wastewater-sourced urea for hydrogen energy-targeted...
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Wastewater-sourced urea for hydrogen energy-
targeted ammonia production
Andrs Chico Proao
A dissertation submitted in partial fulfilment of the requirements for the Degree of
Master of Science in Process and Environmental Systems Engineering
Faculty of Engineering & Physical Sciences
University of Surrey
February 2015
Andrs Chico Proao 2014
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DECLARATION OF ORIGINALITY
"I hereby declare that the dissertation entitled 'Wastewater-sourced urea for
hydrogen energy-targeted ammonia production' for the partial fulfilment of the
degree of MSc in Process and Environmental Systems Engineering, has been
composed by myself and has not been presented or accepted in any previous
application for a degree. The work, of which this is a record, has been carried out by
myself unless otherwise stated and where the work is mine, it reflects personal
views and values. All quotations have been distinguished by quotation marks and all
sources of information have been acknowledged by means of references including
those of the internet."
Andrs Chico Proao 13th February 2015
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ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to my supervisor Dr. Esat Alpay, for his
support and guidance throughout the development of the present study. His
knowledge and willingness to cooperate will always be deeply appreciated.
I would also like to thank to all the people that in one way or another have make my
pass through this journey much easier and enjoyable. To all my lecturers and fellow
colleagues that taught me so much. To all of my friends as well, for supporting me
through this time and make me feel welcome from the first day.
Finally and most importantly, I would like to thank my family. I have no words to
express my gratitude to you all. This work is dedicated to you, for the courage, love
and patient that you bring every day to my life. Dedicated to my loved father,
mother, and brothers.
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ABSTRACT
The dependence of the global economy on fossil fuels sets a series of energy and
environmental concerns for our present and future development. In spite of all the
efforts for reducing our consumption of fossil fuels, our needs for hydrocarbons are
only expected to increase. Therefore, decisions to promote a shift towards
renewable and more environmental-friendly sources of energy need to be made.
Hydrogen has been considered as a suitable candidate for replacing fossil fuels in the
future. Nonetheless, the success of a future hydrogen economy depends on today's
efforts for providing cheaper and renewable sources of energy.
The present project focuses in providing a general overview of the available
requirements, technologies, opportunities and limitations, associated to the use of
urine as a potential energy vector for supporting renewable urea and hydrogen
production. The potential use of urine as an energy resource was analyzed on the
basis of the available information found in previous research. Moreover, urine's
requirements for energy applications, as well as the available pre-treatment and
treatment processes to support such purpose, were discussed. Compatible urine-
treatment technologies were combined to propose alternatives for sourcing urea
from urine. Finally, the integration of urine-treatment processes with hydrogen
production technologies was covered.
Current collection systems for urine, urine's variability and occurring urea hydrolysis,
represent the major barriers associated with urea recovery from urine. Nonetheless,
such limitations can be overcome when urine separate collection systems and urine
storage are taken into account.
There is a real possibility for sourcing urea from urine and for using this renewable
resource in energy applications. Indeed, there are many available technologies that
could provide the right foundations for future developments related to the energy
use of urine. Nonetheless, major breakthroughs are still required in order to expand
the use of this type of technologies. Moreover, technical and economic feasibility
studies are required for defining areas of interest for future research.
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TABLE OF CONTENTS
DECLARATION OF ORIGINALITY ...................................................................................... i
ACKNOWLEDGEMENTS .................................................................................................. ii
ABSTRACT ...................................................................................................................... iii
LIST OF FIGURES ........................................................................................................... vii
LIST OF TABLES .............................................................................................................. ix
ABBREVIATIONS AND NOMENCLATURE ........................................................................ x
1. INTRODUCTION ...................................................................................................... 1
1.1. Problem statement ......................................................................................... 1
1.2. Aims and objectives ........................................................................................ 3
1.3. Overall outline ................................................................................................. 4
2. LITERATURE REVIEW .............................................................................................. 6
2.1. Ammonia in a future hydrogen economy ........................................................... 6
2.1.1. Introduction ................................................................................................. 6
2.1.2. Ammonia as a hydrogen carrier .................................................................. 7
2.2. Urea as a source for ammonia and hydrogen .................................................... 8
2.3. Urine as a renewable source of urea .................................................................. 8
2.3.1. Composition of urine ................................................................................... 8
2.3.1.3. Variations in urine ................................................................................... 10
2.3.2. Urine current collection and treatment .................................................... 11
2.3.2.1. Wastewater composition........................................................................ 11
2.3.2.3. Wastewater Treatment Processes .......................................................... 13
2.3.3. Separate collection of urine ....................................................................... 16
2.3.4. Decomposition of urea in urine ................................................................. 17
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2.3.4.1. Accelerated urea hydrolysis .................................................................... 19
2.4. Source-separated urine treatment alternatives ........................................... 19
2.4.1. Hygienisation ............................................................................................. 21
2.4.2. Volume reduction ...................................................................................... 22
2.4.3. Stabilization ................................................................................................ 23
2.4.4. Nitrogen recovery ...................................................................................... 24
2.4.5. Precipitation ............................................................................................... 27
2.5. Alternatives for hydrogen production from urea and ammonia ...................... 28
2.5.1. Thermo-chemical applications for hydrogen generation from urea-
containing streams ................................................................................................... 29
2.5.2. Ammonia and urea fuel cells ......................................................................... 35
2.5.3. Hybrid systems for urea electrolysis .......................................................... 38
3. WORK PERFORMED ............................................................................................. 39
3.1. Potential urea sourcing from urine ................................................................... 39
3.1.1. Initial composition or urine........................................................................ 40
3.1.2. Urea decomposition in urine ..................................................................... 42
3.1.3. Shortcomings of urine as a source for urea ............................................... 43
3.1.4. Requirements for urine-sourced urea and ammonia ................................ 44
3.2. Alternatives for urea and ammonia sourcing from urine ................................. 44
3.3. Urea and ammonia recovery process for hydrogen production ...................... 45
3.3.1. Process identification for urea recovery from urine wastewaters ............ 45
3.3.2. Integration of urea recovery units and hydrogen production................... 48
4. RESULTS AND DISCUSSION .................................................................................. 49
4.1. Potential urea sourcing from urine ................................................................... 49
4.1.1. Initial composition of urine ........................................................................ 49
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4.1.2. Decomposition of urea in wastewater urine ............................................. 53
4.1.3. Shortcomings of urine as a source for urea ............................................... 56
4.1.4. Requirements for urine-sourced urea and ammonia ................................ 61
4.2. Alternatives for urea and ammonia sourcing from urine ................................. 62
4.3. Urea and ammonia recovery process for hydrogen production ...................... 65
4.3.1. Process identification for urea recovery from urine wastewaters ............ 65
4.3.2. Integration of urea recovery units and hydrogen ..................................... 78
5. CONCLUSIONS AND FUTURE WORK .................................................................... 85
5.1. Conclusions.................................................................................................... 85
5.2. Future work ................................................................................................... 87
6. REFERENCES ......................................................................................................... 91
APPENDIX 1 ................................................................................................................ 105
Characteristics of flushing water ........................................................................... 105
APPENDIX 2 ................................................................................................................ 106
Concentration of urea and ammonia in urine ....................................................... 106
Concentration of ammonia/ammonium in urine .................................................. 107
Amount of water in urine ...................................................................................... 108
Concentration of phosphorus ................................................................................ 108
Urea hydrolysis calculations .................................................................................. 109
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LIST OF FIGURES
Figure 1. Distribution of raw materials and technologies for worldwide hydrogen production (Corbo,
Migliardini and Veneri, 2011). .................................................................................................................. 6
Figure 2. Approximate distribution of water-soluble components in urine expressed in % w/w
(Jnsson et al., 2005). ............................................................................................................................... 9
Figure 3. Composition of domestic wastewater contaminants. Adapted from Tebbutt (1998). ........... 12
Figure 4. Example of eutrophication occurred in waters with high nutrients content Rembrandt
Gardens-London. .................................................................................................................................... 12
Figure 5. General flow diagram of the wastewater treatment process (U.S. Environmental Protection
Agency, 1995; Gomes, 2009; Templeton and Butler, 2011). ................................................................. 14
Figure 6. Typical stages involved in nutrients treatment processes (Gomes, 2009; Templeton and
Butler, 2011; Thames Water, 2011) ....................................................................................................... 15
Figure 7. Distribution of the energy consumption in a WWT facility. Adapted from Pirnie (2005). ...... 16
Figure 8. Scheme of the available source-separated urine treatment alternatives (Maurer, Pronk and
Larsen, 2006). ......................................................................................................................................... 20
Figure 9. Scheme for nitrogen recovery from urine using ammonia stripping (Behrendt et al., 2002). 25
Figure 10. Schematic process for ammonia recovery from urine through ammonium sulphate
production (Antonini et al., 2011). ......................................................................................................... 26
Figure 11. Scheme of a urea-to-hydrogen process unit (Wu et al., 2013). ............................................ 30
Figure 12. Scheme of a membrane reactor for hydrogen generation from ammonia(Abashar, Al-
Sughair and Al-Mutaz, 2002; Garca-Garca et al., 2008). ...................................................................... 31
Figure 13. Scheme of the preheating and gaseous separation process for a) the feed, and b) urea
hydrolysis process (Rahimpour, Mottaghi and Barmaki, 2010). ............................................................ 32
Figure 14. Scheme of wastewater treatment loop (Rahimpour, Mottaghi and Barmaki, 2010)............ 33
Figure 15. Schematic representation of a general USCR process (Rollinson, Rickett, et al., 2011). ....... 34
Figure 16. Basic scheme of the tubular cell with a packed bed for ammonia decomposition and solid
oxide as electrolyte (Wojcik et al., 2003). .............................................................................................. 36
Figure 17. Scheme of a fuel cell that uses urea and KOH as an electrolyte ........................................... 37
Figure 18. Schematic representation of the shortcomings associated with the use of urine's urea as a
precursor for ammonia production. ....................................................................................................... 44
Figure 19. Description of pre-treatment and recovery processes(Maurer, Pronk and Larsen, 2006) ... 47
Figure 20. Scheme of the relationship between ammonia concentration and pH in fresh and stored
urine (Ciba-Geigy, 1981; Kirchmann and Pettersson, 1995; Udert et al., 2003; Jnsson et al., 2005;
Maurer, Pronk and Larsen, 2006). .......................................................................................................... 54
Figure 21. Scheme of requirements for urea and ammonia sourcing for later energy applications ..... 62
Figure 22. Scheme of classification of urine treatment technologies .................................................... 63
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Figure 23. Available alternatives for urine pre-treatment and for urea and ammonia recovery
processes. ............................................................................................................................................... 66
Figure 24. Schematic representation of alternative 1. ........................................................................... 70
Figure 25. Schematic representation of alternative 2. ........................................................................... 71
Figure 26. Description of UV pre-treatment process. ............................................................................ 73
Figure 27. Description of acidification pre-treatment process. ............................................................. 73
Figure 28. Description of acidification pre-treatment process. ............................................................. 74
Figure 29. Potential layout for option 1 (a) and option 2 (b). ................................................................ 75
Figure 30. Alternatives for urine-sourced urea and ammonia, for hydrogen production. .................... 80
Figure 31. Urine treatment process integrated with USCR for hydrogen production. .......................... 81
Figure 32. General Scheme of the processes that involve thermal process, separation and
simultaneous reaction/separation in a hydrogen-selective membrane reactor. .................................. 82
Figure 33. Scheme of the comparison between the processes required for hydrogen production from
urea and ammonia feedstock. ................................................................................................................ 84
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LIST OF TABLES
Table 1. Concentration of phosphorus and nitrogen for domestic wastewater and effluents discharge
to watercourses (The Council of the European Communities, 1991; Booker, Priestley and Fraser, 1999;
Boggs, King and Botte, 2009). ................................................................................................................ 13
Table 2. General parameters in urea decomposition for fresh/stale urine mixtures at T> 20 :C (Liu et
al., 2008b). .............................................................................................................................................. 19
Table 3. Doses of UV light required to reduce by a single order of magnitude, populations of microbial
groups (Koutchma, 2009). ...................................................................................................................... 22
Table 4. Operating conditions for ammonia stripping/recovery (Behrendt et al., 2002)....................... 25
Table 5. Urea adsorption on modified zeolites for urine (Wernert et al., 2005). .................................. 27
Table 6. Operation parameters for maximum hydrogen yield in a USCR process (Rollinson, Rickett, et
al., 2011). ................................................................................................................................................ 35
Table 7. Composition of urine for flow analysis and simulation models in wastewater (Jnsson et al.,
2005). ..................................................................................................................................................... 40
Table 8. Criteria-based selection matrix for determining urine's basic components ............................ 41
Table 9. Composition of urine for different collection sources and storage conditions (Ciba-Geigy,
1981; Kirchmann and Pettersson, 1995; Jnsson et al., 1997; Udert et al., 2003; Maurer, Pronk and
Larsen, 2006). ......................................................................................................................................... 43
Table 10. Criteria-based selection matrix for determining urine's basic components .......................... 49
Table 11. General composition of urine for energy applications (Jnsson et al., 2005) ........................ 51
Table 12. General composition of urine for non-energy applications (Jnsson et al., 2005) ................. 52
Table 13. Percentage of ammonia for urine in different collection locations (Ciba-Geigy, 1981;
Kirchmann and Pettersson, 1995; Udert et al., 2003; Jnsson et al., 2005; Maurer, Pronk and Larsen,
2006) ...................................................................................................................................................... 53
Table 14. General composition of urine in fresh and stored urine (Jnsson et al., 2005) ..................... 56
Table 15. General drawbacks associated with urea sourcing from urine .............................................. 57
Table 16. General classification of available urine-treatment technologies .......................................... 64
Table 17. Results of the comparison between option1 and option 2. ................................................... 76
Table 18. Composition of drinking water (Jnsson et al., 2005) . ........................................................ 105
Table 21. Molecular weights used for urea molecular weight calculation. .......................................... 106
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ABBREVIATIONS AND NOMENCLATURE
WWT Waste water treatment
w/w Weight to weight ratio
v/v Volume to volume ratio
RO Reverse Osmosis
USCR Urea steam catalytic reforming
UV Ultraviolet
CL Continuous light
PL Pulsed light
IBA Isobutyicaldehyde
IBDU Isobutyraldehyde-diurea
PEM Proton exchange membrane
Ts Total solids
T Temperature
IBDU Isobutyraldehyde-diurea
IBA Isobutyicaldehyde
GHG Greenhouse gases
p.e. Population equivalent
SCR Steam catalytic reforming
EAOP Electrochemically assisted oxidation process
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1. INTRODUCTION
1.1. Problem statement
Fossil fuels are the cornerstone of the world's present economy, and our current and
future development strongly depends on such fuels. However, adverse
environmental impacts, oil reserves depletion and constantly varying prices
associated to the use of fossil fuels, generate a complicated future energy scenario
(Penner, 2006). Regardless of all the efforts that have been performed in order to
reduce our dependence on hydrocarbons, the consumption of fossil fuels, far from
being scaled down, is expected to increase drastically over the next years (U.S.
Department of Energy [D.O.E], 2007). Indeed, while fossil will cover at least 75 % of
the world's primary energy demands in the next decades (International Energy
Agency, 2012), production peaks and reserves depletion will represent a major
challenge for fulfilling future energy requirements (Mason, 2007; Shafiee and Topal,
2009; International Energy Agency, 2012). Therefore, a change of course towards
renewable and low-environmental footprint energy sources is essential to provide
future energy security, to envision sustainable development and to prevent
irreversible environmental damages derived from the use of fossil fuels. Under this
premise, the present project aims to explore new energy alternatives that consider
environmental-friendly renewable sources. Particularly, the potential use of urine
from wastewaters as a resource for supporting cheaper and renewable hydrogen for
energy applications will be discussed.
Over the past two decades hydrogen has been studied as one of the potential
candidates for replacing fossil fuels in transportation and energy generation (Dillon
and Heben, 2001; Midilli et al., 2005). However, hydrogen's high storage and
transport expenses, represent the most important barrier in the road to a future
hydrogen economy (Greene et al., 2008; Zhang, 2009; Zhang and Mielenz, 2011).
One of the alternatives to overcome hydrogen's storage and transport limitations, is
the storage of hydrogen within chemical carriers (Graetz, 2008). Ammonia has been
proposed as an effective hydrogen carrier, and it seems up-and-coming for an initial
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transition towards a hydrogen economy (Thomas and Parks, 2006). Despite the
potential of ammonia as a hydrogen carrier, ammonia production costs and the fact
that it is produced from non-renewable sources, represent major challenges yet to
be overcome (Saika et al., 2006; Klerke et al., 2008). Indeed, cheaper and more
environmental-friendly ammonia production is required for supporting a future
hydrogen economy.
Within this context, the present study explores the potential use of domestic
wastewater urea as a renewable and low-cost ammonia precursor for energy
applications. There are no studies to date regarding energy uses of domestic
wastewater-sourced urea through ammonia and hydrogen production. Nonetheless,
a few laboratory-scale applications for energy production from wastewaters via
electrochemical cells and fuel cells have been developed (Lan, Tao and Irvine, 2010;
Kuntke et al., 2012; Wang et al., 2012; Kim et al., 2013; Santoro et al., 2013).
Moreover, previous undertakings have separately covered the development of
technologies for nutrients recovery from urine with agricultural purposes; and
energy applications of urea aqueous solutions via hydrogen production (Rahimpour,
Mottaghi and Barmaki, 2010; Rollinson, Rickett, et al., 2011). Despite these
important achievements, information regarding the use of urine in urea-to-ammonia
processes is still limited. The opportunity to use urea from urine as an ammonia
precursor definitely opens the possibility to cheaper ammonia production; hence, it
provides a potential contribution for achieving a future hydrogen economy. The
potential positive impacts of this research project are enhanced when considering
that urea from urine is the main responsible for the presence of nitrogen in
wastewaters. Furthermore, nitrogen is a contaminant that needs to be removed
from wastewaters in order to comply with environmental regulations (Larsen et al.,
2001). Indeed, near 75 % of the nitrogen found in wastewaters is currently removed
in wastewater treatment (WWT) plants, using expensive and energy-intensive
processes (Pirnie, 2005). Moreover, a different approach that can potentially address
both environmental and energy concerns, represents a novel and worthwhile
studying.
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Considering that every day, billions of litres of wastewaters are collected from
households (Department for Environment Food and Rural Affairs, 2012), the present
dissertation could help to generate a paradigm shift in the way we look at
wastewaters, so they can be envisioned as resources, rather than wastes to be
treated and disposed. Furthermore, wastewater use in energy applications provides
the opportunity to include social, environmental, energy and economic components
at the same time; which will enhance the reach of urine-to-urea technologies,
especially in communities with scarce wastewater treatment systems and limited
access to energy.
1.2. Aims and objectives
The present project aims to provide an overview of the potential use of wastewater
urine as a renewable source of urea for later energy generation purposes. Moreover,
this dissertation attempts to present an initial and general understanding of the
challenges, opportunities and technological alternatives involved in urea recovery
from urine wastewater, and its transformation into a valuable energy vector; which
can be used as a reference for future undertakings in the renewable energies and
wastewater treatment areas.
The present work has been developed in different stages that address the use of
wastewater urine as a potential renewable source for ammonia, the available
processes and technologies that could be suitable for urea sourcing from urine
wastewaters, and the integration of urine treatment processes with urea-to-
ammonia technologies. The objectives for this study cover the aforementioned
stages of the project, and they are:
To estimate an initial composition of urine that includes relevant components
for wastewater urea sourcing
To analyze the potential decomposition of urea in urine wastewaters
To describe possible limitations associated with the potential use of urine as
a source of urea
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To identify general requirements for urine-sourced urea as an ammonia
precursor
To perform a general analysis of processes and technologies with potential to
treat wastewater urine for urea recovery and for urea-to-ammonia
production
To propose a suitable process with potential to treat urine-containing
effluents in order to produce ammonia
To analyze the suitability for integrating urine wastewater treatment systems
into urea-sourced ammonia technologies for hydrogen production
To recommend future work that enhances urine-to-urea processes for
applications that favour ammonia production with energy purposes
1.3. Overall outline
The present dissertation was structured in order to provide a broad overview of the
potential use of urine as a renewable urea source for later energy applications, and
in order to fulfil the proposed aims and objectives. The structure of the present work
includes:
Chapter 2 (Literature review). This chapter regards the literature review of the
present dissertation and it includes some general insights to the use of ammonia in
energy applications. Moreover, information regarding the components, collection
and decomposition of urine is covered. In order to understand current developments
that could be useful for energy applications that consider urine as a resource,
current urine treatment processes, urea-to-ammonia technologies, and ammonia-to-
energy alternatives, will be described.
Chapter 3 (Work performed). This chapter presents the methodology that was
carried out in order to undertake the present dissertation. Moreover, it will describe
the considerations and methods applied for describing, comparing and analysing
different alternatives with potential to be applied in wastewater urine urea-to-
energy applications.
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Chapter 4 (Results and discussion). In this section the findings of the research are
presented, focusing mainly on results derived from qualitative and comparative
analysis of the information found in the literature. This chapter also focuses on the
analysis of the findings, expanding the information related to the presented results
and explaining the different implications derived from the results.
Chapter 5 (Conclusions and Future Work). This part of the work presents the most
important findings and shortcomings aligned with the attainment of the proposed
aims and objectives. Given the novel character of the present work, there are many
areas in which further studies could be undertaken. This chapter also focus on such
areas and mention possible areas of interest for future research.
Chapter 6 (References). This chapter includes all the bibliographic references used
for the development of the present dissertation.
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2. LITERATURE REVIEW
2.1. Ammonia in a future hydrogen economy
2.1.1. Introduction
The development of a worldwide hydrogen economy has been envisioned as an
alternative to address global energy and environmental challenges, as well as an
alternative to change our dependency on fossil fuels (Eberle, Felderhoff and Schth,
2009). Indeed, the potential use of hydrogen in transportation stands out as one of
its more appealing features (Christensen et al., 2006). Hydrogen is found in nature as
a part of other chemical compounds and it is rarely found free on earth. Because of
this, hydrogen is an energy carrier that needs to be obtained through chemical
transformations (Crabtree, Dresselhaus and Buchanan, 2004; Corbo, Migliardini and
Veneri, 2011). Approximately 96% of the hydrogen that is used in the world comes
from fossil fuels. Indeed, natural gas represents almost 50% of the raw materials
used in hydrogen production. Moreover, steam reforming is the most popular
technology for transforming natural gas into hydrogen. Therefore, hydrogen
production uses non-renewable sources and it generates CO2 emissions to the
atmosphere. The technologies and raw materials involved in hydrogen production
are detailed in Figure 1 (Corbo, Migliardini and Veneri, 2011).
Figure 1. Distribution of raw materials and technologies for worldwide hydrogen production (Corbo, Migliardini and Veneri, 2011).
48
30
18
4
0
10
20
30
40
50
60
Natural Gas-Steam reforming
Oil-Oxidation Coal-Gasif ication Water-Electrolysis
%
Raw material-Technology
Distribution of raw materials and technologies for worldwide hydrogen production (%)
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2.1.2. Ammonia as a hydrogen carrier
Ammonia has been regarded as one of the most promising hydrogen carriers,
specially for the early stages of a transition towards a hydrogen economy (Thomas
and Parks, 2006). The interest in ammonia responds to ammonia's worldwide
availability, its hydrogen storage capacity, its production costs and mature
production technologies (Satyapal et al., 2007; Klerke et al., 2008). The most
interesting asset of ammonia's use as a carrier is the already existing distribution
system for ammonia, which includes thousands of kilometres of currently operating
pipelines and vehicles for ammonia storage and transport (Christensen et al., 2006;
Thomas and Parks, 2006). Despite the potential of ammonia as a hydrogen carrier,
storage-related issues, toxicity and production costs are matters of concern in
ammonia's suitability as a hydrogen carrier (Thomas and Parks, 2006). While
ammonia storage and toxicity-related problems can be addressed effectively,
production and storage costs reduction needs major efforts to be overcome (Saika et
al., 2006; Klerke et al., 2008).
Ammonia's production costs depend on the cost of raw materials and on the
efficiency of production processes (Thomas and Parks, 2006). Currently, the majority
of the ammonia in the global market is synthesized from nitrogen and natural gas-
sourced hydrogen, through the Haber-Bosch process (Schlgl, 2003; Glvez,
Halmann and Steinfeld, 2007). Moreover, considering a scenario where typical
ammonia to hydrogen conversions reach less than 70% (Schlgl, 2003), and where
natural gas prices are constantly increasing (Thomas and Parks, 2006); lower
ammonia production costs are required to support a hydrogen economy. Many
efforts have been performed to improve the efficiency of ammonia synthesis by
using different metal-based catalyst (Jacobsen, 2000; Schlgl, 2003). In contrast, a
limited number of alternatives to replace the broadly use of natural gas, for cheaper,
renewable and more environmental-friendly raw materials have been developed
(Darvell et al., 2003; Klerke et al., 2008). Therefore, future research must be
conducted in order to find technologies for ammonia production from cheap
renewable resources and with the minimum environmental footprint possible.
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2.2. Urea as a source for ammonia and hydrogen
Even though urea is extensively used as a fertiliser (Lan, Tao and Irvine, 2010), it is
also a potential energy vector with capacity to store hydrogen and possibility to be
transformed into ammonia and hydrogen through thermal and catalytic processes
(Rahimpour, Mottaghi and Barmaki, 2010). Moreover, urea's production cost,
stability, straightforward transport and storage, high density, and a fully developed
industry; make urea a potential source for ammonia and hydrogen production.
Moreover, urea can be sourced renewably from a wide variety of sources, including
human urine (Rollinson, Jones, Dupont and Martyn Twigg, 2011).
2.3. Urine as a renewable source of urea
Human urine is a potential renewable source of urea that is daily discharged into the
sewerages. In addition, when considering that an adult can produce as much as 11
[kg] of urea per year, urea's energy potential can be envisioned. Indeed, 11 [kg] of
urea can generate the same energy as 18 [kg] of liquid hydrogen in a fuel cell (Lan,
Tao and Irvine, 2010). Consequently, there is an opportunity to use a currently
wasted resource for energy applications. Urea is the most abundant solute in human
urine and it is also the major responsible for nitrogen contamination in wastewaters
(Larsen and Gujer, 1996; Wilsenach and Van Loosdrecht, 2003; Yang and Bankir,
2005). Moreover, urine is a natural and renewable resource available worldwide,
regardless of economical and social conditions (Heinonen-Tanski et al., 2007).
2.3.1. Composition of urine
Urine is a mixture of water and water-soluble that is the main responsible for the
nitrogen and phosphorus found in wastewaters (Jnsson et al., 2005). Indeed, near
81% of the nitrogen and 47 % of the phosphorus found in domestic wastewaters,
come from urine (Larsen and Gujer, 1996; Jnsson et al., 1998; Wilsenach and Van
Loosdrecht, 2003; Larsen et al., 2009). In spite of the importance of urine for
wastewaters in terms of nitrogen and phosphorus, it only represents nearly 1% of
wastewaters volumetric flow (Beal et al., 2007).
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Water is by far the most abundant component of urine. Indeed, water can represent
approximately 98 % in weight, of urine (Jnsson et al., 2005). Besides water, urine
has several water-soluble compounds in small quantities. Out of these water-soluble
compounds, urea is the most abundant. Furthermore, near 94 % of the nitrogen
present in urine, comes from urea (Wilsenach, Schuurbiers and van Loosdrecht,
2007). Because of its nitrogen content, urine wastewaters (urine-containing water
effluents) have been studied for energy and agricultural applications (Heinonen-
Tanski et al., 2007; Pronk and Kon, 2009; Kuntke et al., 2012).
Urine's composition is available from a variety of sources in the literature; however,
big differences in composition can be found between such sources. Indeed, the
amount of nitrogen that comes from urea could be estimated between 8 and 20 [gN2-
urea/lurine] (Ciba-Geigy, 1981; Jnsson et al., 2005; Maurer, Pronk and Larsen, 2006;
Vinners et al., 2006; Wilsenach, Schuurbiers and van Loosdrecht, 2007). The
previous range of concentration for nitrogen provides an idea of the variation of
urine in terms of composition. An approximate distribution of water-soluble
components in urine is presented in Figure 2.
Figure 2. Approximate distribution of water-soluble components in urine expressed in % w/w (Jnsson et al., 2005).
The composition of urine can vary greatly among different groups of people (Schouw
et al., 2002). Indeed, people's habits, urine collection, urine transport and storage
71%
6%
4%
16%
2% 1%
Approximate distribution of water-soluble components in urine
Total Nitrogen
Total Phosphorus
Total Sulphur
Total Potassium
Zinc
Other metals
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conditions, modify urine's composition (Rauch et al., 2003; Wilsenach and Van
Loosdrecht, 2003; Heinonen-Tanski et al., 2007; Liu et al., 2008a).
2.3.1.1. Micropollutants
Micropollutants are organic compounds that are present in very low concentrations
in urine, and consequently, in wastewaters. Despite their low concentrations, such
substances can have adverse effects over the environment. There are different types
of micropollutants that can be found in wastewaters. For example, human
hormones, pharmaceuticals and food additives are typically found in such waters
(Federal Office for the Environment FOEN, 2012; Eggen and Swiss Federal Institute of
Aquatic Science and Technology, 2014).
It is not a common practice to have micropollutants removal units in wastewater
treatment facilities. As a result, most of these substances are discharged into
watercourses (Federal Office for the Environment FOEN, 2012). However, there are
suitable alternatives for micropollutants elimination at industrial scale. In fact,
ozonation and the use of activated carbon have proved to be useful in reducing the
levels of micropollutants in wastewater streams (Eggen and Swiss Federal Institute of
Aquatic Science and Technology, 2014). Other alternatives yet to be scaled-up
regards the use of nanofiltration, reverse osmosis, oxidation processes and ferrates
(Federal Office for the Environment FOEN, 2012).
2.3.1.2. Heavy Metals
The concentration of heavy metals in urine is very low, thus such components are
usually not taken into account for urine composition (Jnsson et al., 1998). The
particularly low concentration of heavy metals in urine has motivated urine's direct
use in crops irrigation (Heinonen-Tanski et al., 2007). Indeed, urine has lower heavy
metals levels than some common inorganic fertilizers (Jnsson et al., 1997).
2.3.1.3. Variations in urine
Urine's composition and flowrate can experience variations during the day due to a
series of factors such as age, different intakes of water, and people's mobility and
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habits (Schouw et al., 2002; Rauch et al., 2003). Moreover, morning peaks, which
evidence a particularly high amount of urine being discharged into the sewerages, is
crucial for WWT plants sizing and design (Henze et al., 2002; Larsen et al., 2009).
Finally, urine is not produced at a constant flowrate, it is rather excreted at random
pulses and during different periods of time (Rauch et al., 2003).
2.3.2. Urine current collection and treatment
Urine is a component of wastewaters; thus, it is collected in toilets and it is
transported through the sewerage system along with grey waters, faecal material,
rain waters and solid contaminants (Heinonen-Tanski and van Wijk-Sijbesma, 2005).
Moreover, wastewater treatment is required to safely dispose effluents coming from
human or industrial sources, in a way that does not compromise public health,
affects the environment, or interfere with water-associated economical activities
(Nelson and Murray, 2008; Gomes, 2009).
2.3.2.1. Wastewater composition
Wastewaters are a complex mixture of water, organic and inorganic compounds that
varies greatly in composition from one place to another. Although almost 99.9 %
w/w of wastewater is water itself, the remaining 0.1 % contains a diversity of
contaminants that need to be removed for safety and environmental issues. Such
contaminants comes from human excreta (faeces and urine), toilet paper, food
wastes and from other sources that can be picked up during wastewater collection
and transportation (Beal et al., 2007; Templeton and Butler, 2011). The complexity of
wastewater composition is not only attached to the variety of compounds, but it is
also related with the variability of domestic wastewater itself. Indeed, people's
lifestyle and eating habits, medical conditions, household piping systems, among
influence wastewater's composition (Tebbutt, 1998). A general composition of the
contaminants present in wastewater is presented in Figure 3.
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Figure 3. Composition of domestic wastewater contaminants. Adapted from Tebbutt (1998).
2.3.2.2. Nitrogen and phosphorus concentration limits
In terms of water pollution control, nitrogen and phosphorus levels are extremely
important (Larsen et al., 2007). Indeed, an excess of nutrients can alter the oxygen
content of watercourses, trigger abundant algal activity and it has a toxic effect on
ecosystems (Chevalier et al., 2000). The presence of an excessive amount of nitrogen
and phosphorus in watercourses, is also known as eutrophication (Kargi and Uygur,
2003; Paerl, 2006). An example of eutrophication is presented in Figure 4.
Figure 4. Example of eutrophication occurred in waters with high nutrients content Rembrandt Gardens-London.
30%
70%
Wastewater contaminants in percentage, % w/w
Inorganic Organic
Sediments, saltsand metals
65% Protein
10% Fats
25% Carbohydrates
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Local authorities need to comply with environmental regulations regarding nitrogen
and phosphorus concentration limits in water discharges. Therefore, and in order to
remove the mentioned components from wastewater streams, a series of treatment
processes are required. The concentration limits and the required reduction of
nitrogen and phosphorus for a safe water discharge into watercourses are presented
in Table 1.
Table 1. Concentration of phosphorus and nitrogen for domestic wastewater and effluents discharge to watercourses (The Council of the European Communities, 1991; Booker, Priestley and
Fraser, 1999; Boggs, King and Botte, 2009).
Component
Typical
concentration
in domestic
wastewaters
[mg/l]
Maximum concentration for
effluent discharge in regulations
[mg/l]
Minimum
required
reduction
[%] 10,000 - 100,000
[p.e.]
> 100, 000
[p.e.]
Total nitrogen 40 15 10 70-80
Total phosphorus 10 2 1 80
2.3.2.3. Wastewater Treatment Processes
Wastewater treatment is a process with several stages, designed to remove
contaminants from water in order to comply with existing standards or
environmental regulations (Jnsson et al., 1998). Before wastewater streams get into
the treatment process, preliminary removal of big size contaminants, paper, plastics,
gravel and sand takes place, in order to prevent damages in the equipment and to
make the overall treatment process easier (Templeton and Butler, 2011).
As it is presented in Figure 5, a wastewater treatment system includes several
operations that make use of physical, chemical and biological separation processes,
that ultimately allow a safe effluents discharge. In terms of operation principles and
type of contaminants removed, wastewater treatment processes are grouped under
three categories: preliminary, secondary and tertiary treatments (Outwater, 1994;
Templeton and Butler, 2011). Due to the interest of the present work in urea
sourcing, only the nutrients removal stage will be explained in detail regarding WWT
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plants (U.S. Environmental Protection Agency, 1995; Gomes, 2009; Templeton and
Butler, 2011).
Figure 5. General flow diagram of the wastewater treatment process (U.S. Environmental Protection Agency, 1995; Gomes, 2009; Templeton and Butler, 2011).
Within the primary treatment, oily substances and 60 % of the total suspended
settleable solids get removed, mainly by means of sedimentation or through other
physical separation methods (U.S. Environmental Protection Agency, 1995; Gomes,
2009; Templeton and Butler, 2011). Through secondary treatments, dissolved and
colloidal organic compounds are biodegraded and transformed into biomass.
Moreover, within this stage, nutrients removal (nitrogen and phosphorus) takes
place (U.S. Environmental Protection Agency, 1995). Nutrients removal takes place in
WWT plants during secondary treatments. Nitrogen (N) and phosphorus (P) are the
main nutrients present in wastewaters, and they need to be separated at this point.
Moreover, each one of the mentioned components requires specific bacteria and
individual treatment processes in order to be successfully removed. As a result,
nutrients removal processes involve a number of stages and operation variables that
make them complex to control and to operate. As a result, these removal processes
require high capital and operation costs (Gomes, 2009; Templeton and Butler, 2011;
Thames Water, 2011). A typical nutrient removal process is presented in Figure 6.
Finally, tertiary treatment regards the disinfection of water streams before effluents
return to watercourses. In a WWT facility, sludge is collected through the overall
Screening Grit
Removal
Clarifier
Wastewater
Liquid
Overflows
Solid
Underflows
Biotreatment
(Activated
sludge) Clarifier
Liquid
OverflowsNutrients
RemovalDesinfection
Discharge
to
Receiving
waters
Solid
Underflows
Sludge
Anaerobic
Digester
DryingLand
disposal
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operation as a by-product. Nonetheless, it can be used to produce methane for later
energy use, or it can be treated with chemical, physical or biological processes to be
safely disposed (Templeton and Butler, 2011; Thames Water, 2011).
Figure 6. Typical stages involved in nutrients treatment processes (Gomes, 2009; Templeton and Butler, 2011; Thames Water, 2011)
2.3.2.3.1. Energy consumption in a wastewater treatment facilities
Nitrogen and phosphorus removal require aeration for their removal in secondary
treatments (Templeton and Butler, 2011). In terms of energy consumption, aeration
is the most energy-intensive process in wastewater treatment operations. Indeed,
and in accordance with Figure 7, aeration can represent between 45 % to 75 % of the
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total energy requirements in a WWT plant (Pirnie, 2005; Rosso, Stenstrom and
Larson, 2008).
Figure 7. Distribution of the energy consumption in a WWT facility. Adapted from Pirnie (2005).
2.3.3. Separate collection of urine
Because of the bacteria-based digestion processes used in WWT plants to remove
nitrogen, the urea that is initially present in human urine is decomposed, and finally
lost to the atmosphere without any agricultural or energy use (Gomes, 2009).
Separate collection of urine provides an alternative to facilitate urea recovery for
future agricultural or energy purposes (Jnsson et al., 1998; Jin et al., 2013).
The idea of having a separate collection of urine has been studied over the past two
decades, as an alternative to cooperate with sustainable developments in
wastewater treatment operations (Berndtsson, 2006; Larsen et al., 2009). It basically
involves the use of NoMix toilets to collect urine and faecal material in a separate
way (Udert, Larsen and Gujer, 2006). Even though source separation has been
considered to have a bigger impact in rural communities, it is also applicable in
urban and populated areas (Nelson and Murray, 2008).
67%
13%
2%
18%
Energy requirements (%)
Aeration Solids Handling
Preliminary treatment Other
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2.3.3.1. Advantages of separate collection systems for urine
Separated urine collection systems present several benefits compared to regular
mixed collection systems. For instance, separate collection manage more
concentrated stream of phosphorus and nitrogen than regular wastewater collection
systems, hence it provides with the opportunity of more efficient and less energy-
intensive WWT processes (Wilsenach and Van Loosdrecht, 2003). In addition,
differentiated collection reduces the total wastewater flow that needs to be
processed in WWT facilities. Indeed, toilets that separately collect urine can save up
to 80% of flushing water (Larsen et al., 2001). Furthermore, when urine is collected
separately in appropriate systems, approximately 1.34 litres of flushed urine per day,
and per person can be gathered. Urine represents 75% of the mentioned discharge
and flushing water stands for the remnant 25% of the effluent. (Larsen and Gujer,
1997; Wilsenach and Van Loosdrecht, 2003). Lower water flowrates also require less
energy in pumping operations and treatment processes, which has a positive impact
in terms of the overall energy used for wastewater processing (Jnsson et al., 1998).
In spite of the advantages that separated collection can provide for wastewater
treatment and nutrient recovery processes, the scarce use of such systems and the
transportation of separated effluents remains still a challenge. Indeed, separate
collection systems have been used mainly in pilot applications and it still does not
count with all the support needed from sanitary industry. In addition, if separate
collected urine is not treated on-site, urine transport to centralize WWT facilities
represents additional costs. Therefore, on-site treatment is the most suitable option
for processing separately collected urine. Moreover, in order to enhance the use of
separated collection technologies, cooperation between industry, consumers and
local authorities is required (Larsen et al., 2009).
2.3.4. Decomposition of urea in urine
When urine is collected, transported or stored under non-sterile conditions,
occurring microbes hydrolyse urea from urine and transform it into ammonia,
ammonium compounds and carbonates. The mentioned process is often referred as
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urea hydrolysis, ureolysis or urea decomposition (Udert et al., 2003; K. Udert, Larsen
and Gujer, 2003; Udert, Larsen and Gujer, 2006). Due to urea hydrolysis in urine,
hydrolyzed urine presents different levels of urea, nitrites/nitrates, calcium,
magnesium and phosphates, compared to fresh urine (Ronteltap et al., 2010).
Because of this degradation processes, the pH of the water/urine mixtures turns
basic and it increases until it is stabilized at a pH value of 9 (Liu et al., 2008a).
Moreover, when the mentioned pH value is reached, magnesium and calcium salts,
specially struvite and hydroxyapatite, precipitate (Udert et al., 2003).
The biggest change in urine's composition due to urea hydrolysis regards the
nitrogen compounds (Hellstrm, Johansson and Grennberg, 1999). Indeed, when
fresh urine is collected, around 90 to 94 % of the nitrogen comes from urea (Jnsson
et al., 2005; Vinners et al., 2006; Wilsenach, Schuurbiers and van Loosdrecht, 2007;
Hug and Udert, 2013)(Hug and Udert, 2013). On the contrary, when urine is
transported and stored, near 94 % of the available nitrogen is present as ammonium
and ammonia. The mentioned change represents the evidence of urea hydrolysis
(Jnsson et al., 2005). Urea hydrolysis in urine follows the reaction (Mobley and
Hausinger, 1989):
CO(NH2)2 + 2H2O NH3 + NH4+ + HCO3
[1]
Microbial ureases do not prosper at low pH values; however, they quickly develop in
neutral pH conditions such as the one found in human urine, where bacteria only
require a few days in order to completely transform urea into ammonia (K. M. Udert,
Larsen and Gujer, 2003). Enzyme active bacteria hydrolysis of urea can be described
by Michaelis-Menten kinetics. In spite of pipeline existing bacteria, fresh urine that
passes quickly through pipelines will not have enough time to be hydrolyzed (K. M.
Udert, Larsen and Gujer, 2003). Urea hydrolysis in urine is time dependent and a
higher degree of hydrolysis involves higher amounts of ammonia and higher pH
values. Nonetheless, it is not common to find a direct correspondence between pH
and hydrolysis-generated ammonia for separated urine collection systems (K. Udert,
Larsen and Gujer, 2003).
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Non-enzymatic hydrolysis of urea at low temperatures is a slow process with a half
life of more than 3 years, for 38 :C. Therefore, enzymatic hydrolysis is the most
important urea decomposition path in wastewater streams and urine collection
systems (Udert et al., 2003). The general conditions for urea hydrolysis in urine, for
temperatures higher than 20 :C are presented in Table2.
Table 2. General parameters in urea decomposition for fresh/stale urine mixtures at T> 20 C (Liu et al., 2008b).
Type of sample Added stale
urine [% v/v]
pHFINAL Temperature
[ C]
Time for urea
hydrolysis [h]
Fresh/Stale
mixture > 10 9 > 20 60
2.3.4.1. Accelerated urea hydrolysis
When fresh urine is diluted with flushing water in a 1:4 ratio, approximately 21 days
are required, at room temperature, to hydrolyze all urine's urea (Udert et al., 2003).
In order to accelerate ureolysis in fresh urine, mixtures of fresh and stale urine can
be used. Indeed, when fresh urine is collected in agitated stored urine-containing
tanks, urea can be hydrolyzed in approximately 1 day (Udert et al., 2003). Moreover,
urease enzyme can also be added to fresh urine in order to accelerated the ureolysis
process (Kabadasli et al., 2006; Liu et al., 2008b).
2.4. Source-separated urine treatment alternatives
There are several technologies that can be applied in order to treat source-separated
urine. Maurer, Pronk and Larsen (2006) proposed to organize such treatment
technologies under seven different categories that consider the treatment's main
purpose: hygienisation, reduction of volume, stabilisation of urine, phosphorus
recovery (P recovery), nitrogen recovery (N recovery), nutrients removal and
micropollutants treatment. Figure 8 presents the mentioned categories and the
available technologies for each case.
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Figure 8. Scheme of the available source-separated urine treatment alternatives (Maurer, Pronk and Larsen, 2006).
From all of the alternatives for urine treatment presented in Figure 8, only
evaporation and storage have been broadly addresses and implemented. Indeed, all
of the other treatment processes have not gone beyond laboratory-scale
Treatment
alternatives
Hygienisation Storage
Volume
reduction
Evaporation
Freeze-
thaw
Reverse
osmosis
Stabilisation
Acidification
Nitrification
P
Recovery
Struvite
Precipitation
N
Recovery
Ammonia
stripping
IBDU
Precipitation
Ion
exchange
Nutrients
Removal
Anammox
process
Micropollutants
Treatment
Ozonation
Electrodialysis
Nanofiltration
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applications, and further research is still needed in order to envision applications at
larger scales (Maurer, Pronk and Larsen, 2006).
2.4.1. Hygienisation
Urine has generated interest as a renewably-sourced fertilizer, with potential to be
used in food crops. Nonetheless, urine's agricultural applications have presented
concerns about the sanitary risks associated with this application (Heinonen-Tanski
et al., 2007). Indeed, pathogens, infectious particles and faecal material can be found
in collected urine; which represents a sanitary risk for urine-fertilized food crops
consumers (Hglund et al., 1998). Within this context, urine sterilization has been
envisioned as an alternative to eliminate pathogens and faecal-related
microorganisms from urine. Among the different alternatives for urine sterilization,
urine storage is perhaps the only one that had been covered in depth (Maurer, Pronk
and Larsen, 2006). In addition, the use of ozone and ultraviolet (UV) light are
alternatives to urine sterilization. Nonetheless, such methods are not usual in
wastewater treatment and they have not been tested for urine sterilization (Maurer,
Pronk and Larsen, 2006; Gomes, 2009).
2.4.1.1. Storage
Storage is one of the alternatives to sterilize urine and to reduce its concentration of
pathogens. Indeed, urine's bacteria density is drastically reduced within four months
of storage (Hglund et al., 2000). Moreover, when urine is stored for six months at
20 :C, it is safe to use it as a fertilizer for food crops. Storage is effective for sterilizing
urine, as long as temperature is carefully controlled and monitored (Hoglund,
Stenstrom and Ashbolt, 2002). In spite of the benefits of the mentioned technology,
during urine storage, precipitation of minerals (struvite) takes place, and pipelines
blockages occur (Doyle and Parsons, 2002).
The amount of collected urine and the capacity for storage are issues to be
considered in urine storage. Indeed, continuous collection and storage generate big
volumes of urine that require either on-site treatment, or transport to a centralized
treatment plant (Dalemo et al., 1997; Hellstrm, Johansson and Grennberg, 1999).
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2.4.1.2. UV light sterilization
UV light has been broadly used in water disinfection and its inactivating effect over
microorganisms has been useful in the food industry, and for water treatment
(Shannon et al., 2008). Indeed, pulsed and continuous light rich in UV-C light (200-
280 nm) have the potential to kill microorganisms in order to sterilize surfaces and
fluids. Both continuous (CL) and pulsed (PL) UV light treatments are lethal for
microorganisms (Gmez-Lpez et al., 2007).
One of the major advantages of the mentioned light-associated treatment
technology is that it can be applied easily to sterilize continuous processes. The
transmittance of UV light is high in water (near 99.77 % at 0.1 cm), and it reduces
with an increment in the solids content (Koutchma, 2009). Furthermore, the dose
and the exposure time are key parameters in PL sterilization processes, and they
should be adjusted according with the type of fluid or surface to be treated, and the
type of microorganism to be eliminated (Wang et al., 2005). The UV inactivation
dosages for different microorganisms can be found in the literature, and some of
them are presented in Table 3. Nonetheless, there is no information available
regarding UV inactivation of urease active bacteria in urine.
Table 3. Doses of UV light required to reduce by a single order of magnitude, populations of microbial groups (Koutchma, 2009).
Microorganism Dosage [J/m2]
Enteral bacteria 20-80
Yeast 23-80
Cocci and micrococci 15-200
2.4.2. Volume reduction
Volume reduction technologies for urine are focused more in water reuse and
recovery, rather than urea or ammonia recovery. Among the different alternatives
for volume reduction in urine, water evaporation is perhaps, the simplest water
recovery technique. Moreover, water evaporation in urine has been previously
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addressed as an option for water recycling in space flights applications. Indeed,
compressed vapour distillation, membrane evaporation processes, air evaporation
and urine lyophilisation, have been covered in the literature as potential processes
for water evaporation in urine (Thibaud-erkey et al., 2002; Holder and Hutchens,
2003; Maurer, Pronk and Larsen, 2006).
Reverse osmosis (RO) has also been presented as a technology that allows volume
reduction in urine. Nonetheless, typical reverse osmosis processes are not efficient
for separating urea from wastewaters. Indeed, urea hydrolysis to ammonia and
existing membrane/solute interactions, complicate urea rejection through RO. One
alternative for removing hydrolyzed urea from wastewaters, considers RO processes
at low pH values (Lee and Lueptow, 2001). Reverse osmosis can be also coupled with
direct osmosis pre-treatment units and osmotic distillation, to concentrate non-
volatile solutes in water. Indeed, osmotic distillation improves urea separation from
wastewaters when membrane distillation is carried out at constant temperature
(Cath et al., 2005).
2.4.3. Stabilization
The stabilization of urine aims to prevent microbial-caused degradation,
volatilization and precipitation processes in urine wastewaters (Maurer, Pronk and
Larsen, 2006).
2.4.3.1. Acidification
Acidification represents a straightforward alternative to prevent urea hydrolysis in
urine. Indeed, dosages of 60 meq of sulphuric or acetic acid are enough to control
urea hydrolysis (Hellstrm, Johansson and Grennberg, 1999). The use of acidification
for preventing urea hydrolysis is related to the inactivation of urea hydrolyzing-
bacteria at low pH values (Maurer, Pronk and Larsen, 2006). When acidification
alternatives are used, urine could remain unhydrolyzed under storage for
approximately 8 months. In order for acidification to be cost-effective, it must be
performed as a preventive stabilization method (Hellstrm, Johansson and
Grennberg, 1999; Maurer, Pronk and Larsen, 2006).
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2.4.3.2. Urease inhibition
Urease inhibition provides a straightforward alternative for preventing urea
hydrolysis in urine. There are several compounds that can inhibit ureolysis, and
quinones, hydroxami acids and phosphorylamides, have proved to be helpful for
such purposes in agricultural applications (Mobley and Hausinger, 1989; Rollinson,
Jones, Dupont and Martin Twigg, 2011). Moreover, N-(n-butyl) thiophosphoric
triamide (NBPT) has been studied as urease inhibitor for controlling ammonia
emissions derived from urea hydrolysis in animal wastes, and for preventing urea
hydrolysis in agricultural fertilizers. Moreover, when NBPT is added in amounts
representing around 0.25 % (w/w) of the present urea, urease can be successfully
inhibited and ammonia losses are controlled (Varel, Nienaber and Freetly, 1999;
Gioacchini et al., 2002).
2.4.3.3. Self cleaning surfaces
Self cleaning surfaces represent a relatively new alternative with potential to prevent
bacterial growth in wastewater applications. This type o technology involves the use
of self-cleaning surfaces made out of superhydrophobic nanopolymers. Nonetheless,
issues related with contamination, wear resistance, and its real application in toilets
and pipelines are to be considered (Lee et al., 2007; Li, Reinhoudt and Crego-Calama,
2007; Larsen et al., 2009).
2.4.4. Nitrogen recovery
There are several technologies that allow nitrogen recovery from urine-containing
streams. Indeed, for such recovery purposes, stripping, adsorption, ion exchange and
precipitation processes and technologies can be considered.
2.4.4.1. Ammonia stripping
Ammonia stripping is one of the possible technologies for nitrogen recovery from
urine, when nitrogen is present as ammonia. Indeed, ammonia stripping from urine
can be performed using air and a packed column that operates at low pressures and
high temperatures (Behrendt et al., 2002). Moreover, in order to maximise ammonia
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recovery, urea hydrolysis needs to take place in a reactor before the stripping
process. Finally, the ammonia stripping unit could be alternatively connected to an
absorber, as it is presented in Figure 9, in order to finally recover nitrogen as an
ammonia-rich aqueous solution that contains 10 % of ammonia. The operating
conditions for the ammonia stripping and recovery process are presented in Table 4
(Behrendt et al., 2002).
Figure 9. Scheme for nitrogen recovery from urine using ammonia stripping (Behrendt et al., 2002).
Table 4. Operating conditions for ammonia stripping/recovery (Behrendt et al., 2002).
Parameter Stripper Absorber
Temperature [C] 40.0 20.0
Pressure [bar] 0.4 5.0
Nitrogen recovery from urine through ammonia air stripping has also been used to
produce liquid fertilizers in pilot applications. Indeed, hydrolyzed low-phosphorus
urine could be conditioned with sodium hydroxide before the stripping process. As it
is presented in Figure 10, the resultant air/ammonia gaseous stream can be treated
later with sulphuric acid in an absorption column to produce liquid ammonium
sulphate, a fertilizer with commercial value (Antonini et al., 2011).
Reactor Stripper
Fresh
urine
Hydrolyzed
UrineNH3 +
Air
Air
Aqueous stream
1 2
1 2 Absorber3
3
AirWater
Ammonia
solution
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Figure 10. Schematic process for ammonia recovery from urine through ammonium sulphate production (Antonini et al., 2011).
2.4.4.2. Nitrogen recovery through ion exchange
Nitrogen can be recovered from wastewaters in the form of ammonium ions (NH4+),
with ion exchange technologies that uses zeolites and polymeric exchangers as
cationic ion exchangers (Jorgensen and Weatherley, 2003). Indeed, in domestic
wastewaters with ammonium concentrations of 150 g/m3 or less, zeolites such as
mordenite, can remove around 90 % of the present NH4+ (Nguyen and Tanner, 1998).
In addition, mordenite and clinoptilolite-containing zeolites are effective adsorbents
for ammonia in aqueous streams (Englert and Rubio, 2005). The performance of
zeolites for ammonium removal depends mainly on wastewater flowrates and
components, zeolites' particle size, zeolite/wastewater contact time, and the
presence of contaminants like sodium ions and organic compounds (Nguyen and
Tanner, 1998). Both ammonia and ammonium removal with zeolites from aqueous
solutions have been broadly covered in the literature (Wang and Peng, 2010).
2.4.4.3. Nitrogen recovery through urea adsorption
Adsorption is a straightforward and cost effective alternative for wastewater
treatment. Moreover, the choice of an appropriate adsorbent is a key factor in the
effectiveness of the wastewater treatment process (Wang and Peng, 2010). Among
the available adsorbents, zeolites (aluminosilicates) have received special attention
over the last years in the wastewater treatment area. Such interest responds to their
Stripper
Treated low
P urine
NH3 +
Air
Air
1
1 Absorber2
2
Air H2SO4
Ammonium
Sulphate
Liquid
effluent
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Andrs Chico Proao 6295371 27
worldwide availability, they particular physicochemical properties and the possibility
to modify their structure (Wang and Peng, 2010).
Zeolites and activated carbon have been proposed as potential adsorbents for urea,
especially in medical applications regarding patients with renal failure (Wernert et
al., 2005). Indeed, zeolites and activated carbon can separate uremic toxins via
adsorption. While both, zeolites and activated carbon could be used for urea
removal, each adsorbent can address different requirements. Activated carbon for
instance, can separate around 14 % of urea in urine; however, it does not have
selectivity for molecule's size, and many other molecules besides urea are adsorbed.
On the contrary, zeolites are size-selective adsorbents that discriminate between
high and low molecular weights. Therefore, zeolites adsorb more urea than activated
carbon for urea-targeted adsorption. Sodium-containing zeolites, for instance, are
urea-selective adsorbents at 37 :C (Wernert et al., 2005). Moreover, the conditions
for urea recovery in the mentioned zeolite are presented in Table 5.
Table 5. Urea adsorption on modified zeolites for urine (Wernert et al., 2005).
Parameter Value/Characteristic
Zeolite Sodium-containing modified stibilite
Urea concentration in urine [mol/l] 0.0086
Temperature [C] 37
2.4.5. Precipitation
The precipitation of nitrogen compounds is also an alternative for nitrogen recovery
with possibility to be applied in urine or wastewater streams.
2.4.5.1. IBDU precipitation
Urea recovery from urine can be undertaken through precipitation. Indeed, urea
from urine can react with isobutyicaldehyde (IBA) to precipitate isobutyraldehyde-
diurea (IBDU). Furthermore, the mentioned reaction requires an excess of IBA, low
pH values and temperatures of around 60:C. The produced IBDU is commonly used
as a fertilizer (Behrendt et al., 2002).
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2.4.5.2. Struvite precipitation
While phosphorus is frequently identified as a worthwhile compound to be
recovered, nitrogen is not usually recognized as such. There are however,
alternatives that allow the recovery of almost all of the phosphorus and part of the
nitrogen that are found in wastewaters. An example of such recovering options is
the production of struvite from wastewater, to obtain solid fertilizers. Struvite is also
known as magnesium ammonium phosphate and it is obtained via phosphate and
ammonia precipitation in the presence of magnesium salts. The precipitation
reaction to obtain struvite is described as it follows (Doyle and Parsons, 2002):
Mg2+ + NH4+ + PO4
3 + 6H2O MgNH4PO4 6(H2O) [2]
The resulting struvite is formed by crystals that contain a N:P:Mg molar ratio of
1:1:1. Moreover, struvite precipitation is controlled by the pH, concentration of
reactants, temperature and presence of additional ions (Booker, Priestley and Fraser,
1999; Bouropoulos and Koutsoukos, 2000). Struvite precipitation can result
challenging for small-scale applications because of the necessary dosage of
magnesium salts for assure precipitation. In such cases, the use of a magnesium
sacrifice electrode could be an effective and cheap alternative for magnesium
dosage(Hug and Udert, 2013). Additional treatment processes and technologies that
were not covered previously, are available in the literature (Maurer, Pronk and
Larsen, 2006).
2.5. Alternatives for hydrogen production from urea and ammonia
Among the different technological alternatives with potential to transform urine
sourced urea into a valuable and useful energy vector, thermo-chemical applications
and electrochemical developments need to be considered (Rahimpour, Mottaghi and
Barmaki, 2010; Rollinson, Jones, Dupont and Martyn Twigg, 2011; Rollinson, Rickett,
et al., 2011; Wu et al., 2013).
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2.5.1. Thermo-chemical applications for hydrogen generation
from urea-containing streams
In terms of thermo-chemical processes, there are mainly two different approaches
that could be considered for producing hydrogen from urea. The first approach
requires two different stages, urea hydrolysis and ammonia cracking, that separately
address ammonia formation and ammonia decomposition into hydrogen
(Rahimpour, Mottaghi and Barmaki, 2010; Wu et al., 2013). The second approach
considers only one catalytic process (urea steam catalytic reforming) to obtain
hydrogen from urea (Rollinson, Rickett, et al., 2011).
2.5.1.1. Hydrogen production from urea through hydrolysis and catalytic
cracking
Urea thermal hydrolysis has been used successfully to produce ammonia in batch
and continuous processes (Park, Lee and Rhee, 2009; Sahu et al., 2010). Indeed, the
general method for producing hydrogen from urea through thermo-chemical
processes involves a series of steps, which can be grouped under two main reaction
mechanisms that are endothermic. The first one, regards the transformation of urea
to ammonia via carbamate compounds; and the second one involves ammonia
decomposition into hydrogen and nitrogen by thermal catalytic cracking (Rahimpour
and Asgari, 2008; Rahimpour, Mottaghi and Barmaki, 2010; Wu et al., 2013). Initially,
urea is hydrolyzed to ammonia in the presence of a catalyst (Sahu, Gangadharan and
Meikap, 2011) through the following reactions (Schell, 1979; Sahu et al., 2010):
Urea hydrolysis to ammonium carbamate
CO(NH2)2 + H2O NH2COONH4 [3]
Decomposition of ammonium carbamate:
NH2COONH4 2 NH3 + CO2 [4]
High temperatures and high urea concentration in the feed enhances ammonia
production in the previous reaction (Sahu, Gangadharan and Meikap, 2011).
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Once that urea has been transformed to ammonia, ammonia decomposition into
hydrogen takes place. This reaction is temperature dependent, and it usually
requires temperatures above 400 C to occur (Garca-Garca et al., 2008). Ammonia
decomposition generally takes place over a nickel-aluminum oxide catalyst according
to (Abashar, Al-Sughair and Al-Mutaz, 2002; Rahimpour, Mottaghi and Barmaki,
2010; Wu et al., 2013):
2NH3 NH3 + 3H2 [5]
The use of new and more efficient catalysts, such as ruthenium-based catalysts,
favors improving ammonia conversions (Garca-Garca et al., 2008).
The use of urea thermal hydrolysis processes combined with hydrogen-selective
membrane reactors, is a promising alternative to produce hydrogen from urea-
containing streams (Rahimpour, Mottaghi and Barmaki, 2010; Wu et al., 2013). Wu
et al. (2013) described a process for hydrogen production from urea aqueous
solutions, that included urea hydrolysis and ammonia catalytic decomposition
processes. The mentioned development considered the possibility to feed the
produced hydrogen into a proton exchange membrane (PEM) fuel cell, and
integrated waste heat recovery (Wu et al., 2013). The mentioned process for
hydrogen production is presented in Figure 11.
Figure 11. Scheme of a urea-to-hydrogen process unit (Wu et al., 2013).
Mixer
Urea
+
H2O
ExchangerUrea
Hydrolyser
NH3
SeparatorExchanger
NH3
Ammonia
Cracker
H2
Separator
H2
Burner
Waste
gas
Air
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2.5.1.2. Membrane reactor for hydrogen production
The use of membrane reactors represents an hybrid approach that includes
considers chemical reaction and separation processes simultaneously and within the
same equipment (Rahimpour, Mottaghi and Barmaki, 2010). Indeed, the use of
membrane reactors allows to integrate a selective membrane and a catalytic reactor
into one equipment that is applicable for ammonia-to-hydrogen production
processes (Rahimpour and Asgari, 2008). Benefits like lower operation temperatures,
higher conversions, bigger yields at low temperatures, and the practicality of
requiring only one equipment, make this technology more attractive than other
hydrogen generation systems. Ruthenium-based catalyst and membranes with
palladium walls have helped to improve the conversion process (Garca-Garca et al.,
2008; Rahimpour and Asgari, 2008). As it is presented in Figure 12, a sweep gas is
normally used in membrane reactors in order to shift the reaction's equilibrium to
the products side (Rahimpour and Asgari, 2009).
Figure 12. Scheme of a membrane reactor for hydrogen generation from ammonia(Abashar, Al-Sughair and Al-Mutaz, 2002; Garca-Garca et al., 2008).
2.5.1.3. Wastewater treatment loop with membrane reactor production
Rahimpour, Mottaghi & Barmaki (2010), presented a wastewater treatment loop
process to separate urea and ammonia from industrial effluents. The mentioned
process included a membrane reactor and hydrogen was the final product of the
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system. Several stages and processes are considered in this complex wastewater
treatment loop.
Initially, the treatment loop is fed with a urea aqueous stream that contains water
(H2O), ammonia (NH3), carbon dioxide (CO2) and urea. As it is presented in Figure 13
a), the feeding stream is preheated before it undergoes a separation process in a
desorption unit, where the gaseous components are separated at low pressures
(Rahimpour, Mottaghi and Barmaki, 2010).
Figure 13. Scheme of the preheating and gaseous separation process for a) the feed, and b) urea hydrolysis process (Rahimpour, Mottaghi and Barmaki, 2010).
As it presented in Figure 13 b), the liquid effluent of the desorber contains urea and
water only and it is heated to be fed into a reactor (hydrolyser) for urea thermal
hydrolysis to takes place. Therefore, ammonia is produced in this stage. Finally, as it
is presented in Figure 14, the ammonia that is obtained after the wastewater
treatment loop undergoes heating and compression (36 *atm+ and 550 :C) and is fed
into a membrane catalytic reactor for producing hydrogen (Rahimpour, Mottaghi
and Barmaki, 2010).
DesorptionPreheating
Feed (H2O (l),
NH3, Urea, CO2)
NH3, CO2, H2O(v)
Urea,
H2O(l)
Heating
Urea +
H2O (l)
Liquid-phase
Hydrolyzer
H2O(l) + Urea(ppm)
H2O(l) +
Urea(ppm)
NH3, CO2, H2O(l)a) b)
LP
Steam
HP
Steam
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Figure 14. Scheme of wastewater treatment loop (Rahimpour, Mottaghi and Barmaki, 2010)
2.5.1.4. Urea steam reforming (USCR)
Technologies such as steam reforming allow to envision the transformation of urea
into hydrogen using only one catalytic unit. Even though the in