thermal treatment of pulp and paper mill biosludge and digestate to enhance their anaerobic ...

143 Pages·2015·2.1 MB·English

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
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.
ii
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.

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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
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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

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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
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Table 5.4: Model kinetics parameters for the thermal hydrolysate samples; A= (mL
biogas/ mg COD input), µ = (mL biogas/ (mg COD input*h)), λ= (h), A is the sum of
m tot
A and A ; *Due to the limited amount of sample points, those parameters had a large
1 2
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
vii
compared to the current mill;*** this is calculated as the reduction on sludge production
compared to the current mill. ............................................................................................ 89

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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
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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
x