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Enhancing Anaerobic Digestion of Pulp and Paper Mill Biosludge Using Thermal Treatment in a Bench-Scale System

By

Xian Meng Huang

A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science (M.A.Sc.)

Graduate Department of Chemical Engineering & Applied Chemistry

University of Toronto

Copyright by Xian Meng Huang 2015

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Enhancing Anaerobic Digestion of Pulp and Paper Mill Biosludge Using Thermal Treatment in a Bench-Scale System

Masters of Applied Science

2015

Xian Meng Huang

Department of Chemical Engineering and Applied Chemistry, University of Toronto

Abstract

This study examines the feasibility of using a bench-scale anaerobic digestion (AD) system

to treat pulp and paper mill biosludge. Three thermal treatment methods were studied: pre-

treatment, intermediate treatment and post treatment. Thermal pre-treatment of biosludge

resulted in overloading of the bench-scale reactor, causing the pH to drop significantly; this

in turn caused reactor acidification. Intermediate thermal treatment of digestate had a

positive impact on the digestibility of the biosludge; the specific methane yield increased by

76% and 29% in two separate bench-scale experiments. Post-treatment of digestate followed

by further digestion was studied in BMP tests, and it was determined that the final specific

methane yield increased by 43%. Overall, anaerobic digestion of pulp and paper mill

biosludge has been proven to be successful at the bench-scale level, and thermal post-

treatment methods have been shown to be effective at enhancing the digestibility of the

biosludge.

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Acknowledgments

I would like to thank my supervisors Prof. D. Grant Allen and Prof. Elizabeth Edwards for

their guidance and support throughout this project. I would also like to thank Dr. Torsten

Meyer, Mr. Nigel Guilford, Mr. Paul Jowlabar, Mrs. Sofia Bonilla Tobar, and other members

of the Allen and Edwards Lab for their help and support.

I greatly appreciated the funding received from the Ontario Government as well as the

Natural Sciences and Engineering Research Council of Canada (NSERC).

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Table of Contents Abstract ..................................................................................................................................... ii

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

List of Figures ......................................................................................................................... vii

List of Tables ........................................................................................................................... ix

Nomenclature ........................................................................................................................... xi

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

1.1 Background ................................................................................................................ 1

1.2 Hypothesis and Objectives ......................................................................................... 4

1.3 Thesis Organization ................................................................................................... 4

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

2.1 Anaerobic Digestion of Pulp and Paper Mill Biosludge ............................................ 6

2.1.1 Biochemical Methane Potential (BMP) Test .................................................... 10

2.1.2 Co-digestion of Pulp and Paper Mill Biosludge with Other Substrates ........... 12

2.2 Pre-treatment Methods to Enhance the Anaerobic Digestion of Pulp and Paper Mill Biosludge ............................................................................................................................ 14

2.2.1 Chemical ........................................................................................................... 15

2.2.2 Mechanical ........................................................................................................ 16

2.2.3 Microwave and ultrasound ............................................................................... 18

2.2.4 Thermal ............................................................................................................. 20

2.2.5 Pre-treatments Summary .................................................................................. 22

2.3 Significance of Current Research ............................................................................ 23

3. Materials and Methods .................................................................................................... 24

3.1 Bench-Scale Anaerobic Digestion System .............................................................. 24

3.2 Medium Formulation and Reactor Start-up ............................................................. 27

3.3 Thermal Treatment of Biosludge ............................................................................. 28

3.4 Measurement of Physical and Chemical Properties ................................................. 30

3.4.1 Chemical Oxygen Demand ............................................................................... 30

3.4.2 Solids Analysis ................................................................................................. 32

3.4.3 Biogas Composition ......................................................................................... 33

3.5 Biochemical Methane Potential Tests (BMP Tests) ................................................ 33

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3.6 Experimental Design ................................................................................................ 35

4. Bench-Scale Reactor Set-Up and Troubleshooting ......................................................... 37

4.1 Walk-in Fume Hood and Bench............................................................................... 37

4.2 Bench Scale Reactor Troubleshooting ..................................................................... 38

4.2.1 Biogas Production Measurements .................................................................... 38

4.2.2 Scum Formation................................................................................................ 40

4.2.3 Reactor Sampling.............................................................................................. 42

4.3 Summary of Bench-Scale Reactor Set-up ................................................................ 44

5. Experimental Results ....................................................................................................... 45

5.1 Thermal Treatment of Biosludge / Digestate ........................................................... 45

5.2 Bench-Scale Reactor Runs with Biosludge ............................................................. 46

5.2.1 Baseline Runs ................................................................................................... 47

5.2.2 Initial Thermal Treatment ................................................................................. 48

5.2.3 Intermediate Thermal Treatment ...................................................................... 50

5.2.4 Post Digestion Thermal Treatment Followed by Further Digestion ................ 53

5.2.5 Bench-Scale Reactor Runs Summary ............................................................... 56

5.3 Biochemical Methane Potential (BMP) Tests .......................................................... 59

5.3.1 Effect of thermal treatment, microbial acclimation, sludge thickening............ 59

5.3.2 Feasibility of thermally treating the solid portion of digestate ......................... 62

6. Discussion ....................................................................................................................... 65

6.1 Reactor set up and operation .................................................................................... 65

6.2 Baseline Runs ........................................................................................................... 66

6.3 Thermal pre-treatment run ....................................................................................... 66

6.4 Intermediate and post thermal treatment runs .......................................................... 68

6.5 Effect of scale........................................................................................................... 70

6.6 BMP tests ................................................................................................................. 71

6.7 Operating strategy and summary ............................................................................. 72

7. Engineering Significance ................................................................................................ 73

8. Conclusion ....................................................................................................................... 74

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