The anaerobic digestion of sugar beet pulp

300 Pages·2014·10.73 MB·English

University of Southampton Research Repository
ePrints Soton
Copyright © and Moral Rights for this thesis are retained by the author and/or other
copyright owners. A copy can be downloaded for personal non-commercial
research or study, without prior permission or charge. This thesis cannot be
reproduced or quoted extensively from without first obtaining permission in writing
from the copyright holder/s. The content must not be changed in any way or sold
commercially in any format or medium without the formal permission of the
copyright holders.

When referring to this work, full bibliographic details including the author, title,
awarding institution and date of the thesis must be given e.g.
AUTHOR (year of submission) "Full thesis title", University of Southampton, name
of the University School or Department, PhD Thesis, pagination
http://eprints.soton.ac.uk
UNIVERSITY OF SOUTHAMPTON

FACULTY OF ENGINEERING AND THE ENVIRONMENT

THE ANAEROBIC DIGESTION OF SUGAR BEET PULP

By

Sri Suhartini

Supervisors: Prof. Charles Banks and Dr. Sonia Heaven

Thesis submitted for the degree of Doctor of Philosophy

May 2014

UNIVERSITY OF SOUTHAMPTON
ABSTRACT
FACULTY OF ENGINEERING AND THE ENVIRONMENT
Civil and Environmental Engineering
Thesis for the degree of Doctor of Philosophy
THE ANAEROBIC DIGESTION OF SUGAR BEET PULP
Sri Suhartini
World-wide there are substantial quantities of sugar beet pulp, which arises as a residue after the
processing of whole beet to extract sugar for refining as a foodstuff or for use in fermentation, in
particular for the production of ethanol for the biofuel market. In both cases the resulting pulp residue is
still rich in pentose sugars and fibre, and the research considered anaerobic digestion (AD) as a potential
technology for the conversion of this material into renewable energy in the form of biogas. To determine
the best operational conditions for biogas production both mesophilic and thermophilic digestion options
were considered. Both were tested using 4-litre working volume mixed digesters operated with semi-
continuous feed over a minimum of three hydraulic retention times (HRT). The first long term trial used
mesophilic temperatures (37 oC±0.5 oC) at applied organic loading rates (OLR) from 2-5 g volatile solids
(VS) l-1 day-1. This resulted in a specific methane yield of ~0.31 lCH g-1 VS day-1 with a biogas methane
4
content of 51.05%. VS destruction was ~90% at all loadings, and increasing the loading resulted in an
increase in volumetric biogas and methane production without significant loss in specific yields. The
major limitation found was not in the biochemical conversion but in dewatering of the digestate, the
characteristics of which were assessed using capillary suction time (CST) and frozen image centrifugation
(FIC). At the higher loading there was also the appearance of a stable foam which made the digesters
difficult to operate as this could block the gas outlet, leading to pressure increases and the loss of
digestate by ‘blow out’.
In the same digesters at mesophilic temperatures antifoam was tested to assess if this could offer a
solution by suppressing foam formation. In practice this required unusually high doses of the reagent and,
in continued use, these appeared to have an inhibitory effect on the digestion process. Dilution of the
feedstock to the digester was also tested but showed no beneficial effects on dewaterability or foaming.
As a post-treatment alternative cellulolytic enzymes were added to the digestate, but had no effect on
improving dewaterability. Trace element (TE) supplementation to the digesters was, however, shown to
eliminate the occurrence of foaming and also gave a slight improvement in dewaterability. TE
supplementation reduced the polymer dose required for dewatering as determined by the CST test, and
eliminated polymer dosing when dewatering was by centrifugation. Digestate dewaterability could also
be improved in a post-digestion one- and two-stage chemical treatment with the use of chemical
coagulants/flocculants alone or combined.
The second long-term trial compared mesophilic (37 oC±0.5 oC) and thermophilic (55 oC±0.5 oC)
digestion over 3 HRT using duplicate digesters fed at OLR of 4 and 5 g VS l-1 day-1. The digesters were
operated without water addition. The thermophilic digesters gave higher biogas and methane productivity
and were also able to operate stably at the higher OLR, whereas the mesophilic digesters showed signs of
instability. Digestate dewaterability was assessed by the CST and FIC tests and the likelihood of stable
foam forming was assessed using a foaming potential test. The results showed thermophilic operation
performed better even at the higher loading and gave a digestate with superior dewatering characteristics
and with very little foaming potential. Using a combination of CST tests, filtration tests, Frozen Image
Centrifugation, SEM and grading centrifugation it was concluded that the poor dewaterability seen in
mesophilic digestate was due to the presence of extracellular polymer substance (EPS) leading to blinding
of the filter by fine particulate materials.
The carbon, energy and nutrient (CEN) footprint was estimated for mesophilic and thermophilic
digestion in which the process was coupled with combined heat and power (CHP) and biogas upgrading
to biomethane. The results showed that the energy input for thermophilic digestion was higher than for
mesophilic although this could be compensated for by the increased specific methane yield at the higher
loadings modelled. There was also no significant difference in the emissions savings or in the quantities
of nutrients recycled in the digestate. The model indicated that the use of CHP gave a higher net energy
yield compared to biogas upgrading, but this of course is dependent on there being an economic use for
the heat produced.

Keywords: anaerobic digestion, sugar beet pulp, biogas, trace elements; digestate, dewatering, foam
formation, carbon footprint, energy footprint, nutrient footprint
i

CONTENTS
ABSTRACT ....................................................................................................................... i
CONTENTS ..................................................................................................................... iii
LIST OF TABLES .......................................................................................................... vii
LIST OF FIGURES ......................................................................................................... xi
DECLARATION OF AUTHORSHIP .......................................................................... xvii
ACKNOWLEDGEMENTS ........................................................................................... xix
DEFINITIONS AND ABBREVIATIONS .................................................................... xxi

CHAPTER 1. INTRODUCTION ..................................................................................... 1
1.1. Background ........................................................................................................... 1
1.2. Research Aims and Objectives .............................................................................. 3

CHAPTER 2. LITERATURE REVIEWS ........................................................................ 5
2.1. Sugar Beet Pulp ..................................................................................................... 5
2.2. Composition of SBP .............................................................................................. 6
2.3. Potential Uses of SBP............................................................................................ 9
2.4. Anaerobic Digestion ............................................................................................ 10
2.4.1. Review of Anaerobic Digestion ................................................................ 10
2.4.2. Factors Influencing Anaerobic Digestion.................................................. 11
2.4.2.1. Temperature ................................................................................. 11
2.4.2.2. pH ................................................................................................. 12
2.4.2.3. Substrate Composition and Concentration .................................. 12
2.4.2.4. Toxicity ........................................................................................ 13
2.4.2.5. Organic Loading Rate (OLR) ...................................................... 13
2.4.2.6. Retention Time ............................................................................ 14
2.4.2.7. Mixing intensity ........................................................................... 14
2.4.2.8. Nutrients ....................................................................................... 15
2.4.3. Performance of AD in a Single- and Two-stage System........................... 17
2.5. Biochemical Methane Potential Test ................................................................... 19
2.6. Previous Work on Anaerobic Digestion of SBP ................................................. 23
2.6.1. Single-Stage Anaerobic Digestion of SBP ................................................ 23
2.6.1.1. Mesophilic Anaerobic Digestion of SBP ..................................... 23
2.6.1.2. Thermophilic Single-stage Anaerobic Digestion of SBP ............ 24
2.6.2. Two-Stage Anaerobic Digestion of SBP ................................................... 24
2.7. Potential Advantages of Thermophilic Anaerobic Digestion .............................. 27
2.8. Dewaterability ..................................................................................................... 28
2.8.1. Review of Dewaterability Test .................................................................. 28
2.8.2. Factors Affecting Dewaterability .............................................................. 29
2.8.3. Treatments for Improving Dewaterability ................................................. 31
2.8.3.1. Chemical Treatments ................................................................... 31
2.8.3.2. Thermal Treatments ..................................................................... 35
2.8.3.3. Mechanical Treatments ................................................................ 36
2.8.3.4. Microbial Treatments ................................................................... 38
2.8.3.5. Enzymatic Treatments ................................................................. 38
2.9. Foaming ............................................................................................................... 42
2.9.1. Review of Foaming ................................................................................... 42
2.9.2. Factors Causing Foaming .......................................................................... 42
2.9.3. Problems Related to Foaming ................................................................... 43
iii
2.9.4. Previous Studies on Reducing Foaming ................................................. 44
2.10. Sludge Ageing ................................................................................................... 46
2.11. Carbon, Energy and Nutrient Footprint ............................................................. 48
2.11.1. Review on Carbon, Energy and Nutrient (CEN) Footprint ................... 49
2.11.1.1. Carbon Footprint ..................................................................... 49
2.11.1.2. Energy Footprint ..................................................................... 50
2.11.1.3. Nutrient Footprint ................................................................... 53
2.11.2. Previous Studies on CEN Footprint in AD System .............................. 54
2.11.3. Application of digestate to land: Regulations ....................................... 55
2.12. Conclusions ....................................................................................................... 56

CHAPTER 3. MATERIALS AND METHODS............................................................. 59
3.1. Materials .............................................................................................................. 59
3.1.1. Feedstocks ................................................................................................. 59
3.1.2. Digester inoculums .................................................................................... 59
3.1.3. Digester design .......................................................................................... 59
3.1.4. Trace element solution .............................................................................. 60
3.2. Analytical Methods ............................................................................................. 61
3.2.1. General ...................................................................................................... 61
3.2.2. Gravimetric Analysis ................................................................................ 61
3.2.2.1. Total Solid and Volatile Solid (TS/VS) ....................................... 61
3.2.3. Chemical and electrochemical analysis ..................................................... 62
3.2.3.1. pH ................................................................................................ 62
3.2.3.2. Alkalinity ..................................................................................... 62
3.2.3.3. Total Ammonia Nitrogen ............................................................. 63
3.2.3.4. Total Kjeldhal Nitrogen ............................................................... 64
3.2.3.5. Gas Chromatograph (GC) determination of volatile fatty acid
(VFA)........................................................................................... 65
3.2.3.6. Trace Element Analysis ............................................................... 65
3.2.3.7. Fibre analysis ............................................................................... 66
3.2.3.8. Elemental Composition ............................................................... 67
3.2.3.9. Calorific value (CV) .................................................................... 67
3.2.3.10. Sugar analysis ............................................................................ 67
3.2.3.11. COD ........................................................................................... 67
3.2.3.12. Potassium ................................................................................... 68
3.2.3.13. Phosphorus ................................................................................. 69
3.2.3.14. Turbidity Analysis ..................................................................... 70
3.2.3.15. Conductivity .............................................................................. 70
3.2.4. Gas Analysis .............................................................................................. 70
3.2.4.1. Gas Composition ......................................................................... 70
3.2.4.2. Gas Volume ................................................................................. 71
3.2.5. Dewaterability Characteristics Analysis ................................................... 72
3.2.5.1. Capillary Suction Time (CST) Test ............................................. 72
3.2.5.2. Filtration Test ............................................................................... 73
3.2.5.3. Frozen Image Centrifuge (FIC) Test ........................................... 73
3.2.5.4. Foaming potential test .................................................................. 74
3.2.6. Scanning Electron Microscopy ................................................................. 74
3.3. Biochemical Methane Potential (BMP) .............................................................. 75
3.4. Anaerobic digestion trials on SBP ...................................................................... 75
3.4.1. Digester operational parameters ................................................................ 75
iv
3.5. Digester experimental runs .................................................................................. 77
3.5.1. Kinetic study under mesophilic conditions and influence of trace
elements ..................................................................................................... 77
3.5.2. Digestion of SBP with water dilution ....................................................... 78
3.5.3. Residual Biogas Production ...................................................................... 78
3.5.4. Assessment of digestion with antifoam addition ....................................... 78
3.5.5. Comparison of mesophilic and thermophilic digestion ............................. 78
3.6. Carbon, Energy, and Nutrient Footprint Analysis ............................................... 79

CHAPTER 4. SUGAR BEET PULP DIGESTION ........................................................ 81
4.1. Research aim ....................................................................................................... 81
4.2. Characterisation of SBP ...................................................................................... 81
4.3. Biochemical methane and biogas potential of SBP ............................................. 83
4.3.1. Kinetics of the BMP .................................................................................. 86
4.3.2 Effect of inoculum age on the BMP test .................................................... 88
4.4. Kinetic study under mesophilic conditions and influence of trace elements ...... 89
4.4.1. Effect of different OLR on digestion performance ................................... 89
4.4.2. Effect of loss of digestate on AD of SBP .................................................. 97
4.4.3. Effect of TE supplementation.................................................................... 99
4.4.4. Residual Biogas Production .................................................................... 103
4.5. Digester-based studies aimed at reducing foaming ........................................... 105
4.5.1. Effect of water dilution on foaming in digestion of SBP ........................ 105
4.5.2. Trials using anti-foam ............................................................................. 108
4.6. Comparison of mesophilic and thermophilic digestion of SBP ........................ 111
4.7. Overall discussion and conclusions from digester studies ................................ 124

CHAPTER 5. DEWATERABILITY CHARACTERISTICS AND FOAMING
OCCURRENCE IN AD OF SBP .......................................................... 127
5.1 Chapter Summary ............................................................................................... 127
5.2. Dewaterability characteristics of digestate ........................................................ 128
5.2.1. Dewaterability of fresh mesophilic digestate .......................................... 128
5.2.2. Effect of physical pre-treatments on dewaterability of digestate ............ 129
5.2.2.1. Digestate dilution ....................................................................... 129
5.2.2.2. Effect of feedstock dilution ........................................................ 134
5.2.2.3. Digestate ageing study ............................................................... 135
5.2.2.4. Freeze/Thaw treatment .............................................................. 139
5.2.3. Effect of chemical pre-treatment on digestate dewaterability ................. 144
5.2.3.1. One-stage chemical treatment .................................................... 144
5.2.3.2. Two-stage chemical treatment ................................................... 152
5.2.4. Enzymatic treatment ................................................................................ 164
5.2.5. Evolution of dewaterability characteristics in mesophilic and
thermophilic digestion ............................................................................. 166
5.3. Foaming ............................................................................................................. 168
5.3.1. Antifoam serum bottle test ...................................................................... 168
5.3.2. Defoaming test ........................................................................................ 172
5.3.3. Effect of digestion temperature on foaming occurrence ......................... 174
5.4. Digestate structure and components .................................................................. 176
5.4.1. Effect of centrifugation on separation of mesophilic digestate ............... 177
5.4.2. Effect of centrifugation on separation of mesophilic and thermophilic
digestates .................................................................................................. 180
v
5.4.3. Effect of heat treatment on mesophilic digestate .................................... 183
5.4.4. Effect of removal of the non-cellular light fraction ................................ 184
5.5. Nutrients and PTE in digestate components ..................................................... 187
5.6. General conclusions .......................................................................................... 193

CHAPTER 6. CARBON, ENERGY AND NUTRIENT FOOTPRINT ....................... 195
6.1. Chapter Summary .............................................................................................. 195
6.2. System boundaries ............................................................................................. 195
6.3. Energy potential of mesophilic and thermophilic AD based on experimental
results ................................................................................................................ 196
6.3.1. 1st semi-continuous AD of SBP in mesophilic conditions with
different OLR ........................................................................................... 196
6.3.2. 2nd semi-continuous AD of SBP at mesophilic and thermophilic
condition with different OLR .................................................................. 197
6.4. Anaerobic digestion scenario modelling (CEN footprint from AD of SBP) .... 198
6.4.1. Basic parameters and input variables ...................................................... 198
6.4.2. Main Scenarios ........................................................................................ 199
6.4.3. Energy balance for electricity and heat production (AD + CHP) ........... 200
6.4.4. Energy balance for biogas upgrading to biomethane (AD + biogas
upgrading) ................................................................................................ 202
6.4.5. Comparison of energy balance for electricity and biomethane
production ................................................................................................ 203
6.4.6. GHG emissions ....................................................................................... 205
6.4.7. Effect of OLR and temperature on energy balance and GHG
emissions .................................................................................................. 208
6.4.7.1. Energy balances .......................................................................... 208
6.4.7.2. GHG emissions ........................................................................... 211
6.5. Overall energy and GHG emissions balances from waste to field .................... 213
6.5.1. Energy and emissions in digestate transport and application .................. 213
6.5.2. Overall energy and emissions balances ................................................... 214
6.5.2.1. Energy balances .......................................................................... 215
6.5.2.2. GHG emissions ........................................................................... 217
6.5.3. Operation without digestate utilisation ................................................... 219
6.6. Conclusions ....................................................................................................... 221

CHAPTER 7. CONCLUSIONS AND RECOMMENDATIONS ................................ 223
7.1. Characterisation and Anaerobic Digestion Trials ............................................. 223
7.2. Dewaterability Trials ......................................................................................... 224
7.3. Foaming Trials .................................................................................................. 225
7.5. CEN Footprint Study ......................................................................................... 227
7.6. Recommendations ............................................................................................. 228
7.7. Future Work ...................................................................................................... 228
LIST OF REFERENCES .............................................................................................. 229
APPENDICES .............................................................................................................. 255
Appendix 1. Two-stage chemical treatment (dosage of polymer on CST test) ....... 255
Appendix 2. Manual ADtool .................................................................................... 257
Appendix 3. Modelling: AD + CHP ......................................................................... 268
Appendix 4. Modelling: AD + Biogas upgrading .................................................... 271
Appendix 5. Modelling: AD without digestate application ..................................... 274
vi
LIST OF TABLES

Table 2.1. Agricultural area and production of sugar beet in the UK ............................. 5
Table 2.2. Comparison of single- and two-stage processes .......................................... 18
Table 2.3. Process parameters from studies on single- and two- stage anaerobic
digestion of sugar beet pulp ......................................................................... 26
Table 2.4. Problems created by foaming in bioprocess ................................................ 44
Table 2.5. Lifetime and global warming potentials (GWPs) for different time
horizons of the three most important GHGs ................................................ 50
Table 2.6. Parameters and upper limit values for digested materials for land
application in England and Wales (PAS 110:2010) ..................................... 56
Table 3.1. Concentration of trace elements in stock solution ....................................... 60
Table 3.2. FICODOX-plus Composition ...................................................................... 68
Table 3.3. Ferroin Indicator Composition ..................................................................... 68
Table 3.4. Composition of colour reagent for phosphorus determination .................... 70
Table 3.5. Operational parameters for AD of SBP ....................................................... 77
Table 3.6. Parameters data required in CEN footprint .................................................. 79
Table 4.1. Characteristics of sugar beet pulp ................................................................ 82
Table 4.2. VFA concentration and pH value at end of BMP test (average value) ........ 86
Table 4.3. Kinetic constants from modelling ................................................................ 87
Table 4.4. Biogas and methane production, VFA concentration, and pH value at
the end of BMP test of SBP (average value for triplicates) ......................... 89
Table 4.5. Average steady state values of performance indicators for duplicate
digesters at different OLR (between days 130 and 411) .............................. 91
Table 4.6. Average steady state values of performance indicators for duplicate
digesters with and without TE additions (average over last 30 days of
operation) ................................................................................................... 100
Table 4.7. Calculation of specific residual biogas and methane production ............... 104
Table 4.8. Steady state values of performance and stability indicators for duplicate
digesters in mesophilic and thermophilic condition (average over last 30
days of operation). ...................................................................................... 113
Table 5.1. Results of CST and FIC tests on digestate samples at different dilutions . 130
Table 5.2. Results of CST and FIC test on digestate samples from mesophilic and
thermophilic digesters at different dilutions............................................... 132
Table 5.3. Experimental design for the ageing study flask trial. ................................. 135
Table 5.4. Results of CST and FIC tests before and after F/T treatment .................... 140
vii