The Design and Build of Biodigester Toilet

SCHOOL OF MECHANICAL ENGINEERING
Project 1587:
The Design and Build of a Biodigester Toilet
James Bass
Nishanth Cheruvu
Natasha Rayan
Charlie Savory
Kieren Sheehan
Supervisors: Dr Cristian Birzer and Dr Paul Medwell
24 October 2014
Word count: 29,797

Executive Summary
Preventable diseases caused by unsafe sanitation practices, and respiratory issues created by burning
solid fuels for cooking, heating, and lighting kill millions of people in developing communities every
year. Providing improved sanitation facilities, and replacing solid fuels (such as wood and dung) with
clean burning modern fuels can improve quality of life for billions of people around the globe, and save
millions of lives each year. A biodigester toilet is a single solution to both of these major issues; it
provides an integrated waste management facility that will convert human excreta into clean burning
biogas, which can be used for cooking, heating, and lighting.
Research was conducted to obtain the background knowledge required to design a biodigester system
that would be capable of successfully producing biogas, while also providing an alternative to unsafe
sanitation practices. A dual tank digester design was chosen, to provide a clarification tank as a
precursor to effluent post-treatment. A thorough risk assessment was performed before construction
and testing of a prototype was conducted. Sponsorship from Barrow and Bench Mitre 10 Malvern,
Caroma and Lynair Logistics enabled the project team to source parts within the project budget, and
construct the prototype. Testing was undertaken at Urrbrae Agricultural High School to determine
whether the system was capable of effectively isolating waste and producing biogas.
The prototype effectively separated feedstock from human contact, and harnessed the anaerobic di-
gestion process to produce biogas. As methane is the primary constituent of biogas, its concentration
was measured throughout the testing period. Results showed an increase in methane concentration,
however the testing period was concluded before flammable biogas was produced. All data indicated
that the anaerobic digestion process was progressing as expected, and it is likely that flammable biogas
would have been produced, given a longer testing period.
i

Acknowledgements
The team would like to thank the following individuals and organisations for their contributions.
Project Supervisors
Dr. Cristian Birzer and Dr. Paul Medwell
Sponsors
Barrow and Bench Mitre 10 Malvern
The University of Adelaide School of Mechanical Engineering
Lynair Logistics
Caroma
Special Thanks
The staﬀ of Urrbrae Agricultural High School
The staﬀ of Barrow and Bench Mitre 10 Malvern
Rob Patterson
Michael Hatch
ii

Statement
This work contains no material which has been accepted for the award of any other degree or diploma
in any university or other tertiary institution and, to the best of our knowledge and belief, contains
no material previously published or written by another person, except where due reference has been
made in the text.
The project team consents to this copy of their report, when deposited in the University Library, being
available for loan and photocopying.
iii

Contents
Executive Summary i
Acknowledgements ii
Signed Statement iii
Nomenclature x
1 Introduction 1
1.1 Report Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Background 3
2.1 Sanitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Solid Fuels and Household Air Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 The Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Project Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3 Technical Background 13
3.1 Human Waste Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Single Appropriate Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3 Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4 Biodigester Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 Existing Biodigester Toilets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4 Scope, Objectives and Timeline 31
4.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2 Core Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3 Extension Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.4 Project Timeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5 Design Formation 34
iv

5.1 Standards and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2 Overall Design Speciﬁcation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3 Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.4 Essential Design Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.5 Conceptual Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6 Final Design 54
6.1 Final System Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.2 Number of End Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.3 Materials Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
6.4 Waste Collection System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.5 Gas Collection System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
7 Risk Assessment 71
7.1 Likelihood Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
7.2 Consequence Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
7.3 Risk Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
7.4 Heirarchies of Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.5 Risk Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
8 Prototype Construction and Cost 76
8.1 Part Sourcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
8.2 Construction and Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
8.3 Personal Protective Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
8.4 Costing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
9 Testing and Operation Procedures 82
9.1 System Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
9.2 Prototype Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
9.3 Feedstock Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
9.4 System Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
9.5 Continuous Process Digester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
9.6 Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
9.7 Biogas Collection and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
9.8 Safe Operating Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
10 Results and Discussion 89
10.1 Gas Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

10.2 Methane Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
10.3 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
10.4 Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
10.5 Portability Demonstration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
10.6 Completion of Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
11 Future Work 98
11.1 Extension Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
11.2 Design Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
12 Conclusion 100
Appendix A Project Timeline 109
Appendix B CAD Drawings 114
Appendix C Australian Standards for Polyethylene Pipes 126
Appendix D Australian Standard Gas System Design Factors 130
Appendix E Stirrer CAD Drawings 132
Appendix F Risk Assessment 148
Appendix G Project Cost Matrix 153
Appendix H Sponsorship Prospectus 155
Appendix I Project Hours Spent by Individual Team Members 158
Appendix J SupelTM Sampling Bag Data Sheet 165
Appendix K Picarro Gas Analyser Data Sheet 168
Appendix L Testing Numerical Results 171

List of Figures
2.1 The proportion of the population using improved sanitation (WHO and UNICEF, 2012) 4
2.2 Pit latrine with squatting slab (Furniss, 2011) . . . . . . . . . . . . . . . . . . . . . . 6
2.3 A Chinese shared pit latrine without a platform, slab or seat (Rivard, 2005) . . . . . . 6
2.4 Hanging toilet in Port Haitien, Haiti (Stauffer, 2014) . . . . . . . . . . . . . . . . . . . 7
2.5 Indication of household solid fuel use globally (Chartsbin (2007) using data from WHO
(2007)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Effect of solids retention time and temperature on volatile solids reduction in a labora-
tory scale anaerobic digester (Wang et al., 2007) . . . . . . . . . . . . . . . . . . . . . 19
3.2 Fixed dome biodigester (Weir, n.d.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3 Floating drum biodigester (Thai Biogas Energy Company, 2008) . . . . . . . . . . . . 23
3.4 Plastic tube plug flow biodigester. Adapted from Energypedia (2014) . . . . . . . . . . 23
3.5 Dismountable FRP biodigester model (Cheng et al., 2014) . . . . . . . . . . . . . . . . 25
3.6 Biodigester created from existing water tanks in Cambodia (Engineers Without Borders,
2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.7 ARTI bioidigester: A prefabricated plastic product based on the existing floating drum
design (Zu, 2005) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.8 EWB Challenge biodigesting toilet (Ashley et al., 2011) . . . . . . . . . . . . . . . . . 27
3.9 Prototype design with flexible membrane gas collection (Coffee et al., 2009) . . . . . . 29
3.10 Prototype design with gasometer gas collection (Coffee et al., 2009) . . . . . . . . . . . 29
5.1 Concept Design 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.2 Concept Design 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5.3 Concept Design 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.4 Concept Design 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.5 Final concept design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
6.1 Drawing of inlet assembly (dimensions in mm) . . . . . . . . . . . . . . . . . . . . . . 58
6.2 Final attached lid for second tank in the system . . . . . . . . . . . . . . . . . . . . . . 59
vii

6.3 First tank attached gas connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
6.4 Gas connection valve on second tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
6.5 A typical bioball shape (Foster and Smith, 2014) . . . . . . . . . . . . . . . . . . . . . 61
6.6 Attached tank flange with neoprene seal . . . . . . . . . . . . . . . . . . . . . . . . . . 62
6.7 Attached ball valve and barb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.8 Attached ball valve, barb, and suction hose . . . . . . . . . . . . . . . . . . . . . . . . 63
6.9 Overall connection between two tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.10 Outlet tap attached to existing 25 mm diameter threaded hole . . . . . . . . . . . . . 64
6.11 Connection between gas collection membrane and pipe network . . . . . . . . . . . . . 65
6.12 1m3 biogas collection membrane used in the final design . . . . . . . . . . . . . . . . . 67
6.13 Scrap material used for insulation layer . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.14 Black plastic layer for heat absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.15 Final Stirrer Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
9.1 Tedlar bag filled with gas sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
10.1 Change in methane concentration over testing period . . . . . . . . . . . . . . . . . . . 90
10.2 Change in methane concentration for different substrates (Sulistyo et al., 2012) . . . . 91
10.3 Change in system pH over testing period . . . . . . . . . . . . . . . . . . . . . . . . . . 92
10.4 Temperature measurements compared to BOM readings . . . . . . . . . . . . . . . . . 94
A.1 Project Gantt Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
A.2 Project Gantt Chart continued . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
A.3 Project Gantt Chart continued . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
B.1 Overall CAD model of prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
E.1 Overall CAD model of stirrer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

List of Tables
3.1 Chemical constituents of biogas (Favre et al., 2009) . . . . . . . . . . . . . . . . . . . . 17
3.2 Biogas production for diﬀerent animal feedstocks (Junfeng et al., 2005) . . . . . . . . . 18
3.3 C/N ratio of some organic materials (Karki and Dixit, 1984) . . . . . . . . . . . . . . 20
5.1 Relevant Australian Standards (Davidson et al., 2013) . . . . . . . . . . . . . . . . . . 35
5.2 Relevant recommendations for biogas installations relating to a small scale biodigester
toilet (Davidson et al., 2013) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.3 Concept Design 1 design criteria analysis . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.7 Evaluation matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.8 Design feature summary table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.9 Final concept design design criteria analysis . . . . . . . . . . . . . . . . . . . . . . . . 53
6.1 Properties of PE100 pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7.1 Consequence scale - risks to project success (The University of Adelaide, 2012) . . . . 72
7.2 Consequence scale - safety risks (The University of Adelaide, 2012) . . . . . . . . . . . 73
7.3 Risk matrix (The University of Adelaide, 2012) . . . . . . . . . . . . . . . . . . . . . . 73
7.4 Risk management required (The University of Adelaide, 2012) . . . . . . . . . . . . . . 74
8.1 Sponsorship summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.2 Prototype cost summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.3 Recycled component alternatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
A.1 Project Review Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
A.2 Major Milestones, Review Gates and Due Dates . . . . . . . . . . . . . . . . . . . . . . 110
ix

Nomenclature
AUD Australian Dollar(s)
BOM Bureau of Meteorology
textitC Design factor
CoP Code of Practice
C/N Carbon to nitrogen ratio
CAD Computer Aided Design
Dm min Minimum mean outside diameter
FRP Fiber Reinforced Plastic
GACC Global Alliance for Clean Cookstoves
L Litre
LPG Liqueﬁed Petroleum Gas
m Metre
MAOP Maximum Allowable Operating Pressure
MDGs Millennium Development Goals
mm Millimetre
MRS Maximum Required Strength
NGO Non-governmental Organisation
PPE Personal Protective Equipment
ppm Parts per million
PE Polyethylene
PVC Polyvinyl Chloride
x

RT Retention time
SDR Standard Dimension Ratio
SOP Safe Operating Procedure
Tmin Minimum wall thickness
UAHS Urrbrae Agricultural High School
UNICEF The United Nations Children’s Fund
USD United States Dollar(s)
UV Ultra-Violet
VS Volatile Solids
WHO World Health Organisation

Chapter 1
Introduction
Currently, 2.6 billion people worldwide lack access to adequate sanitation facilities, while 3 billion
people are put at risk from harmful air pollution because they rely on burning solid fuels for cooking,
heating and lighting. Over 4.6 million deaths are caused every year from a wide range of health issues
related to poor sanitation and household air pollution (WHO, 2014; WHO and UNICEF, 2014c). When
considering the global distribution of these problems, it is clear that they are present in similar regions
all over the world; primarily the rural areas of developing countries. Some of the most marginalised
people in the world are subject to diseases associated with exposure to human faeces, along with
serious respiratory health issues (including lung cancer) caused by household air pollution. While not
tolerated in more developed nations, crippling poverty means billions of people living in developing
countries are subject to these conditions every day. Developing a single, cheap solution that will
provide improved means of sanitation while reducing reliance on solid fuels has the potential to save
the lives of millions, and improve the lives of billions, every year.
The design and build of a biodigester toilet is a humanitarian project aimed at providing improved
sanitation facilities, and reducing air pollution for the billions of people aﬀected by these issues.
For this project, a toilet is integrated with a biodigester - a device that stores and ‘digests’ organic
material while also producing biogas, a mixture of primarily methane (CH4) and carbon dioxide (CO2).
Designing a biodigester to accommodate a toilet enables it to become an integrated waste management
system, and provides a means of safe human waste disposal. The biogas produced from the digestion
of human waste can be used as a cleaner burning alternative to solid fuels for cooking, heating, and
lighting. Thus, both issues of unsafe sanitation and household air pollution may be addressed through
implementation of a functional biodigester toilet.
While the concept and use of the biodigester is widespread and well documented, there remain inade-
quacies in the literature pertaining to a system designed solely for use with human waste. Combined
biodigester and toilet systems have been designed and tested in the past, however experimental results
1

CHAPTER 1. INTRODUCTION 2
from these existing systems indicate that neither design, nor execution, were suitable for the system
to have practical applications. Several explanations as to why the systems were ineffective have been
suggested in the literature, but not subsequently implemented into an improved design. An improved
design will better address the health, sanitation, and energy challenges prevalent in many develop-
ing regions. Furthermore, as biodigester systems produce useful secondary products from primary
waste, there is scope for application in developed countries. Research and development into improved
biodigester toilet designs for developed countries could alleviate concerns of growing waste volume,
energy shortages, and climate change. The integration of biodigester toilets into modern waste man-
agement and energy consumption practices could promote greater self-sufficiency, and environmental
sustainability.
1.1 Report Structure
A detailed outline of the global distribution of poor sanitation practices and the use of solid fuels
for cooking, heating, and lighting is presented within the Background chapter of this report. Also
included in this chapter is an outline of the major health problems related to these practices. The
problems outlined in this chapter helped guide the development of the project aim.
After the project aim is identified, the Technical Background chapter presents research into existing
technologies that provide a means of achieving the overall project aim. Of particular focus is methods
of human waste management as well as waste collection systems.
The Scope, Objectives, and Timeline chapter outlines detailed objectives for the project, specifically
relating to the design, build and subsequent testing of a biodigester toilet. A timeline of the project
is also presented in this chapter.
The information presented in the initial chapters is then used in the Design Formation chapter to
develop several concept designs of biodigester toilets. These designs are evaluated against a list of
design criteria. Based on the analysis of the concept designs, a final design is further developed in the
Final Design chapter.
The Risk Assessment, Prototype Construction and Cost, and Testing and Operation chapters outline
the various stages in the production of a prototype biodigester toilet, based on the final design. A risk
assessment was required to ensure construction and testing could be performed safely.
Preliminary testing results, and a discussion of their significance, are presented in the Results and
Discussion chapter. The extent to which the prototype was able to achieve all core project objectives is
assessed in this chapter. Based on these preliminary testing results and the overall effectiveness of the
prototype, possible design modifications and additions are discussed in the Future Work chapter.

Chapter 2
Background
Household air pollution produced by burning solid fuels, and inadequate sanitation are two major
issues facing the developing world. Both problems cause specific health, social, and environmental
issues. Significant improvements have been made in both these areas over the last fourteen years since
the conception of the millennium development goals in 2000 (WHO, 2012) with scope for substantial
progress in the future. Presented in this chapter is a discussion of these problems, along with informa-
tion about biodigesters, biogas, and the contribution they make to alleviating inadequate sanitation,
and household air pollution.
2.1 Sanitation
Currently 2.6 billion people worldwide do not have access to adequate sanitation facilities, resulting
in the contraction of diseases that are responsible for more than two million deaths every year (WHO
and UNICEF, 2014c). Additionally, there are a number of non-fatal diseases associated with poor
sanitation that significantly reduce quality of life. These issues associated with inadequate sanitation
are primarily present in developing communities. Therefore, the development of sanitation systems
that are readily available in affected regions will help improve the quality of life of billions of people
and save a significant number of lives every year.
2.1.1 Location of Current Practices
The distribution of global access to improved sanitation facilities is shown in Figure 2.1. The definition
of an improved sanitation facility is presented in Section 2.1.3. It can be seen that Africa and the
Indian Subcontinent are the worst affected, where most of the countries have less than 50% access.
China, South-East Asia, and Latin America are also affected, though not to the same extent.
3

CHAPTER 2. BACKGROUND 4
Figure 2.1: The proportion of the population using improved sanitation (WHO and UNICEF, 2012)
The global divide of access to improved sanitation is also disproportionately split between the rural
and urban regions. Currently 79% of people living in urban areas have access to improved sanitation
facilities. Conversely, only 47% of people living in rural areas enjoy the same access. Therefore 1.8
billion people worldwide are unable to use improved sanitation facilities, primarily due to the cost of
sanitation facilities in these areas (WHO and UNICEF, 2012). Open defecation occurs primarily in
rural areas; worldwide, 950 million rural residents are forced to practice open defecation compared to
100 million people living in urban locations (WHO and UNICEF, 2012). Open defecation is mainly
concentrated in India, whose population makes up over 60% of the total world population practicing
this type of unimproved sanitation (WHO and UNICEF, 2012).
2.1.2 Health Issues
There are a number of diseases and associated conditions that can arise from the practice of unimproved
sanitation and subsequent contact with human excreta. These include diarrhoea, cholera, ﬂuorosis,
guinea worm disease, hepatitis A, schistosomiasis, trachoma, and typhoid (UNICEF, 2014). Diarrhoea,
schistosomiasis, trachoma, and typhoid are commonly considered the most damaging and widespread
conditions (UNICEF, 2014).

Diarrhoea
A condition causing the loss of water and electrolytes in a person, leading to dehydration and
sometimes death. With four billion cases occurring annually and 1.8 million deaths (1.6 million
being children under five years old), it is the main health problem associated with poor sanitation
practices (UNICEF, 2014).
Schistomsomiasis
A disease caused by parasitic worms that penetrate the skin of people who come into contact with
contaminated water. It affects 200 million people every year, with 20 million suffering serious
consequences and approximately 200,000 dying annually (Fenwick, 2012; UNICEF, 2014). It is
estimated that adequate sanitation could reduce infection rates by 77% (UNICEF, 2014).
Typhoid
A bacterial infection that can result in headaches and nausea. It affects 12 million people
annually, and is contracted by consuming contaminated food or water (UNICEF, 2014).
Trachoma
An infectious bacterial disease which causes a roughening of the inner surface of the eyelid leading
to pain and possible blindness. Approximately six million people are currently blind because of
this disease. It is estimated that adequate sanitation could reduce infection rates by up to 25%
(UNICEF, 2014).
2.1.3 Definitions and Practices
The WHO classifies sanitation facilities in two broad terms; improved sanitation and unimproved
sanitation facilities. Improved sanitation facilities “...hygienically separate human excreta from human
contact.” (WHO and UNICEF, 2014a). Common toilets that meet this criteria include the western
style flush toilet, flush and pour systems into a pit-latrine (shown in Figure 2.2), and septic tanks.

Figure 2.2: Pit latrine with squatting slab (Furniss, 2011)
Unimproved sanitation facilities and practices typically do not separate human excreta from human
contact. Latrines without a squatting slab, platform or seat, and hanging toilets that dispose of waste
directly into a river or similar body of water are typical examples of unimproved facilities. They are
pictured in Figures 2.3 and 2.4, respectivley. Shared sanitation facilities are also classiﬁed as unim-
proved sanitation. Shared sanitation facilites are “...sanitation facilities of an otherwise acceptable
type that are shared between two or more households.” (WHO and UNICEF, 2014b)
Figure 2.3: A Chinese shared pit latrine without a platform, slab or seat (Rivard, 2005)

Figure 2.4: Hanging toilet in Port Haitien, Haiti (Stauffer, 2014)
2.2 Solid Fuels and Household Air Pollution
Burning solid fuels, or ‘biomass’, for cooking, heating, and lighting creates significant household air
pollution, and is a serious global issue. Approximately three billion people worldwide rely on burning
solid fuels such as wood, charcoal, dung, and crop residues for their cooking, heating, and lighting
requirements (WHO, 2014). The majority of these people live in developing countries in Africa and
Asia (Rehfuess et al., 2011) where access to improved fuels is restricted by economic, and social
factors. Burning solid fuels releases harmful emissions such as carbon monoxide (CO), carbon dioxide
(CO2), oxides of nitrogen (NOX), and particulate matter into the surrounding atmosphere. When this
is performed indoors it can cause significant household air pollution, especially in poorly ventilated
buildings; this causes serious health effects that result in the deaths of 4.3 million people annually
(WHO, 2014). There are also negative environmental and social effects associated with biomass
burning. These include contributions to global greenhouse gas emissions, and gender inequalities.
The eight United Nations Millennium Development Goals (discussed further in Section 2.3.1) are
goals that when achieved, will significantly improve the lives of the worlds most vulnerable people.
Reducing household air pollution produced from solid fuels will make a direct contribution to achieving
MDGs 1,3,4,5 and 7 (WHO, 2014).
2.2.1 Locations of Solid Fuel Usage
The global distribution of solid fuel use is shown in Figure 2.5. It can be seen that the problem is
concentrated in developing countries in Africa and Asia. Over 86% of the population in most African
countries, and especially those in the Sub-Saharan Africa region, use solid fuels (GACC, 2014). In
most parts of Asia the average rate is lower at approximately 52-66% (GACC, 2014), but still highly

significant. The death rate in each country logically follows the proportion of solid fuel use of the
population. In Sub-Saharan Africa, between 400-600 people per million die due to solid fuel usage
while in Asia, this is between 200-300 people per million (Ezzati et al., 2005).
Figure 2.5: Indication of household solid fuel use globally (Chartsbin (2007) using data from WHO
(2007))
2.2.2 Health Implications
There are a large number of health issues that arise from smoke inhalation and household air pollu-
tion. Some of the most common issues are pneumonia, chronic obstructive pulmonary disease, and
lung cancer, which represent 12%, 22% and 6% of the total 4.3 million annual deaths associated with
household air pollution, respectively (WHO, 2014). Acute lower respiratory infections including pneu-
monia are especially vicious, having the greatest effect on young children. Over half of the pneumonia
related deaths worldwide in children under five years of age are caused by household air pollution
produced during the combustion of solid fuels (WHO, 2014).
Other issues that seriously affect quality of life, but are not necessarily fatal, include cataract contrac-
tion (which can result in blindness), asthma, and burns (WHO, 2014). While it is difficult to compare
these issues to the fatal conditions, the negative effect they have on the ability of people to function in
life, and of developing countries to improve their situation, cannot be underestimated. Overall, it can
be seen that the problems resulting from solid fuel use, which causes harmful household air pollution,
are some of the most serious global health issues today.

2.2.3 Environmental Implications
As well as the health related problems discussed in Section 2.2.2, there are a number of significant
environmental issues brought about by the burning of solid fuels. These include contributions to the
greenhouse effect, and deforestation.
The inefficient performance of most cook stoves in the developing world contributes to the greenhouse
effect. In these devices, most fuels undergo a significant degree of incomplete combustion resulting
in the emission of black carbon (soot) into the atmosphere. Soot is one of the largest contributors
to climate change, following CO2 and methane (CH4) (Bond and Sun, 2005). It is estimated that
household solid fuel burning accounts for 18% of these emissions globally (Bond and Sun, 2005).
2.2.4 Social Implications
Women and children are often given the task of gathering fuel for cooking, lighting, and heating
(Parikh, 2011; WHO, 2014). This activity can take a significant period of time, and limits the time
available for schooling, income generation, and other opportunities for economic development (WHO,
2014). The fact that these tasks are often limited to daylight hours only exacerbates the problem.
According to the World Health Organization (2014), women and children also face serious risk of
injury and violence while gathering fuel.
As women are often responsible for household cooking, they are more exposed to air pollution created
in cooking and heating practices (WHO, 2014). Along with women being disproportionately affected
by the use of solid fuels, more than 50% of worldwide deaths among children under five years old can
be directly attributed to household air pollution created by solid fuel use (WHO, 2014).
2.2.5 Modern Fuels and Clean Cookstoves
The main alternatives to biomass burning are modern fuels and clean cookstoves. The term ‘modern
fuels’ encompasses liquefied petroleum gas (LPG), kerosene, ethanol, biodiesel, and biogas. Modern
fuels are superior to solid fuels as they produce fewer harmful emissions (Rehfuess et al., 2011). This
largely eliminates most of the health, environmental, and social issues associated with solid fuel use.
Clean cookstoves are an intermediate measure that still burn biomass, but achieve similar advantages
as using modern fuels.
The main obstacles to modern fuel uptake are affordability, and availability (Foell et al., 2011). For
this reason uptake is significantly higher in wealthier urban areas, where the availability of fuels is
higher due to the centralised location. In rural areas the clean cookstove is often a more attractive
alternative than modern fuels, due to the lower costs and widespread biomass availability (Foell et al.,

2011). Modern fuel uptake is also affected by cultural preferences. In many cases, even when modern
fuels are readily available and affordable, existing practices will be maintained exclusively, or a mix of
the two options applied (Masera et al., 2000). The motivations behind this are varied, including the
preference for smoke as a mosquito repellent, and cultural practices such as using flat pans for cooking
traditional tortillas in Mexico (Masera et al., 2000). Biogas is one modern fuel that has a history of
widespread uptake in developing countries.
By 2007, 26.5 and four million domestic biogas generators (or ‘biodigesters’) were present in China
and India, respectively (Surendra et al., 2013). The Netherlands Development Organisation (SNV),
has also installed over 500,000 domestic biodigesters across Asia and Africa (Surendra et al., 2013).
Biodigester programs have been set up by governments in many developing countries to promote biogas
production (Buysmanc and Mol, 2013). In these cases, a local biodigester market was created through
initial financial and technical training. High construction costs have prevented these markets from
becoming entirely self-sustainable, and currently most people are still partly reliant on government
assistance to purchase a biodigester (Buysmanc and Mol, 2013). While this reliance on government
assistance is obviously a weakness in the programs, they have been highly successful in terms of the
quality and scale of biodigester dissemination (Buysmanc and Mol, 2013). Clearly, biogas is a modern
fuel that has a history of uptake in developing countries, and as such is considered an excellent potential
replacement for solid fuels.
2.3 The Connection
Based on the information presented in Figures 2.1 and 2.5, it is clear that the countries with the
highest population proportions using unimproved sanitation facilities also have high incidences of
solid fuel use. These countries are some of the most poverty stricken in the world (Socioeconomic
Data and Applications Center, 2005). Therefore, people living in these areas are likely subjected to a
combination of the serious health issues presented by poor sanitation practices, respiratory problems
created by household air pollution, and minimal means to improve their situation due to the poverty
distribution within their country.
As the problems outlined in Sections 2.1 and 2.2 are primarily concentrated in the same poverty
stricken areas, it makes sense to develop a single, cheap solution to both major issues. This way, a
single method can be used to minimise the impact of problems arising from both unsafe sanitation
practices, and solid fuel use. Defining one solution would also prove easier to implement and integrate
into the regions where it is most required. Having a single solution to both these issues will also make
significant inroads into progress towards the Millennium Development Goals.

2.3.1 Sanitation, Solid Fuels and the Millennium Development Goals
The eight United Nations Millennium Development Goals (MDGs) were created in 2000 to quanti-
tatively measure and target the progress of developing nations. Almost all of these goals relate in
some way to improving sanitation and modern fuel usage in the developing world. All the Millenium
Development Goals, with the exception of Goal 2 and Goal 8, are especially relevant.
The United Nations Millennium Development Goals (United Nations, 2014)
1. To eradicate extreme poverty and hunger
The use of modern fuels eliminates the need to collect traditional solid fuels which can often be
a highly time consuming process. Saving time allows the pursuit of income generating activities,
and education.
2. To achieve universal primary education
3. To promote gender equality and empower women
Solid fuel usage was shown to disproportionately affect women; reducing the use of solid fuels
will significantly act to address this inequality.
4. To reduce child mortality
Household air pollution from solid fuel usage disproportionately affects children to a significant
degree, as shown in Section 2.2.2. Modern fuels produce less household air pollution, and
therefore help to address this goal. In addition, improving sanitation practices will reduce the
incidences of children contracting diseases from unsuitable sanitation facilities.
5. To improve maternal health
The use of modern fuels will reduce the exposure of women to household air pollution. According
to WHO (2014), reducing household air pollution will help to achieve this MDG.
6. To combat HIV/AIDS, malaria and other diseases
Providing improved sanitation facilities will significantly reduce the devastating diseases associ-
ated with poor sanitation, while use of clean burning modern fuels will help reduce incidences
of health problems related to household air pollution.

7. To ensure environmental sustainability
There are a number of environmental issues associated with the use of solid fuels, explored
in Section 2.2.3. Reducing household air pollution will negate many of these environmental
problems. Providing improved sanitation facilities will also reduce incidences of open defecation,
making for cleaner water bodies.
8. To develop a global partnership for development
2.4 Project Aim
Based on the information presented in this chapter, it is clear that poor sanitation and solid fuel use
are two independent problems causing serious negative eﬀects for billions of people worldwide. Both
problems are typically concentrated in the same developing countries, and often aﬀect the same people.
It is clear that developing a single solution to both of these problems will have a positive impact on
billions of lives worldwide, and has the potential to prevent up to 4.3 million deaths each year. This
leads to the overall aim of the project:
To develop a single appropriate technology that may be implemented in developing communities in
order to alleviate the dangers associated with unsafe sanitation practices and the household burning
of solid fuels.

Chapter 3
Technical Background
3.1 Human Waste Management
It is necessary to consider various waste management techniques in order to develop an appropriate
technology that will help alleviate the dangers associated with unsafe sanitation practices. The term
‘waste management’ comprises practices relating to the treatment and subsequent recycling or disposal
of human waste.
3.1.1 Harmful Pathogens and Health Implications
A pathogen is a broad term for any infectious virus, bacteria, parasite or fungi that may cause disease
to the host organism. They are present in human and animal excreta, contaminated food, industrial
facilities, along with other sources (Wang et al., 2007). Pathogens from human excreta enter the
human body through a number of pathways including direct transmission from inadequate sanitation
facilities, contaminated water sources and contaminated crop ﬁelds (WHO and UNICEF, 2012).
Feachem et al. (1980) explains how there is a large range of bacterial pathogens that can grow and
reproduce in excreta under diﬀerent environmental conditions. Common bacteria include salmonellae,
shigella, vibrios, pathogenic E. coli, Yersinia and campylobacter (Feachem et al., 1980). Bacteria can
remain active for long periods. They become dormant in low temperatures but are likely to become
inactivated under high temperatures. Diarrhoea or gastroenteritis are common symptom of bacterial
infection.
Destruction of these pathogens is a key priority for waste management systems. Human exposure to
harmful pathogens at any stage during the waste management process could result in severe health
implications. Most pathogens in excreta can be minimised by employing one or more various treatment
methods.
13

CHAPTER 3. TECHNICAL BACKGROUND 14
3.1.2 Wastewater Treatment Methods
Wastewater management is a collection of processes that remove the contaminants from wastewater
and sewage. The objective of wastewater management is to convert potentially harmful sewage waste
into a safe product which can be returned to the environment.
3.1.2.1 Sedimentation
As described by Wang et al. (2007), sedimentation is a process involving the separation of dense
suspended particles in a mixture from a lower density fluid, and is often the first phase in a water
treatment process. In sedimentation tanks, solids accumulate at the bottom of the tank to form a
sludge. This process is usually followed by a secondary decantation procedure to separate the sludge
from the fluid.
3.1.2.2 Aerobic Treatment
Aerobic treatment is a process during which biodegradable matter is broken down in the presence of
oxygen, and is commonly referred to as aerobic digestion. Organic matter is oxidised and decomposed
by micro-organisms which feed on the organic material. The basic procedure consists of aerating the
waste in order to oxidise the solids, then allowing the sludge to begin sedimentation. Once settled,
water is decanted, and digested solids are removed or pumped back into the system. During the
oxidation process, organic mass is broken down into carbon dioxide (CO2) and water (H2O), nitrates,
sulphates and energy in the form of heat (Wang et al., 2007).
Odours are minimised during storage and sludge quantities are reduced by removing volatile solids
during aerobic digestion. Aerobic treatment processes are used by many wastewater treatment facilities
due to shorter retention times. One drawback of aerobic digestion is the external energy requirement.
Energy is required to pump recycled bacteria from the settled solids back into the system, along with
providing a continuous oxygen supply to the system (Wang et al., 2007).
3.1.2.3 Anaerobic Treatment
Anaerobic treatment utilises the anaerobic digestion process which breaks down biodegradable matter
in the absence of oxygen (Lettinga, 1995). The process is known to occur naturally in some soils
and lakes where oxygen is restricted, and can also be induced by enclosing organic matter within
a gas-tight vessel to eliminate the supply of oxygen. This gas-tight vessel is commonly referred to
as a ‘biodigester’. Under suitable conditions, the organic material is digested by naturally occurring
anaerobic bacteria which significantly reduces pathogen content of the material (Mata-Alvarez et al.,

2000). In addition to reducing pathogen content, anaerobic digestion produces a flammable gas by-
product, commonly known as biogas (Caruana and Olsen, 2012). The production of biogas offers
a unique advantage of anaerobic treatment over other treatment methods; biogas can be used for
cooking, heating and lighting, as well as electricity generation.
The main drawback of anaerobic digestion is the temperamental nature of the anaerobic bacteria.
They are highly sensitive to fluctuating environmental conditions, and if they are not retained within
the system, organic compounds will not be effectively broken down. This will result in ineffective
pathogen treatment and a low biogas yield (Smith et al., 2005).
3.1.2.4 Decomposition
Decomposition, or composting of organic materials is another method of treating potentially harmful
waste products whilst producing a useful by-product. Bacteria and organisms decompose organic
matter into compost. In regards to human waste composting, the end product has minimal odour,
levels of pathogens which are safe for human handling, and may be applied to gardens and crops as a
nutritional soil conditioner and fertiliser (Wang et al., 2007).
Composting is advantageous in locations with a lack of landfill availability for waste disposal, as the
composted product takes up much less space than the primary organic material. As the end product
is a nutritional fertiliser, it can also be used in local agriculture operations. As the composting system
is low cost and effective, it may be appropriate to implement subsequent to anaerobic digestion so
that any exploitable energy by-products are extracted first (Jenkins, 2005).
3.1.3 Toilets
Fundamentally, a toilet is a sanitation facility designed to separate human waste from human contact
by transporting excreta to a location where it is less exposed. Traditionally, wastes were removed from
the human interface using dry systems which collected excreta in a large container or trench. These
systems are still commonly used in rural regions and in a majority of the developing world (Jenkins,
2005). Modern toilets in developed countries are wet systems which use a flush mechanism to remove
the wastes from human exposure, and transport it to a treatment facility.
The standard flushing toilet is not regarded as self-sustainable from a waste management perspective.
In most cases, flushing toilets simply transport waste from the human body to a sewer or septic tank,
the contents of which are eventually transported to a wastewater management facility for further
treatment. Once the water is treated, often with antibacterial chemicals, it is released back into the
environment. The solid matter is occasionally recycled into fertiliser but often discarded in landfills. In

some cases the ﬂushing toilet is linked to a self-contained waste treatment unit or septic system which
allows for waste management on site (Jenkins, 2005). Self-contained waste management systems have
potential for environmental sustainability and also lower costs as the waste management processes can
be conducted at or near the toilet site and do not necessarily require as much infrastructure, water,
or treatment methods.
3.2 Single Appropriate Technology
As introduced in Section 2.4, the overall aim of the project is “To develop a single appropriate technol-
ogy that may be implemented in developing communities in order to alleviate the dangers associated
with unsafe sanitation practices and the household burning of solid fuels.”. Improving sanitation prac-
tices using a single technical solution requires the integration of a waste management method with a
toilet. This way, waste is separated from human contact at the source using the toilet, and is treated
by the integrated waste management system. Of the waste management systems considered in Sec-
tion 3.1, anaerobic digestion is the only method that will reduce dependence on solid fuels and the
subsequent prevalence of harmful household air pollution, via the production of clean burning biogas.
Designing a combined biodigester toilet thus establishes a self-contained waste management facility
which generates a clean burning modern fuel, and achieves the overall aim of the project.
3.3 Anaerobic Digestion
A biodigester here will be deﬁned as a vessel in which anaerobic digestion takes place. The literature
relevant to the design and operation of a biodigester can be split into two major sections; the anaerobic
digestion process and existing biodigester technology.
Anaerobic digestion is a complex microbial process involving 4 chemical stages:
1. Hydrolysis: The chemical reduction of complex organic molecules (feedstock) into simple monomers
such as amino acids, fatty acids and simple sugars (Wang et al., 2007).
2. Acidogenesis: The bacterial breakdown of the simple monomers into volatile fatty acids (Wang
et al., 2007).
3. Acetogenesis: The bacterial conversion of volatile fatty acids into acetic acids. Carbon dioxide
and hydrogen sulphide are also produced in this stage (Wang et al., 2007).
4. Methanogenesis: The bacterial conversion of acetates into methane and carbon dioxide, the
primary constituents of biogas (Wang et al., 2007). It is also during this stage that the waste
stabilisation occurs, reducing odours and pathogenic concentration (Lettinga, 1995).

Oxygen toxicity occurs when oxygen molecules form free radicals in a cellular environment. These free
radicals are highly reactive and hence toxic to all cells. Unlike aerobic bacteria, anaerobic bacteria do
not possess the enzymes required to defend themselves against these free radicals (Parkin and Owen,
1986). It is therefore necessary for oxygen to be excluded from all stages of anaerobic digestion for
the processes to be performed correctly.
3.3.1 Feedstock
Feedstock for anaerobic digestion is the primary organic material which is broken down by the anaer-
obic bacteria. A number of factors such as the temperature, hydraulic retention time, pH, carbon
nitrogen (C/N) ratio and volatile solids (VS) content of the feedstock affect the rate of anaerobic
digestion. Manure from livestock such as cattle and pigs is commonly used as a feedstock. Systems
operating with these feedstocks are referred to as wet digesters as they require additional water to be
added. Dry digestion systems that do not require water also exist; these use plant based feedstock
such a coffee husks, maize, vegetables and purpose grown crops (Favre et al., 2009).
3.3.2 Anaerobic Digestion Products
The constituents of biogas produced by anaerobic digestion are outlined in Table 3.1. It can be seen
that methane and carbon dioxide are the primary constituents, contributing to approximately 95% of
the mixture. It is this high concentration of flammable methane which makes biogas useful as a fuel
source.
Table 3.1: Chemical constituents of biogas (Favre et al., 2009)
Gas Component Concentration Range Mean Value
Methane (CH4) 45-75% 60%
Carbon Dioxide (CO2) 25-55% 35%
Water Vapour (H2O) 0-10% 3-10%
Nitrogen (N2) 0.01-5% 1%
Oxygen (O2) 0.01-2% 0.3%
Hydrogen (H2) 0-1% <1%
Ammonia (NH3) 0.01-2.5mg/m3 0.7%
Hydrogen Sulphide (H2O) 10-10000mg/m3 <500mg/m3
The solid digested waste, known as effluent, is another useful by-product. Anaerobic digestion removes
a significant amount of pathogens from the primary feedstock leaving a product rich in nutrients (Mata-

Alvarez et al., 2000; Wang et al., 2007). The use of the biodigester effluent as a plant fertiliser has
resulted in substantial improvements to basic farming practices in many communities (Junfeng et al.,
2005).
3.3.3 Technical Factors
The rate at which anaerobic digestion is performed is dependent on a number of technical factors.
It is these factors which therefore determine the rate of biogas production and the extent to which
pathogen content is reduced, making them important considerations for the design and operation of
a biodigester.
Volatile Solids:
Volatile solids (VS) are the organic compounds which are reduced by the anaerobic digestion process,
the VS content can be considered the ‘digestible’ proportion of the feedstock (Wang et al., 2007). VS
reduction is often used as a measure of the extent to which anaerobic digestion has occurred. At a
constant temperature and pH, the biogas potential of a feedstock is primarily a function of its VS
content. Table 3.2 provides the VS% and biogas production potential of different waste feedstocks. It
should noted that this biogas potential is significantly influenced by animal diet; hence, actual values
of biogas production can vary significantly (Amon et al., 2007).
Table 3.2: Biogas production for different animal feedstocks (Junfeng et al., 2005)
Feedstock VS%
Biogas Yield
(L/kg)
Daily Production
(kg/day)
Daily Biogas
Production (L/day)
Human 25 30 0.6 18
Cow 18 25 12 300
Chicken 20 100 0.1 10
Pig 20 25 2 50
As shown in Table 3.2, the average human will produce 18 L of biogas per day. It is estimated that
a single person in a developing nation requires between 150 to 300 L of biogas daily (Deublein and
Steinhauser, 2010). It is obvious that a population cannot be completely self-sustainable from the
energy provided by human waste, however it can make up a significant proportion of a populations
total energy demand.

Temperature:
For waste treatment purposes anaerobic digestion is typically performed in one of two temperature
ranges; mesophilic, between 30◦C and 38◦C, or thermophilic, between 49◦C and 57◦C. Each range
contains a different species of anaerobic bacteria that is responsible for the methanogenesis conver-
sion; mesophiles are present in the mesophilic range and thermofiles in the thermophilic range. Figure
3.1 shows that with decreasing temperature the time required to reach the maximum volatile solids
reduction is increased, indicating that lower temperatures result in a slower rate of anaerobic diges-
tion. Outside their respective temperature ranges, mesophile and thermophile activity reduces and
eventually ceases as the bacteria perish. It has been found that mesophiles are able to survive in
temperatures as low as 15◦C however the rate of digestion at these temperatures is negligible (Wang
et al., 2007).
Figure 3.1: Effect of solids retention time and temperature on volatile solids reduction in a laboratory
scale anaerobic digester (Wang et al., 2007)
Both mesophilic and thermophilic digestion extract roughly the same amount of biogas from feedstock,
however thermophilic reactions are faster due to a higher energy input (Vindis et al., 2009). Both
reaction types are also very sensitive to rapid temperature changes, suggesting a need for insulation
to dampen the effect of fluctuating temperatures (Chae et al., 2008).

Retention Time:
The retention time (RT) is the length of time the organic material remains within the system. The
required RT is directly related to the temperature inside the biodigester. Advanced multistage biodi-
gester designs achieve required retention times for maximum VS reduction as low as ﬁve days by using
the high temperature thermophilic process. Single stage mesophilic biodigesters such as those typically
used in the developing countries require a retention time between 30 and 60 days (Suryawanshi et al.,
2013).
pH:
pH aﬀects the methanogenesis stage of anaerobic decomposition, which is most productive between
pH 6.8 to 7.5 (Environmental Protection Agency, 2012). Activities below a pH of 6 and above a pH
of 8 will hinder and potentially cease the digestion process (Karki and Dixit, 1984). During the initial
set up of an anaerobic reaction, when the acetogenesis stage is approaching completion, the acetic
acid produced can create conditions as low as pH 5.5 (Wang et al., 2007). This initial acidic period
is balanced after methanogenesis is complete and ammonia is produced, increasing pH (Wang et al.,
2007).
C/N Ratio:
If the ratio of carbon to nitrogen (C/N) in the feedstock is too high (> 60), nitrogen will be consumed
rapidly during the acidogenesis and acetogenesis stages, and will not be available to react with the
remaining carbon as required in methanogenesis (Parkin and Owen, 1986). If the ratio is too low (<
2), excess nitrogen will lead to a high concentration of ammonia thus increasing the pH which can
then inhibit methanogenesis (Parkin and Owen, 1986). The ideal C/N for the production of biogas
is 25, though ratios between 5 and 40 are acceptable (Parkin and Owen, 1986). Table 3.3 shows that
C/N ratios of cow and pig manure are close to the optimal value of 25. Humans and chickens have
lower C/N ratios that are still within the acceptable range.
Table 3.3: C/N ratio of some organic materials (Karki and Dixit, 1984)
Feedstock C/N Ratio
Human 8
Cow 25
Pig 18
Chicken 8

3.4 Biodigester Designs
An extensive range of biodigester designs currently exist, each for its own specific application. These
include large-scale processing plants for all types of biomass, medium-scale designs for farms or restau-
rants and small single-stage designs predominant in developing countries. The primary focus of this
review is the single-stage designs, as their simplicity and relatively low cost make them applicable in
developing regions of the world.
Small-scale designs vary in a number of different ways according to shape, size, complexity and ma-
terials. Nonetheless, it is possible to categorise most designs into one of three models; fixed dome,
floating drum or plug flow. Additionally, designs can be classified by their construction techniques;
prefabricated or permanent structure. On-site permanent biodigesters have historically been the most
reliable and widely implemented, however recent improvements in prefabricated technologies are seeing
the emergence of these as a viable alternative.
3.4.1 Fixed Dome
The fixed dome biodigester (Figure 3.2) is the most simple and reliable of the three major designs. It
originated the 1950s and is now common throughout China and Africa (Amigun and Stafford, 2011).
It usually consists of a cylindrical structure for waste storage with a dome-shaped gas collection area
situated above. A displacement pit is included to collect digested slurry. The design relies on pressure
created by the collection of biogas to force the slurry out of the digester and into the displacement
pit.
Figure 3.2: Fixed dome biodigester (Weir, n.d.)
Fixed dome digesters have an expected lifespan of 20 years as there are no moving parts or corrosion
prone surfaces, leaving few potential sources of failure (SNV, 2007). Cement and brick are the most
common construction materials, used for their durability and suitable thermal properties. Fixed
dome digesters are often buried underground, providing additional insulation and reducing spatial
requirements.

Amigun and von Blottniz (2010) note that the average cost of a fixed dome digester constructed in
South Africa is 860 USD, which is significantly cheaper than 1420 USD required for a floating drum
digester in the same location. Similarly in India the price for a 3m3 fixed dome system was 450 USD
cheaper than a floating drum digester of the same size (Singh and Sooch, 2002).
Construction is difficult and labour intensive, usually taking three people at least two days and requir-
ing the supervision of a qualified technician (Rwanda Utilities Regulatory Agency, 2012). Gas leakage
is also an issue as it is difficult to create a completely gas-tight environment from cement and brick.
Also, as the rate of biogas production from anaerobic digestion is not constant, the fixed volume for
gas collection provides a variable pressure output, complicating combustion applications.
3.4.2 Floating Drum
Floating drum biodigesters (Figure 3.3) are common in India, where over 4 million models are currently
in operation (Kaniyamparambil, 2011). The design consists of an underground chamber, similar to
that of the fixed dome digester, with a metal drum above. This drum moves up and down in a guiding
jacket depending on the volume of biogas held in the system.
As the volume of the gas collection system is able to adapt to the variable gas production a relatively
constant gas pressure can be achieved from this system which is desirable from a combustion perspec-
tive. The volume of gas held within the system can also easily be determined by the height at which
the drum is raised.
A floating drum biodigester is more expensive compared to fixed dome and plug flow digesters, pre-
dominantly due to the cost of the large metal drum. Regular maintenance adds additional costs and
labour that are not required for fixed dome or plug flow digesters. Rust must be removed from the
drum as well as regular painting to prevent corrosion. Dried slurry must be regularly removed from
the metal drum surface to ensure the drum can move freely. Even when these maintenance procedures
are adhered to, the average lifespan of a floating drum digester in tropical regions approximately five
years (SNV, 2007).

Figure 3.3: Floating drum biodigester (Thai Biogas Energy Company, 2008)
3.4.3 Plug Flow
Plug flow biodigesters (Figure 3.4) are plastic membranes, typically polyethylene, with length to width
ratios of approximately five (Mart’i-Herrero and Cipriano, 2012). Manure is transferred lengthwise
along the digester with no mixing between different heights or widths. In this way the ejected effluent
is guaranteed to be the most digested waste.
Figure 3.4: Plastic tube plug flow biodigester. Adapted from Energypedia (2014)
The advantage of plug flow digesters is that they are portable and inexpensive. The plastic membrane
is usually placed in a trench during operation and can be easily emptied and transported if required.
Xuan et al. (1997) estimates the costs of a 4 m3 plug flow digester to be 50 USD in Vietnam, which
is on average six to seven times cheaper than other local fixed and floating drum alternatives.
Polyethylene is weak and can be punctured easily by a number of means including a stray animals
(Mart’i-Herrero and Cipriano, 2012). Additionally, as top half of a plug flow digester is located

above ground, it is poorly insulated and susceptible to temperature fluctuations. Kanwar and Guleri
(1994) analysed the performance of a fixed dome and plug flow type biodigestser of the same capacity,
concluding that the daily average biogas production of the plug flow digester was 33% less than the
fixed dome.
3.4.4 Prefabricated Technologies Versus Permanent Structures
Permanent brick and concrete biodigester structures have been the most commonly implemented
biodigester systems since the inception of the technology, however portable, prefabricated designs are
emerging to offer solutions to the lack of related with traditional permanent designs. The motivation
behind these prefabricated biodigesters is to produce “...technically reliable, highly adaptable, easily
transportable, and reasonably priced” products (Cheng et al., 2014).
Specific situations where traditional biodigester technologies are inappropriate:
• Locations with high ground water levels, such as coastal areas where constructing on-site con-
crete, stone or brick digesters is difficult.
• Remote areas, such as mountain regions, where providing and transporting conventional con-
struction materials is difficult.
• Sites with inadequate conventional construction materials and a specialized labour force.
• Residential areas that are rebuilt as a result of land reform measures, thus affecting the perma-
nent site locations of conventional digesters.
These issues prompted the Chinese National Development and Reform Commission to release a report
on biodigester designs which concluded that “...traditional brick and concrete-based digesters do not
meet the requirements for commercialization and large-scale implementation, whereas prefabricated
biogas digesters are promising technologies for dissemination” (El-Mashad and Zhang, 2010). Cur-
rent prefabricated designs can be divided into two categories; bag digesters and composite material
digesters. Bag digesters are predominantly variations of the typical polyethylene plug flow digesters.
These biodigesters are more suited to rural areas where there is less chance of damage and greater
spatial availability (Cheng et al., 2014).
A common type of composite material digester is the fiber-reinforced plastic model (FRP). This model
is based on the fixed dome digester but its lightweight construction makes it portable and durable
with a high rate of productivity (Jiang et al., 2010). Currently the costs of these designs is high,
however with market growth and economies of scale this is expected to decrease substantially (Cheng
et al., 2014). A dismountable FRP is shown in Figure 3.5.

Figure 3.5: Dismountable FRP biodigester model (Cheng et al., 2014)
Another example of composite material digester is the modified water tank design (Figure 3.6). These
designs use existing water tanks to reduce costs yet still provide the portability and reliability of FRP
designs (Jiang et al., 2010).
Figure 3.6: Biodigester created from existing water tanks in Cambodia (Engineers Without Borders,
2011)
The ARTI model (Figure 3.7), created by an NGO in Maharashtra, India, is a composite material
digester based on the traditional floating drum digester. It uses cut-down high-density polyethylene
tanks for the digester and drum. The design costs only approximately 200 USD, significantly cheaper
than current steel and brick models, though currently the design can only be constructed on a very
small scale (ARTI, 2014).

Figure 3.7: ARTI bioidigester: A prefabricated plastic product based on the existing floating drum
design (Zu, 2005)
After review of the different biodigester models, it is obvious that prefabricated systems offer more
opportunities for implementation in the developing world over permanent fixed dome and floating
drum designs. They do not require large areas of land or holes to be created, and can simply be
transported to a particular location, and relocated when required. These advantages also extend to a
biodigester toilet system; a portable design would enable widespread implementation without the need
for large land areas or specialist construction techniques. The system could be constructed offsite by
an NGO or similar party, and provided to a community with little cost on available space.
3.5 Existing Biodigester Toilets
Currently, several successful combined biodigester toilet systems exist, although they are almost ex-
clusively permanent structure designs which share many of the disadvantages of traditonal permanent
biodigester technology. One such design was developed during an Engineers Without Borders (EWB)
challenge. A group of undergraduate students from the University of Adelaide designed and built a
portable biodigester toilet system, however it proved to be unsuccessful. It is clear there exists a lack
of successful, inexpensive and portable designs that incorporate both a toilet and biodigester with the
goal of providing a solution to unsafe sanitation and producing usable biogas.
3.5.1 Engineers Without Boarders Challenge (2011)
In 2011, a group of undergraduate engineering students supported by EWB were presented with a
series of health, energy and environmental issues that faced the village of Devikulam in India. It was
essential that their proposed solution to these issues be cheap, simple, safe, and have a positive impact
on the lives of the people of Devikulam. The team decided that a biodigester toilet system would

be a suitable solution to a lack of clean cooking sources and health issues related to open defecation
(Ashley et al., 2011).
The group’s final design (Figure 3.8) utilised two large (5.5 m x 1.8 m x 1.5 m) concrete and brick
tanks, emptied biannually. A slab with toilet facilities was placed over the top of these tanks, providing
a convenient location for waste disposal. The design also included an animal waste trough, so farm
manure could be disposed of in the same system. A large flexible membrane was used to collect the
biogas produced and the tanks were buried in the ground to provide insulation.
Figure 3.8: EWB Challenge biodigesting toilet (Ashley et al., 2011)
This design provides an excellent method of utilising both human and animal waste to produce biogas,
and addresses the issue of unsafe sanitation and its associated health problems by providing a toilet
facility. However, as the system is a permanent structure built from concrete and brick, it is still
restricted by the disadvantages of traditional biodigester technology, namely portability and space
requirements. While it addresses the needs of a single village it is not suitable as a global solution as
it cannot be easily manufactured and distributed throughout the developing world.
3.5.2 Adelaide University Honours Project 777 (2009)
In 2009, under the supervision of Dr. Steven Grainger and Dr. Colin Kestell, a group of Mechanical
Engineering students from The University of Adelaide designed and constructed a biodigester toilet
system. The overarching goal of the project was to effectively sanitise “human waste so that its effluent
is safe for reuse, producing a form of fuel that can be used to cook meals and aid in the daily lives of

users and must cost nothing to run.” (Coffee et al., 2009). The project team worked in conjunction
with two students from The University of Douala, Cameroon, and the system was designed to be
implemented specifically in this region.
The criteria guiding the design of the prototype were (Coffee et al., 2009);
• The system must fit in a highly populated environment
• The system must effectively treat human waste to reduce the spread of waterborne diseases
• There must not be any stagnant water that provides malaria carrying mosquitoes to breed
• A sustainable fuel that can be used for heating and cooking must be produced
• The system must not require electrical input for operation
• The system must be simple to operate
• The system must be inexpensive
The final design utilised two polyethylene water tanks, a series of plumbing and gas fittings and a
marine toilet. The two polyethylene water tanks served as the main anaerobic digestion chamber,
while the marine toilet utilised a manual pump to input waste into the system. The marine toilet
pump also allowed the toilet to be situated at ground level.
The biodigester used crushed bricks to increase the surface area available for anaerobic bacteria to
cultivate (Coffee et al., 2009; Stephenson, 1987). Two tanks were used to act as a baffle system,
increasing the retention time of the system (Coffee et al., 2009). The tanks could also be isolated, so
that the entire system was portable. To collect any biogas produced, both flexible membranes (Figure
3.9) and a floating drum style gasometer (Figure 3.10) were employed, both of which experienced
complications.

Figure 3.9: Prototype design with flexible membrane gas collection (Coffee et al., 2009)
Figure 3.10: Prototype design with gasometer gas collection (Coffee et al., 2009)
There were several issues with the design and testing procedure, which resulted in little biogas being
collected (Coffee et al., 2009):
• Biogas production was insignificant
• Conditions were too cold for anaerobic digestion (< 10◦C)
• Retention time was too short
• Hand pump for toilet was susceptible to clogging
• Flexible gas collection membrane continually leaked at seals and pipe connections

• Gasometer collection method created back pressure issues, driving biogas back into the system
Based on these issues the following improvements were suggested (Coffee et al., 2009):
• Insulating the system to control the internal temperature and promote anaerobic digestion
• Enlarging the system to increase retention time
• Use a solar water purification system for post-processing of effluent
• Investigate pre-processing of waste to prevent the pumps susceptibility to blockage
• Use purpose built tanks to prevent gas leaks
Honours Project 777 provided an excellent basis for the development of new biodigester toilet designs
by creating a portable and inexpensive design. However, significant improvements are required to
overcome the complications with the effectiveness of the design.
With consideration to the overall project aim, the best single solution to poor sanitation practices and
household air pollution is a combined biodigester toilet system. It provides a method of separating
excreta from human contact at the source, and produces clean burning biogas which can replace
solid fuels. The anaerobic treatment process significantly reduces the pathogens present within the
waste, and creates a safe product that can be used as a fertiliser. Different biodigester designs were
evaluated in Section 3.4, and it was concluded that prefabricated, portable biodigesters offer a number
of advantages over traditional, permanent structures. A review of existing biodigester toilet designs
revealed that although they do exist, many are not portable and require large, permanent structures.
The portable systems that do exist have not been effective, and significant improvements to their
design can be made. Based on the current state of technology, there is a clear need for the design and
construction of a portable biodigester toilet system.

Chapter 4
Scope, Objectives and Timeline
4.1 Scope
Through successful design, construction, and implementation, a biodigester utilising human waste as
a feedstock provides a stand-alone waste management facility that does not require a municipal waste
system. A biodigester toilet also provides a means of producing biogas for cooking, heating, and
lighting. Providing a waste management facility, and a means of producing biogas in one system will
improve the quality of life for millions of people in the developing world. Billions of people living in
wealthy countries enjoy the use of improved sanitation facilities and clean burning cooking gas, and a
biodigester toilet will enable citizens of developing nations to enjoy the same standard of living.
The scope of the project encompasses the design, build, and preliminary testing of a prototype biodi-
gester toilet system. Beyond these achievements, future work may involve developments to the design
or material selection for economic mass production, and implementation into developing communities.
This would be done in conjunction with the development of education programs detailing the correct
operation of a biodigester toilet, to maximise system effectiveness, and guarantee the safety of those
using the system.
4.2 Core Objectives
Objectives were formulated within the scope of the project and defined in a measurable manner for
assessment of project success. The objectives and measurements of success are listed below.
1. Design and build a portable toilet that meets the definition of a Shared Sanitation Facility,
as outlined by the WHO/UNICEF Joint Monitoring Program (JMP) for Water Supply and
Sanitation.
31

CHAPTER 4. SCOPE, OBJECTIVES AND TIMELINE 32
Measurement: If the system ensures hygienic separation of human excreta from human contact,
and is shared by the equivalent of two or more households (WHO and UNICEF, 2013), then this
goal will be achieved.
2. Include a functioning biodigester component in the design that is capable of harnessing the
human waste collected in order to produce biogas.
Measurement: The success of this goal will be based on the biogas production rate (litres of
biogas per kilogram of feedstock). A minimum numerical target of 10L per kg of feedstock was
set after reviewing literature on typical biogas production rates.
3. Integrate the toilet with the biodigester to create a portable biodigester toilet unit.
Measurement: The system will be designed to accommodate a toilet attached to the inlet
pipe. The system will also be disassembled, moved, and reassembled during the testing phase
to demonstrate portability.
4. Ensure the design is acceptable for implementation and use in Australia by adhering relevant
Australian standards.
Measurement: A detailed analysis of the design in regard to the relevant Australian standards
on sanitation and gas production and storage will be performed. Other relevant and insightful
standards will also be identified.
5. Demonstrate a viable use for the gas generated by the biodigester.
Measurement: The goal will be met if the application of biogas is successfully demonstrated
by the use of typical equipment such as a cook stove or lamp.
4.3 Extension Goals
1. Design and build a suitable cubicle to house the toilet.
Measurement: If a cubicle is designed and constructed to a suitable standard, determined by
qualitative analysis, then the goal will be met.
2. Research and design possible methods of effective post-treatment for both liquid and solid com-
ponents in order to ensure the effluent exiting the system poses no health or environmental
risks.
Measurement: Qualitative analysis of the effluent to detect pathogens and other harmful
components will be performed in order to ensure that it is of an acceptable quality.

CHAPTER 4. SCOPE, OBJECTIVES AND TIMELINE 33
4.4 Project Timeline
The timeline for the design and build of a biodigester was deﬁned using a Gantt chart (see Appendix
A). This Gantt chart listed all major milestones and their due dates, as well as a strict timeline to be
followed by the team in order to achieve these milestones. Review gates were listed in a table (Table
A.1 in Appendix A). These review gates were designed to ensure that the team stayed up to date
with all requirements. Major milestones and their completion dates were also listed. These are shown
in Table A.2 of Appendx A.

Chapter 5
Design Formation
The design formation phase of the project involved identifying the requirements of the system for it
to provide the maximum benefit to the end user while meeting the core project objectives outlined in
Section 4.2. A list of design criteria was produced to assess the effectiveness of a design at meeting these
objectives. Several conceptual designs were proposed and evaluated against these design criteria.
5.1 Standards and Recommendations
To ensure the design was safe, effective and robust, it had to comply with relevant Australian standards.
Designing the system to Australian standards also ensured that it could be ethically implemented in
communities lacking in strict safety guidelines. Recommendations from a consultation report produced
for the Australian Pork Association provided a Code of Practice (CoP) for on-farm biogas production,
and use on piggeries (Davidson et al., 2013). These were followed for the project to ensure that all
relevent standards were met, and there were no major safety issues with the design. The CoP was
specifically written for biogas installations implemented in large-scale piggeries, therefore most of the
recommendations were irrelevant. A description of the standards relevant to a small scale biodigester
toilet are presented in Table 5.1. Additionally, the recommendations provided by the Australian Pork
Association CoP that could apply to the design and build of a small scale biodigester are shown in
Table 5.2.
34

CHAPTER 5. DESIGN FORMATION 35
Table 5.1: Relevant Australian Standards (Davidson et al., 2013)
Standard Description
AS 2885 (2008)
Applies to steel pipelines, and associated piping and components that
are used to transmit single and multi-phase hydrocarbon fluids, such as
natural and manufactured gas, liquefied petroleum gas, natural
gasoline, crude oil, natural gas liquids, and liquid petroleum products.
AS 4041 (2006)
Sets out minimum requirements for the materials, design, fabrication,
testing, inspection, reports, and pre-commissioning of piping subject to
internal pressure or external pressure or both. Specific requirements
are given for piping constructed of carbon, carbon-manganese, low
alloy and high alloy steels, ductile and cast iron, copper, aluminium,
nickel, titanium, and alloys of these materials.
AS 4130 (2009)
Specifies requirements for polyethylene pipes for the conveyance of
fluids under pressure. Such fluids include, but are not restricted to:
water, wastewater, slurries, compressed air, and fuel gas. Fuel gas
includes natural gas, liquefied petroleum gas (LPG) in the vapour
phase, and LPG/air mixtures.
AS/NZS 3814
(2010)
Provides minimum requirements for the design, construction, and safe
operation of Type B appliances that use town gas, natural gas,
simulated natural gas, liquefied petroleum gas, tempered liquefied
petroleum gas, or any combination of these gases either together, or
with other fuels.
AS 1375 (1985)
Sets out the safety principles relating to the design, installation, and
operation of industrial appliances that involve the combustion of gas,
or oil, or other fuel in air suspension, or the generation of combustible
vapours in such appliances. It is clear that both open and enclosed
flares are industrial appliances that involve the combustion of gas, so
AS 1375 is applicable to both.
AS 5601.1 (2010)
This standard contains the mandatory requirements, and means of
compliance for the design, installation, and commissioning of gas
installations that are associated with the use or intended use of fuel
gases such as natural gas, LP Gas, biogas, or manufactures gas.

Table 5.2: Relevant recommendations for biogas installations relating to a small scale biodigester toilet
(Davidson et al., 2013)
Relevant Area Recommendation
Materials selection,
digester design
Low levels of hydrogen sulphide present in biogas can corrode
some materials. All plastics are suitable for contact with manure,
however Polyvinyl Chloride (PVC) piping must be UV resistant.
Copper, and steel (with the exception of stainless steel) should
never be used.
Safety
Digesters must be fitted with a hydraulic pressure relief, and
vent stack or equivalent component.
Safety
Waste storage structures must be tightly sealed to avoid
exposure to effluent.
Safety, pipeline design
A shutoff valve must be included in front of any component that
utilizes the biogas (eg. Generators) in a gas line. This valve must
shut automatically when the component ceases operation.
Environmental
protection
Biogas installation must seek maximum recovery of methane
within the feedstock to prevent uncontrolled release to the
atmosphere.
Safety Biogas appliances must have the Gas Safety Certification Mark.
Environmental
protection
Biogas installation must have an emergency flare system. This
will prevent venting of biogas into the atmosphere. The flare
must be capable of handling the entire volume of biogas
contained within the digester.
Materials selection,
pipeline design, gas
storage
All plastics apart from PVC and Polypropylene (PP) can be
used for biogas storage and conveyance. PVC can be used if it is
UV resistant. PP can be used if no fat is present in effluent.
Copper, brass, butyl rubber, and steel (with the exception of
stainless steel) should never be used.
Safety, pipeline design
Biogas pipelines should be operated at pressures less than 100
kilopascals (kPa) for transfer distanced of less than 4000m.
Safety
All piping components subject to pressure above atmospheric
pressure must have a pressure relief valve.
Pipeline design
Pipelines transferring biogas must have a constant minimum
slope of 2%, and must have provisions for condensate removal.

Safety
No open flames should be within six metres of plant, and
appropriate warning signs should be in place.
Safety, gas storage
Pressure free membrane bags fitted with condensate removal and
over-pressure release valves, located in the open, attached to the
ground and protected from damage with a suitable restraining
system are acceptable.
5.2 Overall Design Specification
The project objectives outlined in Section 4.2 determined the requirements necessary for the design to
meet. The toilet was to meet the WHO definition of a shared sanitation facility, as outlined in Section
2.1. Therefore, the system had to adequately separate faeces from human contact, and be designed for
shared use (WHO 2013). The system also had to incorporate a biodigester component that produced
biogas for use as an alternative to solid fuels, and be capable of safely storing this gas.
The biodigester and sanitation facility were required to be a single portable system that could be
easily transported, adressing the inherant problem with traditional fixed dome and floating drum
biodigesters which are typically permanent brick structures installed below ground level. A portable
system is particularly suitable for refugee camps (where construction materials are often in short supply
(Fenner et al., 2007)), building sites (which are typically only temporary sites), high-density urban
areas in developing countries without access to proper sanitation, and where large-scale infrastructure
redevelopment commonly occurs (Mara and Alabaster, 2008).
Post treatment of the feedstock was an important extension objective of the project. This encompassed
recycling any water remaining from digestion, and ensuring effluent exiting the system posed no health
or environmental risks. The anaerobic digestion process is typically sufficient to completely remove
most harmful pathogens from faecal matter (Masse et al., 2011). However, post treatment is necessary
to safely dispose of waste with a higher degree of certainty, especially in areas where disposal occurs
in waterways used for drinking sources.
5.3 Design Criteria
Design Criteria were employed to evaluate initial concept solutions and were fundamental in guiding
the design process for selection of the final design. The criteria were chosen and weighted to best
represent the needs and environmental conditions of the end user. To cover a wide range of possible
end users, it was assumed that the design would be implemented in developing communities. For

these regions, an appropriate technology approach was developed to accommodate a limited technical
understanding and resources available in these areas. There are a number of examples, such as in
Chinhoyi, Zimbabwe (Chinyama, 2013) and Dar es Salaam, Tanzania (Tumwine et al., 2002) where
large, traditional sewage systems were installed with help from external agencies. These facilities
could not be properly constructed and maintained by the local population, and were subsequently
abandoned. Thus, an appropriate technology approach was employed to avoid similar issues.
Focus on an appropriate humanitarian engineering approach to technology in developing countries has
gained prominence in recent years. Murphy et al. (2009) described it as technology that:
• Meets the essential basic needs of the end user
• Is sound and flexible
• Meets local capabilities through materials and resources
Following these ideologies ensured that end user’s dependence on external intervention would be
reduced, thus achieving a more sustainable solution. The following design criteria were chosen in
accordance with these principals:
Function
The ability of the biodigester toilet to effectively separate human waste from human contact
while producing meaningful amounts of biogas.
Cost
Cost must be minimised in order for it to be affordable for implementation in developing com-
munities.
Constructability
Materials required for the design should be sourced locally where possible. Construction must
also be possible with basic skills and without excessive labour.
Acceptability
The design must be easy and intuitive to use. It must also accommodate the existing cultural
practices of the end users.
Reliability
A reliable design enables the end user to be less dependent on external intervention for mainte-
nance, and more committed to ensuring long term use.
Portability
A portable biodigester toilet enables the system can be constructed off-site, or relocated if
required.

In order to quantify the extent to which a design met these criterion, a numerical score for each
criterion was given. An initial score of zero was assigned; if the design contained a feature giving a
major advantage or disadvantage to the criterion two points were added or subtracted respectively.
Similarly one point was added or subtracted for a minor advantage or disadvantage respectively. Each
criterion was considered equally weighted, and the individual criterion scores were added to produce
a final score for each design.
5.4 Essential Design Features
Before concept designs were generated, a number of design features were identified as being essential
to ensuring an effective design. These features were common to each concept design.
Growing Medium
Crushed bricks or similar materials increase the surface area on which anaerobic bacteria can
cultivate, creating a higher density of bacteria, in turn accelarating the anaerobic digestion
process (Stephenson, 1987). It is not essential that the growing medium be crushed bricks to
achieve this result, rather, any material that would sink to the bottom of the digester to create
a larger surface area would suffice. Crushed bricks were specified in this case as they are a low
cost material that is widely available.
Outlets
Outlets were to be included on the digester to release feedstock once it had undergone the diges-
tion process. These outlets were to be large enough to allow both solid and liquid components
to exit the system.
Inflatable Membrane
An inflatable membrane was to be used for gas collection. This is a simple and versatile collection
system that can easily be incorporated into any design. It is also portable, greatly simplifying
the process of extracting gas samples for analysis. Considering practical applications of the
gas, portability is desirable as the gas will likely be more useful at a location separate to the
biodigester toilet. A disadvantage of inflatable membranes is that they are easily damaged and
require additional protection to reduce the risk of leaks.
Biodigester Type
As discussed in Section 3.4.3, plug flow digesters are inferior to fixed dome and floating drum
models in terms of biogas production and thus were immediately eliminated from design consid-
eration. Poor insulating properties result in a requirement of external heat addition to maintain
conditions favourable for anaerobic digestion. The fragile nature of a polyethylene bag system

also contributed to this decision, as the design is intended for rural, developing communities,
where free roaming livestock is common, and the risk of puncture is likely. These factors indi-
cate that the plug flow biodigester model is not suitable at achieving the project aim as poor
biogas production will not reduce dependence on solid fuels. Thus, fixed dome or floating drum
digesters are the remaining suitable types to be considered.
5.5 Conceptual Designs
After a set of design criteria and essential design features were identified (Sections 5.3 and 5.4), four
concept designs were created. These concepts varied in regards to the location of waste input and
toilet, the number of digestion vessels, and the implementation of either fixed or floating drum sub-
systems.
5.5.1 Concept Design 1
Figure 5.1: Concept Design 1
The first concept design (Figure 5.1) utilises a single tank to digest the waste in a fixed dome. When
compared to a dual tank system, a single tank reduces both cost and construction complexity as fewer
parts and less space is required. It features an inlet pipe that starts at the top of the tank and continues
to the base. This bottom feeding system allows the new waste to flow directly to the anaerobes on

the growing medium at the base of the tank, while the older semi-digested waste is pushed upwards.
Bringing the fresh waste in contact with the bacteria present on the growing medium allows for more
eﬀective gas production (Stephenson, 1987). Additionally the bottom feeding pipe, if always below
the liquid level, will prevent gas ﬂowing back up the inlet. The additional length of pipe increases the
risk of blockages.
The toilet is located at the base of the digestion tank, which is desirable in terms of accessibility,
but will require a pumping mechanism to transport the feedstock to the inlet pipe at the top of the
digestion tank. This complicates construction, maintenance, and adds to costs. The evaluation of this
design in regards to the design criteria is shown in Table 5.3.

Table 5.3: Concept Design 1 design criteria analysis
Criteria Concept Design 1
Function (-1)
• Single tank does not allow settling of solid and liquid components for
water recycling (-2).
• Biogas output from a fixed dome is of variable pressure, causing difficul-
ties with combustion (-2).
• Bottom feeding inlet pipe reduces gas back-flow issues (+2).
• Bottom feeding inlet accelerates gas production (+1).
Cost (0)
• Single tank reduces number of parts required and costs (+1).
• Requires pumping mechanism (-1).
Constructability
(+1)
• Single tank requires fewer parts reducing construction time (+2).
• Pump complicates construction (-1).
Acceptability (0)
• Pump requires maintenance and power (-2).
• Single tank requires less space (+1).
• Toilet at ground level is desirable (+1).
Reliability (+1)
• Fewer tank connections reduce potential leaks (+1).
Portability (-2)
• Single large tank more difficult to transport than two smaller tanks of
the same combined size (-2).
Total (-1)

The second concept design (Figure 5.2) is a two tank system. The use of two tanks increases the
portability of the design, as it is easier to transport two small tanks as opposed to a single large tank
of the same volume. The second tank also allows for the settling of solid and liquid components of
the feedstock to subsequently be treated by a water filtration system. The filtered water can then be
recycled for use in flushing the toilet or safely released into the environment.
The toilet is located at ground level with the inlet pipe entering at the base of the digestion tank. This
method has the advantages of directly feeding the waste into the anaerobes on the growing medium,
and preventing gas flowing back out of the toilet. It also minimises the increased risk of blockages that
existed in Design Concept 1 by reducing the length of inlet pipe. As the feedstock inside the digestion
tank will be above the water level of the toilet, feedstock backflow issues will need to be overcome.
The process involved with this second conceptual design is as follows:
1. The inlet pipe takes the feedstock to the bottom of the first tank.
2. Gas collects at the top of this tank and flows into the collection system.
3. After half the total retention time of the system has passed, the valve connecting the two systems
is opened, and the effluent is allowed to flow into the second tank.
4. The waste is stored in the second tank for the same duration that it is stored in the first,
completing the total retention time of the system. The second tank is also used as a settling
tank for the liquid to then be collected in the filtration system.
5. While the original effluent is being treated in the second tank, the first tank is refilled through

daily use. The first tank will fill once the second tank digestion has completed the designed
retention time.
6. Once the retention time is reached, the sedimentation tank is emptied. The connecting valve
between the two tanks is then opened to allow waste to flow into the now empty second tank
and the cycle continues.
Table 5.4: Concept Design 2 design criteria analysis
Criteria Concept Design 2
Function (+1)
• Second tank allows for implementation of settling and filtration system
(+2).
• Biogas output from a fixed dome is of variable pressure, causing difficul-
ties with combustion (-1).
• Filtration system allows water to be recycled (+2).
• Inlet pipe may cause backflow issues (-2).
Cost (-3)
• Two tanks require additional fittings (-1).
• Filtration system is expensive (-2).
Constructability
(-2)
• Filtration system is diffucult to construct (-1).
• Additional connections for dual tanks increase construction time (-1).
Acceptability
(+1)
• Filter requires frequent maintenance (-2).
• Reduced water input (+2).
• Toilet at ground level is desirable for user (+1).
Reliability (-1)
• Extra connections increases likelihood of gas and liquid leaks (-1).
Portability (+2)
• Two small tanks more portable than one large tank (+2).
Total (-2)

The third conceptual design is a dual tank system, however it differs from Concept Design 2 as the
second tank is a floating drum design used as a secondary digestion tank and gas storage vessel. The
major advantage of the floating drum tank over the fixed drum is constant gas pressure which is ben-
eficial for gas burning applications. The floating drum increases the complexity of both construction
and maintenance, as the drum is required to rise and fall freely with varying gas production. Gas
backflow issues can also arise with a floating drum collection system as identified in Section 3.5.2 due
to the weight of the floating drum, which has the potential to push the gas back into the first tank.
The secondary tank in this design still allows for the settling and potential recycling of the liquid
component of the feedstock.
In this design, the toilet and inlet are located at the top of the first digestion tank. This may introduce
some acceptability issues as users would prefer the toilet be at ground level, however it does provide
the major advantage of gravity feeding waste into the system which removes the need for any pumping
mechanism. A support framework would be required to support both the user and toilet on top of the
digestion tank, adding to costs. The process involved with this conceptual design is as follows:
1. Feedstock enters the first tank through the inlet tube.
2. Biogas is generated and collected in the first tank.
3. When the feedstock reaches a certain height in the tank, it is released into the second tank.
4. Gas is passed onto the second tank through a pipe connecting the tops of both the tanks.

The Design and Build of Biodigester Toilet

The Design and Build of Biodigester Toilet

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