the effect of pretreatment methods on methane yield and nutrient solubilization during anaerobic

122 Pages·2014·3 MB·English

by  Alexander Scott Hill

THE EFFECT OF PRETREATMENT METHODS ON METHANE YIELD AND
NUTRIENT SOLUBILIZATION DURING ANAEROBIC DIGESTION OF
MICROALGAE

A Thesis
presented to
the Faculty of California Polytechnic State University,
San Luis Obispo

In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Civil and Environmental Engineering

by
Alexander Scott Hill
June 2014
© 2014
Alexander Scott Hill
ALL RIGHTS RESERVED

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COMMITTEE MEMBERSHIP

TITLE: The Effect of Pretreatment Methods on Methane Yield
and Nutrient Solubilization During Anaerobic Digestion
of Microalgae

AUTHOR: Alexander Scott Hill

DATE SUBMITTED: June 12, 2014

COMMITTEE CHAIR: Dr. Trygve Lundquist, Associate Professor
Civil & Environmental Engineering Department

COMMITTEE MEMBER: Dr. Yarrow Nelson, Professor
Civil & Environmental Engineering Department

COMMITTEE MEMBER: Dr. Gregory Schwartz, Assistant Professor
Bioresource Agriculture Engineering Department

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ABSTRACT
The Effect of Pretreatment Methods on Methane Yield and Nutrient Solubilization
During Anaerobic Digestion of Microalgae
Alexander Scott Hill
Microalgal biomass is a candidate feedstock for biofuel production. To improve the
sustainability of algae biofuel production, following biofuel recovery, the biomass
nutrients should be recycled for additional algae growth. Anaerobic digestion of algae or
oil-extracted algae is a means of recovering carbon and other nutrients, while offsetting
algae production electricity demand. The major limiting factor in microalgae digestion is
the low biodegradability of the cell walls. In the present study, various pretreatment
technologies were tested at bench scale for their ability to improve raw, non-lipid-
extracted algae biodegradability, which was assessed in terms of methane yield, volatile
solids destruction, and solubilization of N, P, and K. The microalgae were harvested by
sedimentation from outdoor wastewater-fed raceways ponds operated in coastal southern
California. Four pretreatment methods (sonication, high-pressure homogenization,
autoclaving, and boiling) were used on the algae slurries, each followed by batch
anaerobic digestion (40 days at 35oC). Biomass sonication for 10 minutes showed the
highest methane yield of 0.315 L CH / g VS , which is a 28% increase over the
4 IN
untreated control. Conversely, autoclaved algae slurry inhibited methane production
(0.200 vs. 0.228 L CH / g VS for the treatment and control). A preliminary energy
4 IN
balance indicated that none of the pretreatments led to a net increase in energy conversion
to biomethane. However, pretreatment did increase the initial N and P solubilization
rates, but, after digestion, the ultimate N and P solubilization was nearly the same among
the treatments and controls. After 40 days of digestion, solubilization of N, P, and K
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reached, respectively, 50-60% of average total Kjeldahl N, 40-50% of average total P,
and 80-90% of average total K. Descriptive first-order models of solubilization were
developed. Overall, certain pretreatments marginally improved methane yield and
nutrient solubilization rate, which cast doubt on the efficacy of, or even the need for,
algae biomass pretreatment prior to anaerobic digestion.

Keywords: batch anaerobic digestion, pretreatment, solubilization, digestate, NPK, lysis,
sonication, high pressure homogenization, heat treatment, L CH /g VS
4 IN
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ACKNOWLEDGMENTS
I would like to thank:
My Parents for their support
Kaylinn for putting up with my antics…although it doesn’t end here!
Trygve Lundquist for his guidance
Greg Schwartz for his support and enthusiasm
Yarrow Nelson for his diligence in reviewing my thesis
Ruth Spierling for her wholehearted commitment to the end goal and relentless desire to
improve me as a scientist and engineer
Matt Hutton for is insightful knowledge of the project, the real world, obscure mountain
adventures, and Internet sensations
Braden Crowe for his dedication to San Clemente
Mike Chang, Justin Kraetsch, Eric Nicolai, Chad Boggess, and Nate Olivas, for trudging
through this together
Perry Ng for his mastery of the total phosphorus test
Serena Lee for her statistical help
Amy and Kay for their administrative guidance
Shelley Blackwell for her commitment to the logistical nature of the project
All of the ENVE 405 helpers directly involved in my project: Chris Apple, Paul
Camarena, David Sng, Cameron Whipple, James Pilkington, Krissy Stroud, Elai Fresco,
Christian Bowen, Anne Guzman, as well as all of the additional help I was fortunate to
glean from the massive organized band of interdisciplinary wonder
The Department of Energy for funding such an exciting project
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Table of Contents
List of Tables ...................................................................................................................... x
List of Figures ................................................................................................................... xv
List of Equations ............................................................................................................. xxv
1 Introduction ................................................................................................................. 1
2 Background ................................................................................................................. 4
2.1 Anaerobic Digestion ........................................................................................... 4
2.2 Benefits of Microalgae for Biofuel Production .................................................. 5
2.3 Historical Experiments Involving Anaerobic Digestion of Microalgae ............. 6
2.4 Anaerobic Digestion Enhancement ..................................................................... 6
2.5 Nutrient Recycling .............................................................................................. 8
2.6 Variables Affecting Anaerobic Digestion of Microalgae ................................. 10
2.7 Rationale of the Present Study .......................................................................... 11
3 Methods..................................................................................................................... 13
3.1 Overview of Experiments ................................................................................. 13
3.1.1 Collection and Storage of Algae ............................................................... 15
3.1.2 Collection and Storage of Digester Inoculum ........................................... 18
3.1.3 Sonication ................................................................................................. 19
3.1.4 High Pressure Homogenization ................................................................ 20
3.1.5 Autoclaving ............................................................................................... 23
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3.1.6 Boiling....................................................................................................... 24
3.2 Digester Setup ................................................................................................... 25
3.2.1 Experimental Overview and Sample Identification .................................. 28
3.3 Analytical Procedures ....................................................................................... 30
3.3.1 Overview of Sample Day Breakdown ...................................................... 30
3.3.2 Solids Concentration ................................................................................. 32
3.3.3 Biogas Volume and Composition ............................................................. 32
3.3.4 Chemical Oxygen Demand (COD) Determination ................................... 34
3.3.5 pH/Alkalinity Determination .................................................................... 36
3.3.6 Nitrogen Determination ............................................................................ 36
3.3.7 Phosphorus Determination ........................................................................ 38
3.3.8 Potassium Determination .......................................................................... 40
4 Results and Discussion ............................................................................................. 41
4.1 Effect of Pretreatment on Methane Yield and Nutrient Solubilization............. 41
4.1.1 Degree of Cell Disruption ......................................................................... 42
4.1.2 Specific Methane Yield............................................................................. 46
4.1.3 Net Energy Balance .................................................................................. 50
4.1.4 Effect of Organic Loading on Degradation Rate ...................................... 54
4.1.5 Neutral to Negative Effects of High Heat on Biodegradability ................ 57
4.1.6 Nitrogen Solubilization ............................................................................. 59
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4.1.7 Phosphorus Solubilization ........................................................................ 65
4.1.8 Potassium Solubilization ........................................................................... 76
4.2 Descriptive Modeling........................................................................................ 77
4.2.1 TAN Model Generation ............................................................................ 78
4.2.2 DRP Model Generation............................................................................. 81
5 Conclusions ............................................................................................................... 84
5.1 Pretreatment Effect on Specific Methane Yield ............................................... 84
5.2 Pretreatment Effect on Nutrient Solubilization ................................................. 85
5.2.1 Nitrogen Solubilization ............................................................................. 85
5.2.2 Phosphorus Solubilization ........................................................................ 85
5.2.3 Potassium Solubilization ........................................................................... 86
5.3 Model Creation ................................................................................................. 86
5.4 Limitations of the Study.................................................................................... 87
5.5 Future Research ................................................................................................ 87
6 Works Cited .............................................................................................................. 90
7 Appendices ................................................................................................................ 95

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List of Tables
Table 1. Partial list of various pretreatment technologies that have been applied to
increase biogas yields of anaerobic digestion of microalgae. ................................. 7
Table 2. The methane yield and TAN concentrations were normalized by initial
volatile solids. Source: (Sialve B., 2009) .............................................................. 10
Table 3. Overview of all five experiments and their respective logistical
information. ........................................................................................................... 15
Table 4. Analytical tests and the corresponding stage during experimental setup at
which point the samples were pulled. The orange markers relate to when
the samples were pulled as seen in Figure 14. ..................................................... 27
Table 5. Overview of all five experiments and their respective sample
identification. TS in experiment 3 refers to “total solids.” ................................... 29
Table 6. Analytical procedures that were performed in order to track digester
health, biogas production, and nutrient release. The third party laboratory
used for the potassium testing was the U.C. Davis Analytical Laboratory. ......... 30
Table 7. Summary of pretreatment methods and means of cell disruption ...................... 41
Table 8. Comparison of the degree of cell disruption by various pretreatments,
based on soluble COD release. All untreated samples had a sCOD/tCOD
ratio of 3-5% (not shown). .................................................................................... 44
Table 9. Comparison of specific methane yields of all five experiments. Values
highlighted in yellow were extrapolated from data (see explanation
below). Sample coding is explained by the blue headers. UAS means
untreated sample. The culture volumes were either 125 mL or 1.2 L and
x