Study of the anaerobic degradation of the algae produced in a post-treatment of an anaerobic ...

80 Pages·2014·2.83 MB·English

Study of the anaerobic degradation of the algae produced in
a post-treatment of an anaerobic membrane bioreactor
effluent

Suzanne Van den Poel

Supervisors: Prof. dr. ir. Jeriffa De Clercq, Prof. Alberto Bouzas Blanco

Master's dissertation submitted in order to obtain the academic degree of
Master of Science in de industriële wetenschappen: chemie

D epartment of Industrial Technology and Construction
C hairman: Prof. Marc Vanhaelst
F aculty of Engineering and Architecture
Academic year 2013-2014

Acknowledgments
First of all, I would like to thank the entire Calagua group for welcoming me in Valencia and for
helping me when I had any questions.
Special thanks go to Silvia Greses for monitoring me, correcting my thesis and working with me
every day.
Thank you for dr. Alberto Bouzas Blanco and dr. ir. Jeriffa De Clercq for reading my work and
giving advice.
I would also like to thank dr. Sc. Greta Diricks for helping me with my Erasmus to Valencia.

Suzanne Van den Poel
May 2014, Valencia, Spain
Abstract
The anaerobic wastewater treatment gets more and more attention because of the ability to
produce biogas. One of the disadvantages however is the presence of nutrients in the effluent.
These nutrients can be removed by algae who use them to create more biomass. The excess of
algae can then also be anaerobically digested to produce biogas.
The cell wall of the algae however prevents bacterial attack and a pre-treatment must be
implemented to improve the biogas production.
In this work, different pre-treatments (mechanical, thermal, ultrasounds, etc.) were performed
and the best one was chosen.
The influence of other factors such as the age of the algae, the ratio of algae and bacteria and the
addition of another waste stream on the biogas production was also researched.
Table of contents
Acknowledgments
Abstract
List of Abbreviations
1 Introduction ................................................................................................................................................................ 1
1.1 Anaerobic wastewater treatment ............................................................................................................ 2
1.1.1 Biochemical reactions ......................................................................................................................... 3
1.1.1.1 Hydrolysis ........................................................................................................................................... 4
1.1.1.2 Acidogenesis ....................................................................................................................................... 4
1.1.1.3 Acetogenesis ....................................................................................................................................... 4
1.1.1.4 Methanogenesis ................................................................................................................................ 5
1.1.2 Microorganisms ..................................................................................................................................... 6
1.1.3 Sulphate reduction in the anaerobic digester ........................................................................... 7
1.1.4 Influencing factors ................................................................................................................................ 9
1.1.4.1 Nutrient balance ............................................................................................................................... 9
1.1.4.2 Temperature ................................................................................................................................... 10
1.1.4.3 pH ......................................................................................................................................................... 10
1.1.4.4 Hydraulic and solids retention time ...................................................................................... 10
1.1.4.5 Inhibiting components ................................................................................................................ 11
1.1.4.6 Hydrogen partial pressure ........................................................................................................ 13
1.2 Microalgae ...................................................................................................................................................... 13
1.2.1 Composition of microalgae ............................................................................................................ 13
1.2.2 Removal of N and P using microalgae ....................................................................................... 14
1.2.3 Digestion of microalgae ................................................................................................................... 15
1.2.3.1 Concentration of the microalgae ............................................................................................. 15
1.2.3.2 C/N ratio ........................................................................................................................................... 16
1.2.3.3 Lipids .................................................................................................................................................. 16
1.2.3.4 Cell wall degradability ..................................................................................................................... 16
1.2.4 Pre-treatments for cell wall degradation of microalgae .................................................... 17
1.2.4.1 Thermal pre-treatment ............................................................................................................... 17
1.2.4.2 Ultrasonic pre-treatment ........................................................................................................... 19
1.2.4.3 Microwave pre-treatment ......................................................................................................... 20
1.2.4.4 Energy balance ............................................................................................................................... 23
1.2.5 Substrate to inoculum ratio ........................................................................................................... 23
2 Goal .............................................................................................................................................................................. 26
i
3 Materials and methods ........................................................................................................................................ 27
3.1 Pilot plant ........................................................................................................................................................ 27
3.2 Bioprocess control ...................................................................................................................................... 28
3.3 Pre-treatments .............................................................................................................................................. 31
3.3.1 Concentrating algae........................................................................................................................... 31
3.3.2 Thermal pre-treatment .................................................................................................................... 31
3.3.3 Ultrasonic pre-treatment ................................................................................................................ 31
3.3.4 Mechanical pre-treatment .............................................................................................................. 32
3.4 Analytical methods ...................................................................................................................................... 33
3.4.1 pH and conductivity .......................................................................................................................... 33
3.4.2 Alkalinity and volatile fatty acids ................................................................................................ 33
3.4.3 Total COD ............................................................................................................................................... 34
3.4.4 Soluble COD .......................................................................................................................................... 35
3.4.5 Total nitrogen and total soluble nitrogen ................................................................................ 35
3.4.6 NH +, NO , NO , PO 3- and SO 2- ..................................................................................................... 35
4 3 2 4 4
3.4.7 Total phosphorus ............................................................................................................................... 36
3.4.8 Soluble phosphorus ........................................................................................................................... 36
3.4.9 S2- .............................................................................................................................................................. 37
3.4.10 Total Solids ........................................................................................................................................... 37
4 Results and discussion ........................................................................................................................................ 39
4.1 Comparison of different pre-treatments ............................................................................................ 39
4.1.1 First experiment ......................................................................................................................................... 39
4.1.2 Second experiment .................................................................................................................................... 41
4.2 Thermal algae pre-treatments ............................................................................................................... 43
4.3 Old versus new algae .................................................................................................................................. 48
4.4 S/I ratio ............................................................................................................................................................ 51
4.5 Addition of an extra waste stream ........................................................................................................ 53
5 Conclusion ................................................................................................................................................................ 57
6 References ................................................................................................................................................................ 58
7 Appendix ................................................................................................................................................................... 60
7.1 Results of analyses on algae after pre-treatments (experiment 1) ......................................... 60
7.2 Graphs pre-treatment experiment 1.................................................................................................... 64
7.3 Results of analyses on algae after pre-treatments (experiment 2) ......................................... 67
7.4 Graphs pre-treatment experiment 2.................................................................................................... 70
ii
List of Abbreviations
 AA: Amino Acids
 ADM1: Anaerobic Digestion Model no. 1
 AF: Anaerobic Filter
 AFBR: Anaerobic Fluidised Bed Reactor
 AMPTS II: Automatic Methane Potential Test System II
 AnMBR: Anaerobic Membrane Reactor
 BMP: Biomethane Production
 BOD: Biological Oxygen Demand
 CIP: Clean-In-Place
 COD: Chemical Oxygen Demand
 COD : soluble Chemical Oxygen Demand
S
 COD : total Chemical Oxygen Demand
T
 DESSAS: Design and Simulation of Activated Sludge Systems
 DS: total Dissolved Solids
 FORSU: stream of organic urban waste solids
 HRT: Hydraulic Retention Time
 IWA: International Water Association
 LCFA: Long Chain Fatty Acids
 MO: Microorganisms
 MPA: Methane Producing Bacteria
 MS: Monosaccharide
 N : soluble Nitrogen
S
 N : total Nitrogen
T
 P : soluble Phosphorous
S
 P : total Phosphorous
T
 S/I ratio: Substrate to Inoculum ratio
 SAnMBR: Submerged Anaerobic Membrane Bioreactor
 SRB: Sulphate Reducing Bacteria
 SRT: Solids Retention Time
 TEM: Transmission Electron Microscopy
 TS: Total Solids
 TSS: Total Suspended Solids
 UASB: Upflow Anaerobic Sludge Blanket reactor
 VDS: Volatile Dissolved Solids
 VFA: Volatile Fatty Acids
 VS: Volatile Solids
 VS : soluble Volatile Solids
S
 VSS: Volatile Suspended Solids
 VTS: Volatile Total Solids
1 Introduction
Wastewater can be cleaned using a lot of different techniques. First of all there is the natural
purification of rivers and streams. The concentration of the pollution in the water may not be too
high however, because then this function will disappear.[1]
Before discharging the water into the rivers or surface waters, it will pass through a wastewater
treatment plant. Depending on the source of the wastewater (urban, industrial, ...), different
techniques are used.[1]
Most of the time, aerobic wastewater treatments are utilized. It consists out of three major
steps[1]:
 Primary treatment: removal of the large solids and other undesirable substances like oil,
fat and sand.
 Between the primary and secondary treatment, phosphates can be removed by adding
FeCl , Al (SO ) or Ca(OH) .
3 2 4 3 2
 Secondary treatment: biological removal of the soluble and colloidal organic matter.
Bacteria convert organic components into carbon dioxide and water in the presence of
oxygen. In this step inorganic components such as nitrogen and phosphorus can be
removed as well. A distinction needs to be made between autotrophic and heterotrophic
bacteria. Heterotrophic bacteria use organic carbon for their growth. The organic carbon
is used as an electron donor and for the synthesis of new biomass. Autotrophic bacteria
use inorganic carbon such as CO for the synthesis of new biomass, and other
2
components such as ammonium and nitrites as electron donors. Both types of bacteria
need nutrients for their growth, but heterotrophic bacteria need less nutrients than
autotrophic bacteria.[2] In the aerobic wastewater treatment, both autotrophic and
heterotrophic bacteria are present.
 Tertiary treatment: removal of the remaining pollution of the water. This step contains
the removal of heavy metals and pathogens.
When using an anaerobic wastewater treatment, the secondary treatment is performed in the
absence of oxygen and only anaerobic microorganisms are present. Some advantages and
disadvantages are[1]:
 Advantages:
Less sludge is produced in comparison with aerobic techniques (0.1-0.2 kg sludge per kg
of Chemical Oxygen Demand (COD) anaerobically versus 0.2-0.4 kg sludge per kg COD
aerobically) because the micro-organisms have a slower metabolism and therefore build
less organic matter. This also implies a cost reduction for sludge processing.

The methane in the biogas can be burned, which gives energy. Each ton of COD removed
produces around 350 m³ of methane. Burning this amount gives 35 MJ/kg gas.

1
 Disadvantages:
For a quick and efficient anaerobic digestion, temperatures of 30°C and higher are
needed. Most waste waters have a temperature lower than 30°C and need to be heated.
For waste waters with a biological oxygen demand (BOD) of approximately 7500 mg/l,
the biogas production is enough to cover the energy needed for heating. If the BOD
concentration is lower, there is a net energy requirement.

Hydrogen sulphide is also formed by sulphate reducing bacteria during the anaerobic
digestion. If the process is not controlled, an unpleasant smell can be detected. This
phenomenon is not a global one, but typical for Valencia where a lot of sulphates are
present in the water.

The slow metabolism of the anaerobic microorganisms also implies that a lot more
surface is needed for the anaerobic reactor and digester to reach the same amount of
organic matter removal as aerobic treatments. To solve this disadvantage, research on
membranes is being done.

Since only anaerobic microorganisms are present in the anaerobic reactor, less nutrients
will be removed and the effluent will still contain a lot of phosphates and nitrogen.
Therefore, a post-treatment needs to be implemented.
An example of such a post-treatment is the use of algae, which remove the nutrients from the
effluent to build new biomass.
The excess of algae are then also digested in the anaerobic digester. The biodegradability of the
microalgae however is very poor and a pre-treatment is necessary to improve the solubility of
the algae and the biogas production.
Improving the biogas production is the main goal of this work and will be studied by performing
different pre-treatments on the microalgae, changing the substrate to inoculum (sludge) ratio,
adding other waste streams and comparing the biogas production of old versus new algae.
The general anaerobic wastewater treatment is first extensively discussed because the
microalgae undergo the same processes during the digestion. Next, the algae and their pre-
treatments are discussed.
1.1 Anaerobic wastewater treatment
During the anaerobic wastewater treatment, organic material is converted into biogas, i.e., a
mixture of methane (CH ), carbon dioxide (CO ) and other components with low concentrations.
4 2
The general composition of biogas is shown in Table 1.[1]

2
Table 1: Composition of biogas[1]
Component Symbol Concentration [%]
Methane CH 55-75 (average 65)
4
Carbon dioxide CO 25-40 (average 35)
2
Hydrogen H 1-5
2
Nitrogen N 2-7
2
Hydrogen sulphide H S 0-11
2
Volatile organic Traces
components
1: The concentration of H S in the biogas formed in Valencia will be higher because a lot more
2
sulphates are present in the wastewater and sulphate reducing bacteria will convert these
sulphates to hydrogen sulphide.
The methane formation from biomass occurs through the following general reaction[3]:

The anaerobic digestion is more complex than this reaction shows: it is a sequence of different
biological processes and different groups of microorganisms are used.
The biochemical reactions that take place during the anaerobic digestion are explained next.
1.1.1 Biochemical reactions
A considerable fraction of the initial Chemical Oxygen Demand (COD) may not be anaerobically
biodegradable. Therefore, it is very important to distinguish between available degradable COD
(substrate) and total COD.[4]
The model includes three biological steps: - acidogenesis or fermentation
- acetogenesis
- methanogenesis
It also has an extracellular disintegration step and an extracellular hydrolysis step.[4]
The individual phases are carried out by different groups of microorganisms which are partly
syntrophic. (One species lives of the products of the other species.)[4]
All the biochemical processes are represented in Figure 1.
3