contents

Abstract ............................................................................................................................... ii

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

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

List of Tables ..................................................................................................................... vi

List of Figures .................................................................................................................... ix

List of Abbreviations ........................................................................................................ xii

Nomenclature ................................................................................................................... xiii

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

1.1. The Challenges with Pulp and Paper Mill Biosludge Management .................. 1

1.2. Thermal Treatment Assisted Biosludge Anaerobic Digestion .......................... 2

1.3. Research Objectives .......................................................................................... 3

1.4. Document Structure ........................................................................................... 4

2. Literature Review......................................................................................................... 6

2.1. Pulp and Paper Mill Biosludge .......................................................................... 6

2.2. Anaerobic Digestion of Pulp and Paper Mill Related Biosludge ...................... 8

2.3. Sludge Treatment Technologies to Enhance Anaerobic Digestion ................. 10

2.4. Thermal Treatment .......................................................................................... 11

2.4.1. Thermal Pretreatment of Biosludge ...................................................... 12

2.4.2. Digestate Thermal Treatment (Interstage Treatment) ........................... 13

2.4.3. Thermal Hydrolysate Recycle............................................................... 15

2.5. Summary of the Existing Literature ................................................................ 16

3. Materials and Methods ............................................................................................... 18

3.1. Biosludge and Digestate Collection and handling .......................................... 20

3.2. Bench-scale Thermal Reactor ......................................................................... 22

3.3. Biomethane Potential (BMP) Test .................................................................. 23

3.4. Modelling of Biogas Production ..................................................................... 25

3.5. Sludge Characterization .................................................................................. 27

3.5.1. Sludge Fractionation ............................................................................. 27

3.5.2. Caustic Extraction ................................................................................. 27

3.5.3. Total and Volatile Solids and Suspended Solids .................................. 28

3.5.4. Chemical Oxygen Demand (COD) ....................................................... 28

3.5.5. Carbohydrates ....................................................................................... 28

3.5.6. Tannin and Lignin ................................................................................. 29

3.5.7. Particle Size Analysis ........................................................................... 29

3.6. Large Batch-scale Experiments ....................................................................... 29

4. Anaerobic Digestion of Untreated and Thermally Treated Biosludge and Digestate 31

v

4.1. Objectives ........................................................................................................ 31

4.2. Results and Discussion .................................................................................... 32

4.2.1. Untreated Biosludge and Digestate ....................................................... 32

4.2.2. Effects of Biosludge Thermal Pretreatment .......................................... 35

4.2.3. Thermal Treatment of Digestate ........................................................... 38

4.2.4. Carbohydrate and Tannin/Lignin Solubilization Due to Thermal

Treatment .......................................................................................................... 49

4.2.5. Biogas Production from Large Batch-scale Experiments ..................... 53

4.3. Summary and Conclusions .............................................................................. 55

5. Anaerobic Digestion of Digestate Thermal Hydrolysate and Solids Residue ........... 58

5.1. Rationales and Objectives ............................................................................... 58

5.2. Results and Discussion .................................................................................... 60

5.2.1. Anaerobic Digestion of Hydrolysate Produced by Thermal Treatment of

Digestate ........................................................................................................... 60

5.2.2. Anaerobic Digestion of the Hydrolysate and Solids Residue ............... 66

5.2.3. Co-digestion of Pulp Mill Biosludge and Hydrolysate ......................... 69

5.3. Summary and Conclusions .............................................................................. 74

6. A Preliminary Economic Analysis............................................................................. 76

6.1. Design Assumptions ........................................................................................ 77

6.1.1. Biosludge Inflow into the Treatment Processes.................................... 77

6.1.2. Major Unit Operations .......................................................................... 78

6.2. Process Description ......................................................................................... 83

6.2.1. Anaerobic Digestion without any treatment (NT) ................................ 83

6.2.2. Anaerobic Digestion with Thermal Pretreatment (TP) ......................... 83

6.2.3. Anaerobic digestion with Digestate Treatment (DT) ............................ 84

6.2.4. Digestate Thermal Treatment with Hydrolysate Recycle (DTH) ......... 85

6.2.5. Summary ............................................................................................... 86

6.3. Preliminary Costs and Economic Analysis ..................................................... 87

6.4. Conclusions ..................................................................................................... 90

7. Conclusions and Recommendations .......................................................................... 91

7.1. Conclusions ..................................................................................................... 91

7.2. Recommendations ........................................................................................... 92

References ......................................................................................................................... 94

vi

List of Tables

Table 4.1: Properties of the biosludge samples used in this section of study; all analysis

preformed in triplicates ..................................................................................................... 32

Table 4.2: Properties of the digestate samples used in this section of study; the digestate

samples were produced by Xian Huang; all analysis were preformed in triplicates. ....... 33

Table 4.3: The properties of treated and untreated biosludge 1; all analysis were

preformed in triplicates. .................................................................................................... 35

Table 4.4: The biogas production modeling for the treated and untreated Biosludge 1

using the modified Gompertz equation ............................................................................. 38

Table 4.5: Thermal treatment of Digestate 1 and Digestate 2; Experiments 1, 2 and 3

studied the effects of temperature; Experiments 2 and 5 studied the effects of retention

time; Experiments 3 and 4 studied the effects of steam explosion; .................................. 39

Table 4.6: The biogas production modeling for Biosludge 2, and Digestate 1 with and

without treatment using the modified Gompertz equation ............................................... 48

Table 4.7: The biogas production modeling for Biosludge 3, Digestate 2 with and without

treatment, and the co-digestion mixtures using the modified Gompertz equation ........... 49

Table 4.8: Properties of the initial biosludge for both runs .............................................. 53

Table 4.9: The biogas production modeling for the two large batch-scale experiments

using the modified Gompertz equation; *this lag phase (λ) is calculated after the

re-inoculation. ................................................................................................................... 54

Table 5.1: Properties of Digestate 3; Digestate 3 was produced by Xian Huang; all

analysis were preformed in triplicates. ............................................................................. 60

Table 5.2: Thermal treatment conditions for Digestate 3 ................................................. 61

Table 5.3: Effect of thermal treatment and caustic extraction on the organic solubilization;

all analysis were preformed in triplicates. ........................................................................ 62

vii

Table 5.4: Model kinetics parameters for the thermal hydrolysate samples; A= (mL

biogas/ mg COD input), µm= (mL biogas/ (mg COD input*h)), λ= (h), Atot is the sum of

A1 and A2; *Due to the limited amount of sample points, those parameters had a large

confident interval so were considered not reliable. .......................................................... 66

Table 5.5: Biogas production from the different fractions ............................................... 68

Table 5.6: Properties of Biosludge 4................................................................................. 69

Table 5.7: The BMP tests preformed to investigate the effects of co-digesting biosludge

and the thermal hydrolysate .............................................................................................. 70

Table 6.1: Inflow biosludge properties; *it was assumed VSS/TSS=0.9; **it was assumed

COD/VS=1.5. .................................................................................................................... 78

Table 6.2: Assumed design criteria for the DAF thickener .............................................. 79

Table 6.3: Assumed design criteria for thickening centrifuge;* it is assumed that the

digestate is easier to dewater comparing to biosludge [4]. ............................................... 79

Table 6.4: Anaerobic digestion data based on the bench-scale experiments .................... 80

Table 6.5: Design criteria for dewatering centrifuge ........................................................ 82

Table 6.6: Economic factors ............................................................................................. 82

Table 6.7: Process outputs summary ................................................................................ 86

Table 6.8: Capital costs, and operation and maintenance costs for the different processes

........................................................................................................................................... 88

Table 6.9: Economic analysis for boiler option;* biogas is used to replace the purchase of

natural gas;** this is calculated as the reduction on polymers or dewatering aids

compared to the current mill;*** this is calculated as the reduction on sludge production

compared to the current mill. ............................................................................................ 89

Table 6.10: Economic analysis for CHP option; * biogas is used to replace the purchase

of natural gas;** this is calculated as the reduction on polymers or dewatering aids

viii

compared to the current mill;*** this is calculated as the reduction on sludge production

compared to the current mill. ............................................................................................ 89

ix

List of Figures

Figure 1.1: Simplified block diagrams of each process investigated in this study ............. 4

Figure 2.1: Typical process flow diagram for a conventional pulp and paper mill

wastewater treatment plant ................................................................................................. 7

Figure 2.2: Simplified flow chart showing the stages of anaerobic digestion, adapted from

[15]. ..................................................................................................................................... 9

Figure 3.1: Overall experimental layout for part 1 of the research ................................... 19

Figure 3.2: Overall experimental plan for part 2 of the research ...................................... 20

Figure 3.3: 10 L large batch-scale batch digester [31] ...................................................... 22

Figure 3.4: Bench-scale thermal reactor set-up ................................................................ 23

Figure 3.5: Large batch-scale experimental set-up; (the digester and thermal reactor) .... 30

Figure 4.1: BMP test for the untreated biosludge and digestate samples ......................... 35

Figure 4.2: BMP test for the treated and untreated biosludge and biogas production

models using the modified Gompertz equation; all measurements were performed in

triplicates. .......................................................................................................................... 37

Figure 4.3: Solids solubilization due to thermal treatment for Digestate 1and Digestate 2;

SE refers to steam explosion; all measurements were performed in triplicates. .............. 40

Figure 4.4: COD solubilization due to thermal treatment for Digestate 1and Digestate 2;

SE refers to steam explosion; all measurements were performed in triplicates. .............. 41

Figure 4.5: The particle size distribution for the untreated Biosludge 2, Digestate 1, and

Digestate 2; data presented in % volume. The numbers on the graph are the mode particle

size in microns. ................................................................................................................. 42

x

Figure 4.6: Particle size distribution for Digestate 1; SE refers to sample subjected to

steam explosion. The numbers on the graph are the mode particle size in microns. ........ 43

Figure 4.7: The BMP results for the treated and untreated Digestate 1, and Biosludge 2;

all measurements were performed in triplicates. SE: steam explosion. ............................ 44

Figure 4.8: The BMP results for the treated and untreated Digestate 2, and Biosludge 3;

all measurements were performed in triplicates. .............................................................. 45

Figure 4.9: The BMP results for biosludge and treated digestate mixtures; all

measurement was performed at triplicates. ....................................................................... 46

Figure 4.10: The synergistic effect of the co-digestion .................................................... 47

Figure 4.11: Ratio of soluble carbohydrate (carb) and tannin/lignin with respected to

SCOD in Biosludge 3, treated and untreated Digestate 1 and Digestate 2; SCOD is also

included; SE refers to steam explosion; all measurements were performed in triplicates.51

Figure 4.12: The concentration of soluble and caustic extracted carbohydrate for the

treated and untreated 60-day digestate; SCOD is also included; all measurements were

performed in triplicates. .................................................................................................... 52

Figure 4.13: Biogas production from the large batch-scale pretreatment and interstage

digestate treatment ............................................................................................................ 54

Figure 5.1: The BMP results for thermal hydrolysate at different treatment conditions; all

measurement was performed at triplicates. ....................................................................... 64

Figure 5.2: The BMP results for A) the digestate treated at 210o C for 30 min and the

corresponding hydrolysate and solids residue with Digestate 3 as reference; B) the

digestate treated at 190o

C for 15 min and the corresponding hydrolysate and solids

residue with Digestate 3 as reference; all measurement was performed at triplicates. ..... 67

Figure 5.3: The BMP results for A) Biosludge 4 and the untreated digestate mixtures; B)

Biosludge 4 and the 210˚C 30 min hydrolysate mixtures; and C) Biosludge 4 and the

190˚C 15 min hydrolysate mixtures; all measurements preformed in triplicates. ............ 72

Figure 5.4: The synergistic effect of co-digestion; the calculated SBP is the proportional

addition of the SBP of every component in the mixture. .................................................. 73

xi

Figure 6.1: Current wastewater treatment plant ................................................................ 77

Figure 6.2: CambiTM

thermal treatment process ............................................................... 81

Figure 6.3: Process train for NT ....................................................................................... 83

Figure 6.4: Process train for TP ........................................................................................ 84

Figure 6.5: Process train for DT ....................................................................................... 85

Figure 6.6: Process train for DTH..................................................................................... 86

xii

List of Abbreviations

BCTMP = Bleached chemi thermo mechanical pulp

BMP = Biomethane potential

COD = Chemical oxygen demand

DAF = Dissolved Air Flotation Thickening

DT = Anaerobic digestion with digestate thermal treatment

DTH = Digestate thermal treatment with hydrolysate recycle only

NT = Anaerobic digestion without any treatment

RMSE = Root mean square error

SCOD = Soluble chemical oxygen demand

SE = Steam explosion

TCOD = Total chemical oxygen demand

TP = Anaerobic digestion with thermal pretreatment

TS = Total solids

TSS = Total suspended solids

VS = Volatile solids

VSS = Volatile suspended solids

WAS = Waste activated sludge

xiii

Nomenclatures

A = Ultimate biogas production potential (mL biogas/mg COD input)

BD = Bio-digestibility derived by Equation 2 (%)

SBP = Specific biogas production (mL biogas/mg COD input)

e = Exponential; equal to 2.718282

µm = Maximum biogas production rate (mL biogas/ (mg COD input*h))

or mL biogas/ (mg COD input*day)))

λ = Lag phase (h or day)

t = Time

y = Biogas production (mL biogas/mg COD input)

1

1. Introduction

1.1. The Challenges with Pulp and Paper Mill Biosludge

Management

The pulp and paper industry is one of the largest wastewater producers in the world,

producing approximately 90 billion tons of wastewater globally annually [1]. This

wastewater is treated through a train of processes including an activated sludge process.

The activated sludge process produces waste activated sludge (WAS), also known as

biosludge, as a by-product.

The pulp and paper industry produces a large amount of biosludge that is expensive to

dispose of. It is estimated that the disposal of biosludge costs more than $230 million

globally annually which accounts for up to 60% of the total wastewater treatment plant

operating cost [2, 3]. This high cost is due to the high water content of the sludge.

Conventionally, biosludge is handled with either land filling or incineration. However,

the high water content in the sludge leads to high tipping fees and transportation fees, and

the additional need of fossil fuels in order to incinerate the biosludge. Moreover, the

tipping fees and cost of fossil fuel are expected to increase in the future due to the

decrease in the number of landfill site and the increase in the fossil fuel price. The

effective treatment of pulp and paper mill biosludge has become a problem to the

industry.

2

1.2. Thermal Treatment Assisted Biosludge Anaerobic Digestion

Anaerobic digestion is an attractive biosludge treatment strategy which further digests the

biosludge and produces biogas (approximately 70% methane and 30% CO2) that can be

used as an energy source. However, anaerobic digestion has not been widely adapted in

the Canadian pulp and paper industry. The bioflocs can reduce the rate of anaerobic

digestion [4], and some mill related toxic compounds such as hydrolyzed lignin, tannin

and resin acid present in biosludge can hinder or even prevent the anaerobic digestion [5,

6]. Therefore, the anaerobic digestion of biosludge requires a long retention time which

leads to the need of large digesters and consequently higher installation and maintenance

costs. To overcome this limitation, sludge pretreatment is required to enhance the sludge

digestibility [7].

Various sludge treatments including thermal, mechanical and chemical treatment have

been studied in the past. Among them, thermal treatment has shown to be one of the most

effective treatment processes according a number of studies. Furthermore, the mill

usually has waste heat that may be used to fuel thermal treatment [5]. Therefore, thermal

treatment has the greatest potential to be implemented in a pulp and paper wastewater

treatment plant.

Conventionally, thermal treatment is arranged as a pretreatment process that is treating

biosludge prior to entering the anaerobic digester. However, some studies that have

focused on the treatment of municipal biosludge suggest that the treatment efficiency

may be improved by replacing the pretreatment process with the interstage digestate

treatment process [6] [7] [8]. The interstage process removes a portion of digestate

from the digester, thermally treats the digestate, and then re-injects the treated digestate

3

back into the digester. However, based on the best of our knowledge, there is not study

investigating the thermal treatment of the pulp and paper mill digestate.

The treated digestate can be re-digested in two ways. One option is to recycle the entire

treated digestate back to the digester. This option utilizes the full potential of treated

digestate. However, the constant recycle of treated digestate may lead to the

accumulation of the refractory solids and overtime reduce the thermal treatment

efficiency. To overcome this problem, the second option is to first separate the treated

digestate into hydrolysate and solids residue and only recycle the hydrolysate. By doing

so, the problem with refractory solids accumulation can be eliminated, and the solids

retention time in the digester does not increase. However, to evaluate the potential of the

second option, a comprehensive study is needed to investigate the anaerobic digestion of

thermal hydrolysate and solids residue individually, and the co-digestion of thermal

hydrolysate and biosludge.

To the best of our knowledge, this is the first study: investigating the effects of thermal

treatment on the pulp and paper mill digestate digestibility; investigating the anaerobic

digestion of the corresponding thermal hydrolysate and solids residue produced from the

digestate thermal treatment; and systematically comparing the different thermal treatment

configurations using biosludge from a pulp and paper mill.

1.3. Research Objectives

The primary goal of this research is to develop a suitable thermal treatment-assisted

biosludge anaerobic digestion process for the pulp and paper industry. It is hypothesized

that: the thermal treatment of digestate is more advantageous than the thermal

pretreatment of biosludge in enhancing biogas production. The objective of this study is

to compare different thermal treatment configurations in terms of the overall biogas

4

production enhancement based on bench- and batch-scale experiments. Specifically, this

study focuses on three types of thermal treatment: 1) thermal pretreatment of biosludge

prior to entering the anaerobic digester; 2) interstage digestate thermal treatment with

treated digestate re-injection; 3) and interstage digestate thermal treatment with thermal

hydrolysate re-injection. Simplified diagrams of each process are presented in Figure 1.1.

Figure 1.1: Simplified block diagrams of each process investigated in this study

1.4. Document Structure

This thesis is divided into seven chapters. This chapter introduces the background and

motivation behind the study. Chapter 2 provides a literature review on the relevant topics

related to this study, including the production of biosludge, the anaerobic digestion of

sludge, the various pretreatment methods, and the thermal treatment method. Chapter 3

describes the overall experiment layout as well as the specific tests used in this study.

Chapter 4 presents the results and a discussion on the anaerobic digestion of untreated

AD

TT

Biosludge

Treated Biosludge

Digestate

Conventional thermal

pretreatment

AD

Biosludge

Waste Digestate

Digestate thermal treatment with treated

digestate recycling

TT AD

Biosludge

Solids Residue

TT

Dewater

Dewater

Dewater

Hydrolysate

Digestate thermal treatment with

hydrolysate recycling

Dewater

Returned Digestate

Dewater

5

and treated biosludge and digestate. Chapter 5 presents the results and a discussion on the

anaerobic digestion of thermal hydrolysate and solids residue. Chapter 6 provides a

preliminary economic analysis of the three thermal treatment processes. Chapter 7

summarizes key conclusions and provides a discussion on the possible future works.

6

2. Literature Review

Pulp and paper mill wastewater treatment plants produce biosludge and the biosludge

disposal has been a problem for the industry (Section 2.1). Anaerobic digestion of

biosludge is an attractive solution but has not been widely used in the Canadian pulp and

paper industry because of the limitations associated with it (Section 2.2). To overcome

these limitations, various types of sludge treatment processes (Section 2.3) have been

developed to assist the anaerobic digestion process. Among them, thermal treatment has

been identified to be one of the most effective treatment methods (Section 2.4). There are

different methods to couple thermal treatment with anaerobic digestion, including

thermal pretreatment (Section 2.4.1), and interstage digestate treatment with entire or

particle reinjection (Section 2.4.2 and 2.4.3).

2.1. Pulp and Paper Mill Biosludge

Pulp and paper mill wastewater treatment plants generate biosludge. Figure 1 presents a

block diagram of a typical wastewater treatment plant. After the initial screening to

remove the debris, the mill effluent enters a primary clarifier which removes the

suspended solids in the effluent by sedimentation or flotation. The resulting primary

sludge from this process consists of fibres, finers and fillets [9]. The overflow then enters

the activated sludge process in which dissolved organic matter is converted into CO2 and

water by aerobic microorganism respiration [1]. The by-product, biosludge, as a result of

bacteria growth is later separated in a secondary clarifier.

7

Screening

Primary clarifier Secondary clarifierActivated Sludge Process

Mill effluents

Overflow

Water treatment

Overflow

Disposal

Primary sludge Biosludge (secondary sludge)

Dewater

Figure 2.1: Typical process flow diagram for a conventional pulp and paper mill wastewater treatment plant

Biosludge is diluted slurry with water content as high as 98% and naturally difficult to

dewater due to the presence of colloid materials and extracellular polymeric substances

[10]. Primary sludge, on the other hand, can be easily dewatered due to its fibrous nature.

Therefore, biosludge is usually mixed with primary sludge to increase the dewaterability

of the former.

However, as fibre recovery becomes more efficient, the amount of primary sludge is

expected to decrease. Also, the more stringent environmental regulations demand a

greater removal of organic contaminants. As a result, a more extensive activated sludge

process that produces more biosludge is required. Ultimately, as the primary sludge to

biosludge ratio decreases, the dewaterability of the overall sludge decreases [11]. The low

dewaterability of the biosludge makes the subsequent sludge handling processes, either

land filling or incineration, costly [12].

8

2.2. Anaerobic Digestion of Pulp and Paper Mill Related

Biosludge

Biosludge consists mainly of microorganisms that are surrounded by a macromolecule

matrix, also called extracellular polymeric substances. From a chemical standpoint, the

composition of biosludge is similar to that of the bacteria consisting of biopolymers

including proteins, lipids and carbohydrates. For pulp and paper mill specific biosludge,

the biosludge can also contain up to 50% of wood-related biopolymers including lignin,

cellulose and hemicelluloses [13].

The organic nature of biosludge allows it to be further digested by anaerobic digestion.

Anaerobic digestion is an attractive process that can recover energy from wet sludge by

producing biogas (60 to 70% methane by volume) [14]. The process has been widely used

to treat the municipal sludge waste due to its comparative advantages including: low

biomass production; fuel generation (methane); and sludge volume reduction.

The anaerobic digestion of sludge can be roughly divided into five stages, and each stage

has different metabolic interactions involving various microbial communities [14]

(Figure 2.2). In the disintegration stage, large organic particles including bioflocs and

fibre, are disintegrated into smaller particulates or dissolved molecules. This is preceded

to the hydrolysis stage, where the complex organic molecules are further hydrolyzed into

their monomer form; for example, protein is hydrolyzed into amino acid. These two steps

take place outside of the cell but are catalyzed by hydrolysis enzymes produced by the

microorganisms [15]. The remaining three steps take place inside the cell. The acidogenic

bacteria degrade glucose, amino acids and fatty acids into organic acids, hydrogen, and

carbon dioxide. Then, the organic acids are converted into acetate, hydrogen, and carbon

9

dioxide by acetogenic bacteria. Finally, the methanogenic bacteria use the acetate and

hydrogen to produce methane [15].

Figure 2.2: Simplified flow chart showing the stages of anaerobic digestion, adapted from [15].

A few studies have been conducted to evaluate the anaerobic digestion of pulp and paper

mill biosludge. Puhakka et al. [3] investigated the anaerobic digestion of Kraft mill

biosludge in the semi-continuous pilot scale. With a 25 day retention time, a specific

biogas yield of 0.222 mL biogas/mg VS fed and a 40% VS removal was achieved.

Karlsson et al. [16] studied the anaerobic digestion of biosludge from six different pulp

and paper mills in both batch and continuous scale. They concluded that the specific

methane yield of the different biosludge varies between 0.1 to 0.2 mL methane/mg VS

(approximately 0.14 to 0.29 mL biogas /mg VS). Comparing those values to municipal

Biosludge particles: bioflocs, fibresand large particulates

Smaller organic particles: polypeptide, carbohydrate and lipid

Organic monomers: glucose, amino acid and fatty acid

Organic acids, hydrogen and carbon dioxide

Acetate, hydrogen, and carbon dioxide

Methane, water and carbon dioxide

Disintegration

Hydrolysis

Acidogenesis

Acetogenesis

Methanogenesis

Affected bythermal

treatment

10

biosludge, 0.146 to 0.217 mL methane/mg VS [17], the pulp and paper mill biosludge has

a good potential for biogas generation.

Although the anaerobic digestion of biosludge has the potential to address both the

economic and environmental problems associated with biosludge disposal, there is

virtually not full-scale operation in the Canadian pulp and paper industry. The main

challenge with anaerobic digestion of pulp and paper mill biosludge is the long retention

time required for the digestion [4]. The cell walls of microorganism and extracellular

polymeric substances can act as barrier to the hydrolytic enzyme produced by the

anaerobic microorganism; thus, the disintegration and hydrolysis are usually the

rate-limited step for the anaerobic digestion of biosludge [15]. Another challenge

specifically posing to the pulp and paper mill biosludge is that of the presence of

wood-related biopolymers in biosludge. Edalatmanesh et al. [13] characterized the

organic composition of two pulp and paper mill biosludge samples and found that the

wood-related biopolymers including lignin, cellulose and hemicelluloses contributed up

to 50% of sludge dry mass. The wood-related biopolymers, especially lignin, are

refractory to anaerobic digestion [18]. An economic study on the anaerobic digestion of

sludge at a pulp and paper mill by Elliott and Mahmood [4] suggested a 9-year payback

period for implementing anaerobic digestion in a pulp and paper mill which was,

unfortunately, considered not acceptable by the industry at that time.

2.3. Sludge Treatment Technologies to Enhance Anaerobic

Digestion

In order to overcome the slow and incomplete biosludge digestion, numerous sludge

disintegration methods have been developed including mechanical, thermal, chemical and

biological treatment. The objective of these treatments is to abacterially (except for

11

biological treatment) disintegrate and hydrolyze the sludge particles so that the anaerobic

microorganisms in the subsequent digestion can more easily access the organic matter

(Figure 2.2). Coupling sludge treatment with anaerobic digestion could make the overall

process more feasible for the pulp and paper industry.

Meyer and Edwards [5] provide a comprehensive review on the anaerobic digestion of

pulp and paper mill biosludge including a discussion on the biosludge pretreatment

technologies. Five approaches, namely: ultrasound and microwave, thermal,

hydrodynamic, chemical, and biological treatments were identified to have potential

relevance for pulp and paper mill sludge. Among them, thermal treatment is especially

promising, since the high temperature applications in mills produces surplus steam that

may be coupled with thermal treatment.

Carrere et al. [17], and Elliott and Mahmood [4] also review the sludge pretreatment

technologies to enhance anaerobic digestibility. They both suggest that thermal treatment

may be energy intensive, but provides the most sludge solubilization and biogas

production enhancement.

2.4. Thermal Treatment

Thermal treatment subjects a substrate to high temperatures under pressure for a defined

length of time. The heat and pressure ruptures the cell walls and decouples the long-chain

polymers. Thermal treatment is conventionally coupled with the anaerobic digestion of

biosludge as a pretreatment process meaning treating the biosludge before entering the

anaerobic digester. However, numerous studies have explored alternative thermal

treatment strategies including digestate thermal treatment [19, 20, 6].

12

2.4.1.Thermal Pretreatment of Biosludge

Thermal pretreatment of biosludge subjects the biosludge usually to temperatures usually

ranging between 150 to 200o C for 15 to 60 min [17, 4]. A few studies focus on the pulp

and paper mill biosludge, and many studies focus on the municipal biosludge. The

optimal thermal pretreatment conditions for municipal biosludge seem to be around 170o

C for at least 15 min.

Many studies aim to optimize thermal treatment by finding the optimal treatment

conditions. Stuckey and McCarty [21] studied the effect of treatment temperatures

ranging from 150 to 225o C with a retention time of 1 h on the biosludge anaerobic

digestibility. They found that the biodigestibility increases with increasing treatment

temperature up to a maximum digestibility at a temperature of 175o C. Beyond that

temperature, significant inhibition was observed. The biogas production at the optimal

treatment conditions was 27% greater than that of the control. Li and Noike [22]

investigated the effect of treatment temperatures ranging from 62 to 175oC and retention

times from ranging 15 to 120 min on the biosludge anaerobic digestibility. The optimal

treatment conditions were found to be 170o C for 60 min. Full-scale installations, such as

Cambi process, also operated at temperatures ranging from 165 to 180o C with a 30-min

retention time [23, 24].

Some studies also suggest that the initial digestibility of biosludge plays a role in the

effectiveness of thermal pretreatment. Wood et al. [25] subjected Kraft mill WAS and

sulfite mill WAS to thermal pretreatment at 170o C for 1 h. The Kraft mill WAS, which

was four times less digestible than sulfite mill WAS, yielded a 280% increase in biogas

production after thermal pretreatment, while only a 55% increase was achieved for sulfite

mill WAS. Carrere et al. [26] investigated the effect of thermal treatment on the

13

digestibility of six biosludge samples at temperatures ranging from 60o to 210

o C for a

retention time of 30 min. They found that the biogas production enhancement is not only

a function of solubilization but also a function of initial digestibility. The lower the

sludge digestibility leads to a greater biogas production enhancement after thermal

treatment. It seems the thermal treatment is more effective to treat the difficult-to-digest

biosludge.

2.4.2.Digestate Thermal Treatment (Interstage Treatment)

Since thermal treatment is more effective for difficult-to-digest sludge, the thermal

treatment of digestate should greatly enhance the anaerobic digestibility of the digestate.

Digestate by definition is the residue remains after the anaerobic digestion of biosludge

(content at the bottom of a digester); therefore, it can be considered the most refractory

portion of biosludge. Only a few studies have investigated the effect of thermal treatment

of municipal digestate; no study has conducted using the pulp and paper mill related

digestate.

Pierides [6] studied the thermal treatment of municipal digestate using a continuous-flow

thermal reactor with steam injection as the heating mechanism. The operating

temperatures were varied between 25 and 220

o C, and the retention times were varied

between 10 and 180 sec. Municipal digestate, biosludge and primary sludge were tested

in this study. For digestate, the treatment conditions of 220o C for 60 sec produced the

best results with a 100% increase in the ultimate biodegradation. For biosludge, the

optimal treatment conditions were observed at 220o

C and 10 sec which led to an 80%

increase in the ultimate biodegradation. No improvement was observed for the primary

sludge used in the study.

14

Garzon-Lopez [27] used the same thermal reactor to study the effect of thermal treatment

operating parameters including pH, retention time and temperature on the solubilization

and digestibility of digestate. The operating temperatures were varied from 140 to 260o C;

the retention times were varied from 10 to 30 sec; and the pH was varied between 2 to 12.

The best improvements in terms of an increase in methane production were obtained

under mild alkaline conditions with a pH between 7 and 9.5 and at a relatively high

temperature of 250o C. The study also indicated that a large portion of the soluble

organics were refractory.

Pinnekamp [28] investigated the effect of thermal treatment temperatures (from 120 to

220o C) and sludge types (municipal digested sludge, biosludge and primary sludge). A

pilot-scale experimental setup was used in this study. The digested sludge (digestate)

before thermal treatment had a substantially lower biogas yield compared to biosludge

and primary sludge. After thermal treatment at 180o C, the biogas yield increased by 270%

which was comparable to the biogas yield for untreated primary sludge.

Nielsen et al. [7] compared the different configurations of thermal treatment and

anaerobic digestion using municipal biosludge on a batch-scale basis. Pretreatments at

80o, 130

o, 170

o C, and 170

o C at a pH of 10 were compared with the interstage digestate

treatments at the same conditions. The overall anaerobic digestion time for all the trials

was kept the same at 40 days. The results show that the pretreatment at 80o C had no

effect on methane production while a 20% increase was observed for the interstage

treatment. Pretreatment at 130o C, 170

o C, and 170

o C at a pH 10 considerably increased

the methane production within the first 4 days but the ultimate methane production was

improved by only 13%, 9% and 2%, respectively. For interstage treatments, a 29%

improvement in the ultimate methane yield was observed at the treatment temperature of

15

170o C. Thus, it was concluded that the interstage treatment is preferred over the

pretreatment.

Takashima [8] also looked at the different configurations of thermal treatment and

anaerobic digestion in continuous bench scale experiments. The thermal treatment was

performed at 120o C for 1 h for all of the configurations. Three configurations were

investigated: pretreatment of biosludge followed by anaerobic digestion; post-treatment

of digestate followed by reinjection back to anaerobic digester; interstage treatment of

digestate followed by sending the treated digestate to a different digester. The results

show that the post-treatment and interstage were more effective in terms of biogas

production enhancement than pretreatment. The average rate of biogas production for the

control, pretreatment, post-treatment and interstage case was 0.78, 0.791, 0.913 and 0.966

L/day, respectively.

2.4.3.Thermal Hydrolysate Recycle

Rather than anaerobically digesting the entire portion of treated sludge containing

hydrolysate and solids residue, an alternative approach is to separate the hydrolysate from

the solids residue and only recycle the hydrolysate. Hydrolysate recycling has several

advantages including: requiring short anaerobic hydrolytic retention time and eliminating

any problem with the solids particle recycling. The separation of treated sludge has been

recommended in a number of published studies.

Zheng [29] proposed a thermal treatment system that first separates the thermally treated

digestate into their solids and liquid portion using a centrifuge. The centrate (hydrolysate)

is then sent to an anaerobic filter reactor for anaerobic digestion. This process requires an

anaerobic filter reactor with a small volume which is more attractive to a wastewater

16

treatment plant where space is limited. However, the anaerobic digestion of hydrolysate

was not investigated in that study.

Wood [12] also proposed the separation of the treated sludge into hydrolysate and solids

residue, and only recycling the hydrolysate to an up-flow anaerobic sludge blanket. The

purpose of the separation is to avoid clogging of the up-flow anaerobic digester. However,

once again the anaerobic digestion of hydrolysate was not investigated in this research.

Bougrier et al. [30] compared the biogas volume produced from the soluble and

particulate phases of biosludge treated at temperatures ranging from 20 to 190o C for 30

min. For untreated biosludge, 91% of the biogas came from the particulate phase. After

thermal treatment at 190o C, only 43% of the biogas came from the particulate phase. It

was also found that the organic matter present in the soluble phase was digested more

completed and at a faster rate than that in the particulate phase.

2.5. Summary of the Existing Literature

Pulp and paper mill wastewater treatment plant produces a large amount of biosludge that

is costly to dispose of. The high disposal cost is due to the difficult-to-dewater nature of

the biosludge. To more effectively handle this biosludge, anaerobic digestion is an

attractive method that can recover energy from wet sludge as well further digests sludge

to reduce the volume.

Unfortunately, the slow and incomplete sludge digestion results in a long retention time

required, so the biosludge anaerobic digestion has not been widely used in the Canadian

pulp and paper industry. To overcome this problem, various treatment processes have

been proposed to partner with biosludge digestion. Among them, thermal treatment

17

which is conventionally arranged in a pretreatment configuration has shown to have the

greatest treatment potential on biosludge.

Numerous studies suggest that thermal treatment is more effective to treated

difficult-to-digest sludge. Unfortunately, no study has investigated the thermal treatment

of pulp and paper mill digestate; a few studies has focused on municipal digestate and

they suggest that interstage digestate thermal treatment is more preferable than

pretreatment of biosludge in terms of biogas production enhancement.

Anaerobic digestion of treated digestate can be carried out in two ways: first, the entire

treated digestate is recycled; an alternative approach is to separate the hydrolysate from

the suspended solids and only recycled the hydrolysate. The hydrolysate recycling has the

advantage of requiring shorter retention times and eliminating any problems with the

refractory solids particle accumulation in the digester. However, the anaerobic digestion

of hydrolysate produced from thermal treatment of pulp and paper mill digestate has not

been systematically investigated.

18

3. Materials and Methods

This research consists of two parts. In part one, the anaerobic digestion of biosludge and

digestate with or without the thermal treatment was investigated in order to compare the

performance of biosludge pretreatment and digestate thermal treatment in terms of biogas

production. In part two, the anaerobic digestion of hydrolysate and solids residue

produced by thermally treating digestate was investigated individually in order to

evaluate the potential of hydrolysate recycling for anaerobic digestion. The experimental

layout is illustrated in Figure 3.1and Figure 3.2.

The biosludge and digestate were collected and handled as described in Section 3.1.

Thermal treatment was performed using the thermal reactor described in Section 3.2.

Section 3.3 describes the biomethane potential (BMP) test which was used to evaluate the

biogas production from the different substrates (the treated or untreated sludge). The

biogas production data was then modeled as described in Section 3.4. Alongside with

BMP test, the sludge was characterized using the analytical methods described in Section

3.5. Finally, large batch scale experiments were conducted using the set-up described in

Section 3.6.

19

Biosludge 1

Effects of biosludge

pretreatment

170° C for 1 h

Digestate 1

Effects of digestate treatment

180° C for 0 min

210° C for 0 min

190° C for 0 min

190° C for 30 min

Characterization+

Bench-scale digestion

(BMP test)

Characterization+

Bench-scale digestion

(BMP test)

Biosludge 3

Mixing at 1:2, 1:1, and 0.5:1

COD ratio

Characterization+

Bench-scale digestion

(BMP test)

Effect of co-digestion Digestate

2

210° C for 0 min steam explosion

190° C for 30 min

Characterization+

Bench-scale digestion(BMP test)

Biosludge 2 as reference

Large batch scale experiments:

pretreatment and digestate treatment

Biogas production monitoring and

analyses

210° C for 0 min steam explosion

Figure 3.1: Overall experimental layout for part 1 of the research

20

Biosludge 3

210° C for 30 min, (optimal treatment)

190° C for 15 min, (sub-optimal treatment)

Bench-scale digestion

(BMP test)

Digestate 3

Effect of hydrolysate

digestion

170° C for 0,15 and 30 min

210° C for 0, 15, and 30 min

190° C for 0, 15, and 30 min

Characterization+

Bench-scale digestion

(BMP test)

Centrifuge at 5000 rpm for

15 min

Hydrolysate samples

Biosludge 4

Mixing at 1:4, 1:6 and 1:8 COD ratio

Centrifuge at 5000 rpm for

15 min

Hydrolysate samples

Solidresidue

Digestion of solid

residue

Co-digestion ofBiosludge

and hydrolysate

Digestion of treated sludge

Determine the optimal treatment conditions

Figure 3.2: Overall experimental plan for part 2 of the research

3.1. Biosludge and Digestate Collection and handling

Biosludge sample were shipped from a Canadian integrated pulp and paper mill. The

treatment plant uses primary and secondary (aerobic) treatment to treat wastewater

generated from sulfite pulping, mechanical pulping (BCTMP), and paper and board

production. The biosludge in this research refers to the sludge generated from the

secondary treatment. Samples were stored in a fridge at 4o C and were processed within

2-3 days upon their arrival. Four sets of biosludge samples were used in this study.

Biosludge 1 was shipped on February 2014. Biosludge 2 was shipped on May 2014,

Biosludge 3 was shipped on July 2014, and Biosludge 4 was shipped on March 2015.

21

The anaerobic bacteria cultures used as inocula in both the BMP tests and the large batch

scale experiments were obtained from an up-flow anaerobic sludge blanket reactor

operating in the same wastewater treatment plant. The cultures were received in a

granular sludge form. The sample was stored in a fridge at 4o C after purging with

nitrogen gas.

The digestate samples were produced by anaerobically digesting biosludge using a 10 L

batch anaerobic digester as shown in Figure 3.3. Three batches of digestate were

produced. Digestate 1 was anaerobically digested for 60 days, Digestate 2 was

anaerobically digestate for 36 days, and Digestate 3 was anaerobically digested for 30

days. For all runs, the ratio of biosludge COD to the input inoculum VSS was 1.6 g /g.

Details regarding the operation of large batch-scale digester and the production of

digestate can be found in Huang el al. [31]. Digestate samples were stored in a fridge at

4o C and processed within 48 h after removing from the digester.

22

Bench-scale

Anaerobic Digester

(10L)

Wet Tip

Gas Meter

Biogas

37°C Water Bath

Water Bath feed to reactor jacket

Reactor jacket water return

Stirrer Motor

Biogas Exhaust

Figure 3.3: 10 L large batch-scale batch digester [31]

The thermally treated sludge samples (biosludge or digestate) would lose their biological

activity and could be treated as organic chemicals. They were stored in sterilized glass

containers at 4o C.

3.2. Bench-scale Thermal Reactor

The thermal treatment experiments were performed using a 144 mL tubular batch reactor

constructed from 316 stainless steel Swagelok® parts (Figure 3.4). Similar reactors have

been in past studies [32, 33, 34]. For each thermal treatment experimental trial, 100 mL

of biosludge or digestate was transferred into the reactor, and the head space of the

reactor was purged with nitrogen gas to prevent oxidation effects. The reactor was then

23

mounted onto a metal frame and heated using a Lindberg Blue M® tubular furnace. The

temperature was monitored using a type K thermocouple and controlled by manually

opening and closing the furnace lid. In all experiments, the time required to increase the

sludge from ambient temperature to the target temperature was 5±1 min. After thermal

treatment, the temperature was brought down by one of two ways: submerging the reactor

into cooling water (required less than 1 min to cool to room temperature); or releasing the

reactor content rapidly to a flash tank at ambient pressure, which is also known as the

steam explosion. After steam explosion, the temperature instantaneously dropped to the

boiling point of the effluent (or about 100 oC).

144 mL Tubular Reactor

Quick Release Ball Valve

Type K Thermocouple

Pressure Indicator

Interior of Lindberg

Blue M® furnace

Figure 3.4: Bench-scale thermal reactor set-up

3.3. Biomethane Potential (BMP) Test

The digestibility of biosludge or digestate was tested using the BMP tests. This test is a

standardized batch anaerobic digestion test. The volume of biogas produced during the

incubation period is measured, and the rate of biogas production can be calculated from

the volume of the biogas produced over a specific time interval. An incubation time of 30

days is usually selected since a full-scale digester will operate at a retention time of less

than 30 days. However, in this study a longer inculcation time (approximately 50 days or

24

35 days) was selected that aimed to capture the full extent of digestion. The BMP assay

performed in this study is based on that performed by Wood et al. [12].

The substrate assays were prepared in a glove bag in an atmosphere consisting of 80%

nitrogen and 20% CO2. Each assay bottle (160 mL, with a working volume of 80 mL)

contained a mixture of medium, substrate, and inocula. A fixed volume of 50 mL of

medium which contains all the essential micronutrients for anaerobic cultivation was

added to each bottle. The composition of the medium can be found elsewhere [35]. Then,

110 mg COD of the biosludge or digestate with or without treatment was added as

substrate. Each bottle was inoculated with 60 mg VSS of anaerobic granular sludge. The

ratio of substrate to inocula, therefore, was 1.8 g COD substrate/ g VSS inocula. The

mixture was then diluted to 80 mL using Milli-Q water. All bottles were incubated at the

mesophilic optimal growth temperature of 37o C. The biogas produced was measured

using a 5 mL or 25 mL glass syringe.

Alongside with the substrate assays described above, negative control and positive

control assays were prepared. In these two cases, the substrate was replaced with Milli-Q

water for the negative control assays and a mixture of 50% COD glucose, 45% COD

sodium acetate, and 5% COD sodium propionate was used for the positive control assays.

The negative control accounts for the background gas production that is not related to the

substrate digestion. In order to determine the biogas production from the substrate only,

25

the biogas production from the substrate assay is subtracted by the biogas production

from the negative control assay. The specific biogas production (SBP) from substrate is

then calculated using Equation 1:

(1).

The positive control determines the maximum activity of the inocula in other words the

maximal biogas production from the specific inocula. The biodigestibility (BD) can then

be calculated using Equation 2:

(2).

3.4. Modelling of Biogas Production

Empirical non-linear regression models were used to model the cumulative biogas

production data. The aim of biogas production modeling was to better understand how

thermal treatment affects the biogas production kinetics. In particular, the non-linear

regression modeling can be used to predict the ultimate biogas production potential,

calculate the maximum biogas production rate, and estimate the lag phase duration of

anaerobic digestion.

26

The modified Gompertz equation [36] was used to describe the biogas production

behavior, under the assumption that the biogas production kinetics has only one phase.

The modified Gompertz equation is shown in Equation 3:

(3),

where y is the total biogas accumulation (mL biogas/ mg COD input) at time t (h); A is

the ultimate biogas production potential (mL biogas/ mg COD input); µm is the maximum

biogas production rate (mL biogas/ (mg COD input*h)); λ is the lag phase (h); and e is

the base of the natural logarithm that approximately equals to 2.718282.

However, a two-phase (two-phase) biogas production behavior was observed in this study.

As a result, this behavior cannot be accurately modeled using Equation 3. To overcome

this limitation, Equation 3 was modified into Equation 4 to take into account the

two-phase behavior:

(4).

Equation 4 captures the digestion behavior of both readily digestible organics and

delay-digest organics. A1, µm1 and λ1 are coefficients associated with the readily

digestible organics which is broken down during the first phase of digestion. A2, µm2 and

λ2 are coefficients associated with the delay-digest organics which is digested during the

second phase of digestion. To fit the data, Curve Fitting toolbox, a Matlab® built-in

27

application, was used. The root mean square errors (RMSE) method was used to find the

optimum model parameters.

3.5. Sludge Characterization

This section describes the methods used to characterize the biosludge or digestate with or

without treatment. By comparing the treated and untreated samples, we can better

understand how thermal treatment affects the properties of the sludge.

3.5.1.Sludge Fractionation

In this thesis, the “hydrolysate” of a sample is referred to as the soluble portion of the

sample; these two terms are used interchangeably. To obtain hydrolysate, a sample was

centrifuged at 5000 rpm for 15 min; and then the centrate was filtered using a Whatman®

934-AH glass fibre filter paper with a pore diameter of 1.5 µm. The “solids residue” of

the sample refers to the solids remaining in the centrifuge tube. The “total” sample refers

to a sludge sample that was not fractionated. It was analyzed as a whole (containing both

the hydrolysate and solids residue).

3.5.2.Caustic Extraction

To characterize the solids residue of a sample, caustic extraction was performed. The

extraction liberates the organic compounds in flocs and cells into the caustic extractive

which can be further analyzed. The caustic extraction is reported to have the highest

extraction yield and able to extract intercellular organics [37, 38] hence the underlying

reasons as to why it was in this study. The caustic extraction procedure can be found in

Comte et al. [37] with the exception that in the present work only 10 mL of sludge was

28

used for extraction due to the limitation in the sample size. In this study, the caustic

extractive was further analyzed.

3.5.3.Total and Volatile Solids and Suspended Solids

Total solids (TS), volatile Solids (VS), total suspended solids (TSS) and volatile

suspended solids (VSS) were measured in triplicate according to Standard Methods for

the Examination of Water and Wastewater #2540B-E [39]. TS and VS represent the

amount of particles and organic particles present in the entire sludge. TSS and VSS

represent the amount of particles and organic particles present in the solids residue.

3.5.4.Chemical Oxygen Demand (COD)

The COD is defined as the total amount of oxygen required to completely oxidize the

organic compounds in a sample. It provides a rough indication of the organic compounds

that are available for digestion. The total chemical oxygen demand (TCOD) and soluble

chemical oxygen demand (SCOD) was measured using Hach® high range (20 -1500

mg/L COD) digestion vials which is in compliance with Standard Methods for the

Examination of Water and Wastewater #5220D [39]. The COD in the solids residue can

be calculated using the following equation:

(5).

3.5.5.Carbohydrates

Carbohydrates are a digestible biopolymer present abundantly in sludge. Based on past

studies [3, 4], it was hypothesized that the concentration of carbohydrate roughly

indicates how readily digestible a sludge is. Only the soluble and caustic extracted

29

fraction of the samples was measured according to the sulfuric acid phenol method

described in Biochemical Method Ch 1.6 [40].

3.5.6.Tannin and Lignin

Tannin and lignin are less digestible materials present in the wood-related biomaterial.

They may cause inhibition to the digestion process [41, 18]. The soluble and caustic

extracted tannin/lignin were measured using Hach® tannin-lignin test kit which is based

on tyrosine colorimetric method developed by Kloster el al. [42].

3.5.7.Particle Size Analysis

The particle size analysis test was conducted using a Malvern® Mastersizer 2000

equipped with Hydro 2000G. This type of instrument has been used by others to analyze

biosludge particle size distribution [43, 44].

3.6. Large Batch-scale Experiments

The large batch-scale anaerobic digestion experiments were conducted using the 10 L

digester that was also used to produce digestate. A 500 mL Parr® reactor was used to

perform the thermal treatment (Figure 3.5). For the pretreatment experiment, 7.5 L of

biosludge was thermally treated at 190o C for 30 min and then injected into the digester

with 1.9 L of inocula. For the digestate treatment experiment, 7.7 L of biosludge was

digested with 1.8 L of inocula; at day 13, 18, 25 and 31, 2 L of the digestate was removed,

thermally treated at 190o C for 30 min, and then re-injected back into the digester. For

both treatments, the initial ratio of biosludge COD to inoculum VSS was 3.8 g /g. The

performance of the two treatments was compared in terms of the biogas production.

Details regarding to the digester operation can be found in Huang el al. [31]. The large

30

batch-scale experiments were conducted at a higher solids concentration compared to the

BMP tests; thus they were more comparable to a full-scale digester.

Anaerobic

Digester

Wet Tip

Gas Meter

37° C Water Bath

Water Bath

to Reactor Jacket

Stirrer

Motor

Outlet

Inlet Hopper

500 mL

Reactor

Not

Continuous

Feeding;

Manually

Feeding

Gas Exhaust

Figure 3.5: Large batch-scale experimental set-up; (the digester and thermal reactor)

31

4. Anaerobic Digestion of Untreated and

Thermally Treated Biosludge and Digestate

This chapter is devoted to the discussion of the anaerobic digestion of the biosludge and

digestate with or without thermal treatment. The performance of biosludge thermal

pretreatment and digestate thermal treatment was compared in terms of the biogas

production primarily based on the BMP tests. Then, the data from the BMP tests was

used as the basis for the large batch-scale experiments. The overall experimental

procedure is described in Section 3 Figure 3.1.

4.1. Objectives

As previously stated, we hypothesize that the thermal treatment of digestate is more

advantageous than thermal pretreatment of biosludge in terms of the overall biogas yield.

To prove this hypothesis, the specific objectives of this section are:

To investigate the effects of thermal pretreatment under the theoretical optimal

treatment conditions (170o C for 1 h) according to the literature on biosludge

anaerobic digestibility using BMP tests.

To investigate the effects of thermal treatment under various treatment conditions on

the digestate anaerobic digestibility using BMP tests. Effects of temperature,

retention time and steam explosion were studied.

To evaluate the effects of anaerobically co-digesting the thermally treated digestate

with the untreated biosludge at various mixing ratio using BMP tests.

32

To confirm the results obtained from the BMP tests by conducting large batch-scale

experiments.

4.2. Results and Discussion

4.2.1.Untreated Biosludge and Digestate

In this section of the study, three biosludge samples were used that were directly shipped

from the mill wastewater treatment plant. Biosludge 1 was used to determine the effects

of biosludge thermal pretreatment. It was thermally treated, and its digestibility was

characterized using the BMP tests. Biosludge 2 was used as a reference material in the

study of digestate thermal treatment. Its digestibility was characterized using BMP tests.

Biosludge 3 was used in the co-digestion of biosludge and treated digestate. It was mixed

with the treated digestate at various ratios, and the digestibility of the mixtures was

characterized using BMP tests. The properties of the three biosludge are shown in Table

4.1.

Table 4.1: Properties of the biosludge samples used in this section of study; all analysis preformed in

triplicates

Biosludge 1 Biosludge 2 Biosludge 3

Shipment date 27-Feb-14 07-May-14 14-Jul-14

Thickening before treatment NO NO NO

TSS (g/L) 10.8±0.1 NA 13.9±0.4

VSS (g/L) 9.7±0.1 NA 12.62±0.3

VSS/TSS 90% NA 91%

TS (g/L) 13.3±0.3 15.3±0.2 15.6±0.5

VS (g/L) 10.3±0.2 12.4±0.1 12.5±0.4

VS/TS 77% 81% 80%

TCOD (g/L) 14.9±0.2 19.0±0.6 21.1±0.3

SCOD (g/L) 0.7±0.0 NA 0.4±0.1

SCOD/TCOD 5% NA 2%

33

From Table 4.1, A few observations can be made from the values presented. The TSS, TS,

and TCOD varied between 10.8-13.9 g TSS/L, 13.3-15.6 g TS/L, and 14.9-21.1 g COD/L,

respectively. Most of the biosludge solids are organic in nature as the VSS/TSS ratio for

two out of the three samples was greater than 90%. Most of the organic matter was

distributed in the particulate (solids) phase of the sludge as the SCOD/TCOD ratio for

two out of the three samples was lower than 5%. The above observations indicate that the

pulp and paper mill biosludge had a high organic content and a low inorganic content,

which made it a suitable candidate for anaerobic digestion. However, most of the

organics were present in the particulate form and required pretreatment prior to digestion.

The two digestate samples were produced through two anaerobic digestion experiments

preformed by Xian Huang [31]. Digestate 1 was used to determine the effects of digestate

thermal treatment. It was thermally treated, and its digestibility was characterized using

the BMP test. Digestate 2 was used in the co-digestion of biosludge and treated digestate.

It was thermally treated and then mixed with the Biosludge 3 at various ratios. The

properties of the two digestate samples are shown in Table 4.2.

Table 4.2: Properties of the digestate samples used in this section of study; the digestate samples were

produced by Xian Huang; all analysis were preformed in triplicates.

Digestate 1 Digestate 2

Days of anaerobic digestion 61 36

Thickening 2000 rpm30 min 2000 rpm 30 min

TSS (g/L) 38.8±1.5 55.6±0.5

VSS (g/L) 32.7±1.6 41.8±0.64

VSS/TSS 84% 75%

TS (g/L) 51.6±1.2 68.7±0.6

VS (g/L) 38.9±1.0 47.3±0.5

VS/TS 75% 69%

SCOD (g/L) 3.7±0.0 5.7±0.0

TCOD (g/L) 50.3±4.1 64.0±0.5

SCOD/TCOD 7% 9%

34

Digestate 1 and Digestate 2 were produced by anaerobically digesting the biosludge for

61 days and 36 days in the 10 L anaerobic digester, respectively. Before thermal

treatment, both digestate samples were thickened to 40-60 g TSS/L to simulate

high-solid-load thermal treatment. Comparing the biosludge samples, the digestate solids

contained lower amount of organics, as the VSS/TSS ratio for both digestate samples was

less than 85%. This was expected because as the biosludge is digested, the organic matter

is depleted and the digestate becomes more refractory. Also, a greater amount of organic

matter is presented in the soluble phase of the digestate compared to that of the biosludge.

The SCOD of the digestate samples was approximately 10 times greater than that of the

biosludge samples. This was also expected since as the first stage of anaerobic digestion

is hydrolysis, the particulate organic matter in biosludge will hydrolyze and be measured

as the SCOD.

Figure 6 shows the BMP data for untreated biosludge and digestate samples. The three

biosludge samples have different digestibilities. The SBP varied between 0.22-0.29

mL/mg COD input. Compared to the positive control which had an SBP between

0.55-0.59 mL/mg COD input, the BD for the biosludge samples was between 41-50%.

The digestate samples, as expected, were more refractory as the SBP was less than 0.1

mL/mg COD input and the BD was approximatly 15%.

35

Figure 4.1: BMP test for the untreated biosludge and digestate samples

4.2.2.Effects of Biosludge Thermal Pretreatment

The thermal treatment of Biosludge 1 was conducted at 170 oC for 1 h. This treatment

condition was selected based on past studies which suggested that the optimal thermal

treatment conditions ranged from 170-180 oC for 15-120 min [21, 45, 28, 22]. According

to Table 4.3, after thermal treatment, a substantial amount of sludge solids was

solubilized. The SCOD increased from 0.7 g COD/L to 5.3 g COD/L, and the amount of

TSS and VSS decreased by 48% and 43%, respectively, after thermal treatment. These

results are similar to those reported in the literature [30, 46, 26].

Table 4.3: The properties of treated and untreated biosludge 1; all analysis were preformed in triplicates.

Biosludge 1 Treated Biosludge 1

TSS (g/L) 10.8±0.1 5.6±1.6

VSS (g/L) 9.7±0.1 5.2±1.4

TCOD (g/L) 15.8±0.2 13.4±1.3

SCOD (g/L) 0.7±0 5.3±0.1

SCOD/TCOD % 4.43% 40%

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 10 20 30 40 50

SB

P m

L/m

g C

OD

in

pu

t

Day

Biosludge 1

Biosludge 2

Biosludge 3

Digestate 1

Digestate 2

36

Shown in Figure 4.2, the biogas production pattern was altered by the thermal treatment.

Initially, the treated biosludge produced a significantly greater amount of biogas, almost

three times greater, than that from the untreated biosludge for the first 10 days of

anaerobic digestion. This was expected as the high SCOD in the treated biosludge

contributed to this initial rapid biogas production. However, as the digestion progress, the

biogas production from the treated biosludge diminished, while the biogas production

from the untreated Biosludge 1 remained constant. After the 50th

day of digestion, the

SBP for the treated and untreated biosludge was approximately equal to 0.24 mL

biogas/mg COD. Compared to the SBP of the positive control, 0.58 mL/mg COD, the BD

for the treated and untreated biosludge was the same. The results show that the

solubilization of sludge due to thermal pretreatment increased the rate of biogas

production only during the early days of digestion and did not affect the ultimate biogas

production after extended digestion time. For instance, from Figure 4.2, the required

digestion time decrease from 30 days to 20 days using thermal pretreatment. This

reduction in digestion time translates to the reduction of digester size and hence the cost

of the digester. Previous studies reported a similar conclusion [8, 7].

37

Figure 4.2: BMP test for the treated and untreated biosludge and biogas production models using the

modified Gompertz equation; all measurements were performed in triplicates.

In Table 4.4, the results of biogas production modeling using the modified Gompertz

equation are presented. The models show good agreement with the experimental biogas

production data (Figure 4.2). The pretreated biosludge had a greater maximum biogas

production rate (µm) but a smaller ultimate biogas potential (A) than that for the untreated

biosludge. For the pretreated biosludge, the model was unable to predict the biogas

production at later stage of digestion. A secondary biogas production occurred after 35

days of digestion while the model predicted the plateau of biogas. The biogas production

for the pretreated biosludge seemed to consist of two phases (phase one before 30 days,

and phase two after 30 days) and needed to be modeled using the two phase model

(Equation 4). However, because only a few data points were taken after day 30th

, the two

phase model is not reliable. The two phase model was included in Figure 4.2 as reference

but is not further discussed here.

0

0.05

0.1

0.15

0.2

0.25

0.3

0 10 20 30 40 50

SB

P m

L/m

g C

OD

in

pu

t

Day

Biosludge 1 Model

Treated Biosludge 1 Model

Treated Biosludge 1 Two-

Phase Model

Treated

Biosludge 1 at

170 oC for 1h

Biosludge 1

38

Table 4.4: The biogas production modeling for the treated and untreated Biosludge 1 using the modified

Gompertz equation

Biosludge 1 Treated Biosludge 1

BD 51% 51%

SBP (mL/mg COD input) 0.24 0.24

Gompertz parameters

A (mL/mg COD input) 0.26 0.21

µm (mL/(mg COD input*h)) 0.00038 0.000653

λ (h) 41.44 0

RMSE 0.0073 0.019

Whether or not thermal pretreatment can increase the biosludge biogas yield is still

inconclusive in the literature, e.g.: Wood et al. (pulp mill WAS treated at 170o C for 1 h)

[25] observed a 55% increase in biogas production after thermal treatment; on the

contrary, Nielsen et al. (municipal WAS treated at 170o C for 15 min) [7] reported only a

2% increase in biogas production. In this study, based on BMP assays thermal treatment

did not change the ultimate biogas yield.

4.2.3.Thermal Treatment of Digestate

Thickened Digestate 1 and thickened Digestate 2 were thermally treated at various

treatment conditions. Table 4.5 lists the treatment conditions that were preformed. The

effects of temperature, retention time and steam explosion were investigated.

The experiment 1, 2, and 3 studied the effects of treatment temperature ranging from 180

to 210o C. The temperatures were selected in accordance with Pierides [6] who suggested

that the digestate should be treated at a higher temperature (greater than 170o C)

compared to biosludge due to the refractory nature of digestate.

39

Experiment 2 and 5 studied the effects of increasing retention time from 0 min to 30 min.

A retention time of 0 min was used to simulate rapid heating, meaning that the reactor

was cooled to terminate the reaction as soon as the temperature reached the target

temperature. According to literature, thermal treatment with short retention time can

minimize the generation of inhibitory compounds and improve the digestibility of treated

biosludge [32, 27, 6].

The effect of steam explosion compared to submerging the reactor into cooling water was

evaluated using Digestate 1. Two experiments (Experiments 3 and 4) were both

conducted at 210o C for 0 min: one was cooled using cooling water, and the other was

subjected to steam explosion. Both samples were characterized and results were

compared to examine the effects of steam explosion.

Table 4.5: Thermal treatment of Digestate 1 and Digestate 2; Experiments 1, 2 and 3 studied the effects of

temperature; Experiments 2 and 5 studied the effects of retention time; Experiments 3 and 4 studied the

effects of steam explosion;

Exp Sludge Type TSS(VSS)

(g/L)

Temperature

(o C)

Retention time

(min)

Steam

Explosion

1 Digestate 1 38(33) 180 0 No

2 Digestate 1 38(33) 190 0 No

3 Digestate 1 38(33) 210 0 No

4 Digestate 1 38(33) 210 0 Yes

5 Digestate 1 38(33) 190 30 No

6 Digestate 2 56(42) 190 30 No

7 Digestate 2 56(42) 210 0 Yes

Sludge Solubilization

In Figure 4.3, the solids solubilization due to thermal treatment is presented. Thermal

treatment at 190o C for 30 min produced the highest sludge solubilization for both

Digestate 1 and Digestate 2 (Experiments 5 and 6). The degree of sludge solubilization

40

increased with increasing temperature and retention time. However, in the present study,

retention time had a more significant effect on sludge solubilization. By increasing

retention time from 0 to 30 min, the TSS/VSS solubilization increased from 12% to 40%

(Experiments 2 and 5). It was observed that increasing the temperature by 20o C, the

TSS/VSS solubilization only increased from 12% to 22% (Experiments 2 and 3).

Furthermore, steam explosion did not result in any additional increase in sludge

solubilization (Experiments 3 and 4). The effects of thermal treatment were less

pronounced for Digestate 2 possibly due to a higher initial TSS content.

Figure 4.3: Solids solubilization due to thermal treatment for Digestate 1and Digestate 2; SE refers to steam

explosion; all measurements were performed in triplicates.

In Figure 4.4, the COD solubilization due to thermal treatment is presented. A similar

trend compared to the solids solubilization was observed.

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

180˚ C 0

min

190˚ C 0

min

210˚ C 0

min

210˚ C 0

min SE

190˚ C 30

min

190˚ C 30

min

210˚ C 0

min SE

TS

S/V

SS

Red

uct

ion

TSS Reduction

VSS Reduction

Exp# 1 Exp# 7 Exp# 6 Exp# 5 Exp# 4 Exp# 3 Exp# 2

Treated

Digestate 1

Treated

Digestate 2

41

Figure 4.4: COD solubilization due to thermal treatment for Digestate 1and Digestate 2; SE refers to steam

explosion; all measurements were performed in triplicates.

Particle Size Analysis

The particle size distribution (on a percentage volume basis) for the untreated Biosludge

2, Digestate 1, and Digestate 2 is shown in Figure 4.5. The particle size for the digestate

was overall larger than the particle size for biosludge. The peak particle size shifted from

approximately 89 µm for the biosludge to 300 µm for digestate samples. The exact cause

of this observation is still unknown. There are many possible explanations including:

aggregation of sludge particles due to anaerobic digestion; and reduction of small

particles due to the digestion that leaves only the large particles behind.

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0

5

10

15

20

25

Digestate

1

180˚C 0

min

190˚C 0

min

210˚C 0

min

210˚C 0

min SE

190˚C 30

min

Digestate

2

210˚C 0

min SE

190˚C 30

min

CO

D (

mg

/L)

SCOD g/L

SCOD/TCOD %

Exp# 1 Exp# 7 Exp# 6 Exp# 5 Exp# 4 Exp# 3 Exp# 2

42

Figure 4.5: The particle size distribution for the untreated Biosludge 2, Digestate 1, and Digestate 2; data

presented in % volume. The numbers on the graph are the mode particle size in microns.

As thermal treatment ruptured the sludge particle, the average particle size for the treated

Digestate 1 samples reduced (Figure 4.6). It seems that thermal treatment disintegrated

the large particles and the peak particle size shifted back to the peak particle size of the

biosludge (89µm). Once again, the retention time was observed to be the dominant factor

associated with size reduction while an increase in temperature had a minor effect on

particle size reduction (dotted line) in these experiments. Steam explosion, an additional

physical disintegration method, promoted further size reduction (comparing cross and

triangles in Figure 4.6). A similar conclusion was made for the treated Digestate 2

samples (data is not shown here).

88.91

301.68

0

2

4

6

8

10

12

1 10 100 1000

Vo

lum

e %

Paritcle diameter (µm)

Biosludge 2

Digestate 2 (36 day digestion)

Digestate 1 (60 day digestion)

43

Figure 4.6: Particle size distribution for Digestate 1; SE refers to sample subjected to steam explosion. The

numbers on the graph are the mode particle size in microns.

BMP Tests for the Treated and Untreated Digestate

In Figure 4.7, the SBP for the treated and untreated Digestate 1 are shown. For

comparison, the SBP for untreated Biosludge 2 is also plotted as a reference. Overall, the

SBP for all treated Digestate 1 samples were notably higher than that of the untreated

Digestate 1. Despite the difference in the treatment conditions that resulted in the

different degrees of sludge solubilization, all of the treated digestate samples produced a

similar amount of biogas (within 10%) after 35 days of digestion. Compared to the

positive control which had an SBP of 0.59 mL/mg COD input, the BD for the treated

Digestate 1 samples varied from 33 to 37% which was two times higher than that of the

untreated Digestate 1 (BD=15%) and about two-third of that for the untreated Biosludge

2 (BD=51%). Shown in Figure 4.7, the difference in treatment conditions seems to only

affect the early stage rate of digestion. The SBP for the treated Digestate 1 at 190o C for

30 min was the highest during the first 15 days of digestion. The substantial increase in

0

1

2

3

4

5

6

1 10 100 1000

Vo

lum

e %

Paritcle diameter (um)

Digestate 1

180˚ C 0 min

190˚ C 0 min

190˚ C 30 min

210˚ C 0 min

210˚ C 0 min SE

44

digestate digestibility after thermal treatment was also reported in the studies by Pierides

[6], and Garzon-Lopez [27].

As discussed before, steam explosion caused further particle size reduction, but it had

little effect on the sludge digestibility. As seen in Figure 4.7, the SBP for treated

Digestate 1 at 210o C for 0 min with and without steam explosion were within 10% (dash

and dotted line). This indicates that the particle size reduction had minor effects on the

biogas production rate enhancement while the solubilization played a more significant

role in improving SBP.

Figure 4.7: The BMP results for the treated and untreated Digestate 1, and Biosludge 2; all measurements

were performed in triplicates. SE: steam explosion.

To confirm the finding reported in Figure 4.7 as well as preparing treated digestate for the

co-digestion experiment portion, a second set of tests were conducted using Digestate 2

samples. The SBP values for the treated and untreated Digestate 2 samples are presented

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 10 20 30 40 50

SB

P m

L/m

g C

OD

in

pu

t

Day

180˚C 0

min

190˚C 0

min

210˚C 0

min

210˚C 0

min SE

190˚C 30

min

Biosludge 2

Digestate 1

Treated

Digestate 1

45

in Figure 4.8. The overall trend for this set of tests is similar to that of the previous one.

The treatment at 190 oC for 30 min was the most effective treatment in terms of biogas

yield enhancement and the biogas production rate enhancement.

Figure 4.8: The BMP results for the treated and untreated Digestate 2, and Biosludge 3; all measurements

were performed in triplicates.

BMP Tests of the Co-digestion Mixtures

The co-digestion of biosludge and treated digestate was studied using mixtures of

Biosludge 3 and the treated Digestate 2 (treated at 210o C for 0 min) at the ratio of 1:1,

1:2 and 1:0.5 (based on COD). Since treated digestate was proposed to be re-injected

back into the anaerobic digester, it was necessary to establish that the re-injection of

treated digestate would not have any adverse effects on the anaerobic digestion process.

In Figure 4.9, the BMP results for the mixtures are presented. All mixtures produced a

greater amount of biogas than that from the untreated Biosludge 3 or the treated Digestate

2 individually. The optimal mixing ratios in terms of biogas yield enhancement were 1: 2

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 10 20 30 40 50

SB

P m

L/m

g C

OD

in

pu

t

Day

210˚C 0

min SE

190˚C 30

min

Biosludge 3

Digestate 2

Treated

Digestate 2

46

and 1:1 biosludge to treated digestate (in COD ratio). The SBP for these mixtures was 20%

and 84% greater than that for the untreated Biosludge 2 and the treated Digestate 3 at the

35th

day of digestion.

Figure 4.9: The BMP results for biosludge and treated digestate mixtures; all measurement was performed

at triplicates.

This result suggests that the co-digestion seems to have a synergistic effect on the biogas

production. Figure 4.10 shows the measured SBP versus the calculated SBP according to

the rule of mixtures as described in Equation 5:

(5).

The COD1 and COD2 refer to the COD in the feed that contributed to component 1

(biosludge) and component 2 (treated digestate) respectively. If the co-digestion did not

have synergistic or inhibitory effect, the measured SBP should be equal to the calculated

SBP (all points should be on the 45o line). If there were synergistic effects, then the

measured SBP would be greater than the calculated SBP which was shown in Figure 4.10.

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 10 20 30 40 50

SB

P m

L/m

g C

OD

in

pu

t

Day

Mixture 1 (1 Biosludge:2 Treated Digestate)

Mixture 2 (1 Biosludge:1 Treated Digestate)

Mixture 3 (2 Biosludge:1 Treated Digestate)

Treated

Digestate 2

Biosludge

3

47

The cause of this synergistic effect are unknown. It is hypothesized that the dilution of

the inhibitory compounds in the treated digestate by the biosludge may be the reason of

this synergistic effect.

Figure 4.10: The synergistic effect of the co-digestion

If the digestate thermal treatment of 210o C for 0 min was conducted, the following mass

balance could be derived as shown in Figure 5.5. Compared with anaerobic digestion

without any treatment, the overall COD conversion increased from 40% to 60%.

1 g COD of biosludge

0.6 g COD of digestate

Anaerobic digestion 40% COD reduction

0.6 g COD of Treated

digestate

ThermalTreatment

0.4 g COD Remaining

Anaerobic digestion 33% COD reduction

1 g starting COD 0.4 g ending COD

Figure 4.11: Overall COD balance for digestate thermal treatment

Biogas production modeling

0

0.05

0.1

0.15

0.2

0.25

0 0.05 0.1 0.15 0.2 0.25

Mea

sure

d S

BP

mL

/mg

CO

D i

np

ut

Calculated SBP mL/mg COD input

Mixture 1

Mixture 2

Mixture 3

x=y

48

The ultimate biogas potential (A) indicates the degrees of digestibility of a substrate; a

greater biogas potential suggests that the substrate can be digested to a greater extent. The

maximum biogas production rate (µm) indicates how fast the digestible component in a

substrate can be digested. The lag phase (λ) indicates the time required for the bacteria to

process a substrate (shown in Figure 2.2) into biogas; thus a more readily-digestible

material is expected to have a shorter λ. Based on the biogas production modeling

presented in Table 4.6 and Table 4.7, a number of observations were made:

The biosludge had a greater BD (larger A), faster rate of digestion (larger µm), and

needed less time required for inocula to acquire the substrate and produce biogas

(shorter λ) than those of the digestate, as expected, since digestate is the refectory

remains after biosludge anaerobic digestion.

The degree of solubilization represented by SCOD/TCOD ratio did not correlate to

the biodigestibility (BD) or the ultimate biogas potential (A).

The degree of solubilization represented by the SCOD/TCOD ratio seemed to

correlate to the maximum rate of biogas production (µm), i.e. a greater SCOD/TCOD

ratio lead to a greater the maximum rate of biogas production (µm).

The thermal treatment removed the lag phase such that all treated digestate samples

had a phase lag (λ) of zero. This may suggest that the hydrolysate can be directly

digested by the bacteria to produce biogas.

Table 4.6: The biogas production modeling for Biosludge 2, and Digestate 1 with and without treatment

using the modified Gompertz equation

SCOD /

TCOD BD

A (mL/mg

COD input)

µm (mL/(mg

COD input*h)) λ (h) RMSE

Biosludge 2 No

measured 51% 0.308 0.00039 20.2 0.005

49

Digestate 1 8% 15% 0.0926 0.00015 184.9 0.003

Exp#1

180˚C 0 min 20% 34% 0.196 0.00031 0 0.008

Exp#2

190˚C 0 min 21% 33% 0.191 0.00030 0 0.007

Exp#3

210˚C 0 min 29% 37% 0.2044 0.00039 0 0.013

Exp#4

210˚C 0 min SE 32% 33% 0.1804 0.00044 0 0.009

Exp#5

190˚C 30 min 41% 35% 0.188 0.00049 0 0.012

Table 4.7: The biogas production modeling for Biosludge 3, Digestate 2 with and without treatment, and

the co-digestion mixtures using the modified Gompertz equation

SCOD /

TCOD BD

A (mL/mg

COD input)

µm (mL/(mg

COD input*h)) λ (h) RMSE

Biosludge 3 2% 42% 0.248 0.00026 70.9 0.006

Digestate 2 9% 16% 0.096 0.00011 256.6 0.008

Exp#6

210˚C 0 min SE 18% 25% 0.125 0.00021 0 0.014

Exp#7

190˚C 30 min 32% 33% 0.148 0.00034 0 0.007

Mixture 1 13% 45% 0.253 0.00029 3.8 0.006

Mixture 2 10% 45% 0.249 0.00030 29.7 0.005

Mixture 3 7% 42% 0.227 0.00029 30.0 0.004

4.2.4.Carbohydrate and Tannin/Lignin Solubilization Due to

Thermal Treatment

To better understand the effects of digestate thermal treatment on the organic

solubilization, a series of organic composition analyses were performed on the treated

and untreated digestate samples. Carbohydrate and tannin/lignin were two main groups of

compounds investigated.

50

As observed in this study, the SCOD of digestate (both 1 and 2) was substantially greater

than the SCOD of biosludge (Figure 4.12). However, despite the high SCOD, the

digestate was less digestible than biosludge. This implies that the organic matter present

in the digestate hydrolysate was refractory. Moreover, the organic composition analysis

revealed that the concentration of soluble carbohydrate and soluble tannin/lignin in the

digestate hydrolysate reached a steady state of approximately 1:1 ratio.

Thermal treatment increased the SCOD as well as altered the organic compound

distribution in the hydrolysate (Figure 4.12). The most notable change was an increase in

the percentage of carbohydrates in the SCOD while there was little change in the

percentage of lignin/tannin compounds in the SCOD. It is hypothesized that the ratio of

carbohydrate and tannin/lignin may be another factor affecting anaerobic digestion

(indicated by the triangles in Figure 4.12). As this ratio was driven away from the plateau

ratio (which is about 1) by thermal treatment, further digestion could take place to reverse

this ratio back to possibly a plateau. Also, carbohydrate thermally hydrolyzed more easily

at the earlier stage of thermal treatment such that samples subjected to 0 min thermal

treatments had a high carbohydrate to tannin/lignin ratio. Longer retention times

promoted the solubilization of lignin/tannin such that at 30 min retention time samples

had a low carbohydrate to tannin/lignin ratio.

51

Figure 4.12: Ratio of soluble carbohydrate (carb) and tannin/lignin with respected to SCOD in Biosludge 3,

treated and untreated Digestate 1 and Digestate 2; SCOD is also included; SE refers to steam explosion; all

measurements were performed in triplicates.

The carbohydrate distribution in treated and untreated Digestate 1 is shown in Figure 4.13.

Higher temperatures and longer retention times promoted greater carbohydrate

solubilization. However, the sum of thermally hydrolyzed carbohydrate and caustic

extracted carbohydrate increased with increasing treatment temperatures (comparing

Experiments 1, 2 and 3). Retention time had no effect on the sum of thermally

hydrolyzed carbohydrate and caustic extracted carbohydrate (comparing Experiments 2

and 5). This possibly indicated that some non-extractable carbohydrates were converted

by thermal treatment into their soluble forms. For the experimental conditions used in this

study, the extent of carbohydrate solubilization of carbohydrate was only related to the

temperature.

The hydrolysis of carbohydrate can be done by caustic or heating. Caustic hydrolysis is a

chemical reaction that is able to hydrolyze most of the bacterial related polysaccharides

0

0.5

1

1.5

2

2.5

3

3.5

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Biosludge

3

Digestate 2

(36 days

digestion)

Digestate 1

(60 days

digestion)

210˚ C 0

min SE

Digestate 2

210˚ C 0

min SE

Digestate 1

190˚ C 30

min

Digestate 2

190˚ C 30

min

Digestate 1

Ra

tio

of

Ca

rbo

lyd

rate

to

Ta

nn

in/l

ign

in

Co

nce

ntr

ati

on

(g

/g S

CO

D)

Carb (g glucose/g SCOD)

tannin/lignin (g tannic/g SCOD)

Ratio of carbohydrate to tannin/lignin

Exp# 7 Exp# 6 Exp# 5 Exp# 4

52

such as capsular polysaccharides, and some fibrous related carbohydrate such as

hemicelluloses. However, caustic extraction at room temperature cannot be used to

hydrolyze cellulose. Thermal hydrolysis, on the other hand, has a different mechanism

and may be able to hydrolyze cellulose and other caustic non-extractable compounds [47,

48].

Figure 4.13: The concentration of soluble and caustic extracted carbohydrate for the treated and untreated

60-day digestate; SCOD is also included; all measurements were performed in triplicates.

An effective thermal treatment should selectively solubilize the readily digestible

organics while keeping the tannin/lignin intact since some hydrolysable lignin/tannin

compounds inhibit the anaerobic digestion [41]. Based on current study,

short-retention-time thermal treatment seemed to be more advantageous over

long-retention-time thermal treatment in terms of promoting carbohydrate solubilization

while keeping tannin/lignin intact. However, further experiments in the future are needed

to evaluate this hypothesis.

0

5

10

15

20

25

0

0.5

1

1.5

2

2.5

Digestate

1

180˚ C 0

min

190˚ C 0

min

210˚ C 0

min

210˚ C

0min SE

190˚ C 30

min

Ca

rbo

(g

glu

cose

/L)

Caustic extracted

carb (carbohydrate

caustic extracted

from solid residue )

Soluble Carb

(carbohydrate in

thermal hydrolysate)

SCOD (g/L)

Exp# 1 Exp# 5 Exp# 4 Exp# 3 Exp# 2

53

4.2.5.Biogas Production from Large Batch-scale Experiments

To examine the validity of findings the study from the BMP tests, large batch-scale

experiments were preformed in collaboration with Xian Huang [31]. Xian Huang

contributed to 100% of the experimental work, and some reviews and verification; all

data presented in this section were obtained from Xian Huang with permission. The

author contributed to 100% of the data analysis and 100% of the modeling.

Two large batch-scale runs were conducted in this study:

1) The pretreatment run where all biosludge was thermally treated at 190o C for 30 min

before loaded into the 10 L digester.

2) The interstage digestate treatment run where all biosludge was digested in the 10 L

digester for 12 days initially; then 2 L of the digestate was removed and thermally

treated at 190o C for 30 min at day 13, 18, 25 and, 31, and the treated digestate was

then re-injected back into the digester.

In Table 4.8, the initial properties of the biosludge for each run are listed. The biosludge

was first thickened by centrifuging before thermal treatment or anaerobic digestion. The

ratio of biosludge COD to the input inoculum VSS was 3.8 g /g for both runs. The

performance of each run was compared in terms of the biogas production.

Table 4.8: Properties of the initial biosludge for both runs

Thickened biosludge

properties

Initial reactor content after

mixed with inoculum Total

Volume (L) Initial Properties

TS

(g/L)

VS

(g/L)

COD

(g/L)

TS

(g/L) VS (g/L)

COD

(g/L)

Pretreatment run 29.2 23.4 39.6 51.9 39 47.2 9.5

Digestate

interstage run 27.9 23.3 38.5 42.5 32.1 49.2 9.5

54

In Figure 4.14, the biogas production data for both runs is presented. The biogas

production modeling parameters using the modified Gompertz equation are summarized

in Table 4.9.

Table 4.9: The biogas production modeling for the two large batch-scale experiments using the modified

Gompertz equation; *this lag phase (λ) is calculated after the re-inoculation.

A (mL/g COD

input)

µm (mL/(g COD

input*day) λ (day) RMSE

No treatment 125 11.4 0 (fixed at bound) 4.80

Digestate treatment 1 51.3 11.6 0.23 1.19

Digestate treatment 2 24.3 4.59 0 (fixed at bound) 1.61

Digestate treatment 3 16.3 3.64 0.23 0.32

Digestate treatment 4 49 5.14 0.675 0.871

Pretreatment 100 7.24 4.647 2.45

Figure 4.14: Biogas production from the large batch-scale pretreatment and interstage digestate treatment

0

50

100

150

200

250

0 10 20 30 40 50

Bio

ga

s P

rod

uct

ion

mL

/ g

CO

D

Days

4 interstage thermal treatments:

each treated 2 L out of 9.5 L digestate

Re-inoculation

Thermal pretreatment of

biosludge all at the beginning

1

4

3

2

Theoretical biogas production if not

treatment was preformed

55

The dotted line represents the theoretical biogas production modeled with the modified

Gompertz equation, if no treatment was preformed. After each digestate treatment, the

biogas production increased notably. It is also worth noting that after each thermal

treatment the subsequent treatment became less effective than the previous one with the

exception of the 4th

treatment (also shown in Table 4.9). This decrease in treatment

efficiency may be due to the dilution caused by treated digestate recycling. The content in

the reactor became less susceptible to thermal treatment after each treatment.

On the other hand, pretreatment inhibited the biogas production at a high organic

concentration. Minimal amounts of biogas were produced during the first 15 days of

digestion. To overcome this inhibition, the digester was re-inoculated with inocula at day

15. This inhibition is likely due to the drop in pH caused by the accumulation of volatile

fatty acids. Even after re-inoculation, the biogas was not produced at a higher rate or to a

greater amount compared to the theoretical biogas production. The results confirm that

the digestate thermal treatment is the preferred treatment method over the biosludge

pretreatment in terms of biogas production enhancement.

4.3. Summary and Conclusions

This section of research compares the performance of the biosludge thermal pretreatment

and the interstage digestate thermal treatment in terms of the biogas production

enhancement. The anaerobic digestibility of the biosludge and digestate were evaluated in

with BMP tests and in the large batch-scale experiments.

Based on the BMP tests:

1) Biosludge thermal pretreatment increased the maximum rate of biogas production

but did not increase the ultimate biogas potential.

56

2) Digestate thermal treatment improved both the rate and extent of anaerobic digestion,

more specifically:

The degree of sludge solubilization increased with increasing temperature and

retention time. Retention time was the more dominant factor affecting the degree

of solubilization than temperature.

Steam explosion, an additional physical disintegration mechanism, further

reduced the sludge particle size but did not affect the degree of sludge

solubilization and sludge anaerobic digestibility.

Degree of sludge solubilization only correlated to the maximum rate of biogas

production but did not affect the ultimate biogas potential.

The co-digestion of biosludge and the treated digestate seemed to have a

synergistic effect on the biogas production.

Sludge hydrolysate organic composition analysis revealed that the

short-retention-time thermal treatment seemed to be more advantageous over the

long-retention-time thermal treatment in terms of promoting carbohydrate

solubilization while keeping tannin/lignin content intact.

Based on the large batch-scale experiments:

1) Biosludge thermal pretreatment inhibited the biogas production at a high organic

concentration.

2) Interstage digestate thermal treatment for re-injection increased the biogas

production and could be preformed repeatedly, however:

57

Each cycle of digestate thermal treatment became less effective than the

previous one.

Overall, these results imply that the interstage digestate thermal treatment is the preferred

treatment over the biosludge thermal pretreatment, if optimizing biogas production is the

main objective.

58

5. Anaerobic Digestion of Digestate Thermal

Hydrolysate and Solids Residue

This chapter is discussed the anaerobic digestion of the thermal hydrolysate and solids

residue produced by thermally treating the digestate at various treatment conditions. The

hydrolysate and solids residue were separated as described in Section 3.5.1. The

anaerobic digestibility of the hydrolysate and solids residue was evaluated using the BMP

tests. The overall experimental procedure is described in Figure 3.2.

5.1. Rationales and Objectives

In section 4, it was concluded that the interstage digestate thermal treatment was the

preferred treatment than the biosludge thermal pretreatment in terms of optimizing biogas

production. However, there are some potential problems associated with re-injecting the

entire treated digestate for re-digestion including:

1. A constant recycling of treated digestate process will cause a higher concentration of

non-digestible solids within the digester and decrease the thermal reactor efficiency.

2. A larger digester volume is required to accommodate the recycled digestate. The

digester has to scale up accordingly to ensure sufficient solids and hydraulic

retention time.

In order to overcome 1, the digester sediment could be replenished regularly with fresh

sludge [49]. However, stopping the digester for sediments removal is costly and

adversely affects the economic feasibility of the process. For 2, there is not obvious

solution; it is an inherent trade-off for the system.

59

To address the above problems, it is proposed to first separate the thermally treated

digestate into the hydrolysate and the solids residue, and then re-inject only the

hydrolysate back to the digester. The solids residue is removed for further handling. The

advantages of this strategy include: 1) the recycled stream does not cause the

accumulation of the refractory solids in the reactor; 2) the solids loading of the reactor

will not increase, so the digester does not need to be scaled-up to maintain the solids

retention time; 3) anaerobic digestion of hydrolysate is expected to be quicker and more

completed than that of the entire treated digestate. To better understand the potential of

digestate treatment with only thermal hydrolysate recycling, studies were conducted as

described in Figure 3.2.

The specific objectives of this section are:

To investigate the effects of thermal treatment under various treatment conditions on

the hydrolysate anaerobic digestibility.

The effects of temperature and retention time were studied.

To investigate the effects of thermal treatment under various treatment conditions on

the anaerobic digestibility of solids residue.

To evaluate the effects of co-digesting the thermal hydrolysate with the biosludge at

various mixing ratio.

60

5.2. Results and Discussion

5.2.1.Anaerobic Digestion of Hydrolysate Produced by Thermal

Treatment of Digestate

In this section of study, Digestate 3 was used to conduct for the thermal treatment studies.

Before thermal treatment, samples were thickened to simulate a high-solid-load thermal

treatment. Table 5.1 lists the properties of Digestate 3.

Table 5.1: Properties of Digestate 3; Digestate 3 was produced by Xian Huang; all analysis were preformed

in triplicates.

Digestate 3

Days of anaerobic digestion 30

Thickening before treatment 2000 rpm 30 min

TSS (g/L) 35.7±3.3

VSS (g/L) 28.8±2.8

VSS/TSS 81%

TS (g/L) 53.4±0.3

VS (g/L) 37.8±0.1

VS/TS 71%

SCOD (g/L) 5.6±0.1

TCOD (g/L) 55.2±0.3

SCOD/TCOD 10%

Thermal treatment was conducted at the conditions listed in Table 5.2. Treatment

temperatures from 170o C to 210

o C, and retention times from 0 to 30 min were

investigated in this work. For all experiments, increasing the sludge temperature from the

ambient temperature to the target temperature took 5±1 min. In addition, a caustic

extraction treatment was performed as a comparison to thermal treatment. The caustic

extraction procedure is described in Section 3.5.2. The anaerobic digestibility of the

caustic extractive was characterized using the BMP test.

61

Table 5.2: Thermal treatment conditions for Digestate 3

Sludge Solubilization

The organic solubilization due to thermal treatment is represented in Table 5.3. As

expected, higher temperatures and longer retention times led to greater organic

solubilization. The caustic extraction resulted only in a modest level of solubilization that

was comparable to that of the thermal treatment at 190o C for 0 min. The solubilization of

carbohydrate, COD and tannin/lignin seemed to follow a similar trend with respected to

the temperature and retention time. The treatment at 210o C for 30 min led to a reduction

in soluble carbohydrate compared to the treatment at 210o C for 15 min. This may be the

result of carbohydrate degradation at extreme treatment conditions.

Sample Treatment Retention Time (min)

Digestate 3 No treatment

Digestate 3 170˚ C 0 15 30

Digestate 3 190˚ C 0 15 30

Digestate 3 210˚ C 0 15 30

Digestate 3 Caustics 1N NaOH 240

62

Table 5.3: Effect of thermal treatment and caustic extraction on the organic solubilization; all analysis were

preformed in triplicates.

Retention

time

SCOD (g/L) Soluble carbohydrate

(g glucose/L)

Soluble tannin/lignin (g

tannic acid/L)

Digestate 3 5.7±0.1 0.38±0.01 0.60±0.02

Caustic 11.7±0.2 1.20±0.02 0.62±0.00

170˚ C 0 min 9.1±0.2 0.87±0.11 0.73±0.01

15 min 15.1±0.2 1.62±0.06 1.16±0.01

30 min 16.9±0.1 1.75±0.06 1.33±0.00

190˚ C 0 min 10.4±0.1 1.21±0.03 0.88±0.01

15 min 17.7±0.0 1.81±0.01 1.42±0.02

30 min 22.8±0.3 2.23±0.09 1.71±0.00

210˚ C 0 min 13.0±0.13 1.48±0.07 1.05±0.00

15 min 24.5±0.2 2.34±0.17 1.96±0.01

30 min 28.0±0.4 2.26±0.16 2.32±0.00

The carbohydrate was solubilized more easily compared to the tannin/lignin at low

treatment intensities (both lower temperature and shorter retention time). One possible

reason is that the carbohydrate is present mostly as the extracellular polymeric substances

of the sludge; thus it can be more readily solubilized at low treatment conditions [30].

BMP Tests for the Hydrolysate Samples

In Figure 5.1A, the BMP results for Digestate 3, hydrolysate and caustic extractive are

presented. The SBP for Digestate 3 and the hydrolysate was 0.063 mL/mg COD input

and 0.028 mL/mg COD input, respectively. The low SBP for the hydrolysate indicates

that the soluble organics present in digestate was refractory. The SBP for the caustic

extractive was 0.11 mL/mg COD input which was lower than that for all of the

hydrolysate samples produced from thermal treatment.

63

The BMP results for the hydrolysate samples produced from treatment at 170o C for 0, 15

and 30 min are presented in Figure 5.1B. The treatment for 30 min retention time

produced the highest SBP, 0.185 mL /mg COD input. A two-phase biogas production

behavior was observed for all of the 170o C thermal hydrolysate. From day 0 to day 9, the

biogas produced at a constant rate (indicated by the slope of the SBP vs. time line); then,

the biogas production stopped from day 9 to day 18; the biogas production restarted after

day 18, and eventually was plateaued. This may indicate that there are two distinct groups

of organics undergoing anaerobic digestion.

The BMP results for the hydrolysate samples produced from treatment at 190o C for 0, 15

and 30 min are presented in Figure 5.1C. The treatment with 30 min retention time had

the highest SBP, 0.222 mL biogas/mg COD input. The two-phase biogas production

behavior was observed only for the treatments with 0 and 15 min retention time. The

treatment with 30 min retention time followed a one-phase biogas production behavior.

The BMP results for the hydrolysate samples produced from treatment at 210o C for 0, 15

and 30 min are presented in Figure 5.1D. The treatment with 30 min retention time had

the highest specific biogas yield, 0.242 mL biogas/mg COD input. The one-phase biogas

production behavior was observed for the treatments with 15 and 30 min retention time.

The treatment with 0 min retention time still followed the two-phase biogas production

behavior.

64

Figure 5.1: The BMP results for thermal hydrolysate at different treatment conditions; all measurement was

performed at triplicates.

According to the BMP results, the hydrolysate produced at relatively low treatment

intensities (at low temperatures and shorter retention times) had a two-phase biogas

production behavior. This suggests that there might be two distinct groups of organic

matter present in those hydrolysate: a) readily digestible compounds which contributes to

the early stage biogas production; and b) delay-digestible compounds that requires the

bacteria to acclimatize to them. At the high treatment intensities (at 190o C for 30 min,

210 o

C for 15 min and 210 o

C for 30 min), the hard-to-digest (group ‘b’) compounds

were not longer present. Those compounds were likely degraded, or transformed into

readily digestible organics under high treatment intensity. It may be that the readily

digestible compounds (group ‘a’) were at least partly the degradation products of the

hard-to-digest compounds. This two-phase behavior has not been reported in the past

studies.

-0.05

0

0.05

0.1

0.15

0.2

0.25

-5 5 15 25 35 45 55

SBP

mL/

mg

CO

D in

pu

t

Day

A: No Treatment and Caustic Extractive

Digestate 3

Digestate 3 Hydrolysate

Caustic Extractive

-0.05

0

0.05

0.1

0.15

0.2

0.25

-5 5 15 25 35 45 55

SBP

mL/

mg

CO

D in

pu

t

Day

B: Treatment at 170˚ CDigestate 3 Hydrolysate

0 min

15 min

30 min

-0.05

0

0.05

0.1

0.15

0.2

0.25

-5 5 15 25 35 45 55

SBP

mL/

mg

CO

D in

pu

t

Day

C: Treatment at 190˚ CDigestate 3 Hydrolysate0 min15 min30 min

-0.05

0

0.05

0.1

0.15

0.2

0.25

-5 5 15 25 35 45 55

SBP

mL/

mg

CO

D in

pu

t

Day

D: Treatment at 210˚ CDigestate 3 Hydrolysate

0 min

15 min

30 min

65

Modeling of the Biogas Production from the Hydrolysate Samples

To better understand the kinetics behavior of biogas production, the one-stage biogas

production behavior was modeled by the modified Gompertz equation (Equation 3). In

this study, to adequately capture the two-phase biogas production behavior described

earlier. Equation 4 was used that accounts for the readily digestible organics and the

delay-digest organics. The readily digestible organics contributed to the first stage of

digestion, and the delay-digest organics contributed to the second stage of digestion.

Experimental data showed good agreement with the model prediction except for the case

of Digestate 3 and Digestate 3 hydrolysate (Table 5.4). This was likely due to the low

biogas production of these two samples that likely resulted in a large experimental error.

Due to the limited sample points during the transition phase from stage 1 to stage 2 (day

15 to 25), the parameters associated with the stage 2 especially µm2 and λ2 have a

relatively large confident interval so are considered not reliable and are not discussed

here.

The µm did not seem to correlate to the treatment conditions. The Atot, on the other hand,

increased with increasing treatment temperatures and retention times. The maximum

ultimate biogas yield was observed at 210o C for 30 min. The A1 was higher than A2 for

most of the case (with the exception to the caustic extraction). This indicated that the

delay-digest organics were only a small fraction of the total organics.

66

Table 5.4: Model kinetics parameters for the thermal hydrolysate samples; A= (mL biogas/ mg COD input),

µm= (mL biogas/ (mg COD input*h)), λ= (h), Atot is the sum of A1 and A2; *Due to the limited amount of

sample points, those parameters had a large confident interval so were considered not reliable.

Stage 1 Stage 2

Atot RMSE

Sample A1 µm1 λ1 A2 µm2* λ2*

Digestate 3 0.065 7.79E-05 20 N/A 0.065 0.005

Digestate 3 hydrolysate Unable to model using the Gompertz Equation

Caustics 0.066 5.45E-03 171 0.047 1.42E-03 672 0.113 0.009

170o C 0 min 0.09 4.00E-04 0 0.039 4.63E-03 480 0.125 0.004

170 o

C 15 min 0.12 9.07E-04 57 0.056 9.29E-04 528 0.181 0.007

170 o

C 30 min 0.14 8.72E-04 46 0.058 6.42E-04 440 0.194 0.005

190 o

C 0 min 0.09 8.50E-04 76 0.053 1.95E-03 568 0.138 0.007

190 o

C 15 min 0.15 6.41E-04 26 0.046 6.19E-04 442 0.192 0.006

190 o

C 30 min 0.22 6.93E-04 28 N/A 0.219 0.005

210 o

C 0 min 0.10 8.67E-04 64 0.055 1.86E-03 584 0.160 0.007

210 o

C 15 min 0.22 6.50E-04 22 N/A 0.219 0.006

210 o

C 30 min 0.24 7.36E-04 22 N/A 0.241 0.007

5.2.2.Anaerobic Digestion of the Hydrolysate and Solids Residue

The solids residue separated after thermal treatment of digestate is expected to be

refractory and not suitable for anaerobic digestion. It is needed to quantify how much of

the digestible materials are being rejected as the solids residue.

To confirm the hypothesis, the digestate treated at 210o C for 30 min (representing the

optimal treatment conditions in terms hydrolysate biogas production) and 190o C for 15

min (as a sub-optimal treatment condition) were examined. Figure 5.2 shows the SBP of

the total treated digestate samples, and the corresponding hydrolysate and solids residue

for those samples. Comparing total digestate samples, the SBP of the treated digestate at

210o C for 30 min was about 30% greater than that of the digestate treated 190

o C for 15

min. Similarly, in contrast, the hydrolysate samples generated from those two treatments

67

exhibited similar digestibility with SBP of about 0.22 mL/mg COD input. On the other

hand, the solids residue for digestate treatment at 210o C 30 min, was significantly more

refractory (SBP of 0.027 mL/mg COD input) compared to the sample treated at 190o C

15 min (SBP of 0.08 mL/mg COD input). This was expected since the intense treatment

results in more solubilization, leaving behind the highly refractory compounds with low

SBP. Bougrier et al. [30] also reported the similar conclusion that higher treatment

intensities results in more refractory solids residue.

Figure 5.2: The BMP results for A) the digestate treated at 210o C for 30 min and the corresponding

hydrolysate and solids residue with Digestate 3 as reference; B) the digestate treated at 190o C for 15 min

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25 30 35 40

SB

P m

L/

mg C

OD

in

pu

t

Day

A: 210˚ C 30 min Treated Digestate

Hydrolysate

Solid Residue

Digestate 3

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25 30 35 40

SB

P m

L/

mg C

OD

in

pu

t

Day

B: 190˚ C 15 min Treated Digestate

Hydrolysate

Solid Residue

Digestate 3

68

and the corresponding hydrolysate and solids residue with Digestate 3 as reference; all measurement was

performed at triplicates.

With the fractionation technique mentioned in Section 3.5.1, a certain volume of

hydrolysate was obtained. Knowing the SCOD, the amount of COD recovered as

hydrolysate was calculated. Knowing the SBP of the hydrolysate, the biogas production

from the hydrolysate was calculated. Table 5.5 shows the contribution of hydrolysate and

solids residue fractions to the biogas production.

Table 5.5: Biogas production from the different fractions

210˚C 30 min 190˚C 15 min

Fractionation according to Section 3.5.1

Hydrolysate volume (mL /mL of

treated digestate) 0.91 0.825

Solids residue volume (mL / mL of

treated digestate) 0.09 0.175

COD recovery after fractionation

COD recovered as hydrolysate (g

/mL of treated digestate) 26.2 14.6

COD rejected as solids residue (g

/mL of treated digestate) 29.0 40.6

%COD recovered 47% 26%

Biogas production from the different fractions

Hydrolysate SBP (mL/mg COD) 0.223 0.211

Solids residue SBP (mL/mg COD) 0.027 0.08

Biogas from the hydrolysate

(mL/mL of treated digestate) 5.8 3.1

Biogas from the Solids residue

(mL/mL of treated digestate) 0.8 3.2

% of biogas not being digested 12% 51%

With the fractionation procedure used in this study, over 90% of the hydrolysate was

recovered. However, for the 210˚C 30 min treatment, the recovered hydrolysate

69

represented 47% of the total COD; while for the 190˚C 15 min treatment, this value was

only 26% of the total COD. In the case of digestate treated at 210˚C 30 min, only 12% of

biogas was considered wasted due the solids residue rejection after the thermal treatment.

However, for the 190˚C 15 min treatment condition, 51% of biogas was considered

wasted. For the treatment at 210o C for 30 min, by recycling 50% of the COD as

hydrolysate, 80% of digestibility was recovered from the treated digestate.

5.2.3.Co-digestion of Pulp Mill Biosludge and Hydrolysate

In this section, the co-digestion of biosludge and the thermal hydrolysate is discussed.

Biosludge 4 was used to mix with the hydrolysate samples (Table 5.6). The following

types of hydrolysate were studied in this work:

Untreated hydrolysate (supernatant of the untreated digestate): baseline study which

no thermal treatment was preformed;

Hydrolysate obtained by treating the digestate at 210˚C 30 min: optimal treatment

condition in terms of the treated hydrolysate biogas production;

Hydrolysate obtained by treating the digestate at 190˚C 15 min: sub-optimal

treatment condition in comparison for comparison.

Table 5.6: Properties of Biosludge 4

Biosludge 4

Shipment date 20-Mar-15

TSS (g/L) 13.8±0.2

VSS (g/L) 12.5±0.1

TS (g/L) 16.4±0.2

VS (g/L) 14.5±0.1

SCOD (g/L) No measured

TCOD (g/L) 22.0±0.6

70

Biosludge 4 and the hydrolysate were mixed at the ratios of 4:1, 6:1, and 8:1 on the COD

basis. The 4:1 mixing ratio represented the maximum amount of COD that can be

recovered as hydrolysate under the experimental conditions used in this work. The BMP

tests preformed are listed in Table 5.7.

Table 5.7: The BMP tests preformed to investigate the effects of co-digesting biosludge and the thermal

hydrolysate

Mixing ratio Hydrolysate mixed Purposes

Biosludge 4 N/A N/A

Baseline: no mixing

Untreated hydrolysate

(digestate supernatant) N/A N/A

210˚C 30 min hydrolysate N/A N/A

190˚C 15 min hydrolysate N/A N/A

Mixtures 1, 2, and 3 4:1, 6:1 and 8:1

respectively

Untreated

hydrolysate No treatment

Mixtures 4, 5, and 6 4:1, 6:1 and 8:1

respectively 210˚C 30 min

Optimal treatment

condition

Mixtures 7, 8, and 9 4:1, 6:1 and 8:1

respectively 190˚C 15 min

Sub-optimal treatment

condition

BMP tests of the biosludge and hydrolysate mixtures

The BMP results for the co-digestion mixtures are presented in Figure 5.3. The biosludge

and untreated hydrolysate mixtures, Mixtures 1 to 3, had essentially the same SBP

compared to Biosludge 4 (Figure 5.3A). Unexpectedly, the refractory nature of the

hydrolysate did not decrease the biogas production from the mixtures. The exact cause of

this observation is still known. A possible explanation is the untreated hydrolysate has

bacteria activity; thus adding it into the BMP tests may alter the inocula bacterial activity

[50]. The SBP of Mixtures 1-3 was used as the baseline when analyzing Mixtures 4-9.

Mixtures of biosludge and thermal hydrolysates, i.e. Mixture 4-9, on the other hand, had

a greater SBP compared to Biosludge 4 (Figure 5.3B and C). Regardless of the thermal

71

treatment conditions used for hydrolysate preparation or the mixing ratios, the biogas

production for the mixtures of biosludge and treated hydrolysate were similar within

10%.

By comparing the measured SBP and the calculated SBP (using Equation 5) for the

different biosludge and thermal hydrolysate mixtures, it seems that the co-digestion does

not have any synergistic effect on the biogas production (Figure 5.4). After subtracting

the baseline (Mixtures 1-3), for Mixtures 4-9, the measured SBP was only slightly higher

than the calculated SBP (2% higher for Mixtures 4-6 and 5% higher for Mixture 7-9).

Ultimately, it can be concluded that the thermal hydrolysate can be re-injected back into

the digester and co-digested with the biosludge without decreasing the biogas production.

72

Figure 5.3: The BMP results for A) Biosludge 4 and the digestate supernatant mixtures; B) Biosludge 4 and

the 210˚C 30 min hydrolysate mixtures; and C) Biosludge 4 and the 190˚C 15 min hydrolysate mixtures; all

measurements preformed in triplicates.

-0.05

3E-16

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35 40

SBP

mL/

mg

COD

inpu

t

Day

A: Biosludge 4 mixed with the untreated hydroylsate

Biosludge 4

Mix 1 (4:1)

Mix 2 (6:1)

Mix 3 (8:1)

Untreated Hydrolysate

-0.05

3E-16

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35 40

SBP

mL/

mg

COD

inpu

t

Day

B: Biosludge 4 mixed with the 210˚C 30 min hydroylsateBiosludge 4Mix 4 (4:1)Mix 5 (6:1)Mix 6 (8:1)210˚C 30 min hydrolysate

-0.05

3E-16

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35 40

SBP

mL/

mg

COD

inpu

t

Day

C: Biosludge 4 mixed with the 190˚C 15 min hydroylsateBiosludge 4Mix 7 (4:1)Mix 8 (6:1)Mix 9 (8:1)190˚C 15 min hydrolysate

73

Figure 5.4: The synergistic effect of co-digestion; the calculated SBP is the proportional addition of the

SBP of every component in the mixture.

If the thermal treatment was conducted of 210o C for 30 min, the following mass balance

could be derived as shown in Figure 5.5. Compared with anaerobic digestion without any

treatment, the overall COD conversion increased from 40% to 54%.

1 g COD of biosludge

0.6 g COD of digestate

0.3 g COD of solids residue

0.3 g COD of hydrolysate

0.165 g COD of hydrolysate remaining

Anaerobic digestion 40% COD reduction

Thermal Treatment50% solubilization

Anaerobic digestion 42-45% COD reduction

1 g starting COD0.465 g ending (solids residue + remain COD)

Figure 5.5: Overall COD balance for hydrolysate produced at 210o C for 30 min recycling

74

5.3. Summary and Conclusions

To address the problems with the interstage digestate thermal treatment with the treated

digestate re-injection, this chapter explores the option of the interstage digestate thermal

treatment with only the thermal hydrolysate re-injection.

Based on the BMP tests:

1) The anaerobic digestibility of the thermal hydrolysate was affected by the thermal

treatment conditions, more specifically:

Two groups of organics, one is readily digestible and the other is delay-digest,

might be presented in the thermal hydrolysate, which resulted in a two-phase

biogas production behavior. This two-phase behavior has not been reported in

the past studies.

Increasing in the thermal treatment intensities (both temperatures and retention

times) reduced the amount of delay-digest organics in the hydrolysate and

increased the hydrolysate ultimate biogas potential.

The maximum biogas potential was observed at 210o C for 30 min.

2) The anaerobic digestibility of the thermal solids residue was also affected by the

thermal treatment conditions, more specifically:

The solids residue obtained by treating digestate at 210o C 30 min was more

refractory than the one obtained under190o C 15 min treatment conditions.

Since the solids residue after the thermal treatment was proposed to be disposed

of, the treatments at high treatment intensities such as the one at 210o C for 30

75

min is recommended to minimize the amount of digestible COD wasted as the

solids residue.

3) The thermal hydrolysate could be re-injected back into the digester and co-digested

with the biosludge without decreasing the biogas production.

76

6. A Preliminary Economic Analysis

The purpose of the section is to analyze the economic feasibility of the different thermal

treatment-assisted biosludge anaerobic digestion processes at a preliminary basis. The

following processes were considered:

1. Anaerobic digestion without any treatment (NT);

2. Thermal pretreatment of biosludge followed by anaerobic digestion (TP);

3. Thermal treatment of digestate followed by reinjection of the entire treated digestate

for further anaerobic digestion (DT);

4. Thermal treatment of digestate followed by separation of hydrolysate and only

recycling the hydrolysate (DTH).

The design of anaerobic digestion and thermal treatment processes were based on the

bench-scale experimental data. The information regarding the biosludge and digestate

dewatering, however, was obtained from published literature. To develop the economic

analysis, simplified process flow diagrams including major unit operations were drafted

for each of the above processes. Based on the process flow diagrams, the preliminary heat

and mass balances were developed for the sizing of the equipments. Accordingly, the

capital costs and the operating costs were estimated for each of the processes.

77

6.1. Design Assumptions

6.1.1.Biosludge Inflow into the Treatment Processes

Thermal treatment-assisted anaerobic digestion is proposed to be installed in a pulp and

paper mill wastewater treatment plant to more effectively handle the biosludge. The

wastewater treatment plant has a configuration similar to that showed in Figure 6.1. The

plant treats 90,000 m3 of wastewater and produces 50 dry tone of biosludge per day. The

biosludge has approximately 1.5% solids content (15 gTSS/L) that corresponds to 3,300

wet tone (or about 3,300 m3) of biosludge leaving the secondary clarifier per day [51].

ScreeningPrimary clarifier

Secondary clarifierActivated sludge process

Mill effluents OverflowWater

treatment

Overflow

Primary sludge

Biosludge: 50 dry tonne/day

15 g VS/L3300 m3/day

Dewater

Biosludge Treatment

Returned Sludge

Figure 6.1: Current wastewater treatment plant

Currently, the biosludge is mixed with the primary sludge. The mixed sludge is thickened

using gravity belt thickeners to 10-12% solids content, and dewatered in a belt filter press

to 30-32% solids content. The polymers and dewatering aids cost approximately 2.8

million dollar per year. The dewatered sludge is then incinerated in the biomass boiler

along with other wood wastes.

78

In the new processes described earlier, the biosludge is handled separately through a

thermal treatment and anaerobic digestion. Without mixing with the biosludge, the

primary sludge can be dewatered much easily; thus the use of polymers and dewatering

aids may be reduced. According to Amin [52], the primary sludge requires four times less

dewatering aids per dry tone if it is not being mixed with secondary sludge. Anaerobic

digestion can also further digest the biosludge to reduce the amount of biosludge and

produce biogas as an energy source.

The biosludge entering into the biosludge treatment is assumed to have properties

described in Table 6.1.

Table 6.1: Inflow biosludge properties; *it was assumed VSS/TSS=0.9; **it was assumed COD/VS=1.5.

Sludge type Biosludge

Quantity (m3/d) 3,300

Temperature (o C) 10

TSS (g/L) 14

VSS (g/L) 13*

TS (g/L) 15

VS (g/L) 13

COD (g/L) 20**

6.1.2.Major Unit Operations

Thickening Processes

The biosludge enters the process with an initial solids content of 1.5%. It must be

thickened to approximately 4-5% solids before feeding into the anaerobic digester in

order to achieve the appropriated solids loading rate with a reasonable digester volume.

For example, in Ashbridges Bay Wastewater Treatment Plant, biosludge is thickened

their biosludge to 4% solids before being fed them into the digester [53]. The thickening

of biosludge is proposed to be done by the dissolved air floatation thickener (DAF). The

79

DAF is a common thickening process used to thicken biosludge (also used in Ashbridges

Bay Plant).

Before feeding the sludge (biosludge or digestate) to the thermal treatment unit, it is

further thickened to over 15% solids. This is to minimize the thermal reactor volume (a

similar idea is adopted by other commercially available thermal treatment processes [24].

In the proposed process, this is achieved using thickening centrifuges.

For design purpose, it is assumed that the thickening of pulp mill biosludge is similar to

that for the municipal biosludge. The design criteria for the thickening devices are listed

in Table 6.2 and Table 6.3.

Table 6.2: Assumed design criteria for the DAF thickener

DAF Thickener

Design values Literature Values [54]

Sludge Type Biosludge Biosludge

Inflow solids Content (%) 1.5 0.4-2.0

Outflow solids Content (%) 5 2.5-5

Loading rate (kg/m2/h) 4 1.5-20

polymer addition (g/kg) 5 2-5

Solids Capture (%) 90 90-99

Table 6.3: Assumed design criteria for thickening centrifuge;* it is assumed that the digestate is easier to

dewater comparing to biosludge [4].

Thickening centrifuge

Design values

Literature Values

[54]

Sludge Type Biosludge Digestate Biosludge

Inflow solids Content (%) 1.5 Same as biosludge 0.5-1.5

Outflow solids Content (%) 15 Same as biosludge 3-15

Centrifuge capacity (mL/min) 760 Same as biosludge 300-3,000

polymer addition (g/kg) 10 5* 5-10

Solids Capture (%) 90 Same as biosludge 90-99

80

Anaerobic Digestion

The anaerobic digestion of biosludge and digestate is based on the bench-scale

experimental data present in the earlier chapters. The biogas production is based on the

specific biogas production (SBP) that was measured during the experiments (Sections 4

and 5). Then, based on SBP, the VS removal can be calculated by assuming that the

production of 1.42 mole of biogas requires 64 g of COD, and COD to VS ratio is equal of

1.5.

As discussed in Chapter 4, for the untreated biosludge, (Table 6.4), 35% of the VS were

removed after 30 days of digestion. Thus, at least, a 30-day retention time is selected for

anaerobic digestion of the untreated biosludge, i.e. (Table 6.4). In the case of thermal

pretreatment, 34% of the VS removal was achieved with only 20 days of digestion. Thus,

a retention time of 20 days is selected for the anaerobic digestion of thermally treated

biosludge (process TP). In the cases of digestate thermal treatment (DT and DTH), since

the treated material will be mixed with the untreated biosludge, a 30-day retention time is

selected. The sizing of anaerobic digester is according to the study conducted by Gikas

[55].

Table 6.4: Anaerobic digestion data based on the bench-scale experiments

Process NT TP (170o C 1 h) TD (190

o C 30 min)

TDH (210o C 30

min)

Day of

digestion 20 20 20 20

VS

removal % 27% 34% 25% 40%

Day of

digestion 30 30 30 30

VS

removal % 35% 35% 30% 42%

81

Thermal Treatment Unit

The thermal treatment of biosludge and digestate is proposed to be performed using the

CambiTM

process, a commercially available thermal treatment process [24, 56] (Figure

6.2). The process is composed of a pulper, a reactor, and a flash tank. The biosludge with

15-16% solids is first fed the pulper and preheated with the recycled steam from the flash

tank. The preheated biosludge is then transferred to the reactor where it will be heated to

about 170o C with high pressure steam injection. After holding the biosludge at 170

o C

for about 30 min, the treated sludge is discharged into a flash tank. The biosludge at this

point is about 100o C which is too hot for the anaerobic digestion. Dilution water is

usually mixed with the treated biosludge to cool it down to approximately 37o C. In the

case of digestate treatment, the treatment temperature is raised to 210o C. The solids

reduction due to thermal treatment for both biosludge and digestate is 40%.

Thermalreactor

Feed tank

Flash Tank

Biosludge10o C

Treated biosludge37o C

Dilution Water

10o C

Fresh steam injection170o C

Steam Recycle

Figure 6.2: CambiTM

thermal treatment process

Dewatering

Before the final disposal, the digestate is first dewatered to 20% solids to reduce the

volume. The dewatering of the digestate or treated digestate is performed using

82

dewatering centrifuge. The thermally treated digestate is easier to dewater than the

untreated digestate. According to Zheng [32], no polymer is needed to centrifugally

dewater the thermally treated digestate. The design criteria for the dewatering devices are

listed in Table 6.5.

Table 6.5: Design criteria for dewatering centrifuge

Dewatering centrifuge

Design values Literature Values [54]

Sludge Type Digestate Treated Digestate Digestate

Inflow solids Content (%) 3.5 8 1-3

Outflow solids Content (%) 20 20 15-36

Max centrifuge capacity (mL/min) 760 760 40-3,000

polymer addition (g/kg) 5 0 3-10

Solids Capture (%) 90 90 85-94

Other Assumptions

Other economic factors are listed in Table 6.6. Costs values are typical values based on

the current market prices or published full-scale plant reports. The economic analysis

presented here includes the estimation of capital and operation costs. However, as a

preliminary analysis, only major unit operations are included in the cost estimation. Other

miscellaneous costs including installation costs, storage costs, and pumping and utility

installation are not considered in this study. Equipment redundancy and safety factors are

not considered either. For simplicity, payback period is used to assess the economic

feasibility of various processes examined in this study.

Table 6.6: Economic factors

Factors Value Sources

Price of natural gas ($/MMBTU) 3 Market value

Price of electricity ($/kWh) 0.09 Average market value

Polymer cost ($/tone) 3800 [53]

Sludge hauling cost ($/m3) 90 [57]

83

6.2. Process Description

6.2.1.Anaerobic Digestion without any treatment (NT)

Figure 6.3 presents the process train for anaerobic digestion without any treatment. The

biosludge is first thickened to 4-5% solids using the DAF thickener. Then, the thickened

biosludge is fed into the anaerobic digester. The anaerobic digestion produces biogas

which is then fed to the boiler or combine heat and power generator (CHP). A portion of

thermal energy is used to preheat the biosludge to 37o C. The residues in the digester, also

known as digestate, are regularly removed from the digester. Then, the digestate is

dewatered to 20% solids using the dewatering centrifuge before final disposal.

Dissolved Air Floatation Thickening

Thicken biosludge

Feed Preheating/Internal recycle

Preheatedbiosludge

Anaerobic digester

Boiler/CHP

DewateringCentrifuge

Digestate

Biosludge

NetBiogas

Totalbiogas

Dewatered Digestate

Overflow

Centrate

Figure 6.3: Process train for NT

6.2.2.Anaerobic Digestion with Thermal Pretreatment (TP)

Figure 6.4 presents the process train for anaerobic digestion with thermal pretreatment.

The biosludge is first thickened to 15% solids using the DAF thickener and thickening

centrifuge. Then, the thickened sludge is fed to thermal pretreatment process. The treated

sludge is mixed with the dilution stream before entering the anaerobic digester. The

84

anaerobic digestion produces biogas which is then fed to the boiler or CHP. A portion of

thermal energy produced from the boiler or CHP is used to heat up the thermal

pretreatment process. The digestate removed from the digester is then dewatered to 20%

solids using the dewatering centrifuge before final disposal.

Dewateredbiosludge

ThermalPretreatment

Combine

Anaerobic Digester

Boiler/CHP

DewateringCentrifuge

Digestate

Biosludge

Net biogasTotal

biogas

Dewatered Digestate

Return to activated sludge process

Overflow

Return to activated sludge process

ThickeningCentrifuge

Thickening

Thickenedsludge

Dilution stream

Centrate 1

Treatedbiosludge

Figure 6.4: Process train for TP

6.2.3.Anaerobic digestion with Digestate Treatment (DT)

Figure 6.5 presents the process train for anaerobic digestion with digestate thermal

treatment. In this case, the digestate removed from the digester is dewatered to 20%

solids using the dewatering centrifuge. Then, the dewatered digestate is divided into two

parts: one part is disposed; the other part is fed to the digestate thermal treatment. The

treated digestate is then mixed with the fresh biosludge and fed into the digester. The

ratio of recycle to reject is selected so that the mixture of biosludge and treated digestate

is at 37o C. In this case, the ratio of recycle to reject is 3:2.

85

Thickening

Thickenbiosludge

Anaerobic Digester

Boiler/CHP

DewateringCentrifuge

Biosludge

Dewatered Digestate

Return to activated sludge process

Overflow

Digestate Thermal Treatment

Combine

Reject

Return to activated sludge process

TreatedDigestate

Centrate

Recycle

Net biogas

Digestate

Total biogas

Figure 6.5: Process train for DT

6.2.4.Digestate Thermal Treatment with Hydrolysate Recycle

(DTH)

In this process, the digestate is thickened to 15% solids and then fed into the digestate

thermal treatment. The treated digestate is then separated into hydrolysate and solids

residue using the dewatering centrifuge. The hydrolysate is mixed with the fresh

biosludge and re-injected into the digester. The solids residue with 20% solids is

disposed.

86

Thickening

Thickenedbiosludge

Anaerobic Digester

Boiler

Digestate

Biosludge

DewateredDigestate

Return to activated sludge process

Overflow

DigestateThermal Treatment

Return to activated sludge process

DewateringCentrifuge

Treateddigestate

Reject

Hydrolysate

Thickening Centrifuge

Centrate 1

Thickeneddigestate

Combine

Total Biogas Net biogas

Figure 6.6: Process train for DTH

6.2.5.Summary

Table 6.7 presents a summary of the process outputs. Details regarding to the heat and

mass balance, stream tables can be found in Appendix 3. The DT case produces the

lowest amount of final disposal and the highest amount of total and net biogas. The DTH

case produced second highest amount of total biogas, but this process requires more

thermal energy to perform the digestate treatment. However, the DTH case requires the

lowest amount of polymer addition for dewatering. The TP case has the lowest net biogas

production as a large part of biogas is needed to fuel the thermal treatment.

Table 6.7: Process outputs summary

NT TP DT DTH

Final disposal (m3/a) 47,000 33,000 28,000 33,000

Solids in final disposal % 20 20 20 20

Total biogas (MWh/a) 23,000 21,000 36,000 31,000

Net biogas (MWh/a) 11,000 11,000 27,000 15,000

Total polymer addition

(tone/a) 170 290 220 140

87

6.3. Preliminary Costs and Economic Analysis

Based on the heat and mass balance, the major unit operations can be sized accordingly.

The major unit operations and their corresponding capital costs, and operation and

maintenance (O&M) costs are listed in Table 6.8. Cost values are based on published

literature values. The biogas can be utilized by either a boiler or a CHP unit. In the case

of a boiler, only thermal energy is produced. The net biogas production can be seen as a

replacement fuel to natural gas. In the case of CHP, electricity is also produced. Thus, the

electricity generated can be substituted for plant electricity use. The CHP unit is assumed

to have an electric efficiency of 40%.

The anaerobic digester is the largest contributor to the capitals costs. For all the options,

the costs of digester are over 10 million dollars contributing to over 80% and 50% of the

capital costs for the boiler option and CHP option, respectively. The second largest cost

contributor is the centrifuge dewatering unit. Especially for the pretreatment case, the

initial biosludge dewatering adds largely to the capital costs. The CHP option is more

expensive than the boiler option since a CHP unit is almost 20 times more expensive than

a boiler unit.

88

Table 6.8: Capital costs, and operation and maintenance costs for the different processes

Anaerobic digestion without treatment (NT)

Major units Estimated Costs ($x1000) O&M Costs ($x1000/a)

Dissolve air flotation thickener 81 [58] 29 [58]

Anaerobic digester 14,000 [59, 55] 770 [59]

Digestate centrifuge 1,000 [58] 100 [58]

Boiler (CHP) 280 (7,000) [59] 28 (900) [59]

Thermal Pretreatment (TP)

Major units Estimated Costs ($x1000) O&M Costs ($x1000/a)

Dissolve air flotation thickener 81 [58] 29 [58]

Biosludge centrifuge 1,000 [58] 100 [58]

Thermal pretreatment 6200 [24] 150 [24]

Anaerobic digester 10,000 [59, 55] 810 [59]

Digestate centrifuge 1000 [58] 100 [58]

Boiler (CHP) 240 (6,300) [59] 26(840) [59]

Digestate Thermal Treatment (DT)

Major units Estimated Costs ($x1000) O&M Costs ($x1000/a)

Dissolve air flotation thickeners 81 [58] 29 [58]

Anaerobic digester 16,000 [59, 55] 460 [59]

Digestate centrifuge 1100 [58] 110 [58]

Digestate thermal treatment 3,800 [24] 96 [24]

Boiler (CHP) 200 (1,100) [59] 20 (1,400) [59]

Digestate Treatment (hydrolysate only) (DTH)

Major units Estimated Costs ($x1000) O&M Costs ($x1000/a)

Dissolve air flotation thickeners 81 [58] 29 [58]

Anaerobic digester 14,000 [59, 55] 590 [59]

Digestate centrifuge (thicken) 1100 [58] 110 [58]

Digestate centrifuge (dewater) 240 [58] 24 [58]

Digestate thermal treatment 4,900 [24] 120 [24]

Boiler (CHP) 360 (9,200) [59] 41 (1,400) [59]

Table 6.9 presents the economic analysis of the different treatment processes if a boiler is

used to utilizing the biogas. The NT case gives the lowest capital costs followed by the

TP case and the DTH case. The DT case is the most expensive due to the need for large

digester. In terms of the annual O&M costs, the PT case has the highest annual O&M

costs because of the need for large capacity centrifuge units and thermal treatment. The

89

TDH case produces the lowest amount of sludge; therefore it generates the greatest

saving on sludge hauling costs. In terms of the net benefit, the TDH and TD case have the

lowest payback period of 6 years. The NT and PT cases have the lowest net benefit.

Table 6.9: Economic analysis for boiler option;* biogas is used to replace the purchase of natural gas;**

this is calculated as the reduction on polymers or dewatering aids compared to the current mill;*** this is

calculated as the reduction on sludge production compared to the current mill.

Boiler option NT TP DT DTH

Total Capital Costs $x1000 15,000 19,000 21,000 21,000

O&M Costs ($/a)x1000 900 1,200 700 900

Biogas as natural gas ($/a)x1000* 120 110 280 160

-Net biogas Production (MWh/a) 11,000 11,000 27,000 15,000

Saving from dewatering agents ($/a)x1000** 1600 1200 1500 1800

Saving on sludge hauling ($/a)x1000*** 1,100 2,200 2,600 2,200

-COD reduction 40% 40% 52% 54%

Net benefit ($/a)x1000 1,900 2,300 3,600 3,200

Payback period (a) 8 8 6 6

In comparison, the CHP biogas utilization option is also assessed (Table 6.10). However,

due to the high capital and O&M costs for the CHP unit, the economic benefit for the

CHP option is much less than that for the boiler option.

Table 6.10: Economic analysis for CHP option; * biogas is used to replace the purchase of natural gas;**

this is calculated as the reduction on polymers or dewatering aids compared to the current mill;*** this is

calculated as the reduction on sludge production compared to the current mill.

CHP option NT TP DT DTH

Total Capital Costs $x1000 22,000 25,000 32,000 30,000

Total O&M Costs ($/a)x1000 1,800 2,000 2,100 2,300

Biogas as electricity ($/a)x1000* 800 750 1,600 1,100

-Net biogas Production(MWh/a) 11,000 11,000 27,000 15,000

Saving from dewatering agents ($/a)x1000** 1,600 1,200 1,500 1,800

Saving on sludge hauling ($/a)x1000*** 1,100 2,200 2,600 2,200

-COD reduction 40% 40% 52% 54%

Net benefit ($/a)x1000 1,700 2,200 3,600 2,800

Payback period (a) 13 12 9 11

90

6.4. Conclusions

Based on the results of this analysis, the DTH and DT cases had the best economic with a

6-year payback period. In comparison, the TP case actually had the lowest net benefit and

the longest payback period. However, the economic analysis in this study should be

considered only as a preliminary evaluation and used as a way to rank various processes.

There are many possibilities for process and unit operation optimization. One of the ways

is to utilize existing equipments, such as gravity belt thickeners, boilers, etc. Also, rather

than constructing a new plant, upgrading existing anaerobic digester with digestate

thermal treatment may be a more attractive option. Moreover, utilizing waste heat from

the mill as supplemental energy for the thermal treatment is not considered here but can

significantly improve the biosludge treatment economics. Finally, government subsidy

energy programs such as the Feed-in Tariff (FIT) program may be available which will

significantly increase the plant revenue.

91

7. Conclusions and Recommendations

7.1. Conclusions

This research provides a proof-of-concept study regarding to the thermal

treatment-assisted pulp and paper mill biosludge anaerobic digestion. This study

demonstrates that:

Thermal pretreatment of biosludge did not extend the biogas yield and inhibited

biogas production at high organic concentrations; thus it is not preferable.

Digestate thermal treatment increased the rate of digestion at early stage of digestion

and increased the ultimate biogas yield; the co-digestion of biosludge and treated

digestate had a synergistic effect on biogas production; thus it is preferable process if

optimizing biogas yield is the main objective.

Separating the treated digestate and only recycling the thermal hydrolysate for

re-anaerobic digestion would be recommended, if the thermal treatment was

conducted at high intensities such as the one at 210o C for 30 min; Low treatment

intensities resulted in a great amount of digestible COD wasted as the solids residue;

the thermal hydrolysate could be re-injected back into the digester and co-digested

with the biosludge without distorting the biogas production.

A preliminary cost and economic analysis indicated that the digestate thermal

treatment with thermal hydrolysate recycling is the most economical process.

Thus, future studies should be oriented around the digestate thermal treatment rather than

the thermal pretreatment of biosludge, if optimizing biogas production is the main

objective.

92

7.2. Recommendations

This study is still a proof-of-concept study. Further studies will be needed to expend the

understanding of the digestate thermal treatment and to verify the feasibility of such

process in a pulp and paper mill wastewater treatment plant. Recommended future studies

could include the following:

1) In-depth sludge composition study: the biogas production is related to the types of

organics present in the digestate. An in-depth composition analysis should be

conducted to expend our understanding on how thermal treatment alters the digestate

sample and affects the anaerobic digestion. With better knowledge on those concepts,

it is possible to model the biogas production, and even optimize biogas production.

2) Continuous thermal treatment and anaerobic digestion study: current study is based

on BMP experiments which can not fully represent a full-scale installation. A

full-scale installation will operated at continuous basis. Therefore, it is essential to

verify the results from this study using a continuous experimental set-up.

3) Thermal treatment and anaerobic digestion of dewatered digestate: dewatering of

digestate prior to thermal treatment is necessary to make the process more economic

attractive. Reducing the amount of water in the digestate before treatment reduces the

overall volume of the digestate; therefore less energy is needed to heat up the

digestate. Current study used digestate with a solids content of about 5%. However,

it is suggested that digestate should be dewatered to at least 10% before thermal

treatment.

4) Effect of thermal treatment on digestate dewaterability: reducing sludge disposal is

the main benefit of the thermal treatment-assisted anaerobic digestion. Any

93

improvement in dewaterability will significantly improve the plant economics. Both

filtration dewatering and centrifugation dewatering should be addressed.

94

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Thermal Treatment of Pulp and Paper Mill Biosludge and

Digestate to Enhance Their Anaerobic Digestibility

Appendices

by

Lei Chen

ii

Table of Contents

Table of Contents ........................................................................................................... ii

List of Tables ............................................................................................................... iii

List of Figures ................................................................................................................ v

1. Appendix 1: Raw Data ............................................................................................ 1

1.1. BMP Test on Feb 27, 2014: Thermal Pretreatment Study ........................... 1

1.2. BMP Test on May 6-7, 2014: Digestate Thermal Treatment ....................... 2

1.3. BMP Test on July 14: Digestate Thermal Treatment and Co-digestion ...... 5

1.4. BMP Test on December 23, 2014: Anaerobic Digestion of Thermal

hydrolysate ............................................................................................................. 8

1.5. BMP Test on March 20, 2015: Anaerobic Digestion of Solids Residues and

Co-digestion of Thermal hydrolysate and Biosludge .......................................... 11

2. Appendix 2: Experimental Method ....................................................................... 15

2.1. BMP Assay Preparation ............................................................................. 15

2.1.1. Medium Recipe ................................................................................ 15

2.1.2. Recipe for Synthetic Feed ................................................................ 18

2.1.3. Preparing BMP Assay in Glove bag ................................................ 18

2.2. Caustic Extraction Procedure ..................................................................... 20

2.3. Calibration Curves ............................................................................... 21

2.3.1. COD Calibration .............................................................................. 21

2.3.2. Carbohydrate Calibration ................................................................. 22

2.3.3. Tannin/lignin Calibration ................................................................. 23

3. Appendix 3: Economic Analysis .......................................................................... 24

Reference ..................................................................................................................... 27

iii

List of Tables

Table 1.1: Recipes for each BMP assay (Feb 27, 2014) ................................................ 1

Table 1.2: Properties of Inoculums and substrate; COD preformed in triplicates (Feb 27,

2014) .............................................................................................................................. 1

Table 1.3: Biogas production for each assay measured in mL (Feb 27, 2014) .............. 1

Table 1.4: Recipes for each BMP assay (May 6-7, 2014) ............................................. 2

Table 1.5: Properties of Inoculums and substrate; COD preformed in triplicates (May 6-7,

2014) .............................................................................................................................. 2

Table 1.6: Biogas production for each assay measured in mL (May 6-7, 2014) ........... 4

Table 1.7: Recipes for each BMP assay (July 14, 2014) ............................................... 5

Table 1.8: Properties of Inoculums and substrate; COD preformed in triplicates (July 14,

2014) .............................................................................................................................. 5

Table 1.9: Biogas production for each assay measured in mL (July 14, 2014) ............. 6

Table 1.10: Recipes for each BMP assay (December 23, 2014) ................................... 8

Table 1.11: Properties of Inoculums and substrate; COD preformed in triplicates

(December 23, 2014) ..................................................................................................... 8

Table 1.12: Biogas production for each assay measured in mL (December 23, 2014) . 9

Table 1.13: Recipes for each BMP assay (March 20, 2014) ....................................... 11

Table 1.14: Properties of Inoculums and substrate; COD preformed in triplicates (March

20, 2015) ...................................................................................................................... 11

Table 1.15: Biogas production for each assay measured in mL (March 20, 2015) ..... 12

iv

Table 3.1: Stream summary for anaerobic digestion without treatment (NT) ............. 24

Table 3.2: Stream summary for anaerobic digestion with thermal pretreatment ......... 25

Table 3.3: Stream summary for anaerobic digestion with digestate thermal Treatment25

Table 3.4: Digestate thermal treatment with hydrolysate recycle only(DTH)............. 25

v

List of Figures

Figure 2.1: Caustic Extraction Procedure; Extraction was performed in triplicates for all

samples. ........................................................................................................................ 21

Figure 2.2: COD calibration curve............................................................................... 22

Figure 2.3: Carbohydrate calibration curve ................................................................. 23

Figure 2.4: Tannin/lignin calibration curve ................................................................. 24

1

1. Appendix 1: Raw Data

1.1. BMP Test on Feb 27, 2014: Thermal Pretreatment Study

Table 1.1: Recipes for each BMP assay (Feb 27, 2014)

Sample Substrate Total

volume (ml)

Mediu

m (ml)

Inoculums

(ml)

Sample (ml) Synthetic

feed (ml)

Water

(ml)

1 Biosludge 1 80 50 4.9 7.4 0.0 17.7

2 Treated sample biosludge 1 80 50 4.9 13.9 0.0 11.2

3 Negative Control 80 50 4.9 7.4 0.0 17.7

4 Positive Control 80 50 4.9 0 4.5 20.7

5 Negative Control No Inoculums 80 50 0 7.4 0.0 22.6

Table 1.2: Properties of Inoculums and substrate; COD preformed in triplicates (Feb 27, 2014)

TSS/VSS COD

Inoculums TSS g/L VSS g/L Dilution COD mg/L

1 13.5 12.9 Biosludge 1 N/A 14,870

2 13.4 12.2 Treated 2x 7,895

3 12.7 12.3 Positive Control N/A 25,647

Avg: 13.2 12.5

Table 1.3: Biogas production for each assay measured in mL (Feb 27, 2014)

Gas sampling time (day)

ID 1 2 4 7 10 14 17 21 28 35 49

1 6.5 1.2 5.8 2.2 2.5 5 5.2 1.2 4.2 4.5 4

5.5 1.8 3.5 2 3 3 5.5 1.2 5.2 3.5 4

5 1.6 3.2 2.2 2.5 2.7 3.2 3.5 2.5 4.8 4.5

2 6 3 7.2 3.6 3 2 2 0.5 2 1.5 4

7 2.8 8.8 2.5 3.5 1 3.5 0.2 0 1.7 4

6.2 2.1 6.5 5.5 3.2 2 3 0 1 2 4

3 4.5 0.5 3.5 0 1 0 0 0.5 0 0 0.7

4.5 0.8 1.8 1.7 0 1 0.8 0 0.5 0 1

5.5 0.8 0.7 2 2.5 0 0 0 1.1 0 0

4 13.5 4.5 25 12.5 6 7 5.2 1 2.5 0 1

16 3 25 12.5 6.5 4.5 6 0.2 1 0 1.5

2

16 2.5 26 12 7.5 3 6 2.7 0 0 0.2

5 5.5 1 4.2 3.2 4 1.7 3 1.5 2.5 1 2

5.2 1.5 5.5 2 3.5 2.5 3 2 2.2 0.9 2

5 3 5.5 2.2 2.7 5 1 0.8 3.5 0.9 2.2

1.2. BMP Test on May 6-7, 2014: Digestate Thermal Treatment

Table 1.4: Recipes for each BMP assay (May 6-7, 2014)

Sample Substrate Total

volume (ml)

Medium

(ml)

Inoculum

(ml)

Sample

(ml)

Synthetic

feed (ml)

Water (ml)

1 Digestate 1 80.00 50.00 4.86 2.16 0.00 22.97

2 Digestate 1 no inoculums 80.00 50.00 0.00 2.16 0.00 27.84

3 Negative control 80.00 50.00 4.86 0.00 0.00 25.14

4 Positive control 80.00 50.00 4.86 0.00 4.11 21.03

5 180C 0min 80.00 50.00 4.86 5.11 0.00 20.02

6 190C 0min 80.00 50.00 4.86 4.52 0.00 20.62

7 190C 30min 80.00 50.00 4.86 4.64 0.00 20.49

8 210C 0min 80.00 50.00 4.86 4.98 0.00 20.16

9 210C 0min SE 80.00 50.00 4.86 5.76 0.00 19.37

10 Biosludge 2 80.00 50.00 4.86 5.79 0.00 19.35

Table 1.5: Properties of Inoculums and substrate; COD preformed in triplicates (May 6-7, 2014)

TSS/VSS

Inoculums TSS g/L VSS g/L

1 14.05 12.8

2 12.9 12.4

3 12.8 12.3

Avg: 13.475 12.5

COD

Dilution COD mg/L

Digestate 1 N/A 50867

Positive control N/A 26780

180C 0min 2x 21517

190C 0min 2x 24342

190C 30min 2x 22092

210C 0min 2x 23700

210C 0min SE 3.11x 19087

3

Biosludge 2 N/A 19007

4

Table 1.6: Biogas production for each assay measured in mL (May 6-7, 2014)

Gas sampling time (day)

ID 1 2 3 3.5 6.5 7.5 11.5 14.5 17.5 22.5 31 37 44 53

1 0 1.00 0.10 0.00 1.00 0.00 1.00 1.50 1.00 1.40 4.40 1.00 1.00 1.60

0.2 0.50 0.00 0.00 1.00 0.00 0.80 1.20 1.00 2.00 5.80 1.00 0.80 1.50

0 0.50 0.00 0.00 1.00 0.00 0.80 2.00 2.00 1.60 5.20 2.00 0.40 1.80

2 0 0.00 0.00 0.00 1.00 0.00 0.00 0.80 0.80 0.90 4.20 1.50 1.20 0.80

0 0.50 0.00 0.00 1.00 0.00 0.00 0.80 0.50 0.00 4.20 1.00 1.20 1.30

0 0.00 0.00 0.00 1.00 0.00 0.00 0.50 0.80 0.00 3.80 1.50 0.20 0.80

3 0.2 0.50 0.00 0.00 0.50 0.00 0.00 0.20 0.00 0.00 1.70 0.20 1.00 0.60

0 0.50 0.00 0.00 0.50 0.00 0.00 0.80 0.50 0.80 3.00 1.20 0.60 0.80

0 0.50 0.00 0.00 0.50 0.10 0.00 0.20 0.00 0.00 1.00 0.00 1.70 0.40

4 9.5 4.50 6.00 11.00 16.00 3.00 2.50 5.50 2.20 1.00 3.50 0.20 0.40 1.20

9.5 3.50 6.50 11.00 16.50 3.00 2.50 6.00 3.00 0.60 2.40 1.50 1.00 1.30

10 3.00 6.50 12.50 16.50 3.00 2.50 4.50 4.50 1.00 4.00 0.00 0.60 1.60

5 0.89 1.79 0.89 0.89 3.13 0.89 2.50 1.79 1.50 1.79 6.61 1.61 1.20 1.50

0.89 2.23 1.34 0.89 2.86 0.45 2.50 2.23 1.50 2.23 7.14 1.61 1.60 2.00

0.89 1.79 1.34 0.89 2.86 0.89 2.50 2.23 2.00 1.96 4.82 1.34 1.80 2.20

6 1 2.00 1.50 0.50 2.50 0.50 2.50 3.00 2.50 1.20 6.50 1.50 1.60 1.70

1 2.00 1.00 1.10 3.00 0.50 2.00 2.00 1.50 2.50 6.00 1.20 1.50 1.90

1 1.50 1.00 0.80 2.50 1.00 2.20 2.00 1.20 2.60 5.50 1.50 2.20 1.40

7 1 2.50 2.00 1.00 4.50 1.00 3.00 2.00 1.80 0.50 3.80 1.50 2.40 1.80

1.5 2.50 2.00 1.00 4.50 1.00 3.20 1.80 1.50 1.00 3.50 1.00 3.20 2.20

1 2.00 2.00 1.00 4.50 1.00 3.20 2.00 1.80 1.60 5.00 1.00 1.00 2.10

8 1.2 2.00 2.00 1.00 4.00 0.50 2.50 2.20 2.00 1.40 5.80 2.50 1.80 1.70

1.5 2.00 1.50 1.00 4.00 0.50 2.80 2.50 1.20 0.80 4.50 2.00 2.00 2.20

1 2.00 2.00 1.00 4.00 1.20 2.80 2.20 1.50 1.80 5.20 2.00 3.00 1.60

Gas sampling time (day)

ID 1 4 5 6 10 15 21 28 37 44 51

9 2 5.00 0.50 1.60 4.00 3.80 2.60 2.40 2.60 0.80 2.10

2 4.80 0.50 1.60 3.00 4.00 2.50 2.90 2.50 1.00 2.60

1.8 4.20 2.00 0.60 4.00 3.00 3.00 3.00 3.00 0.80 1.70

10 1.8 2.50 1.00 1.60 4.50 5.00 6.00 6.20 6.00 2.20 2.60

2 3.00 0.00 2.00 4.50 5.80 6.20 5.40 6.20 2.80 2.50

2 2.80 1.00 0.80 4.00 4.80 6.00 4.80 6.00 2.20 2.60

5

1.3. BMP Test on July 14: Digestate Thermal Treatment and

Co-digestion

Table 1.7: Recipes for each BMP assay (July 14, 2014)

Sample Substrate Total volume

(ml)

Medium

(ml)

Inoculum

(ml) Sample (ml)

Synthetic feed

(ml)

Water

(ml)

1 210C 0min SE 80.00 50.00 4.86 4.24 0.00 20.90

2 190C 30min 80.00 50.00 4.86 4.46 0.00 20.68

3 Mixture 1 80.00 50.00 4.86 4.98 0.00 20.15

4 Mixture 2 80.00 50.00 4.86 5.08 0.00 20.06

5 Mixture 3 80.00 50.00 4.86 4.81 0.00 20.32

6 Digestate 2 80.00 50.00 4.86 3.32 0.00 21.81

7 Biosludge 3 80.00 50.00 4.86 5.20 0.00 19.94

8 Postive Control 80.00 50.00 4.86 0.00 4.11 21.03

9 Negative Control 80.00 50.00 4.86 0.00 0.00 25.14

10 NCN-Digstate 80.00 50.00 0.00 3.32 0.00 26.68

11 NCN-PMWAS 80.00 50.00 0.00 5.20 0.00 24.80

Table 1.8: Properties of Inoculums and substrate; COD preformed in triplicates (July 14, 2014)

TSS/VSS

Inoculums TSS g/L VSS g/L

1 14.05 12.8

2 12.9 12.4

3 12.8 12.3

Avg: 13.475 12.5

COD

Dilution COD mg/L

210C 0min SE 2.43x 25967

190C 30min 2.6x 24667

Mixture 1 N/A 22083

Mixture 2 N/A 21650

Mixture 3 N/A 22850

Digestate 2 1.9x 33100

Biosludge 3 N/A 21150

6

Postive Control N/A 26780

Table 1.9: Biogas production for each assay measured in mL (July 14, 2014)

Biogas sampling time (day)

ID 1 2 3 4 6 9 13 19 27 32 40 50

1 2.4 1.50 2.00 1.80 2.60 2.20 2.00 3.70 4.00 1.50 0.00 2.60

2.1 1.70 2.10 2.00 2.60 2.60 1.70 3.20 3.50 1.80 2.80 3.30

1.8 1.90 1.60 1.60 3.00 2.60 2.00 2.20 3.00 1.70 0.50 3.20

2 2.1 1.80 1.40 1.40 1.60 1.80 1.80 3.00 3.00 1.80 1.00 2.20

0.9 2.00 1.50 1.20 1.00 2.40 0.70 4.20 3.80 1.80 0.80 2.00

1.4 2.00 1.60 1.20 1.60 2.80 1.00 3.20 3.00 1.60 0.60 2.60

3 1.6 2.20 1.60 1.20 1.50 2.90 2.00 6.00 5.80 3.60 1.80 4.00

3.1 1.80 1.50 1.20 1.60 3.10 2.20 6.20 6.50 3.80 0.70 3.20

1.4 2.20 1.70 1.20 1.60 2.60 2.10 6.20 6.00 3.20 2.30 3.10

4 1.1 2.20 1.60 0.90 1.70 3.00 2.80 7.00 6.00 3.30 1.20 3.80

1.6 1.60 1.50 1.00 1.60 3.00 2.40 5.80 6.00 3.60 2.10 3.80

2.6 1.40 1.40 1.00 1.80 3.20 2.40 5.80 6.50 3.70 2.00 2.90

5 1.80 1.40 1.20 0.80 1.40 3.00 2.70 5.00 6.00 3.40 1.50 2.40

3.10 1.20 1.20 0.80 1.40 3.20 2.90 5.20 6.00 3.10 1.90 2.40

1.60 2.00 1.30 0.80 1.60 3.00 2.60 5.50 6.00 3.80 2.10 3.40

6 0.4 0.90 0.50 0.40 0.50 1.10 0.00 2.50 3.50 2.20 0.00 3.20

0.4 0.70 0.70 0.40 0.40 1.00 1.30 3.20 3.80 2.30 0.50 2.00

1.1 0.60 0.60 0.40 0.50 1.00 1.80 2.70 3.00 2.90 1.50 2.70

7 2.4 0.80 0.80 0.80 1.00 2.40 2.20 6.00 6.50 4.50 1.80 5.20

3.1 0.60 0.80 0.70 1.00 2.10 2.80 5.00 6.00 3.50 2.80 4.60

3.1 0.90 0.60 0.90 1.00 2.20 2.20 5.20 5.00 3.40 0.00 4.60

8 13.1 2.00 3.80 11.00 12.80 8.50 1.80 9.00 2.00 1.70 0.00 0.10

11.6 2.60 5.50 12.00 11.50 7.00 3.60 8.20 1.50 1.40 0.00 1.00

12.4 1.20 4.00 14.00 13.00 8.50 2.20 9.00 2.00 1.40 0.00 0.80

9 3.1 0.00 0.00 0.00 0.00 0.90 0.00 1.70 0.50 1.10 0.00 0.00

2.8 0.00 0.30 0.00 0.40 0.80 0.00 1.60 1.00 1.50 0.10 0.00

2.1 0.00 0.40 0.00 0.40 0.70 0.00 1.20 0.80 1.40 0.00 0.00

10 2.1 0.00 0.00 0.20 0.30 0.80 1.60 2.60 1.00 2.00 0.00 0.00

2.4 0.00 0.00 0.00 0.00 0.40 0.70 2.20 1.80 2.50 0.00 0.00

1.6 0.00 0.00 0.00 0.40 0.60 0.60 2.20 1.50 2.20 0.00 1.20

11 3.4 0.00 0.40 0.20 1.00 1.70 2.60 3.90 1.20 2.30 0.00 1.20

3.1 0.00 0.00 0.20 1.00 1.70 2.60 3.50 1.80 2.10 0.00 1.80

7

3.1 0.00 0.40 0.20 1.00 1.70 2.20 3.80 1.20 2.00 0.00 1.80

8

1.4. BMP Test on December 23, 2014: Anaerobic Digestion of

Thermal hydrolysate

Table 1.10: Recipes for each BMP assay (December 23, 2014)

Sample Substrate Total volume (ml) Medium

(ml)

Inoculum

(ml)

Sample

(ml)

Synthetic

feed (ml)

Water

(ml)

1 Digestate 3 80.00 50.00 4.0 2.0 0.00 24.05

2 Digestate 3 Hydrolystate 80.00 50.00 4.0 19.3 0.00 6.70

3 170C 0min 64.00 40.00 3.2 14.4 0.00 6.39

4 170C 15min 80.00 50.00 4.0 10.9 0.00 15.10

5 170C 30min 80.00 50.00 4.0 9.8 0.00 16.28

6 190C 0min 80.00 50.00 4.0 15.8 0.00 10.26

7 190C 15min 80.00 50.00 4.0 9.3 0.00 16.72

8 190C 30min 80.00 50.00 4.0 7.2 0.00 18.81

9 210C 0min 80.00 50.00 4.0 12.7 0.00 13.32

10 210C 15min 80.00 50.00 4.0 6.7 0.00 19.30

11 210C 30min 80.00 50.00 4.0 5.9 0.00 20.15

12 Caustic 80.00 50.00 4.0 20.6 0.00 5.42

13 PC 80.00 50.00 4.0 0.0 4.11 21.93

14 NC 80.00 50.00 4.0 0.0 0.00 26.04

Table 1.11: Properties of Inoculums and substrate; COD preformed in triplicates (December 23, 2014)

TSS/VSS

Inoculums TSS g/L VSS g/L

1 17.70 14.60

2 17.35 15.05

3 19.80 16.70

Avg: 18.28 15.45

COD

Dilution COD g/L

Digestate 3 N/A 55.2

Digestate 3 Hydrolystate N/A 5.7

170C 0min 1.5x 9.1

170C 15min 1.5x 15.1

170C 30min 1.5x 16.9

9

190C 0min 1.5x 10.5

190C 15min 1.5x 17.7

190C 30min 1.5x 22.8

210C 0min 1.5x 13.0

210C 15min 1.5x 24.5

210C 30min 1.5x 28.0

Caustic 2.2x 11.7

PC N/A 26.8

Table 1.12: Biogas production for each assay measured in mL (December 23, 2014)

Biogas sampling time (h)

ID 13 38 109 161 209 258 308 348 420 493 591 684 810 930 1050 1218

1 7.7 2 2 0.9 1.5 0.8 3 1.3 2 0.6 2.5 2 1.4 0.4 1.8 2

7.7 2 1.9 1.1 1.4 0.6 3 1.4 1.8 0.6 2.3 2.1 2 1 2.4 2.2

7.5 1.9 2 0.9 1.6 0.7 2.8 1.2 2 0.4 2.2 2.1 1.4 0.4 2 2.2

2 6 1.2 1.2 1.1 2.2 0.6 1.9 0 1.5 0.6 3.6 3 3.2 1.6 2 1.4

6 1.3 1.5 1.2 1.8 0.3 2 0 1.2 0.3 4 2.4 1.2 0.3 2.2 1.4

5.8 1.3 1.1 1 2.2 0.6 1.8 0 1.5 0 3.4 3.2 1.2 0 2.2 1.4

3 7.5 2 2.2 2.4 2.6 0.4 2.2 0 2 1.6 3.4 1.2 0.8 0.2 1.2 1.3

8 2 2.2 2.4 2.7 0.6 2.1 0 1.8 1 3.6 1.2 0.8 0.5 1.4 1.4

7.7 1.9 2.2 2.3 2.6 0.4 1.9 0 1.8 1 3.6 1.2 0.8 0.6 1.4 1.4

4 7.5 3.5 4.5 4 5.6 1.4 2.2 0 1.5 1 7 1.8 1.2 0.5 1.9 2

7.2 3.4 4.7 3.9 5.6 1.2 2.4 0 1.6 1.6 6.3 1.7 1.4 0.4 2 2.2

7.2 3.4 6.4 3.9 5.6 1.2 2.2 0 1.4 0.7 6.5 1.8 1.5 0.1 2 1.6

5 7.2 3.5 5 4 5.6 1.2 2.2 0.4 2.8 4.2 3 1.2 1.2 0.3 2.2 2.2

7.5 3.8 6.2 3.9 5.2 1.3 2.4 0.4 3.2 4 3 1.4 1.4 0.2 1.8 1.6

7 3.8 5.2 3.9 5.6 1.7 2.5 0.2 2.6 3.5 4 1.5 1.2 0.3 1.8 1.6

6 7 3.1 3.8 3.8 4 0.5 2.2 0 1.4 1 3.5 4.7 3.2 0.1 2 2.2

7 2.8 3.5 3.8 4.5 0.7 2.2 0 1.2 0.2 4 5 1.2 0.3 1.6 1.5

7 2.8 3.2 3.7 5 0.9 2 0 1.2 0.8 3.8 4.5 1.4 0.4 1.6 1.2

7 7.5 4.4 4.4 3.8 4.2 3 2.8 0.3 2.7 4.4 2.5 1.2 1.2 0.2 2.2 2.8

7.8 4.3 4.6 3.8 4.2 2.8 2.6 0.4 2.9 3.9 2.8 1.4 1.2 0.4 2 2

7.2 4.4 4.4 3.8 4.5 3 2.6 0.2 3 4 2.5 1.4 0.9 0.2 2.2 2.2

8 8 4.2 5 4.1 4.5 3.2 3.8 2.5 5 0.5 1.5 1 1 0.3 2.2 2

8 4 5 4.3 4.2 3.4 4.2 2.4 5.2 1 1.5 1.2 1.4 0.4 2.4 2

8 4.5 5.4 4 4 3.2 3.8 2.2 5 0.5 2 1.3 1.1 0.6 1.8 2.2

9 7.2 3.8 3.6 3.8 5.5 1.2 2 0.1 1.3 0.3 5 7 1 0.2 1.6 1.4

7.2 4 3.8 3.9 5.5 1.2 2 0.2 1.3 0.3 3 4 1.2 0.3 1.7 1.7

7.2 3.7 3.8 4 5 0.3 2 0 1.3 0.5 2.5 6.2 1.5 0.5 1.8 1.6

10

10 7.5 4.2 4.8 4 4.2 3.2 4 2.6 5.5 0.5 1.5 1.2 1 0.3 1.8 2.8

8.2 4.7 4.6 4 4.2 3.4 3.5 2 6 0.4 1.5 1.6 1.5 0.8 1.6 1.8

8.2 4.7 5.1 3.8 4.5 3.6 3.5 2 6 0.6 1.3 1.2 0.8 0.6 1.9 1.8

11 8.6 4.9 5.1 3.8 4.5 3.8 4.2 2.6 5.8 0.4 1.2 1.2 1.1 0.3 1.8 1.8

8.5 4.5 5 4.1 5 3.7 4.5 2.6 5.5 0.6 1.2 1 0.8 0.5 1.9 1.9

9 4.5 5 4.1 4.5 4.1 4.2 2.6 5.5 0.5 1.2 1.1 1.4 0.8 2.2 2.3

12 7.1 2.7 1.9 0.9 2.5 2.8 5 0.7 1.5 0 1 2.6 4.8 0 2 2

7 3 2 0.9 2 2.2 4.5 1.4 2.1 0 1.2 2.8 5 0 2.2 2

6.8 3 2.4 0.9 2.5 2.3 4.5 1.2 1.8 0 1 2.8 5 0.2 2.4 2.1

13 8.6 11.5 20 14.5 6.2 1 3.3 0 2.2 3.6 6.8 2.1 1.8 0.2 1.4 1.4

11 8 20.5 12 5.5 1.2 3.3 0 3 7 3.5 1.7 1.5 0.3 1.8 1.4

8.2 11 20.5 13 7 1.5 3.2 0 2.7 4.8 6 1.8 0.9 0.8 1.6 1.3

14 8 2.3 0 0.9 1.8 0 2.4 0.4 2.6 0 1.1 0.7 1.4 1.4 1.3 1.6

8 2.3 0 0.9 1.7 0 2 0.1 3 0 1.1 0.6 0.4 0.4 1.4 1.4

8 2.3 0 0.9 1.6 0 1.9 0 3 0 1.1 1.2 1.8 1.8 1.6 1.2

11

1.5. BMP Test on March 20, 2015: Anaerobic Digestion of Solids

Residues and Co-digestion of Thermal hydrolysate and

Biosludge

Table 1.13: Recipes for each BMP assay (March 20, 2014)

Sample Substrate Total volume

(ml)

Medium

(ml)

Inoculum

(ml)

Sample

(ml)

Synthetic

feed (ml)

Water

(ml)

1 Biosludge 4 80.00 50.00 4.0 5.0 0.00 21.00

2 Digestate 3 80.00 50.00 4.0 2.0 0.00 24.01

3 Digestate 3 Hydrolysate 80.00 50.00 4.0 20.6 0.00 5.38

4 210C 30min 80.00 50.00 4.0 3.2 0.00 22.78

5 210C 30min Hydrolysate 80.00 50.00 4.0 6.1 0.00 19.85

6 210C 30min Residue Solids 80.00 50.00 4.0 1.9 0.00 24.13

7 190C 15min 80.00 50.00 4.0 3.1 0.00 22.95

8 190C 15min Hydrolysate 80.00 50.00 4.0 12.2 0.00 13.80

9 190C 15min Residue Solids 80.00 50.00 4.0 3.5 0.00 22.53

10 Mixture 1 80.00 50.00 4.0 8.1 0.00 17.88

11 Mixture 2 80.00 50.00 4.0 7.6 0.00 18.40

12 Mixture 3 80.00 50.00 4.0 6.9 0.00 19.05

13 Mixture 4 80.00 50.00 4.0 5.2 0.00 20.77

14 Mixture 5 80.00 50.00 4.0 5.2 0.00 20.81

15 Mixture 6 80.00 50.00 4.0 5.1 0.00 20.86

16 Mixture 7 80.00 50.00 4.0 6.4 0.00 19.56

17 Mixture 8 80.00 50.00 4.0 6.2 0.00 19.80

18 Mixture 9 80.00 50.00 4.0 5.9 0.00 20.10

19 NC 80.00 50.00 4.0 0.0 0.00 26.00

20 PC 80.00 50.00 4.0 0.0 4.11 21.89

Table 1.14: Properties of Inoculums and substrate; COD preformed in triplicates (March 20, 2015)

TSS/VSS

Inoculums TSS g/L VSS g/L

1 17.70 14.60

2 17.35 15.05

3 19.80 16.70

12

Avg: 18.28 15.45

COD

Dilution COD g/L

Biosludge 4 N/A 22

Digestate 3 N/A 55.2

Digestate 3 Hydrolysate N/A 5.3

210C 30min 1.5x 34.2

210C 30min Hydrolysate 1.5x 17.9

210C 30min Residue Solids Thickened 58.7

190C 15min 1.5x 36

190C 15min Hydrolysate 1.5x 9

190C 15min Residue Solids Thickened 31.7

PC N/A 26.8

Table 1.15: Biogas production for each assay measured in mL (March 20, 2015)

Biogas sampling time (h)

ID 12 39 64 86 113 134 154 183 206 257 304 400 473 589 712 826

1 7.1 1.5 1 1.2 1.5 2.7 1.4 1.3 0.4 4.2 2.3 3.7 4 4.5 2.5 2.8

7.1 1.5 1.2 1.2 1.5 2.7 1.7 1.3 0.4 4.2 2.2 3.6 3.5 4.7 2.5 3.2

7.1 1.7 1 1.2 1.5 2.8 1.4 1.3 0.4 3.6 2.8 4 3.5 3.8 2.5 3.2

2 7 1.1 0.6 0.2 0.4 1.6 0 0.6 0 0.4 1.2 2.3 2.2 1.5 1.1 1.8

7 1 0.5 0.4 0.4 1.6 0 0.6 0 0.4 1.6 2.2 1.2 1.2 1 2.1

7 1 0.7 0.3 0.3 1.5 0.4 0.3 0 0.8 0.9 2.4 1.5 1 0.2 1.6

3 7 1 0.4 0.1 0.4 1.5 0.9 0.8 0.1 2.4 2 1.7 1.5 1 0.5 1.6

7 0.8 0.3 0.2 0.4 1.6 0.8 0.5 0 2.3 1.6 1 1.8 1 0.5 1.5

6.8 1.2 0.2 0.2 0.3 1.8 0.7 0.8 0 3 1.5 0.6 1.2 1.2 0.7 1.6

4 7.1 2 1.2 1.2 1.2 2.6 1.8 1.9 0.4 1.8 0.4 4 4.1 1.7 1 1.7

7.1 2 1.2 1 1.2 2.6 1.8 2 0.6 1.7 0.2 4.1 3.3 1.9 0.4 2.2

7.1 2 1 1 1.2 2.6 1.4 2.2 0.7 1.8 0.2 4 3.7 2 0.5 2.2

5 8 2.6 1.8 1.6 1.8 3.2 2.8 3.2 1.3 2.3 1 5.1 3.5 1.9 0.5 2.6

8 2.8 1.8 1.7 1.8 3.2 2.7 3.2 1.7 2.5 0.9 5.5 3.2 1.5 0.4 2.5

7.5 2.7 1.8 1.4 1.8 3.4 2.6 3.2 1.7 2.8 1 5.7 4 2 0.6 2.5

6 7 1.5 0 0.2 0.5 2.1 0.8 0.6 0 1.2 0.2 3.4 1.5 1 0.5 2

7.2 1.6 0.2 0.2 0.5 2 0.8 0.6 0 0.8 0.3 3.3 1.6 1.5 0.1 1.8

7.1 1.4 0.4 0.2 0.6 2 0.9 0.5 0 0.9 0.1 3.5 1 1.2 0 1.8

7 7 1.4 0.8 0.4 0.9 2.2 1 1.7 0.2 2 1.4 2.8 2 2 1.3 3.4

7 1.4 0.8 0.5 1 2.2 1.2 1.5 0.2 2.2 1.7 3.3 1.7 2 0.8 2.4

7.2 1.4 0.8 0.6 1 2.3 1 1.3 0.3 2.3 1.8 3.6 2 2 0.5 2.4

8 8 2.4 1.6 1.1 1.7 2.6 2.3 2.7 1.7 3.2 1.3 6.1 2 2.5 0.7 2.2

13

7.8 2.5 2 1.4 1.6 2.8 1.9 2.7 1.7 4.2 2.8 3.6 2.5 2.2 1 2.2

8 2.4 1.7 1.4 1.5 2.8 1.9 2.8 1.8 3.2 1 5.8 2.6 2.2 0.7 2.4

9 8 1.2 0.3 0.4 1.1 2.2 1.2 1 0 2.1 0.6 3.5 2.5 2 0.5 2.4

7.8 1 0.5 0.6 1 2.2 1.2 0.9 0 1.7 0.5 3.6 2 2 0.4 1.8

8 1.2 0.5 0.6 1 2.2 1.1 0.9 0 1.7 0.6 3.5 2.5 1.8 0.3 1.9

10 7.2 1.2 1.1 1 1.5 2.6 1.7 1.4 0.6 3.7 1.8 3.7 4 4.7 2.2 2.5

7 1.2 1.3 1.2 1.3 2.8 1.5 1.2 0.6 3.9 2 3.7 3.8 4.5 2 2.5

7.2 1.4 1.4 1.2 1.4 2.7 1.7 1.5 1 4.3 1 3.5 3.8 4.7 2.7 2.5

11 7.5 1.4 1.2 1 1.6 2.6 1.8 1.5 0.8 4.4 1 3.9 3.5 4.1 2.9 2.9

8 1 1.4 1.2 1.6 2.6 1.7 1.5 0.9 4.2 0.9 3.5 3.2 4.3 3 2.9

7.8 1.2 1.3 1.2 1.6 2.8 1.6 1.6 0.9 4.2 1 3.5 3 4.3 2.7 2.8

12 8 1 1.2 1 1.6 2.6 1.4 1.5 0.6 3.2 3 4.2 3.6 4 2.7 3.1

7 1.2 1 1 1.4 2.7 1.7 1.5 0.4 4.2 2.6 4.5 3.9 4 1.8 3

7.5 1.2 1 1 1.6 2.8 1.7 1.4 0.5 3.2 2.7 4.2 4 4.2 2.5 3

13 8 2.2 0.8 1.1 2 2.9 2 1.8 0.6 3.5 3.4 5.9 4.3 4.5 2.5 2.8

7.8 2.3 0.8 1.2 1.4 2.9 1.8 1.9 1 3.6 3.4 4.5 3.9 4.7 2.7 3

8 2.2 1 1.2 1.8 3 2 1.7 0.6 3.4 3.4 4.2 4 4.7 3 3

14 8 2.2 0.6 1.2 1.4 2.8 2 1.8 0.6 3.2 4.2 4.8 3.8 3.5 2.5 3.1

8.5 2.2 1 1.2 1.8 2.9 1.9 1.6 0.8 3.6 3.6 4.5 3.5 3.7 2.2 3

8.5 2.5 0.7 1 1.7 2.7 1.7 2 0.6 3.6 3 5.1 4.2 5 2.7 2.9

15 8.2 2.5 0.6 0.9 1.4 2.5 1.8 1.6 0.4 3.4 2.8 5 3.9 4 2.5 2.8

8.2 1.6 0.6 1 1.4 2.6 2 1.6 0.4 3.5 3 4.5 4 4.5 2.5 3.1

8.2 1.6 0.6 0.8 1.6 2.5 2 1.4 0.5 3.4 3.5 4.7 3.7 4.7 3 3

16 7 2.6 0.8 1.2 1.6 2.4 2.2 2.1 0.6 3.6 3 4.2 4 4 3.5 3

7.5 2.4 0.8 1.1 1.6 2.5 2.2 2 0.8 3.4 3 4.6 4.1 4.8 3.2 3

7.8 2 0.8 1.1 1.6 2.6 2.2 1.8 0.7 3.4 3.3 4.6 4 4.5 3.2 3

17 7.5 2.6 1.1 1 1.8 2.4 2.3 1.8 0.6 4 3.1 4.5 4 4.2 3.3 3.2

7.8 2.4 1 1 1.8 2.5 2.2 1.4 0.7 3.9 3.2 5 4 4.1 2.5 3

7.5 1.9 1.4 1.1 1.8 2.5 2.2 1.6 0.6 3.9 3.8 5 4 3.5 2.9 2.7

18 7.2 2.4 0.8 0.9 1.7 2.2 2.6 1.4 0.8 3.9 3.2 5 4 4 3 2.5

7.5 2.5 0.8 1 1.7 2.3 2.4 1.3 1 3.8 3.2 4.9 3.8 3.5 3 2.8

7.5 2.5 0.6 1.2 1.7 2.3 2.6 1.5 1.2 3.6 3 5.1 3.8 3.5 3 2.8

19 7.5 1.4 0 0 0.2 1.7 0.5 0 0 1.2 0.3 2 1.5 1.1 0 1.2

7.5 1.2 0 0 0 1.6 0.6 0 0 1 0.4 2.5 1.2 1.5 0 1.4

7.8 1.6 0 0 0.3 1.6 0.5 0.2 0 1.2 0.3 2.5 1 1 0 2.1

20 8 9 1.8 6 14.8 7.5 4.4 4.8 2.2 1.9 0 2 5.3 5.1 0.8 1.6

8 9.5 1.2 5 14.8 7.7 4.8 6.2 2.4 1.7 0 1.8 4.5 7 1 1.6

8 8 2 6.2 6.2 11.8 4.8 4.6 1.6 1.9 0 2 4.5 6.5 0.6 1.6

14

15

2. Appendix 2: Experimental Method

2.1. BMP Assay Preparation

2.1.1.Medium Recipe

The nutrient medium is prepared as described below. The medium contains all essential

nutrients for anaerobic digestion, and it also dilutes the assay to minimize the errors and

ensure the reproducibility of the test. The recipe is originally developed by Edwards and

Grbić-Galić [1] and adopted by the Biozone at University of Toronto as the standard

recipe.

Stock Solutions:

Stock solution 1: Phosphate buffer

KH2PO4 27.2 g Stephan’s modification 20.96 g

K2HPO4 34.8 g 42.85 g

Adjust pH to 7.0. Make up to 1 L with distilled H2O (dH2O).

Stock solution 2: Salt solution

NH4Cl 53.5 g

CaCl2.6H2O 7.0 g (or 4.79 g CaCl2.2H2O)

FeCl2.4H2O 2.0 g

Make up to 1 Liter with H2O.

Stock solution 3: Trace Minerals

H3BO3 0.3 g

ZnCl2 0.1

Na2MoO4.2H2O 0.1 g

NiCl2.6H2O 0.75 g

MnCl2.4H2O 1.0 g

16

CuCl2.2H2O 0.1 g

CoCl2.6H2O 1.5 g

Na2SeO3 0.02 g

Al2(SO4)3.18H2O 0.1 g

Add 1 ml concentrated H2SO4 per liter to dissolve all components. Make up to 1 L.

Stock solution 4: Magnesium chloride solution

MgCl2.6H2O 50.8 g/L

Stock solution 5: Redox indicator

Resazurin 1 g/L

Stock solution 6: Saturated bicarbonate

Mix approximately 20 g NaHCO3 in 100 ml H2O. Pour slurry into 160-ml serum bottle,

cover with foil and autoclave. After autoclaving, sparge with O2-free N2 for at least 15

minutes while cooling. Seal with sterile black butyl rubber stopper and crimp. The

preparation will have undissolved NaHCO3 in the bottom.

Stock solution 7: Microvitamins

Biotin 0.02 g

Folic acid 0.02 g

Pyridoxine HCl 0.1 g

Riboflavin 0.05 g

Thiamine 0.05 g

Nicotinic acid 0.05 g

Pantothenic acid 0.05 g

PABA 0.05 g

Cyanocobalamin

(vitamin B12) 0.05 g

Thioctic (lipoic) acid 0.05 g

Coenzyme M 1.0 g

Adjust pH to 7.0 with NaOH. Make up to 1 Liter. Store in one or two ml aliquots frozen.

Dilute the stock 1/100 to get 100x stock. Filter the sterilized 100x stock into sterile

160-ml serum bottle and sparge with sterile O2-free N2 for 15 minutes. Seal with sterile

black butyl rubber stopper and crimp.

Stock solution 8: Amorphous Ferrous Sulfide

17

(NH4)2Fe(SO4)2.6H2O 19.6 g/500 ml

Na2S.9H2O 12.0 g/500 ml

Procedure to make 500 ml: Deoxygenate 2.5 Liters of dH2O with O2-free N2 for > 1 hour.

Weigh out the ferrous ammonium sulfate in a small beaker. Weigh out the sodium sulfide

in a separate small beaker. Bring a 1-L Erlenmeyer flask with stopper and the chemicals

(as powders) into an anaerobic glove box. After gassing the 2.5 L of dH2O, seal the bottle,

and bring it into the glove box. Put 500 ml of dH2O into the 1-L Erlenmeyer flask. Add

the Na2S and mix until dissolved. Add the (NH4)2Fe(SO4)2. A black precipitate forms

immediately. Put the Erlenmeyer into the glove box antechamber and cycle three times to

evacuate the H2S being formed. Returned flask to glove box. Allow precipitate to settle

for 24 hours. Wash by removing (decanting or siphoning) as much as possible of the clear

supernatant, and replacing with about 500 ml of O2-free water. Repeat 3 more times. The

rate of settling decreases as the precipitate is washed and sometimes more than 24 hours

is required. The purpose of washing is to remove any free sulfide in the water. The iron

and the sulfide in the reactants combine in equimolar proportions to form FeS (ferrous

sulfide). Make sure that on the last wash, you resuspend the precipitate such that the total

volume is 500 ml to get the right concentration. Dispense the final 500 ml of slurry into

five 160-ml serum bottles. Seal and crimp in the glove box. Remove and autoclave. The

amorphous ferrous sulfide prepared this way is sterile and anaerobic. The approximate

concentration of FeS in the slurry is 2 g/L (as Sulfide).

Making nutrient medium

The nutrient medium is made from the above stock solutions:

1. Add to 1L of H2O: 10 mL of 1, 10 mL of 2, 2 mL of 3, 2 mL of 4 and 1 mL of 5;

2. Autoclave the mixture;

3. Sprarge the mixture with O2 free N2/CO2 gas for at least 30 min;

4. While sprarge the mixture, keep the mixture in ice water to let it cool;

5. After, cap the mixture and bring the mixture into the glove box;

6. In the glove box, add Stock solution 6 to 8 in the following orders: 10 mL of 7, 10

mL of 8, and then 10 mL of 6;

7. Let the final nutrient medium in the glove box for at least 1 day to let it becomes

truly anaerobic.

18

2.1.2.Recipe for Synthetic Feed

The synthetic feed is the substrate injected into the positive control BMP bottles. It is

composed of the following chemicals:

In 100 mL of autoclaved Milli-Q water, 829.58 mg glucose, 799.21 mg sodium acetate,

282.13 mg sodium propionate and 0.803 ml 100% methanol is added.

2.1.3.Preparing BMP Assay in Glove bag

The BMP assays are prepared in a Glove bag as described below:

1. Connect the glove bag to N2, and N2/CO2 cylinders; also connect the bag to a vacuum

pump; seal all the connection with anaerobic taps;

2. Transfer nutrient medium, substrate samples, inoculum, HCl and NaOH solutions,

and Milli-Q water into the glove bag (note: all of the above items should be

deoxygenated and sealed before bring into the glove bag.);

3. Transfer enough supplies such as beakers, pipettes, graduated cylinders, syringes and

needles into the glove bag (note: each lab may have different supplies one can utilize.

For example if automatic pipette is available, then syringes and manual pipettes are

not needed.); if the supplies is sealed in packages, crake open the packages before

putting into the glove bag;

4. Once everything is in the glove bag, seal the bag opening with anaerobic tapes;

5. Pump the air in the bag out using the vacuum pump, stop the pump when the bag is

almost flat;

19

6. Turn on the N2 cylinder and fill the bag with N2 gas; stop the gas flow when the bag

is a little bit inflated (note: do not over inflate the bag. This may cause leaking);

7. Allow the bag atmosphere to equilibrate for 20 to 30 min;

8. Repeat step 5 to 7 for 2 times;

9. Repeat step 5 to 8 for 2 more times; however instead of using the N2 gas, use the

N2/CO2 gas to fill the bag;

10. After the purging cycles, transfer the medium, substrate samples and inoculum into

the assay bottles followed the recipes as described in Table 1.1, Table 1.4, Table 1.7,

Table 1.10, and Table 1.13;

11. Test the pH of each bottle using pH meter to make sure the pH of the content is

between 6.8 and 7.2 (if the pH is out of that range, adjust the pH using HCl and

NaOH solution.);

12. Cap and seal each BMP bottle, and then take out the bottles to the incubator;

An observation: if the glove bag atmosphere is truly anaerobic, the medium will appear to

be colorless. If not, the medium will turn pink. After purging the bag, it is recommended

to test whether the bag is truly anaerobic before transferring:

1. Transfer a small amount of medium (5-10 mL in each tube) into small tubes and seal

the tubes;

2. Bring the tubes into the glove bag; before transferring solutions into assay bottles,

open one of the tube;

20

3. If the medium in the tube does not change color, continue with the transferring; if the

medium changes color, purge the bag a few more time;

4. Repeat step 3 until the medium is not turning pink.

2.2. Caustic Extraction Procedure

To characterize the organic composition of sludge particles, caustic extraction (alkaline

extraction) was performed to release the organic compounds in the bioflocs into the

solution phase. Based on literature review, this method is reported to have the highest

extraction yield [2, 3]. The caustic extraction procedure developed based on past studies

is illustrated in Figure 2.1. The sludge sample was first diluted to 5 g TSS/L; then the

diluted sample was centrifuged at 5000rpm for 15min. The centrate was then transferred

to a glass container; the solids was re-suspended in 1N NaOH solution and extracted for 4

hr. After 4 hr, the extracted sample was centrifuged again at 5000rpm for 15min. The

centrate was transferred into the same glass container. The final solution contains both

the soluble and extracted organic compounds.

21

Figure 2.1: Caustic Extraction Procedure; Extraction was performed in triplicates for all samples.

2.3. Calibration Curves

2.3.1.COD Calibration

The total chemical oxygen demand (TCOD) and soluble chemical oxygen demand

(SCOD) was measured using Hach® high range (20 -1500 mg/L COD) digestion vials

which is in compliance with Standard Methods for the Examination of Water and

Wastewater #5220D [36]. A calibration curve was developed to validate the Hach

method (Figure 2.2).

The standard solutions were prepared using potassium hydrogen phthalate (KHP). A 1

mg KHP/L solution is equivalent to a 1 mg COD/L solution. A 1g KHP/L stock standard

22

solution was needed initially. All other standard solutions were made by diluting the 1g/L

stock with Milli-Q water accordingly.

Figure 2.2: COD calibration curve

2.3.2.Carbohydrate Calibration

The carbohydrate was measured according to the sulfuric acid phenol method as

described in Biochemical Method [5]. One milliliter of sample was added to a Hach®

incubation tube. Then, 1 mL of 5 vol% phenol and 5 mL of 96% sulfuric acid were added

to the tube and mixed by shaking the tubes for few minutes. The incubation tube

containing the final solution was placed at room temperature for 10 min and then

submerged into a water bath maintained at 25-30o C for another 20 min. After, the

absorbance of the tube was measured using a Hach® DR3900 Spectrophotometer at the

wavelength of 490 nm. Standard solutions were prepared using D-glucose, from 0.01 to

y = 1.0319x + 2.9169

R² = 0.9996

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

KH

P c

on

cen

tra

tio

n (

mg

/L)

Measured Concentration (mg/L COD) given by Hach DRB 3900

23

0.2 g glucose/L. A calibration curve was constructed using the standard solutions, and the

carbohydrate concentration of the sample was determined using the calibration curve

(Figure 2.3).

Figure 2.3: Carbohydrate calibration curve

2.3.3.Tannin/lignin Calibration

The tannin/lignin were measured using Hach® Tannin-lignin test kit which is based on

tyrosine colorimetric Method developed by Kloster el al. [33]. This method cannot

distinguish tannin from lignin; it measures the total amount of tannin and lignin (referred

to as tannin/lignin). A 25 mL of sample were added to a 50 mL centrifuge tubes. Then,

0.5 mL of TanniVer™ 3 Tannin-Lignin Reagent and 5mL of sodium bicarbonate solution

(both provided in the kit) was added. After, the centrifuge tube containing the final

solution was placed at room temperature for 30 min and then the absorbance of the tube

was measured using a Hach® DR3900 Spectrophotometer at the wavelength of 700 nm.

y = 10.446x + 0.0603

R² = 0.9864

0

0.5

1

1.5

2

2.5

0 0.05 0.1 0.15 0.2 0.25

Ab

s (4

90

nm

)

Concentration (g glucose/L)

24

Standard solutions were prepared using tannic acid, from 0.1 to 9 mg tannic acid/L. A

calibration curve was constructed using the standard solution and the tannin/lignin

concentration of the sample was determined using the calibration curve (Figure 2.4).

Figure 2.4: Tannin/lignin calibration curve

3. Appendix 3: Economic Analysis

The stream table is present in Table 3.1 to Table 3.4.

Table 3.1: Stream summary for anaerobic digestion without treatment (NT)

Stream Flowrate (m3/d) State T (

o C) TSS (g/L) VS (g/L)

Biosludge 3300 liquid+solids 10 14 13

Thickened Biosludge 1206 liquid+solids 10 36 34

Overflow 2094 liquid 10 0.5 0.6

Preheated Biosludge 1206 liquid 37 36 34

Digestate 1206 liquid 37 24 22

Dewater Digestate 130 liquid+solids 37 200 186

Centrate 1076 liquid 37 2.7 2.5

y = 0.0918x + 0.0295

R² = 0.9991

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10 12

Co

nce

nra

tio

n (

mg

/L)

Absorbance (700)

25

Total Biogas 11499 gas (37o C) 37

Net Biogas 5595 gas (37

o C) 37

Table 3.2: Stream summary for anaerobic digestion with thermal pretreatment

Stream Flowrate (m3/d) State T (

o C) TSS (g/L) VS (g/L)

Biosludge 3300 liquid+solids 10 14 13

Thickened Biosludge 1206 liquid+solids 10 36.4 34

Upflow 2094 Liquid 10 0.6 0.6

Dewater Biosludge 263 liquid+solids 10 150 135

Centrate 1 241 Liquid 10 5 4

Treated Biosludge 301 liquid+solids 100 66 118

Dilution stream 701 liquid 10 4.7 4.4

Combine feed 1002 liquid+solids 37 23 39

Digestate 1002 liquid+solids 37 20 26

Dewater Digestate 90 liquid+solids 37 200 258

Centrate 2 912 liquid+solids 37 2.2 2.8

Total Biogas 10422 Gas (37 o C) 37

Net Biogas 5263 Gas (37

o C) 37

Table 3.3: Stream summary for anaerobic digestion with digestate thermal Treatment

Stream Flowrate (m3/d) State T (

o C) TSS (g/L) VS (g/L)

Biosludge 3300 liquid+solids 10 14 13

Thickened Biosludge 1206 liquid+solids 10 36 34

Overflow 2094 liquid 10 0.7 0.7

Combine 1322 liquid+solids 38 44 48

Digestate 1322 liquid+solids 38 33 31

Centrate 1093 liquid 38 18 17

Recycle 117 liquid+solids 38 202 190

Treated Digestate 117 liquid+solids 190 121 190

Reject 78 liquid+solids 38 191 190

Total Biogas 17687 gas (37o C) 37

Net Biogas 13447 gas (37

o C) 37

Table 3.4: Digestate thermal treatment with hydrolysate recycle only(DTH)

Stream Flowrate (m3/d) State T (

o C) TSS (g/L) VS (g/L)

Biosludge 3300 liquid+solids 10 14 13

Thickened Biosludge 1206 liquid+solids 10 36 34

Overflow 2094 liquid 10 0.7 0.7

Combine 1325 liquid+solids 38 35 40

Digestate 1325 liquid+solids 38 26 25

26

Thickened Digestate 210 liquid+solids 38 150 143

Centrate 1115 liquid 38 3.1 3.0

Treated Digestate 210 liquid+solids 210 96 143

Hydrolysate 119 liquid 210 17 101

Reject 91 liquid+solids 210 200 198

Total Biogas 15204 gas (37o C) 37

Net Biogas 7570 gas (37

o C) 37

27

Reference

[1] Edwards, E.A. and Grbić-Galić, D., "Anaerobic degradation of toluene and o-xylene

by a methanogenic consortium," Applied and Environmental Microbiology, vol. 60,

no. 1, pp. 313-322, 1994.

[2] S. Comte, G. Guibaud and M. Baudu, “Relations between extraction protocols for

activated sludge extracellular polymeric substances (EPS) and EPS complexation

properties: Part I. Comparison of the efficiency of eight EPS extraction methods,”

Enzyme and Microbial Technology, vol. 38, pp. 237-245, 2006.

[3] Hong Liu and Herbert H.P. Fang, “Extraction of extracellular polymeric substances

(EPS) of sludges,” Journal of Biotechnology, vol. 95, pp. 249-256, 2002.

[4] AHPA, Standards Methods for the Examination of Water and Wastewater., 1998.

[5] S Sadasivam and A. Manickam, “Phenol Sulphuric Acid Method,” in Biochemical

Methods for Total Carbohydrate, New Delhi, New Age International, 1996, pp. 8-9.

[6] M. B. Kloster, “The determination of tannin and lignin,” Journal (American Water

Works Association), vol. 66, no. 1, pp. 44-46, 1974.