The anaerobic digestion of halophytic microalgae
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Date
2015
Authors
Ward, Andrew
Editors
Advisors
Lewis, David Milton
Ball, Andrew
Ashman, Peter John
Ball, Andrew
Ashman, Peter John
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Abstract
The anaerobic digestion of microalgae is a potential environmentally feasible
option for creating a renewable source of energy for industrial and domestic needs.
Microalgae anaerobic digestion is a key unit process that integrates efficiency and
beneficially into the production of microalgae derived biofuels. Anaerobic digestion
culminating in methane fermentation improves the economic viability of microalgae
liquid biofuel production and presents an opportunity for power generation from
wastewater derived microalgae. However the anaerobic digestion of halophytic
microalgae biomass is not straight forward due to several technical restraints
including low concentration of digestible biodegradable substrate, recalcitrant
substrate constituents, cell wall degradability and effects from salinity and associated
metal ions.
To address the quantification of low biodegradable substrate associated with
microalgae cultures, development of a high throughput methodology to determine
the quantification of suspended microalgae biomass content and other water quality
parameters via turbidity measurements was determined. The development of the
new management tool allows faster operational control from a simple turbidity
analysis, reducing time delays to fewer than 5 minutes and avoids expensive
laboratory testing. Further development of this management tool will support the
operational control for biofuel pond management and wastewater treatment plants.
This management tool provides a rapid quantification of biomass and allows
harvesting volumes to be calculated to allow consistent volatile solid and chemical
oxygen demand loading to anaerobic digesters.
The anaerobic digestion of halophytic microalgae biomass however, has a
significant challenge to be mitigated before this technology can be beneficial for the
burgeoning microalgae industry. The halophytic microalgae biomass as a potential
substrate feedstock for anaerobic digestion will have salinities > 35 ppt. To address
this issue the first section of my PhD research focussed on the changes undertaken
in the bacterial community associated with the anaerobic digestion of piggery effluent
under increasing saline conditions with the aim of establishing a saline tolerant
anaerobic digestion inoculum capable of digesting feedstock’s under high salinity
conditions.
Favourable results from this inoculum development study allowed the second
part of the PhD research to be investigated where the anaerobic digestion of
halophytic biomass was investigated utilising the inoculum established from the initial
component of the reported study. Results of the later study demonstrated that a
hyper saline inoculum was achieved and subsequent DGGE fingerprinting of the
bacterial community detected several high salinity methanogens at a salinity of 7%
and validated the establishment of a halo-tolerant anaerobic digestion community.
Establishment of a halo tolerant anaerobic digestion community was further validated
by significant methane production at the high 7% salinities. This inoculum was then
used for all other reported studies.
Another major difficulty associated with the anaerobic digestion of microalgae
is the need to disrupt the cell wall allowing the cell contents to be processed by the
bacterial community. In this study I compared the methane production from lipid
extracted, pre-treated disrupted and non-pretreated Tetraselmis sp. microalgae
respectively. Results demonstrate that a methane production of 122 mL per g VS for
the lipid extracted Tetraselmis sp. biomass. This result demonstrates that after the
extraction of lipid for use in biofuel production residual lipid extracted microalgae
biomass is a viable feedstock for methane production. A methane production of 252
mL per g VS and 248 mL per g VS was reported for the non-disrupted algae and pretreated
disrupted Tetraselmis sp. respectively. This study also identified the ability of
the anaerobic digestion microbial community to undertake cell lyses via microbial
degradation of the Tetraselmis sp. microalgae. Cell lyses by the anaerobic digestion
microbial community can offer a direct conversion pathway for energy production
were whole biomass can be harvested and concentrated and directly fed to the
anaerobic digester without energy intensive pre-treatment or processing being
required.
Investigation was also undertaken to quantify the suitability of anaerobically
digested halophytic Tetraselmis sp. microalgae digestate as a nutrient feed stock to
form a closed loop nutrient system. To determine microalgae digestate suitability I
established that the following factors needed to be observed: growth, lipid content,
and the bacterial community diversity. Microalgae digestate was diluted according to
the concentration of NH₄⁺ content (20, 40, 60, 80 mg/L) and compared against a
standard medium for Tetraselmis sp.. The growth rate on the microalgae digestate
media was not as rapid as the F/2 standard medium and the high microalgae
digestate media concentrations correlated with lower total lipid contents, additionally
acyl carrier proteins (ACP) gene expression rates displayed lower lipid gene
expression within high microalgae digestate treatments. Lastly, higher
concentrations of microalgae digestate were correlated with a higher bacterial
diversity in the bacterial community throughout the investigation. No significant
difference in lipid production and satisfactory growth was recorded for the lower
microalgae digestate treatments. These results confirmed the suitability of microalgae digestate as a suitable nutrient source for use in the production of
Tetraselmis sp. biomass for lipid and biofuel production.
School/Discipline
School of Chemical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2015.
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Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals