Wastewater-sourced urea for hydrogen energy-targeted...

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

Wastewater-sourced urea for hydrogen energy-targeted ammonia production

Andrs Chico Proao 6295371 i

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


Andrs Chico Proao 6295371 ii

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.


Andrs Chico Proao 6295371 iii

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.


Andrs Chico Proao 6295371 iv

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


Andrs Chico Proao 6295371 v

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


Andrs Chico Proao 6295371 vi

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


Andrs Chico Proao 6295371 vii

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


Andrs Chico Proao 6295371 viii

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


Andrs Chico Proao 6295371 ix

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


Andrs Chico Proao 6295371 x

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


Andrs Chico Proao 6295371 1

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



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.



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



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.



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.



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 (%)



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.



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).



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



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



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.



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



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



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



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



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



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



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).



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.



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



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).



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



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).



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



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



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



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).



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).



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).



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



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



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



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

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