Analysis of nutrient requirements for the Anaerobic Digestion of Fischer-Tropsch Reaction Water

188 Pages·2014·2.17 MB·English

Analysis of nutrient requirements for the Anaerobic Digestion
of Fischer-Tropsch Reaction Water

By
Aarefah Mathir

In fulfilment of the MSc Chemical Engineering degree, College of
Agriculture, Engineering and Science, University of Kwa-Zulu Natal

Supervisors: Date of submission:
Dr KM Foxon 02 December 2013
Mr CJ Brouckaert
COLLEGE OF AGRICULTURE, ENGINEERING AND SCIENCE
DECLARATION - SUPERVISOR

I, ……………………………………….………………………., declare that as the candidate’s
Supervisor I agree to the submission of this thesis.

Signed
………………………………………………………………………………

I
Form EX1-5
COLLEGE OF AGRICULTURE, ENGINEERING AND SCIENCE
DECLARATION - PLAGIARISM

I, ……………………………………….………………………., declare that
1. The research reported in this thesis, except where otherwise indicated, is my original
research.

2. This thesis has not been submitted for any degree or examination at any other university.

3. This thesis does not contain other persons’ data, pictures, graphs or other information, unless
specifically acknowledged as being sourced from other persons.

4. This thesis does not contain other persons' writing, unless specifically acknowledged as being
sourced from other researchers. Where other written sources have been quoted, then:
a. Their words have been re-written but the general information attributed to them has been
referenced
b. Where their exact words have been used, then their writing has been placed in italics and
inside quotation marks, and referenced.

5. This thesis does not contain text, graphics or tables copied and pasted from the Internet,
unless specifically acknowledged, and the source being detailed in the thesis and in the
References sections.

Signed
………………………………………………………………………………

II
Acknowledgements
The author would like to acknowledge the following individuals:

K. M. Foxon for providing excellent guidance and supervision throughout the research
D. Teclu for running the UASB reactors needed to provide the seed sludge and for all the long
hours assisting with the analysis, even on short notice
My parents for keeping me inspired and humbled
My husband for your endless patience and much appreciated support

The author would also like to acknowledge:
Pollution Research Group for creating a great environment to work in
Sasol for providing the opportunity to perform this research
University of Kwa-Zulu Natal
III
Executive Summary
Nutrients play an important role in the functioning of microorganisms during anaerobic digestion.
The anaerobic treatment of industrial wastewaters, such as Fischer-Tropsch Reaction Water
(FTRW), requires the addition of nutrients suitable for micro-organisms (micronutrients) since
these wastewaters are devoid of essential metals. However, the dosing of nutrients is only effective
if the metals are in a bioavailable form which in turn is dependent on the chemical speciation of the
system.
This study aimed to investigate and model the influence of precipitation on bioavailability by
considering the extent to which precipitation can sequester metals into forms that are not
bioavailable and the extent to which this sequestration can describe biological effects in an
anaerobic system. Visual MINTEQ and Excel were used to develop a combined mass balance and
chemical-equilibrium speciation model that considered the soluble and the precipitate metal phases.
The model was compared to two sets of experimental analysis. Experiment A included metal
analysis on the sludge and supernatant from glucose and ethanol fed ASBRs while Experiment B
included similar analysis on FTRW fed ASBRs while biological parameters were monitored during
a micro-metal washout experiment.
Precipitation was found to sequester Al, Zn and Fe to a large extent making them non-bioavailable
in Experiment A, while sulphide precipitates were predicted to dominate the metal speciation in
Experiment B. In Experiment A, the organically bound metals phase was also a significant phase
that sequestered metals. Furthermore, the rates of washout of most of the metals (excluding Mg)
were over-predicted, which may have been due to the absence of other solid related phases in the
model. This may also be attributed to kinetic effects in the system. Although there were reasonable
correlations between the model predicted and the experimentally determined concentrations, it is
recommended that the model should include the organically bound phase and consider mass
transfer effects in the system. After 12 cycles without dosing micro-metals in Experiment B, the
biogas production decreased by 43%. A decline in the predicted and determined soluble
concentrations of a variety of metals were observed during this time, suggesting that there may be
an agreement between predicted metals washout and reduction in anaerobic activity. Since the
soluble metal concentrations did not decrease as rapidly as predicted by the model, a lag period
between the two parameters was observed. Therefore, although the model provides an improved
understanding of metal speciation and bioavailability such that recommendations may be made for
prudent micro-metal dosing, further development is required for more accurate representations of
the system.
IV
Table of Contents
Acknowledgements ........................................................................................................................... III
Executive Summary .......................................................................................................................... IV
Table of Contents ............................................................................................................................... V
List of Figures .................................................................................................................................... X
List of Tables ................................................................................................................................. XIII
1. Introduction ................................................................................................................................. 1
1.1 Background and motivation ................................................................................................ 1
1.2 Background into the Field ................................................................................................... 3
1.3 Aims and objectives ............................................................................................................ 5
2. Literature Review ........................................................................................................................ 6
2.1 Source and Properties of Reaction water ............................................................................ 6
2.2 Anaerobic Digestion............................................................................................................ 7
2.2.1 Favourable Conditions for Anaerobic Processing ....................................................... 7
2.2.2 Parameters used to determine efficiency of anaerobic digestion .............................. 13
2.3 Importance of nutrients in anaerobic digestion ................................................................. 14
2.3.1 Treatment of Industrial Streams ................................................................................ 17
2.3.2 Growth and functioning of Microorganisms ............................................................. 19
2.3.3 Settleability of the sludge .......................................................................................... 24
2.4 Bioavailability of Metals ................................................................................................... 24
2.5 Precipitation Chemistry ..................................................................................................... 26
2.6 Methods to determine Bioavailability in a system ............................................................ 28
2.6.1 Analytical Approach ................................................................................................. 28
2.6.2 Chemical Speciation Modelling ................................................................................ 33
2.7 Uptake of metals by the microorganisms .......................................................................... 35
2.8 Micronutrient dosing ......................................................................................................... 36
2.8.1 Dosing Strategy ......................................................................................................... 36
V
2.8.2 Recipes Used ............................................................................................................. 37
2.9 Anaerobic Sequencing batch reactors ............................................................................... 39
2.9.1 Sequencing Batch Reactor operation ........................................................................ 39
2.9.2 Advantages and Disadvantages ................................................................................. 40
3. Research Methodology.............................................................................................................. 42
3.1 Experiment A .................................................................................................................... 44
3.1.1 Experimental setup .................................................................................................... 44
3.1.2 Reactor Operation ..................................................................................................... 46
3.1.3 Influent Composition ................................................................................................ 46
3.1.4 Sampling and Analytical Techniques ........................................................................ 48
3.2 Experiment B .................................................................................................................... 52
3.2.1 Experimental Setup ................................................................................................... 53
3.2.2 Seed sludge source .................................................................................................... 56
3.2.3 Initial operation ......................................................................................................... 57
3.2.4 Stable Operation ........................................................................................................ 57
3.2.5 Washout Experiment ................................................................................................. 58
3.2.6 Reactor Operation ..................................................................................................... 58
3.2.7 Sampling and Analytical Techniques ........................................................................ 60
3.3 Mass Balance-Chemical Speciation Modelling ................................................................ 62
3.3.1 Rationale ................................................................................................................... 62
3.3.2 Model Development .................................................................................................. 62
3.3.3 Assumptions .............................................................................................................. 63
4. Results ....................................................................................................................................... 69
4.1 Experiment A Results ....................................................................................................... 69
4.1.1 Metals Mass Balance ................................................................................................. 69
4.1.2 Sequential Extraction of Sludge ................................................................................ 71
4.1.3 Comparison between Acid Digestion and Sequential Extraction ............................. 73
VI
4.1.4 Mass Balance-Speciation Modelling Results- Experiment A ................................... 74
4.2 Experiment B Results ........................................................................................................ 80
4.2.1 Mass Balance-Speciation Modelling Results- Experiment B ................................... 82
4.2.2 Supernatant Metal Analysis ...................................................................................... 88
4.2.3 Sludge Metal Analysis .............................................................................................. 90
4.2.4 Bioprocess Results Calculation and Summary .......................................................... 93
4.2.5 Biogas production, methane activity and methane recovery ..................................... 96
4.2.6 pH Control ................................................................................................................. 97
4.2.7 Biogas production comparison to alkalinity dosage.................................................. 98
5. Discussion ............................................................................................................................... 100
5.1 Experiment A Discussion ................................................................................................ 100
5.1.1 Metals Mass Balance ............................................................................................... 100
5.1.2 Sequential Extraction of Sludge .............................................................................. 101
5.1.3 Comparison between Acid Digestion and Sequential Extraction ........................... 105
5.1.4 Mass Balance-Speciation Modelling Discussion- Experiment A ........................... 106
5.2 Experiment B Discussion ................................................................................................ 111
5.2.1 Mass Balance-Speciation Modelling Discussion- Experiment B ............................ 111
5.2.2 Supernatant Metal Analysis .................................................................................... 114
5.2.3 Sludge Metal Analysis ............................................................................................ 115
5.2.4 Biogas production, methane activity and methane recovery ................................... 118
5.2.5 pH Control ............................................................................................................... 119
5.2.6 Biogas production comparison to alkalinity dosage................................................ 120
5.2.7 Biogas production comparison to soluble metal concentration ............................... 121
5.2.8 Validity of Model Assumptions .............................................................................. 123
5.2.9 Dosing Strategy ....................................................................................................... 125
6. Conclusions and Recommendations ....................................................................................... 127
7. References ............................................................................................................................... 131
VII
8. Appendix A-List of Micro-nutrient recipes from Literature ................................................... 140
9. Appendix B: Analytical Methods ............................................................................................ 150
9.1 ICP-AES Analysis ........................................................................................................... 150
9.1.1 Sample Preparation ................................................................................................. 150
9.1.2 Standard Solutions preparation ............................................................................... 150
9.1.3 Quality Control........................................................................................................ 151
9.2 Acid Digestion of Sludge ................................................................................................ 152
9.2.1 Apparatus and Reagents .......................................................................................... 152
9.2.2 Method .................................................................................................................... 152
9.2.3 Obtaining the sludge samples for acid digestion ..................................................... 153
9.2.4 Calculating the total amount of metals in the reactor .............................................. 154
9.3 Sequential Extraction ...................................................................................................... 155
9.3.1 Apparatus and Reagents .......................................................................................... 155
9.3.2 Method .................................................................................................................... 156
10. Appendix C: Initial Conditions for Mass Balance-Speciation Modelling .......................... 158
11. Appendix D: Illustration of the Mass balance in the Mass balance-Speciation model ....... 160
11.1 Mass balance for Experiment A ...................................................................................... 160
11.2 Mass balance for Experiment B ...................................................................................... 160
12. Appendix E: Mass balance-Speciation Modelling for Experiment A, Reactor II. .............. 162
12.1.1 Soluble Concentration Changes .............................................................................. 162
12.1.2 Precipitate Formation .............................................................................................. 165
13. Appendix F: Experiment B results for Reactor I ................................................................. 168
13.1 Results Summary ............................................................................................................ 168
13.2 Biogas Production, methane activity and methane recovery .......................................... 168
13.3 pH Control ....................................................................................................................... 169
13.4 Biogas production comparison to alkalinity dosage ....................................................... 170
13.5 Supernatant Metal Analysis ............................................................................................ 172
VIII
13.6 Sludge Metal Analysis .................................................................................................... 173

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