Anaerobic digestion of marine microalgae

258 Pages·2016·6.63 MB·English

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University of Southampton

Faculty of Engineering and the Environment

Bioenergy and Organic Resources Group

Anaerobic digestion of marine

microalgae
By

Keiron Philip Roberts

Thesis for the degree of Doctor of Philosophy
May 2015

Declaration of authorship
I Keiron Philip Roberts declare that this thesis and the work presented in it are my own
and have been generated by me as the result of my own original research.
Title: Anaerobic digestion of marine microalgae.
I confirm that:
1. This work was done wholly or mainly while in candidature for a research degree at this
University;
2. Where any part of this thesis has previously been submitted for a degree or any other
qualification at this University or any other institution, this has been clearly stated;
3. Where I have consulted the published work of others, this is always clearly attributed;
4. Where I have quoted from the work of others, the source is always given. With the
exception of such quotations, this thesis is entirely my own work;
5. I have acknowledged all main sources of help;
6. Where the thesis is based on work done by myself jointly with others, I have made
clear exactly what was done by others and what I have contributed myself;
7. Parts of this work have been published as:
Roberts, K. P., et al. (2016). "Quantification of methane losses from the acclimatisation of
anaerobic digestion to marine salt concentrations." Renewable Energy 86: 497-506.

Signed:………………………………………………………………………………………

Date:………………………………………………………………………………………...

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Abstract
Anaerobic digestion is a simple and energetically efficient way in comparison to some
other biofuel methods of producing renewable energy from a range of biomass types.
Although digestion of micro-algal biomass was first suggested in the 1950s, only a few
studies have been conducted for assessment of its performance. This work assessed the
potential for energy recovery from microalgae via anaerobic digestion for both freshwater
and marine species.
This research screened seven laboratory-grown marine and freshwater microalgal species
(Nannochloropsis. oculata, Thalassiosira . pseudonana, Dunaliella. salina, Rhododomas
sp, Isochrysis. galbana, Chlorella. vulgaris and Scenedesmus sp) and two samples from
large-scale cultivation systems for their suitability as a substrate for anaerobic digestion.
Biochemical methane production and a theoretical maximum growth yield of each species
were employed to offer a means of comparing methane productivity per unit of
cultivation under standard conditions. The data generated were useful in determining
suitable species to culture and digest under continuous operation.
A review of the literature highlighted a gap in the knowledge for the continuous digestion
of different marine micro-algal species, as well as the potential inhibitory effect of high
salinities on the anaerobic digestion process to non-acclimatised systems run under
continuous operation. Addition of total salt ≥ 10g L-1 caused reactor failure, supporting
the findings of the literature review. It was possible, however, to gradually adapt the
inoculum to marine concentrations of chloride salts (31.1 g L-1) with <7% difference in
specific methane production of controls. Addition of sulphate showed competition
between methanogens and sulphate-reducing bacteria with further minor losses in
methane yield. There was up to 60% reduction in SMP for the highest sulphate loaded
reactors, however, the population successfully adapted to sulphate concentrations above
those typically found in seawater and showed gaseous H S productivity in proportion to
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the applied sulphate load. This suggests that the effects of marine concentrations of
chloride and sulphate salts can be overcome by a gradual acclimatisation.
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The selected algal species I. galbana and D. salina were continuously cultivated in a
photobioreactor under low and high sulphate media and continuously digested using the
salt adapted inoculum. The specific methane production for I. galbana and D. salina was
0.19 and 0.23 L CH g-1 VS, with a VS destruction of 32% and 50% respectively.
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Addition of a high SO grown D. salina as a feed resulted in a reduction of SMP to 0.19 L
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CH g-1 VS with an increase in H S production. Loses in total solids and sulphur were
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observed under continuous study due to oxidation of H S and struvite precipitation within
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the reactors, which was not observed under batch analysis. This highlights the importance
in conducting continuous studies over batch, as these effects can be overlooked.

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Acknowledgements
Throughout the emotional rollercoaster that is a PhD I have had the fortune to work with
and meet many fantastic people whom have helped and guided me along in what appears
at times, the long lonely road of academic research.
I would like to give my heartfelt thanks and appreciation to Dr. Sonia Heaven and Prof.
Charles Banks, whom without their perseverance, patience and professionalism in
developing and guiding myself as a researcher, I would never have had the opportunity to
start, or dedication to finish a PhD. Thank you for putting up with my terrible spelling.
My appreciation and thanks to Dr. Yue Zhang for her valuable advice, keen eye for detail
and kind words throughout the PhD.
My thanks and appreciation to Dr. Dominic Mann or all the help you have given me
within the laboratory and Pilar Pascual-Hidalgo for training me on almost every piece of
analytical equipment we have access too. A special thank-you for the lengthy
conversations we used to have around the GC.
I must give a massive thank-you to everyone within the Bioenergy and Organic
Resources Research Group, and my friends within the office for all your help, advice,
proofreading and discussions over coffee/ beer.
Thanks to the EPSRC and the All Gas project for funding me throughout my studies to
enable me to conduct my research and visit distant places.
Thanks to the “Magnolia Army”, Alisdair and the late Phil all of whom encouraged each
other to pursue a PhD and stick with academia, at the last count 5 out of 8 are now Dr’s.
Lastly I would like to give my thanks and congratulate my wife Lauren, and my family,
particularly my father, for putting up with me talking about “poo” all the time. Your
unwavering encouragement, patience and help have helped power me through, and
without you I doubt I would have had the determination to finish.

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Table of Contents
1. Background ................................................................................................................ 1
1.1 Energy crisis ................................................................................ 1
1.2 Biofuels ...................................................................................... 4
1.2.1 First generation biofuels ............................................................ 5
1.2.2 Second generation biofuels ......................................................... 7
1.2.3 Third generation biofuels ........................................................... 8
1.2.4 Anaerobic digestion .................................................................. 9
1.3 Aims and objectives ................................................................ 10
2. Literature review .................................................................................................. 13
2.1 Algae ...................................................................................... 13
2.1.1 Aquatic photosynthesis ..................................................... 14
2.1.2 Nutrients ........................................................................... 18
2.1.3 Commercial applications of macro and microalgae ............. 22
2.1.4 Commercial and large scale algal culturing ........................ 24
2.2 Algae and biofuels .................................................................. 29
2.2.1 Macro algae as a feedstock for biofuels ............................. 31
2.3 Algal species examined ........................................................... 33
2.3.1 Chlorella vulgaris .............................................................. 33
2.3.2 Scenedesmus spp ............................................................... 34
2.3.3 Nannochloropsis oculata .................................................... 35
2.3.4 Isochrysis galbana ............................................................. 36
2.3.5 Dunaliella salina ................................................................ 37
2.3.6 Thalassiosira pseudonana ................................................. 38
2.3.7 Rhodomonas spp ............................................................... 39
2.4 Anaerobic digestion ................................................................ 39
VII