Sustainability of Sewer Systems
for Source Separated Blackwater
Low pressure, vacuum and gravity systems for case Munga

Master’s thesis in Nordic Five Tech Master Programme Urban Water
VILLE TANSKANEN

Department of Architecture and Civil Engineering
CHALMERS UNIVERSITY OF TECHNOLOGY
Gothenburg, Sweden 2018





Master’s thesis 2018:
BOMX02-18-4

Sustainability of Sewer Systems
for Source Separated Blackwater

Low pressure, vacuum and gravity systems for case Munga

VILLE TANSKANEN

Department of Architecture and Civil Engineering
Division of Water Environment Technology
Chalmers University of Technology

Gothenburg, Sweden 2018



Sustainability of Sewer Systems for Source Separated Blackwater
Low pressure, vacuum and gravity systems for case Munga
VILLE TANSKANEN

© VILLE TANSKANEN, 2018.

Supervisors:
Annika Malm, RISE
Sebastien Rauch, Chalmers

Examiner:
Britt-Marie Wilen, Chalmers

Master’s Thesis 2018: BOMX02-18-4
Department of Architecture and Civil Engineering
Division of Water Environment Technology
Chalmers University of Technology
SE-412 96 Gothenburg
Telephone +46 31 772 1000

Cover: Flow chart visualisation of system alternatives

Typeset in LATEX

iv



Sustainability of Sewer Systems for Source Separated Blackwater
Low pressure, vacuum and gravity systems for case Munga
Ville Tanskanen
Department of Architecture and Civil Engineering
Chalmers University of Technology

Abstract
Source separation of sewage into different fractions has potential to mitigate aris-
ing problems from climate change, resource scarcity and urbanisation through con-
trol over wastewater volume produced and simplified treatment and nutrient reuse.
Merely around 30% of the daily domestic wastewater discharge is dirty toilet waste
(blackwater), containing 3/4 of the phosphorus discharge. However a lack of knowl-
edge on reliability and sustainability of blackwater sewer systems exists and the
rate of implementation is limited. This paper analyses and compares sustainability
of three blackwater sewer alternatives in a desk study for a case area described as
rural—urban transition zone to assist considerations for similar cases. Analysis by
selecting and scoring indicators inside predefined framework consisting of five as-
pects: economic, environmental, health and hygiene, socio-cultural, and technical
function criteria. Results aggregated with help of multi-criteria analysis suggest
following ranking in the order of sustainability: vacuum sewer, gravity sewer, low-
pressure sewer, and tank truck collection. Low dilution of wastewater from vacuum
system was recognised as dominant variable in all but socio-cultural aspect where
technology-user nexus presents a challenge for sustainability. Results provide an
established overview on considerations for similar developments, whilst the theory
section functions as objective reference and introduction to specified technologies.

Keywords: decentralisation, wastewater, sustainability, sewer systems, source sepa-
ration, blackwater, low-pressure, vacuum, gravity sewer

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Acknowledgements
It has been a joyous journey to study in the degree programme Urban Water un-
der the Nordic Five Tech alliance universities NTNU and Chalmers, and to have
a possibility to tap into the knowledge base and inspiration of professors in both
universities. My uttermost gratitude goes to co-student Yulang Guo, who’s been
there through all of it. I also want to thank all friends and family for endless en-
couragement with this and other challenges. And I’d like to acknowledge previous
research that this study to largest extent relies on, and the outstanding dedication
of researchers at Research Institutes of Sweden RISE in promoting and studying
innovative technologies such as source separation for wastewater. And I want to
express my gratitude for a chance to participate in working for such an inspiring or-
ganisation. I acknowledge great work done by Mälarenergi, NSVA, Lund University
and their personnel for contributions on the thesis, providing its backbone. Special
acknowledgement goes to organisation Maa- ja vesitekniikan tuki ry, for their much
valued support. Last I want to thank my supervisors Annika Malm and Sebastien
Rauch for their patience and guidance.

Ville Tanskanen, Gothenburg, April 2018

vii





Contents

1 Introduction 1
1.1 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Delimitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Theory 5
2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Wastewater systems and sustainability . . . . . . . . . . . . . . . . . 7
2.3 Source separation and sewage fractions . . . . . . . . . . . . . . . . . 10
2.4 Sustainability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4.1 Framework and criteria . . . . . . . . . . . . . . . . . . . . . . 13
2.4.2 Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.3 Weighting and scoring . . . . . . . . . . . . . . . . . . . . . . 16

3 Method 18
3.1 System alternatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1 Sewage tank system, Alternative 0 . . . . . . . . . . . . . . . 20
3.1.2 Gravity sewer system, Alternative 1 . . . . . . . . . . . . . . . 21
3.1.3 Low pressure sewer system, Alternative 2 . . . . . . . . . . . . 22
3.1.4 Vacuum sewer system, Alternative 3 . . . . . . . . . . . . . . 24

3.2 Economic criteria (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Environmental criteria (2) . . . . . . . . . . . . . . . . . . . . . . . . 28
3.4 Health and hygiene criteria (3) . . . . . . . . . . . . . . . . . . . . . . 30
3.5 Socio-cultural criteria (4) . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.6 Technical function criteria (5) . . . . . . . . . . . . . . . . . . . . . . 34
3.7 Multi-Criteria-Analysis and scenarios . . . . . . . . . . . . . . . . . . 37

4 Results and discussion 39
4.1 Economic criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2 Environmental criteria . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3 Health and hygiene criteria . . . . . . . . . . . . . . . . . . . . . . . . 42
4.4 Socio-cultural criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.5 Technical function criteria . . . . . . . . . . . . . . . . . . . . . . . . 45
4.6 Multi-Criteria-Analysis and scenarios . . . . . . . . . . . . . . . . . . 46
4.7 Method evaluation and recommendation . . . . . . . . . . . . . . . . 48

5 Conclusion 51

ix



Contents

Bibliography 52

A Munga gravity sewer system plan I

B Munga LPS system plan III

C Munga vacuum system plan VI

D Questionnaire VIII

E Equations XI

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Contents

Wastewater
Water influenced by human activities resulting in degenerated quality, origins in
industrial, commercial, agricultural domestic functions or surface runoff

Stormwater
Surface runoff, which can be conveyed combined with sewage or in a separate system

Sewage (also domestic wastewater or municipal wastewater)
Wastewater from housing or similar functions, produced by a community of people

Blackwater
Toilet fraction of sewage containing human waste (urine and feaces), flushing water
and toilet paper

Greywater Washing water fraction of sewage originating from sinks, showers, dish-
washers, laundry and showers

TSS Total Suspended Solids

ULFT Ultra Low Flush Toilet

DFT Dual Flush Toilet

NASS New Alternative Sanitation Systems

SRT Sustainability Reporting Tools

MCA Multi Criteria Analysis

LCA Life Cycle Analysis

MDG Millenium Development Goal

SDG Sustainable Development Goal

WC Conventional Flush Toilet (Water Closet)

mwc meters water column, derivative unit of pressure

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Contents

xii



1
Introduction

Decentralisation and centralisation of wastewater treatment are commonly emerg-
ing, opposing and persistent topics in wastewater treatment dialogue. As centralised
wastewater treatment has been the proven method to control substance and pollu-
tant levels in effluent, the paradigm is slowly shifting around to resource reuse and
meta-level focus on wastewater as a part of metabolism of a human population.
The idea of wastewater as a metabolic product allows a broader view and leads to
the concept of a circular economy where the metabolic product is circulated back
into substrate and energy for metabolism. This concept can be supported both on
decentralised and centralised level and is very much dependent on practical implica-
tions of the existing wastewater management system. Diving deeper into the idea of
wastewater reuse as resource there is another persistent topic that surfaces: Source
separation of wastewater. The idea of source separation of wastewater in a nutshell is
to separate human waste fractions from pathogenically uncontaminated washing wa-
ter fraction. The benefits of doing this are further on discussed in detail but consist
mostly of benefits of a more concentrated wastewater flow that is easily applicable
for nutrient reuse. Generally the reasons for renewed and strengthened focus on al-
ternative meta level concepts of wastewater management, and on source separation,
are connected to emerging and pressing world wide issues such as climate change,
resource scarcity and urbanisation. These reasons demand a re-evaluation of west-
ern culture of water use and an increased attention on sustainability of wastewater
management and pave the foundation for this thesis work.

Among many Grafton et al. (2015) and Davis et al. (2016) have recognised trends
and challenges for sustainability of wastewater management. These include analysis
of flows of energy, water and nutrients through the system; simul-analysing effects
on water-energy-food-climate nexus and development of tools and performance met-
rics for measuring sustainability of systems. "Many of the hottest emerging topics in
the area of sustainability relate to translating abstract or complex knowledge into
components that will help effect real-world change" (Davis et al., 2016). Sharma
et al. (2010), Böhm et al. (2011), Arnold et al. (2013), Larsen et al. (2013), Sapkota
et al. (2014), Naik and Stenstrom (2016), and Siegrist (2017) among others have
highlighted the possibilities of decentralised and hybrid wastewater management
systems as a way for countering the various challenges of sustainability. Among the
recognised challenges by the authors are lack of knowledge on reliability, sustain-
ability and resilience of these systems, due to lack of experience from implemented
systems. As one possible way of organising decentralised wastewater management,

1



1. Introduction

source separation and in specific the separtion of the toilet fraction (blackwater), has
been recognised as a viable method to improve the treatability of wastewater and
the quality of the end product by many, including Jönsson, Nordberg, and Vinnerås
(2013), Larsen et al. (2013), Kjerstadius et al. (2015), Londong, Wätzel, and Giese
(n.d.), McConville et al. (2017), and Kärrman et al. (2017).
This master’s thesis is a desk study on sustainability of source separating blackwa-
ter sewer systems. Studied sewer systems are used for conveying sewage originating
from household toilets (referred to as blackwater further on). Paying detailed at-
tention on blackwater is reasoned by its higher concentration of solids, nutrients,
and contaminants when compared to other household wastewater from washing and
showers (referred to as greywater further on), and thus more challenging consistency
for the sewer system. The method utilised in this sustainability study is indicator
based multi-criteria analysis according to sustainability assessment framework by
Malmqvist et al. (2006). The method is established by recognising relevant indica-
tors applicable for assessment of sewers through literature research and gathering
data for the recognised indicators from pilot-projects and literature sources. The
technologies studied are conventional gravity system, low-pressure system and vac-
uum system for blackwater transport. The following research question is considered
essential: What are the primary variables that contribute to sustainability of a
specific technology for a given case?

1.1 Aim

The aim of the thesis is to:
- Develop and define a method and selection of indicators for analyzing sustainabil-
ity of different blackwater sewer systems in peri-urban context based on previous
research and ongoing pilot projects
- Analyse sustainability of specified sewer alternatives by conducting a case study
on pilot project area Munga in Västerås, Sweden
- Determine factors essentially affecting the overall sustainability of a given technol-
ogy in similar cases

1.2 Objectives

• Define sewer system alternatives
• Define criteria and indicators to be used
• Weight the defined criteria and indicators
• Score the indicators and alternatives for case-study
• Compare sustainability of alternatives for case-study
• Discuss results and key variables influencing end-result of case-study

2



1. Introduction

• Conclude with key variables to be considered for overall sustainability

1.3 Delimitation

The study focuses on three different sewer technologies for blackwater as mentioned
forehand. Accompanying greywater systems are omitted from this study as the
greywater system is considered independent of the chosen blackwater sewer system.
The wastewater systems covered in this study are limited to systems applicable for
peri-urban areas in Sweden as the case study area is defined as such area. This
limitation is done to avoid vastly different environmental conditions, and drivers for
system implementation that apply for other types of areas, such as single housing
units or urban areas implementing source separation.
Lifetime for each sewer system is set to 50 years as an acceptable lifetime of sewer sys-
tem components without major rehabilitation. This is a major simplification causing
limitation as differing solution and material selections would in reality inflict a vary-
ing lifetime, that would further-on affect economic sustainability of specific system
in substantial way. In addition, all systems are considered as new construction not
taking into account any presently installed systems. However this assumption is
in reality invalid as closed tank collection presents accurately the current system
in the study area. Another major limitation of the study comes from areal limi-
tation as transport distance from treatment to reuse location in agriculture which
sets the boundary for the quantitative parts of the study. From reuse perspective
only qualitative properties caused directly by studied systems are considered. The
limitations and their effects are discussed further in chapter 4. Conceptual model
of flow fractions, and systems for source separated blackwater, considered in this
study are shown in figure 1.1.

3



1. Introduction

Figure 1.1: Conceptual model

4



2
Theory

The theory chapter presents the theoretical background and details and assumptions
made on each of the studied wastewater systems, and on sustainability analysis. The
first part of the theory chapter connects the thesis into wider context in the Swedish
effort of advancing circular economy for nutrients. The second part presents different
wastewater systems. The third section covers the different frameworks and criteria
as a background for analysing sustainability of wastewater management projects.

The wastewater collection system typically comprises of drainage systems for both
wastewater and stormwater and these can be combined or separate within the sys-
tem. Requirements for a functional urban water system by include: providing clean
water for all required uses, removing wastewater and promoting hygiene, and remov-
ing stormwater to prevent flooding. The dimensions of the wastewater system thus
need to be adequate for transporting all water delivered to the buildings, and the
stormwater system for a design rainfall event (Butler, 2010; Hellström, Jeppsson,
and Kärrman, 2000).

On a general level wastewater collection systems are coarsely defined as conventional
or un-conventional, dependent on if the system is powered by gravity or needs a
steady input of auxiliary energy to function (Miszta-Kruk, 2016). Another distinc-
tion can be made between traditional or alternative sanitation system, dependant
on if all wastewater is collected combined or some fractions (e.g. toilet wastewater
and washing wastewater) are collected separately within their own parallel sewer
systems.

The categorization of sewer systems adapted from Miszta-Kruk (2016) is shown in
figure 2.1. Gravity sewer systems are considered conventional systems and divided
into two sub-categories either designed to collect storm water combined in one sys-
tem or separate in a parallel system from the industrial and domestic wastewater.
Pressure and vacuum sewer systems are considered as unconventional disposal sys-
tems. In the pressure sewer system exists two categories in which the systems are
divided by the prevailing pressure: Low pressure achieved via centrifugal pumps,
whilst high pressure is achieved with positive displacement pumps. Vacuum sewer
systems can be divided into pure vacuum systems and vacuum systems with siphon
and further categorized according to the location of the vacuum interface (i.e. toilet
or collection chamber).

5



2. Theory

Figure 2.1: Categorization of wastewater sewer systems

2.1 Background

The background of the thesis lies in Vinnova-project MACRO (Mat I Cirkulära
Robusta System), which concentrates in developing recycling of nutrient rich or-
ganic household waste to agriculture. The MACRO-project supports pilot studies
and selection of technology for food waste and wastewater streams in H+ –housing
area in Helsingborg and for separating black and grey wastewater in the peri-urban
area of Munga in Västerås municipality. As a part of this project there is a need
for sustainability analysis of different sewer solutions, with emphasis on technical
evaluation. RISE, Technical Research Institutes of Sweden, has initiated the thesis
project and it is conducted in close cooperation with Mälarenergi and NSVA to gain
practical information of the ongoing pilot projects.
Munga source separation project with low pressure sewer system in Västerås was
initiated by the local water utility Mälarenergi in cooperation with Urban Water
Management AB to provide a sustainable wastewater solution for a peri-urban area
with complex or varying geographical or topographical surface structure. The hous-
ing in the area consists of single-family houses used for permanent accommodation
as well as houses for seasonal recreational use. From the total amount of 279 prop-
erties in the area approximately 47% are used for permanent housing and 53% for
recreational use. The wastewater system development has advanced swiftly due to
the wastewater utility’s pro-active approach and was at moment of writing in con-

6



2. Theory

struction stage. The chosen sewer system for blackwater is a low pressure sewer
system with household specific pump stations complying with system alternative 1
presented in section 3.1.3. The chosen contractor for system construction is Skandi-
navisk Kommunalteknik AB (SKT). According to the contractor additional flushing
posts are added network to guarantee a quick relief in case problems with stagnation
and blockages occur. Majority of housing units rely on three chamber septic tank
treatment systems (installed on 250 properties). The latest regulation for wastew-
ater treatment systems in the area, valid since 2007, has been a requirement for
closed tank system (installed on 29 properties). Free construction consulting is
given to properties with newly renovated system but no compensation is paid for
tank residual value. (Wallsten, 2017)

2.2 Wastewater systems and sustainability

Wastewater in specific contains a mixture of inorganic and organic substances in
varying forms, ranging from coarse grit to fine suspended solids and from colloidal
to soluble matter. In general wastewater is 99.9 % water with merely a fraction of
substances problematic to water systems and potentially valuable as resources. The
concentration and volume of domestic wastewater varies with time and location but
tends to follow a diurnal pattern. Random point sampling and analysis techniques
are thus subject to error and caution is advised when using standard or typical values
for a specific community without control. Specific pollutant sources for domestic
wastewater include human excreta, toilet waste (toilet paper, condoms, sanitary
towels, tampons, disposable nappies, toilet tissue paper, paper towels, miscellaneous
paper and miscellaneous fat), food, and laundry. (Butler, 2010, p. 61-74)

For wastewater management and systems Butler and Parkinson (1997) suggests
three fundamental strategies: To reduce reliance on water as transport medium,
to avoid mixing of industrial waste with domestic wastewater and to avoid mix-
ing stormwater with wastewater, in order to reach sustainability. The first strategy,
closely linked to the topic of source separation, consists of reducing water utilised for
flush the toilet. Using water as transport medium causes multiple problems such as
excessive water consumption, pollution of clean water, dilution of waste and require-
ment for costly end-of-pipe treatment. By reducing water consumption, introducing
low flush toilets or vacuum systems the problem can be tackled. Potential draw-
back, from reducing the amount of carriage water, is however increased possibility
of sedimentation and scaling in sewers. (Butler, 2010, p.593-615) The second and
third mentioned strategies of separating industrial wastewater and stormwater from
domestic wastewater support nutrient recycling as it separates the flow of many
harmful contaminants and on the other hand eliminates an enormous volumetric
flow of nutrient poor wastewater (stormwater).

Life-cycle of sewers has been studied by Akhtar et al. (2015), Vahidi et al. (2016),
and Petit-Boix et al. (2016). The components of sewer life-cycle according to Akhtar
et al. (2015) is presented in figure 2.2. In their study sewer life-cycle was divided into
three parts: Manufacture, Construction and Operation & maintenance. The study

7



2. Theory

however concentrated purely on economic and environmental impacts of sewer pipes
neglecting the social considerations. Vahidi et al. (2016) however found that in-
stallation and material transportation only contribute a small effect of total impact
from pipe material. For sewer systems with pumping the usage phase was found
comparable to the manufacturing phase. The environmental impact of construction
of sewers was estimated to be minimal. Vahidi et al. (2016) instead highlighted that
life cycle environmental performance is only one of the factors that need to be con-
sidered when designing a sewer system. Other factors, such as seasonal temperature
changes, corrosion, safety requirements, cost, trench conditions (geological condi-
tions), and groundwater/soil chemistry were recognised as highly relevant. A major
environmental benefit of re-using excavated material in respect to replacing with
newly extracted materials was found by Petit-Boix et al. (2016). Global Warming
Potential (GWP) of construction phase in kg CO2 equivalents without reuse was
found to increase by 37%.

There is an enormous need for renewing the current wastewater systems, as ur-
ban water systems around the globe are facing an imbalance between improvement
need and actual implementation. This need consists of infrastructure rehabilitation,
upgraded wastewater treatment, and the integration of water management with eco-
logical requirements. (Malmqvist et al., 2006, p.1-6) According to Malm et al. (2013)
the total length of the Swedish wastewater network amounted to 100,900 km in 2013
of which 60,200 km were separated sewer pipes. The emphasis on rehabilitation debt
of current systems and the shear volume of the task presents another rational cause
for replacement with alternative sewer systems. As decentralised source separation
system can potentially provide shorter transport distances and cut down the length
of network in need of rehabilitation.

Figure 2.2: Life-cycle of sewers according to Akhtar et al. (2015)

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2. Theory

Böhm et al. (2011), Larsen et al. (2013), Naik and Stenstrom (2016), and Arnold
et al. (2013) have found benefits in decentralisation of wastewater treatment. Böhm
et al. (2011) recommends a semi-centralised approach treatment of wastewater as
better suited for fast growing urban areas than conventional centralised wastewa-
ter systems. According to Larsen et al. (2013, p. 471-472) there’s a possibility for
urban areas to improve ecosystem services of the surrounding watershed. Source
separation and decentralisation can steer the attention to a local level of solving
sustainability challenges promoting individual involvement in sustainability and cli-
mate change discourse. Feasibility analysis methodology of decentralised solutions
by Naik and Stenstrom (2016) found that in the case study decentralised system
was more economical and energy efficient than a centralised one. The main un-
der laying reason being the pump energy consumption in the studied decentralised
system which amounted to 50% of the aeration energy consumption for treatment
compared to centralised system. Arnold et al. (2013) found that increased area de-
mand, limited capacity, larger elevation variability, and lower discount rates favoured
decentralised wastewater treatment and reclamation for irrigation purposes. They
emphasise to note that the viability of wastewater reclamation is directly linked to
the end-use of treated wastewater, affecting treatment requirement, and complexity
of treatment process.

Reliability and failure of sewer networks for combined wastewater have been previ-
ously studied by Miszta-Kruk (2016) and Kowalski and Miszta-Kruk (2013). Miszta-
Kruk (2016) studied the reliability of pressure, vacuum and gravity sewer systems
based on data of 7 different large scale installations for a three to five year time pe-
riod. Kowalski and Miszta-Kruk (2013) studied the failure intensity of wastewater
networks in selected cities. Due to lack of data on dedicated blackwater systems,
the data from combined wastewater systems is used for assessment of operational
reliability and maintenance need in this study. Scaling in vacuum sewer systems
for source separated blackwater has been studied by Rohde (2016). Findings imply
that the factors influencing scaling in order of descending magnitude are: prior scal-
ing, and leakages, flow velocity, and surface roughness, ambient temperature, and
rinse water hardness. Sievers et al. (2016) has likewise studied vacuum system on a
case study on Flintenbreite, Lübeck (Germany) and characterised source separated
blackwater and greywater from the area.

Theoretical basis for a vacuum system is considered in detail as it is by far the most
uncommon of studied technologies. Vacuum conveyance works through constant
lower than atmospheric pressure (gauge pressure) in the collection network produced
with a vacuum pump. Vacuum sewer systems work within a pressure range of 1 ·
105Pa (1 bar) to 3 ·103Pa (0,03 bar), classified as low vacuum. Pressure range limits
applicability of vacuum sewer as maximum lift or geodetic head is with common
systems between 4 to 6 mwc. (Water Environment Federation, 2008; Islam, 2017)
For simplified system diagram see section 3.1.4 and figure 3.5

For vacuum transport correct construction and hydraulic consideration are extremely
important to guarantee a reliable system, Mäkinen (2016) specifies a saw tooth pro-
file necessary to guarantee the proper transport function. In detail a minimum slope
of 0.2% similar to gravity sewers, but always uphill is recommended by Water En-

9



2. Theory

vironment Federation (2008). Typical failures in vacuum systems include blockages
due to calcium-carbonate encrustations and foreign objects, observed in cruise boat
context according to practical experiences by Heikkonen et al. (2016). Suggested
remedies are frequently repeated acid dilution and manual mechanical cleaning pro-
cedure. Udert, Larsen, and Gujer (2003) suggests that dilution of urine with flushing
water up to a factor of 1:18 can efficiently reduce precipitation potential and there-
fore prevent blockages in pipelines. This method is however often counterproductive
to actual goal of source separation so its use should be carefully considered. In vac-
uum sewer systems with vacuum interface valves at collecting sumps, the interface
accounted for 80% of system failures and of these failures 92% were observed to
relate directly to operation of valve itself or its controller (Miszta-Kruk, 2016).

According to Remy and Jekel (2011) energy demand for vacuum collection of toilet
wastewater is 15 kWh/(pe ∗ a) added with transfer energy use. Use of low-flush
vacuum toilets in with vacuum collection reduces energy needed for flushing with
drinking water by 3.5 kWh/(pe ∗ a). However Remy (2010) suggests that a wide
range of energy demands has been observed for vacuum systems in pilot studies,
energy demands ranging from 3 to 51 kWh/(pe∗a). Available vacuum sewer system
manufacturers at moment of writing include: Evac, Jets, Bilfinger-Berger (Airvac
and Roevac) and Iseki (Redivac).

2.3 Source separation and sewage fractions

The aim of source separation is to separate the different fractions of sewage at
origin. This may be achieved through various technical solutions inside the housing
units such as urine diversion, separating the toilet fraction with separate piping,
or utilising dry toilets. The different fractions of sewage that can be separated are
often described with names implying a characteristic colour of the fraction. The
following definitions are given by Larsen et al. (2013, p. 471-472).

Yellowwater consists the urine excreted and the amount is roughly 1,270 grams ·
person-1 · day-1 with water content of 95%. The remaining 5% consists of dissolved
salts thus making it a saline solution. Urine contains most of the nitrogen excreted
by humans.
Brownwater is the toilet waste excluding the yellowwater fraction. The amount hu-
mans excrete is roughly 200 grams · person-1 · day-1 with a water content of 77%.
Amount of disposed toilet paper is also considerable and it forms 11% of the TSS
load.
Blackwater is a combined waste flow of yellowwater and brownwater that can also
be described as the toilet fraction. The proportion of blackwater flow is 27 % of
average daily domestic wastewater discharge of 148 L · person-1 · day-1 in 11 coun-
tries observed, amounting to 40 L · person-1 · day-1. According to Tidåker et al.
(2006) 74% of wastewater phosphorus originating from Swedish households is found
in blackwater.
Greywater is the remaining fraction of household wastewater and is fecally uncon-
taminated and thus contains minor amount of the total nutrients and suspended

10



2. Theory

solids making it often easily and cost efficiently treatable at close proximity of pro-
duction.

Based on research by Butler, Friedler, and Gatt (1995), Larsen et al. (2013) defines
the diurnal flow pattern for BW to follow general wastewater flow pattern with a
peak discharge of 0.06 L · person-1 ·min-1 happening at 08:00. However the weekly
variation for blackwater is described low when compared with overall variation of
wastewater flows. The appliance wastewater use is presented in table 2.1. This table
shows the data on flushing toilets as known as conventional flushing toilet (WC),
Low flush toilets (LFT), Ultra low flush toilet (ULFT) and Vacuum toilet (VT).

Table 2.1: Wastewater production by appliances

Data source Appliance Average volume (l/use)
Butler and Parkinson (1997) Conventional Flush Toilet (WC) 8.8

Bath 74
Shower 36
Wash basin 3.7
Kitchen sink 6.5
Washing machine 116

Butler and Parkinson (1997) Dual Flush Toilet (DFT) 5.0
Butler and Parkinson (1997) Ultra Low Flush Toilet (ULFT) 1.5
Oldenburg et al. (2008) Vacuum Toilet (VT) 0.7 - 1.2

Applied flush frequencies according to Eveborn et al. (2007), and flush volumes for
toilet types considered in the study are specified in table 2.2. Blackwater properties
used in this study according to Hertel et al. (2015), Tidåker et al. (2006), and Rohde
(2016) are specified in table 2.3.

Table 2.2: Wastewater flush volumes and frequencies

Flush type Flush amount [l] Frequency [1/d]
DFT large flush 6 3
DFT small flush 4 5.5
VT large flush 1 3
VT small flush 1 5.5

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2. Theory

Table 2.3: Blackwater properties

Property Data source Value [kg/ton]
Phosphorus concentration Tidåker et al. (2006) 0.150

Hertel et al. (2015) 0.162
Rohde (2016) 0.160

Average 0.157
Dry matter concentration Tidåker et al. (2006) 8.0

Hertel et al. (2015) 6.0
Rohde (2016) 8.3

Average 7.4

2.4 Sustainability analysis

Dissecting the terminology of sustainability and analysis is thought important as this
clarifies an otherwise vague concept, and supports transparency of chosen method.
Sustainability is commonly defined to comprise of three parts: environmental, socio-
cultural and economical sustainability. History of definition of sustainability starts
from the first recognised written version in the Brundtland-report (Our common
future), as development that full-fills the needs and aspirations of the present gen-
eration, without compromising the possibility of future generations to full-fill theirs
(Brundtland and Khalid, 1987). The definition was further refined to include as-
pects and criteria of environmental, economic and social sustainability by Stedman
and Hill (1992). They stressed the importance of the ecological system as the basic
life support system for all other systems, and highlighted economic sustainability,
being dependent on if ecological principles and boundaries are followed. Social sus-
tainability was also brought forward as a criteria, to ensure equitable distribution of
benefits, and give a possibility of participation in decision processes when personally
affected in a negative way by the proposed development.
More in depth definition of sustainability for water and sanitation has been given by
Grafton et al. (2015) who examine how water is managed in urban environments in
a sustainable manner, see figure 2.3. The definition includes sustainability of infras-
tructure submissive to biosphere’s and society’s sustainability, providing a platform
for economical sustainability. Sustainability can also be defined by Sustainable De-
velopment Goals (SDG’s) from 2015 by United Nations as targets to reach by 2030
as a follow up on the Millenium Development Goals (MDG’s). The relevant SDG’s
defining sustainability in relation to this study are:

• Goal number 6: Ensure availability and sustainable management of water and
sanitation for all.

• Goal number 9: Build resilient infrastructure, promote inclusive and sustain-
able industrialization and foster innovation.

• Goal number 11: Make cities and human settlements inclusive, safe, resilient
and sustainable.

12



2. Theory

Figure 2.3: Sustainability by Grafton et al. (2015)

• Goal number 11: Ensure sustainable consumption and production patterns.

The definition of analysis is to conduct a procedure where a substantial whole is
broken down into parts, and then followed by a synthesis where the separated ele-
ments are combined to form a coherent whole. To analyse and define sustainability
of a system or a development, a framework with criteria and indicators for each cri-
terium has be established. In order to recognise and develop optimal indicators for
source separated sewer systems it is necessary to review prior research on sustain-
ability analysis in the area of water and infrastructure systems. Multiple research
groups have developed, created and suggested frameworks and criteria that can po-
tentially be used as a foundation for analysing the sustainability of source separated
sewage systems. Research by Balkema et al. (2002), Malmqvist et al. (2006), and
Siew, Balatbat, and Carmichael (2016) among others are reviewed as theoretical
background.

2.4.1 Framework and criteria

Balkema et al. (2002) analysed four methodologies for sustainability assessment
of wastewater treatment. Sustainable technology is defined as technology that is
not threatening quantity or quality, including diversity, of environmental resources.
Flexible and subject to change with given time and space. According to authors
sustainability can be assessed in various ways through single indicators (e.g. exergy
analysis, economic analysis), environmental impact (e.g. LCA) or system analysis
with multiple indicators for assessment (e.g. MCA). Another framework considered
for the study is by Siew, Balatbat, and Carmichael (2016), who propose an alterna-
tive framework and method from the commercial assessment frameworks typically
used in infrastructure development.

This framework is described according to Y. J. Siew, C. A. Balatbat, and

13



2. Theory

Figure 2.4: Interaction of system aspects and criteria for sustainability assessment.
Adapted from Malmqvist et al., 2006

G. Carmichael (2013) to consist of six elements:
(1) systems-based criteria selection;
(2) quantitative measurement scales for criteria;
(3) characterizing each criterion by a measure of central tendency and dispersion;
(4) the distinction of additionality;
(5) criteria weighting; and
(6) combining criteria to give an overall score characterised by a measure of central
tendency and a measure of dispersion.

Sustainability criteria for selection and comparison of sanitation systems are listed
by Hellström, Jeppsson, and Kärrman (2000), Malmqvist et al. (2006), and Bracken
et al. (2006), as health, environment, economy, socio-culture and technical func-
tion. The framework for systems analysis of sustainable urban water management
by Hellström, Jeppsson, and Kärrman (2000) gives an approach on how to assess
sustainability of technical systems of infrastructure. This framework is built on the
requirements of Sustainable Urban Water Management - programme by Swedish
Foundation for Strategic Environmental Research (MISTRA) that gives following
supplementary requirements for water system. A water system should:
(a) have a high degree of functional robustness and flexibility,
(b) be adapted to local conditions and
(c) be easy to understand and thus encourage responsible behavior by the users.

As a reference to appreciate the task of system analysis, a list of eight practical
sustainability criteria/indicators and the method for evaluation as proposed by Hell-
ström, Jeppsson, and Kärrman (2000) are presented in table 2.4. Many of the meth-

14



2. Theory

Table 2.4: Priority set of criteria and method for evaluation according to Hellström,
Jeppsson, and Kärrman (2000)

Citerion Method for evaluation

Health and hygiene criterion
Risk for infection Microbial risk assessment

Social and cultural criterion
Acceptance Action research and assessment scales

Environmental criteria
Eutrophication Spreading of toxic com-
pounds to water, spreading of toxic
compounds to arable soil, use of nat-
ural resources

Life-cycle assessment, computer-based
modeling, material-flow analysis, and
exergy analysis

Economical criterion
Total cost Cost-benefit analysis

Functional and technical criterion
Robustness Functional risk analysis

ods for evaluation specified here can be described to require extensive research and
amount of data. Any of the mentioned, for example economic cost-benefit analyses,
are common and widely accepted subjects for whole theses.

2.4.2 Indicators

The sewer system as a whole can be seen to provide a service, which is the pri-
mary goal of any kind of system: to provide an optimal service in a sustainable
manner. Thus the reason for defining criteria, requirements and indicators is es-
sentially to measure the service provided by the sewer system. Optimal service can
further be judged by using specific indicators that have been defined by many for
various occasions and studies. For example, for water supply evaluation industry
standard performance indicators (PIs) have been defined by Alegre et al. (2016).
Lindholm and Nordeide (2000) researched the relevance of criteria for stormwater
system sustainability and suggested that while time taken to gather data correlates
with accuracy of results, the number of indicators should remain low in order to
optimise the usability of a method.
Different indicator frameworks for wastewater systems have been defined by Matos
et al. (2003b), Schilling (n.d.), and Lindholm and Nordeide (2000), however no in-
dustry standard exists. The framework of indicators by Matos et al. (2003b) is
divided into five categories: environmental indicators (1), operational indicators(2),
quality of service indicators (3), physical indicators (4), and economic and financial
indicators (5). Requirements for selecting representative performance indicators for
these categories as described by Matos et al. (2003a) include: clear definitions of

15



2. Theory

indicators, keeping number of indicators low, presentation of whole system, unbiased
and truthful, no overlapping, practicable, auditable, defined by time, defined spa-
tially. The authors highlight that using too few indicators risk misleading through
push-pull effect, often occurring when improvement in one area is linked to deteri-
oration in another. Another framework of indicators is defined by "European Fifth
Framework programme: Computer aided rehabilitation of sewers and storm water
networks (CARE-S)" It established a framework for prioritising sewer rehabilitation
based on performance assessment (Schilling, n.d.). Care-S performance criteria are:
environmental, operational, quality of service, economic and financial, and physical.

2.4.3 Weighting and scoring

Comparing specific criteria and indicators established for differing systems can be
done through scoring and applying specific weighting to emphasise the overall im-
portance of specific wanted properties. Index can thus be formed from an indicator.
Scoring indices and weighting has been researched by Lindholm, Greatorex, and
Paruch (2007) who studied sustainability indicators for alternative urban infras-
tructure systems. According to authors the need for practical methods to compare
sustainability of urban infrastructure alternatives is huge, but care has to be taken
as weighting used will lead to significant consequences for the result. Methods
proposed by Lindholm, Greatorex, and Paruch (2007) to normalize sustainability
indicator values include the following: Aggregating all indicators either to group
indices or into one system index. Use of a reference system to establish a zero level
in specific case, for example nature-based sewerage system as a reference system for
conventional wastewater system.

Three levels of aggregation by Lindholm, Greatorex, and Paruch (2007):

• Level 1: Unaggerated level, if no weighting is done and indicators handled
separately.

• Level 2: Grouping naturally connected indicators together: Ecological indica-
tors, economic indicators and social indicators in separate groups allowing a
group index to be calculated.

• Level 3: All indicators weighted into one index, from level 2 or directly from
level 1.

Example methodology for comparing system alternatives by weighting an scoring
comes from Gothenburg city (Sweden) where system study was used to analyse and
find a sustainable future wastewater and bio-waste management system (Göteborgs
Kretsloppskontoret, 2007). The applied method was a structured system study
according to the conceptual framework by Malmqvist et al. (2006). Scoring was
done by grading the different options on a scale of 0-4 on each indicator, where 4
was highest grade. Comparison of different criteria between alternatives was not
seen possible, and system alternatives were compared only inside the same criteria.
All aspects were included in a multi-criteria analysis (MCA) and weighted according
to their importance to the city of Gothenburg and the stakeholders. Weighting of

16



2. Theory

criteria was done by dividing a total sum of 100 between all criteria, and weighting
of indicators was done by dividing a total sum of 100 between all indicators inside
a criteria. (Göteborgs Kretsloppskontoret, 2007)

17



3
Method

Chosen method for the case study is a Multi Criteria Analysis (MCA) based on
framework from previous research by Malmqvist et al. (2006) and Kärrman et al.
(2012) as presented in section 2.4. The method is defined by Malmqvist et al.
(2006): The conceptual framework utilizes a structured system study by dividing a
studied system into subsystems of technology, organisation, and users which are then
analysed against set sustainability criteria defined as health, environment, economy,
socio-culture and technical function. See section 2.4.1 and figure 2.4 for description
and graphical presentation of the chosen framework.

Figure 3.1: Sustainability of system, criteria and indicators adapted from
Malmqvist et al. (2006)

This study concentrates on the subsystem of technology, but as interaction between
subsystems is continuous, none of the subsystems can be neglected to provide a holis-
tic assessment of sustainability. The approach chosen is to consider the impacts of
a specific technology in a user and organisational setting. Sustainability is analyzed

18



3. Method

against the criteria of economy, environment, health and hygiene, socio-culture and
technical function. To establish this, indicators for each criterion were chosen as
presented in figure 3.1. Selection of indicator(s) for a given criterion is based on
desk study and practical considerations of the case at hand. Theory considered as
background for this is described in section 2.4.2.Indicator data could be obtained
partly as directly applicable data from past projects but mostly refinement of data
was necessary in order to establish comparable variables between the considered
system alternatives (0-3). The refinement as part of data acquisition is considered
part of the method and described in detail in following sections.

3.1 System alternatives

In order to compare and analyse the sustainability of different sewer solutions for
blackwater for the case area the system alternatives are defined as:

• Alternative 0: (Reference) Closed tank collection system

• Alternative 1: Gravity sewer system

• Alternative 2: Low pressure sewer system

• Alternative 3: Vacuum sewer system

To define the alternatives many assumptions and decisions on system structure and
components need to be made. While many factors influencing sustainability are
defined by the specific environment where technology will be applied, there are like-
wise multiple factors that can be heavily influenced during design and construction
process. An optimal solution is however very case dependent as multiple variables
such as housing type, population density, choice of toilet type, topography, soil con-
ditions etc. influence the outcome greatly. To establish a comparison with limited
number of variables many of the fore mentioned factors were predefined in the fol-
lowing alternative descriptions. The factors not specifically predefined are assumed
to remain constant for all the alternatives. The plan for the LPS system (appendix
B, used as a basis for alternatives, is provided as a courtesy of Mälarenergi AB
(Holmgren, 2017-01-18).

Assumptions related to all alternatives:

• Munga residental area consists of 280 households

• 112 properties for housing

• 168 properties for recreational use

• Population per household is 2.6 persons

• Blackwater volumes based on table 2.2

• Blackwater properties according to table 2.3

19



3. Method

Household specific closed tank sewage collection system was chosen as a reference
technology, as it was thought to full-fill the bare minimum requirements of all criteria
for a sewer system. This technology has also been the chosen technology for studied
case area of Munga for past decade (Wallsten, 2017). The chosen reference system
was thus considered a feasible basic alternative when examining the characteristics
of the peri-urban case study area as it also presents a common sewer system in
majority of rural areas of Sweden (Jönsson, Nordberg, and Vinnerås, 2013).

3.1.1 Sewage tank system, Alternative 0

Figure 3.2: Schematic diagram of sewage tank system

The function of a closed sewage tank collection system relies on a gravity sewer
connected to a private sewage tank, forming a closed system. Regular emptying
of sewage tanks by a tanker truck and transportation to treatment facility is thus
necessary. Besides the sewage tanker truck service function, all other considered
system parts are private property.Figure 3.2 presents a schematic diagram of the
system parts.
Components of the system defined and considered in this study include:

• Regular flushing toilets
• Household sewers leading to sewage tanks
• Sewage tanks for closed collection system
• Private vacuum truck service for transportation

The following further specification and assumptions are made for the system com-
ponents of alternative 0:

• Toilets are conventional flushing toilets with flush volume 8.8 l/use

20



3. Method

• Household sewer length is estimated to average 22 meters, 1/3 of the plot
length,

• Sewage tanks are cylindrical in form and have a volume of 6.0 m3

• Sewage tanks are emptied by vacuum truck service

• Vacuum truck capacity is 6.0 m3 and average transport distance is 2 km

• Vacuum truck energy consumption is 8.2 MJ/ton* km transportation Sonesson
(1996)

3.1.2 Gravity sewer system, Alternative 1

Figure 3.3: Schematic diagram of gravity sewer system

A wastewater collection system is often based on a gravitational force and an ele-
vation gradient that transports wastewater from higher elevation to lower elevation
in partially filled conduits. As gravity provides a free driving force for operation
this is a common system that has been used for conventional wastewater systems
since the first implementation of sewer systems. Even as a gravity system is mostly
driven by force of gravity it is often necessary to pump wastewater part of the way as
providing wastewater treatment for each natural drainage area is uneconomic and
technically difficult to organise. (Butler, 2010). In most cases it is also required
to lift wastewater at the treatment facility to succeed with treatment processes so
wastewater pumping is inculded as a system component. Figure 3.3 presents a
schematic diagram of the system parts.

Components of the system defined and considered in this study include:

• Toilets inside properties

21



3. Method

• Household sewers leading to trunk network

• Trunk sewer network (interceptor)

• Pump stations with accessories

• Pressure transfer sewers to treatment facility

System plan presented in appendix A (Munga gravity system plan) functions as the
basis of system specification. The following further specification is made for the
system components of alternative 1:

• Toilets are conventional flushing toilets with flush volume 8.8 l/use

• Household sewer length is estimated to average 33 meters, 1/2 of the plot
length

• Trunk sewer network length is based on the LPS system design for case Munga
(appendix B) and is 10 800 meters

• Amount of pump stations needed for Munga is 9 pcs

• Pressure transfer pipeline length is 3230 meters

3.1.3 Low pressure sewer system, Alternative 2

Figure 3.4: Schematic diagram of low pressure system

In a pressurized sewage collection system wastewater is driven by pressure gradi-
ent generated by pumping. For low pressure sewer (LPS) system pumping stations
with single impeller centrifugal pumps can typically provide the needed pressure.
Achievable maximum head of approximately 120 meters defines a low pressure sys-
tem (Butler, 2010). Often a considerably lower pressure is sufficient to convey

22



3. Method

sewage and as pressure equals energy this is also desired. In Västerås, Munga LPS
system has been selected for blackwater and the implemented design is presented in
appendix B Munga LPS system plan (courtesy of MälarEnergi). Figure 3.4 presents
a schematic diagram of the system parts.

Components of the system defined and considered in this study include:

• Toilets inside properties

• Household sewers to pump stations

• Household pump stations with accessories

• Pressure sewer network with accessories (i.e. valves)

The amount of data available in English for source separated blackwater systems
operating with household specific pump stations (LPS). The practical considerations
for theory are thus considered from experiences from conventional collection systems.
According to Miszta-Kruk (2016) small household dedicated wastewater pumping
stations were found most vulnerable and susceptible to failure and responsible for
over 90% of all failure events in pressurized sewer systems studied, most of these
failures (67%) were due to control unit break down.

System plan presented in appendix B (Munga LPS system plan) functions as the
basis of system specification. The following further specification is made for the
system components of alternative 2:

• Toilets are conventional flushing toilets with flush volume 8.8 l/use

• Household sewer length is estimated to average 22 meters, 1/3 of the plot
length

• Pump sizing follows the design from the contractor SKT (see appendix B for
pump Q-H curve)

• Number of household pump stations in the design equals the number of house-
holds (280 pcs)

• Trunk network length is 10 800 meters

23



3. Method

3.1.4 Vacuum sewer system, Alternative 3

Figure 3.5: Schematic diagram of vacuum system

In vacuum collection system the medium is driven by pressure gradient generated
by a vacuum station. There exists multiple variations of a vacuum sewer system,
main difference being the location of vacuum interface. Vacuum collection system
can be organised with interface valves integrated into toilet units, or separated into
collection sumps, collecting all wastewater through gravity sewers much like in a
LPS system. System considered in this study is of the former type and has vacuum
interface at toilet level, expanding the vacuum network into housing units.

Figure 3.5 presents a schematic diagram of the system parts. The simplified function
of a vacuum sewer system is as follows: When interface valve opens, the lower
than atmospheric pressure (0.6 bar abs) in the pipeline drives the medium in a
slug like form towards the vacuum pump station at a high velocity of 5-6 m/s.
Through multiple interface valve openings and leakages in the system the slugs are
transported to a vacuum collection tank. From the vacuum collection tank sewage
is often transported onward to treatment with a low pressure system. (Butler, 2010,
p.363)

Components of the system defined and considered in this study include:

• Vacuum toilets inside properties with interface valves

• Household vacuum sewers to trunk network

• Vacuum sewer network with accessories (i.e. flushing valves)

• Combined vacuum and pump stations complete with pumps and accessories

24



3. Method

• Pressure transfer sewers to treatment facility
System plan presented in appendix C (Munga vacuum system plan) functions as the
basis of further system specification. The following further specification is made for
the system components of alternative 3:

• Toilets are vacuum toilets with flush volume 1.0 l/use
• Household sewer length is estimated to average 33 meters, 1/2 of the plot

length
• Trunk sewer network length is based on the LPS system design for case Munga

(appendix B) and is 10 800 meters
• Amount of combined vacuum and pump stations needed for Munga is 2 pcs
• Pressure transfer pipeline length is 1440 meters
• Vacuum system is assumed to require 15.0 kWh/(pe*a) for operation and

save 3.5 kWh/(pe*a) by reducing flushing water consumption Remy and Jekel
(2011)

25



3. Method

Table 3.1: Chosen indicators for each criterion and respective unit

Criterion Indicator Unit

(1) Economic (11) Investment cost €

(12) Operation cost €

(2) Environmental (21) Primary energy use for operation kWh

(22) Product quality as fertiliser qualitative

(23) Product ease of handling as fertiliser qualitative

(3) Health and hygiene (31) Risk for microbial infection qualitative

(4) Socio-culture (41) Education need for chosen system qualitative

(42) Maintenance need for households qualitative

(43) Risk of flooding qualitative

(5) Technical function (51) Expandability / adaptability qualitative

(52) Maintenance need for wastewater utility qualitative

(53) Risk of component failure qualitative

The chosen indicators presented in table 3.1, are partly quantitative but mostly qual-
itative by nature. These indicators were deemed to support sustainability assess-
ment of sewers in the chosen case study, as well as be applicable for similar projects
within geographical boundaries of Sweden. The selection process was based on avail-
able data, practicality, and fore presented theoretical background. Implications of
indicator selection are further discussed in section 4, Results and discussion.

The quantitative calculations are based on yearly blackwater production of the per-
son equivalent (PE) from the case study area of Munga, Västerås. See introduc-
tion of area in section 2.1, and equation 3.1 for adjusted person equivalent. The
148 households (53%) labelled as recreational properties are transformed to perma-
nent residencies with a usage percentage of 21,6% throughout the year (Kesämökit
2002. SVT. Tilastokeskus.) The 131 (47%) permanent residencies in the area and
recreational properties transformed to permanent residencies result all-together into
imaginary 163 households in the area. PE number for resulting households is formed
with multiplier of 2.6 persons per household (Statistics Sweden: Average persons liv-
ing in a single household dwelling, 2016).

PEtot = PErecreational + PEpermanent (3.1)

Where,
PEtot = Total adjusted person equivalent
PErecreational = Respective person equivalent according to usage percentage

26



3. Method

PEpermanent = Respective person equivalent according to usage percentage

Yearly blackwater flow QBW for PE of Munga was calculated dependant on toilet
flush volume for alternatives specified in section 2.3 with equation 3.2. As the
specified alternatives included different toilet types, QBW was calculated for both
DFTs and VTs resulting in QBW,DF T and QBW,V T . Data applied is specified in
section 2.2.

QBW = QT F · PE (3.2)

Where,
QT F = Toilet Flush flow [l/h · pe−1]
QBW = Blackwater flow [l/h]

3.2 Economic criteria (1)

Indicator 11, Capital expenditure in MSEK was calculated by forming an investment
cost by addition of system component values and construction costs. Values used for
construction costs and system components for each system is largely based on Urban
Water (n.d.), VeVa - tool for sustainability analysis of water and wastewater systems
in transforming areas. The cost data was adjusted to value year 2016 by using
construction cost index of Sweden (Statistics Sweden) (Byggkostnadsutvecklingen
index 1939=100. Korrigerad 2017-03-17 n.d.).

Cost data used for household components is based on a market overview by VA-
guiden (Marknadsöversikt 2016 n.d.). VA-guiden is an independent organisation
promoting water and wastewater education and information sharing. Cost compo-
nents for each alternative are specified in theory section 2.3. A total of 15% from
total costs was considered to present design and project costs for each system alter-
native. The results for indicator 11 were further valued according to grading and
scoring scale presented in figure 3.6 to establish a score from quantitative assessment.

Figure 3.6: Grading and scoring scale for indicator 11

27



3. Method

Indicator 12, Operation expenditure in kSEK was calculated through the energy
use calculation added with annual maintenance costs. Cost data used for operation
costs is based on Urban Water (n.d.), VeVa - tool for sustainability analysis of water
and wastewater systems in transforming areas. The cost data was adjusted to value
year 2016 by using the construction cost index of Sweden from Statistics Sweden
(Byggkostnadsutvecklingen index 1939=100. Korrigerad 2017-03-17 n.d.). As cost
data was seen rather theoretical lacking consideration to practicality of construction
an additional 25% construction marginal was added. Operation costs for system
alternatives with pumping (0,2) and vacuum transport (3) are based on indicator
21, primary energy use of operation and electricity cost of 0.437 SEK/kWh (Statistics
Sweden, 2013). No changes in electricity price over system lifetime was considered.
The results for indicator 12 were further valued according to grading and scoring
scale presented in figure 3.7 to establish a score from quantitative assessment.

Figure 3.7: Grading and scoring scale for indicator 12

3.3 Environmental criteria (2)

Indicator 21, Primary energy use for operation, was considered in order to grasp
which of the alternatives provide energy-wise the most sustainable solution. The
base data was applied to blackwater volumes in each alternative according to equa-
tion 3.2. For alternatives 0 and 3 previous research data by Sonesson (1996) (section
3.1.1) and Remy and Jekel (2011) (section 3.1.4) was used, providing direct energy
consumption. Equations used for calculating a rough estimation of operational en-
ergy use for alternatives 1 and 2 are specified in equation 3.3. Further equations
E.1, E.2, E.3, E.4, E.5 and E.6 are detailed in appendix E. Pump head of 50 mwc
was used for calculating pressure energy and a transport distance half of total net-
work length as an average. Relative friction factor (ε) 1 mm was used to account
for pipe friction losses developing over time within the sewer system and local head
losses due to pipe bends, valves, intakes, outlets etc. Relative pipe roughness was
defined as relative friction factor divided with pipe internal diameter ε

d
. Completely

turbulent flow regime was assumed with Reynolds number (RE)105 and dynamic
head-loss calculated according to D’Arcy equation using the Moody diagram (see
appendix E for diagram) to determine Darcy-Weisbach friction factor (f = 0.01).
Figure 3.8 shows grading and scoring scale for primary energy usage.

28



3. Method

Etot = Epot + Ekin + Epre + Eloss

Feff

(3.3)

Where,
Etot = Total Energy
Epot = Potential Energy
Ekin = Kinetic Energy
Epre = Pressure Energy
Eloss = Pressure Losses
Feff = Efficiency factors

Figure 3.8: Grading and scoring scale for indicator 21

Indicator 22, Product quality as fertiliser and indicator 23, ease of handling were con-
sidered as a qualitative indicators for environmental criteria. The most important
aspect for product quality was considered to be phosphorus and dry matter concen-
tration of blackwater as it affects the quantity and handling of fertiliser. Data was
taken as average from three literature sources handling source separated blackwater
in vacuum systems by Hertel et al. (2015), Tidåker et al. (2006), and Rohde (2016).
See theory section, table 2.3 for details and data used.
For both indicators 22 and 23, average values for phosphorus concentration (Pavg,V T )
and dry matter concentration (DMavg,V T ) were applied for system alternative 3
(vacuum) directly, and for alternatives 0-2 adjusted for same FU but larger produced
volume of blackwater according to equations 3.4 and 3.5.

Pconc = QBW,V T

QBW,DF T

· Pavg,V T (3.4)

Where,
Pconc = Phosphorus concentration in kg/ton−1

QBW,V T = Blackwater flow from vacuum toilets
QBW,DF T = Blackwater flow from dual flush toilet systems
Pavg,V T = Average phosphorus concentration from literature

DMconc = QBW,V T

QBW,DF T

·DMavg,V T (3.5)

29



3. Method

Where,
DMconc = Dry matter concentration in kg/ton−1

QBW,V T = Blackwater flow from vacuum toilets
QBW,DF T = Blackwater flow from dual flush toilet systems
DMavg,V T = Average dry matter concentration from literature
The results for indicators 22 and 23 were further defined according to grading and
scoring scale presented in figures 3.9 3.10 to establish a score from qualitative as-
sessment.

Figure 3.9: Grading and scoring scale for indicator 22

Figure 3.10: Grading and scoring scale for indicator 23

3.4 Health and hygiene criteria (3)

The health and hygiene criterion is included by applying a qualitative microbial risk
assessment (MRA) to analyse Indicator 31, Risk for microbial infection. Risk was
determined as a product of probability and consequence of occurrence for each con-
sidered exposure pathway. Mechanisms of exposure were considered for each system
alternative including as many mechanisms for users and maintenance personnel.
Point and mechanism of exposure for users:

• Toilet: Exposure during maintenance or use
• Surface water: Uncontrolled discharge from system
• Field crops: Unhygienic fertiliser

30



3. Method

Point and mechanism of exposure for maintenance personnel:
• Toilet: Exposure during maintenance
• Tank/pipe system: Exposure during maintenance
• Truck/pump/vacuum station: Exposure during maintenance

Risk products were calculated for each mechanism by multiplying score from risk
probability with risk consequence. Probability was scored with scale one to five: 1,
less than once; 2, one to two times; 3, three to four times; 4, five to six times; 5,
more than six times annually. Consequence was scored with scale one to five: 1,
mild symptoms; 2, short day lasting symptoms; 3, week long symptoms; 4, month
long symptoms; 5 permanent illness or death
Figure 3.11 presents the matrix used to form a risk product for the recognised risk
mechanisms according to probability and consequence. Total risk was calculated as
average from the sums of user and maintenance risks for each system alternative.
and scored according to figure 3.12.

Figure 3.11: Microbial risk assessment matrix

Figure 3.12: Grading and scoring scale for indicator 31

3.5 Socio-cultural criteria (4)

Selected qualitative indicators for criteria are
• (41) Education need for chosen system

31



3. Method

• (42) Maintenance need for households
• (43) Risk of flooding

Education need (indicator 41) was included as qualitative assessment of education
need, by applying multiple statements for each system alternative to form a compre-
hensive view on educational need that is bound to specific technology alternative.
The general principle is that scoring follows the familiarity of system alternative
as education need for current system applied as reference alternative gains maxi-
mal score due to lack of education need. Both user and operation perspective were
considered for the same statements.
Test statements applied for indicator 41, education need from user and operator
perspective:

• Need for guidance to implement and manage installation of system indepen-
dently

• Need for guidance to start using/operating system correctly
• Amount of new procedures involved with use/operation
• Complexity of system working principle and operation
• Effort required for implementing system

Results for indicator 41 were further defined according to grading and scoring scale
presented in figure 3.13 to establish a score from qualitative assessment.

Figure 3.13: Grading and scoring scale for indicator 41

Indicator 42, Maintenance need for households was included with aim to account
for failure events that demand user attention and initiative to be taken and likewise
events where entry inside household, and or property, can likely disturb users. Due
to lack of available data specific on source separated sewer systems, data from mixed
sewer systems was used as defined by Miszta-Kruk (2016).
Included failure points were:

• (0,1,2,3) household sewer
• (0) collection tank
• (1) connection chamber
• (3) vacuum interface

32



3. Method

Event numbers were calculated according to failures per length of sewer and amount
of units and valued according to figure 3.14.

Figure 3.14: Grading and scoring scale for indicator 42

Indicator 43, Risk of flooding was included as an indicator by applying a qualitative
risk assessment for flooding with different system alternatives. Risk was determined
as a product of probability and consequence of occurrence for each considered flood-
ing mechanism. Mechanisms of flooding were considered for each system alternative
in similar number. Risk products were calculated for each system alternative for the
following points and mechanisms.

Points and mechanisms of flooding:

• Household sewer: Blockage and flooding

• Sewer/collection tank: Blockage and flooding

• Pump/vacuum station/truck: Operational malfunction/pump blockage

Figure 3.15 presents the matrix used to form a risk product for the recognised
risk mechanisms according to probability and consequence. Risk products were
calculated for each mechanism by multiplying score from risk probability between
less than once annually (<1/a) and more than six annually (>6/a), with score from
risk consequence, between one to five in severity. Single risk score then added up to
form total product that was scored according to figure 3.16.

Risk consequences score and definitions were defined as:

1. single sewer surcharging

2. areal sewer surcharging

3. single household flooding

4. multiple households flooding

5. areal system wide flooding

33



3. Method

Figure 3.15: Flooding risk assessment matrix

Figure 3.16: Flooding risk assessment result scoring

3.6 Technical function criteria (5)

Selected indicators for criteria are

1. (51) Expandability / adaptability

2. (52) Maintenance need for wastewater utility

3. (53) Risk of component failure

Indicator 51, Expandability / adaptability was included to account for possible fu-
ture alterations to system structure be it expansion through population growth or
diminishing wastewater flow through degrowth of population. Technical considera-
tion was done by judging the system alternatives against following statements:

• Consequence of diminishing population

• Consequence of growing population

• Possibility to alter treatment location

• Subsystems capable to operate independently

• Technical ease of adding/removing a sewer connection

Results for indicator 51 were further defined according to grading and scoring scale
presented in figure 3.13 to establish a score from qualitative assessment.

34



3. Method

Figure 3.17: Grading and scoring scale for indicator 51

Indicator 52, Maintenance need for wastewater utility was included to account for
maintenance events in the sewer network which are initiated and handled by the
utility without interference from users. Much like for indicator 42, maintenance
need for households, data availability specific from source separated sewer systems
was poor, and data from mixed sewer systems was used as defined by Miszta-Kruk
(2016).

To separate these events from indicator 42 focus was put on events in facilities of
the system that are solely under utility’s control and operation such as in:

• sewer mains

• pump stations

• vacuum stations

Event numbers were calculated according to failures per length of sewer and amount
of units for each alternative and valued according to figure 3.18.

Figure 3.18: Grading and scoring scale for indicator 52

Indicator 53, Risk of component failure was included as an indicator by applying a
qualitative risk assessment for different system alternatives. Risk was determined as
a product of probability and consequence of occurrence for each considered failure
mechanism. Mechanisms were considered for each system alternative in similar
number. Risk products were calculated for each system alternative for the following
points and mechanisms.

Points and mechanisms of failure:

35



3. Method

• Toilet: Blockage

• Household sewer: Blockage

• Sewer main/collection tank: Leakage

• Pump/vacuum station/truck: Breakdown

Figure 3.15 presents the matrix used to form a risk product for the recognised
risk mechanisms according to probability and consequence. Risk products were
calculated for each mechanism by multiplying score from risk probability between
less than once annually (<1/a) and more than six annually (>6/a), with score from
risk consequence, between one to five in severity.

Risk consequences score and definitions were defined as:

1. single household affected

2. multiple households affected

3. prolonged daylong discrepancy

4. prolonged areal discrepancy

5. system failure for all users

Figure 3.19 presents the matrix used to assess the recognised risk events according
to probability and consequence, while single risk scores were then added up to form
total product that was scored according to figure 3.20 while

Figure 3.19: Component failure risk assessment matrix

Figure 3.20: Grading and scoring scale for indicator 53

36



3. Method

3.7 Multi-Criteria-Analysis and scenarios

Decided method applied for weighting of selected criteria and indicators was similar
to Göteborgs Kretsloppskontoret (2007) presented in theoretical background. For
this study included subsystems (criteria) according to theoretical framework were:
Economic, Environmental, Health and hygiene, Socio-cultural and Technical func-
tion. For each considered system alternative aggregation of indicator scores was done
to criteria level, by using weighting as well as total sustainability level, including all
subsystems.
Weights applied were acquired through a questionnaire delivered to expert group
involved in source separation project in Munga, Västerås (,for questionnaire see
appendix D). This weighting was applied to scores formed for each indicator as
described in detail in following sections for each subsystem. The scoring scale was
selected to range from one to five describing 1, very low; 2, low; 3, moderate; 4, high;
5, very high grade for respectful indicator. Weighting scale for MCA according to
questionnaire is presented in figure 3.21. The answer rate for the questionnaire was
3/8 (37,5%).

Figure 3.21: Weighting for MCA according to questionnaire

Scenario 1 (even-weight scenario) analysis was performed, as the answer rate for
weighting questionnaire was low and take size small. Scenario 1 was built with
even weighting for all considered subsystems and indicators within them to test
the sensitivity and effect of weighting. In this scenario all subsystems were given a
weight of 20% and within subsystem (criteria) the weight was distributed evenly.

37



3. Method

Scenario 2 (electric-truck) analysis was done to test applicability of the method for
future situation where advances in truck transport technology would allow fossil
fuels to be replaced by electricity. For scenario 2 original weighting was used to test
outcome of MCA by changing input values for only alternative 0 (tank collection)
for indicators 11 (Capex), 12 (Opex), and 21 (Primary energy consumption). Used
values for electric truck transport were applied from Sen, Ercan, and Tatari (2017)
and Zhao et al. (2016).

38



4
Results and discussion

The results and discussion chapter presents findings of the study for each studied
sustainability subsystem criteria under its own section. Result tables are expanded
with directly following discussion highlighting major finding with each indicator.
Structure is seen efficient in conveying to reader much needed information on back-
ground factors and possible uncertainties influencing result. Results are strictly
applicable only for case study area of Munga but discussion parts can be used as
a valuable reference when considering similar system implementations in other ar-
eas in Sweden. The overall comparison between system alternatives is presented in
section 4.6.

4.1 Economic criteria

Comparison of results in MSEK for alternatives with economic indicator 11, Capital
expenditure are shown in table 4.1 together with weighted scoring. Results in MSEK
during system lifetime and scoring for indicator 12, Operational expenditure are
presented in table 4.2. For indicator 11 in table 4.1 the benefit of reference alternative
0, closed tank collection system over other alternatives is seen as expected but major
finding inside economic criteria. As for alternative 1, gravity sewer system results
show very high investments costs that are expected. For indicator 12 in table 4.2
otherwise rather even result between alternative shows high operational costs for
alternative 0.

For capital expenditure indicator 11, results indicate anticipated and pronounced
difference between extremes of tank collection system and gravity sewer system
due difference with nonexistent and extensive network investment. Results imply

Table 4.1: Economic indicator 11, Capital expenditure results

Alternative Capex [MSEK] Score Weighted
score

Tank (0) 14.9 5 0.30
Gravity (1) 137.2 1 0.06
LPS (2) 78.7 3 0.18
Vacuum (3) 75.0 3 0.18

39



4. Results and discussion

Table 4.2: Economic 12, Operational expenditure results

Alternative Opex [MSEK] Score Weighted
score

Tank (0) 82.8 1 0.09
Gravity (1) 0.4 5 0.45
LPS (2) 0.7 5 0.45
Vacuum (3) 0.5 5 0.45

gravity sewer applicability for Munga suffers most from steep alternating terrain,
requiring multiple pump stations in addition to generally higher unit construction
cost, due to stricter requirements constant slope and construction quality as pointed
out by Water Environment Federation (2008) and Larsen et al. (2013) in section 2.3.
Results show similarity when comparing gravity system to vacuum as Islam (2017),
who suggests vacuum system to have 30% benefit over gravity, whilst study here
shows 50% benefit. A closer look and scrutiny on elements such as valve pricing in
vacuum system is neglected in the study and this is seen as a possible handicap and
beneficial subject for further study.

For operational expenditure indicator 12, results seem at first hand rather extreme
for tank collection system alternative. Method for obtaining cost may cause discrep-
ancy as tank collection cost was evaluated as cost per truck transport for calculated
volume as other cost data based on expenditure per person equivalent. Results are
however supported by sheer logic of and linkage to energy consumption of truck
transport with fossil energy source. While within network based alternatives 1, 2
and 3 the differences are clearly defined by energy use for transport of wastewater.
Recognised further research is related to cost from transport at boundary between
treatment and reuse in agriculture as highlighted by Kjerstadius (2017) among oth-
ers. Unsustainable transport distance and volume of hygienic BW are recognised
having potential to tilt economic comparison around and important further research
area connected to source separated sewers.

4.2 Environmental criteria

Environmental indicators considered for system alternatives together with weighted
scoring, are presented in following tables for 21, Primary energy use of operation in
MWh/FU; 22, Product quality as fertiliser; 23, Ease of handling as fertiliser. For
indicator 21, table 4.3 shows alternatives 0 and 2 yielded clearly higher yearly energy
use than other system alternatives. For indicator 22, table 4.4 product quality results
were logical and as predicted as they portray theoretical phosphorus concentration
for produced BW, thus alternatives with dilute BW scored very low. Likewise for
indicator 23, table 4.5 results indicate that dilute systems result in very low outcome
for ease of handling due to vast volume of dilute sewage produced.

40



4. Results and discussion

Table 4.3: Environmental indicator 21, Primary energy use of operation results

Alternative Primary energy use
(op) [MWh]

Score Weighted
score

Tank (0) 29.5 1 0.09
Gravity (1) 5.2 4 0.35
LPS (2) 14.4 3 0.26
Vacuum (3) 6.4 4 0.35

Table 4.4: Environmental indicator 22, Product quality as fertiliser results

Alternative Product quality Score Weighted
score

Tank (0) very low 1 0.11
Gravity (1) very low 1 0.11
LPS (2) very low 1 0.11
Vacuum (3) high 4 0.43

Table 4.5: Environmental indicator 23, Ease of handling as fertiliser results

Alternative Ease of handling Score Weighted
score

Tank (0) very low 1 0.09
Gravity (1) very low 1 0.09
LPS (2) very low 1 0.09
Vacuum (3) high 4 0.35

For indicator 21, major findings are substantially greater energy use for tank system,
as well as LPS alternative compared to other network based sewer systems. Finding
is supported by energy intensiveness of road transport and higher energy losses with
pumping in longer and smaller diameter pipelines resulting in lower efficiency in
comparison to systems with a centralised pumping approach and larger component
size. Indicators 21 and 12, opex can be seen to slightly overlap as results show similar
order between alternatives and indicator 12 includes energy pricing as a component.
Recognised further research for indicator 21 is related to transport at boundary
between treatment and reuse in agriculture as highlighted previously for economic
criteria, section 4.1. Unsustainable transport distance and volume of hygienic BW
are recognised having potential to overcome environmental benefits gained elsewhere
in the sewer chain.
Method behind indicators 22 and 23 and its connection to flush amounts defined in
section 2.2 can be considered problematic. Indicators show considerable overlapping
due to this and neglect possibilities with reducing flushing amounts from conven-
tional flush toilets. It is however shown that systems yielding high phosphorus

41



4. Results and discussion

concentration in blackwater yield also good environmental sustainability as volume
required to be transported and spread on fields remains lower. Field handling and
spreading of ready fertiliser was outside the scope of this study but considerable
difference between alternatives exists due to five times better phosphorus to water
-ratio and water to dry matter -ratio.

Above mentioned key finding is supported by research by Tidåker et al. (2006) as
effect of dilution through flushing amounts and system design for collection were
listed as main factors for environmental sustainability of blackwater systems. Yet
another aspect for dilution comes from storage and treatment facilities, as according
to Jönsson et al. (2005) this contributes in major way to energy use due to increased
volumes and common treatment method which includes increasing sewage tempera-
ture to gain pathogen removal. Effects of treatment and spreading on sustainability
are outside the scope of this work and present a recognised blind spot for overall
sustainability of compared systems.

Impact of sewer construction on environmental criteria is not considered as overall
effects were deemed minimal by research of Vahidi et al. (2016). However difference
between alternatives is implied by different construction methods, mainly due to
recommended pipeline profile: 0; no sewer main, 1; constant slope, 2; parallel to
ground, and 3; saw-tooth profile. Here the major environmental benefit of re-using
excavated material in respect to replacing with newly extracted materials found by
Petit-Boix et al. (2016) could lift specially solutions powered by external pressure
and independent on strict pipeline profile requirements. Further research is however
required to cover implications on resources needed for construction.

Indicator selection for environment criteria is found to be partly overlapping. Push-
pull effect between indicators as detailed by Matos et al. (2003a) in section 2.4.2
implies an insufficient number of independent indicators. Additional indicator rec-
ommended would be quantitative energy use per kg phosphorus reused and carbon
dioxide emissions per kg phosphorus reused.

4.3 Health and hygiene criteria

Indicator considered for health and hygiene criteria was qualitative risk of microbial
infection. Results for indicator 31, are shown in table 4.1 together with scoring.
As key differences between alternatives it can be seen that two categories emerged
lower risk for alternatives 1 and 3 and higher risk for alternatives 0, gravity and 2,
LPS.

42



4. Results and discussion

Table 4.6: Health and hygiene indicator 31, Risk of microbial infection results

Alternative Risk of microbial
infection

Score Weighted
score

Tank (0) moderate 3 0.50
Gravity (1) low 4 0.67
LPS (2) moderate 3 0.50
Vacuum (3) low 4 0.67

Comparison with indicator 31, risk analysis indicates increased risk for system alter-
natives with user participation in operations such as collection tank filling status in
alternative 0 and low pressure pump station in alternative 2. When neglected this
participation leads to risk of microbial infection by exposure to pathogens through
these exposure points. Specific care with projects such as Munga need to be directed
towards avoiding problems from potential exposure points such as collection tanks
or pump wells with aid of automation, remote monitoring and set alarms in water
utility’s supervisory control and data acquisition (SCADA) system. Recognised lim-
itations on the executed MRA are lack of relevant data from operating systems and
set system boundary that does not consider function or method of BW treatment.
Recommendation for further details of MRA are to apply data from pilot projects
for source separated BW systems where the study may give basis to build on.

4.4 Socio-cultural criteria

Qualitative indicators considered for socio-cultural criteria were 41, Education need
for chosen system; 42, Maintenance need in households; and 43, Risk of flooding.
Result comparisons for indicators and system alternatives are shown in tables 4.7, 4.8
and 4.9. For indicator 41 results showed higher need for education and lower score
for more technically advanced and complex system alternatives 2 and 3 compared
to more commonly used simple technologies in alternatives 0 and 1. For indicator
42 systems relying heavily on user initiated maintenance as tank collection system
alternative 0 scored considerably lower. For indicator 43 previous observation with
user maintenance was pronounced with higher risk of flooding and lower score due
to more likely occurrence.

Table 4.7: Socio-cultural indicator 41, General education need for system results

Alternative Education need Score Weighted
score

Tank (0) very low 5 0.37
Gravity (1) very low 5 0.37
LPS (2) moderate 3 0.22
Vacuum (3) very high 1 0.07

43



4. Results and discussion

Table 4.8: Socio-cultural indicator 42, Maintenance need in households results

Alternative Maintenance need,
households

Score Weighted
score

Tank (0) very high 1 0.09
Gravity (1) moderate 3 0.26
LPS (2) very low 5 0.43
Vacuum (3) low 4 0.35

Table 4.9: Socio-cultural indicator 43, Risk of flooding results

Alternative 43 Risk of flooding Score Weighted
score

Tank (0) high 2 0.08
Gravity (1) moderate 3 0.12
LPS (2) moderate 3 0.12
Vacuum (3) moderate 3 0.12

Both user and utility aspects were considered for indicator 41. The high education
need for vacuum systems is based on the unfamiliarity and rareness of the technology
in municipal sewage applications as it is considered an unconventional sewer system
as defined in theory section 2 by Miszta-Kruk (2016). Key point found to affect
education need is in-house sewer system renovation requiring comprehensive support
to households for implementation. Conventional systems such as tank collection and
gravity get higher score on education need, as little or concern to changes in toilet
behaviour needs to be paid when using a gravity sewer system. As an example
of further research a system alternative where the current existing system would
implemented as part of a hybrid system should be studied. Thus existing in-house
sewers would feed vacuum collection system via collection chambers, with vacuum
interface replacing collection tanks.
For indicator 42 the low maintenance need for alternatives 2 & 3 is mostly due to
maintenance that is in the network domain and thus falls under indicator 52, Utility
maintenance need. Also it is notable that based on previous failure analysis research
by Miszta-Kruk (2016) the small household units at the connection threshold seem
dominant in maintenance need and failure amounts as compared to transfer lines
and larger pump stations in any system. For alternative 0 the bulk of maintenance
need is explained by continuous need of emptying the collection tanks.
For indicator 43 the results imply higher risk of flooding for alternative 0 with
required user participation for collection tank supervision, very much like in case
of indicator 31. This overlap can be seen as a major limitation on indicator 31 as
they address largely same points and events of failure. Even though very local in
nature flooding of collection tank is thought to be socially unsustainable and possible
cause for stigmatisation of specific system. Further research in crustation build up
and flooding for BW systems through large scale pilot experiments is needed and

44



4. Results and discussion

recommended to gain data and knowledge on how this affects large scale flooding
risk.

4.5 Technical function criteria

Results for qualitative indicators considered for technical function criteria are pre-
sented in tables 4.10, 4.11 and 4.12. For indicator 51 results indicate high flexibility
for simple system alternative 0 and low for complex system alternative 3. For indi-
cator 52 results high score for alternative 0 where user initiated maintenance is key.
For indicator 53 results showed high score for simple system and lower for higher
complexity.

Table 4.10: Technical function indicator 51, Difficulty of expansion or contraction
of system results

Alternative Expandability /
adaptability

Score Weighted
score

Tank (0) high 4 0.16
Gravity (1) moderate 3 0.12
LPS (2) moderate 3 0.12
Vacuum (3) low 2 0.08

Table 4.11: Technical function indicator 52, Maintenance need for utility results

Alternative Maintenance need,
utility [qualitative]

Score Weighted
score

Tank (0) very low 5 0.23
Gravity (1) moderate 3 0.09
LPS (2) very high 1 0.05
Vacuum (3) very low 5 0.23

Table 4.12: Technical function indicator 53, Risk of component failure results

Alternative Risk of component
failure

Score Weighted
score

Tank (0) very low 4 0.45
Gravity (1) low 3 0.34
LPS (2) low 3 0.34
Vacuum (3) low 3 0.34

The expandability and adaptability indicator 51 is of interest on many levels as it
addresses the common need for updating and renewing the sewer system. Especially

45



4. Results and discussion

when considering diminishing sewage flows, common in many country-side regions,
systems that do not rely heavily on volumes of water for conveying waste fraction
show tremendous potential in terms of adaptability. In this aspect tank collection
system gains advantage with ease of alterations as a single household system with
new modules added or old ones removed without further consideration.

Maintenance need for the utility, indicator 52 is according to the results clearly
heightened for the low pressure system. The simple reason for this network design
where each household has its own pump station, adding up to theoretical 280 pump
stations in Munga area. Research data according to Miszta-Kruk (2016) shows that
household pumping stations have a high tendency for blockages. For this indicator
limitation comes from overlap with indicator 42, maintenance need for in households.

Indicator 53 shows more complex systems introduce a higher risk of system failure as
discovered in research by Göteborgs Kretsloppskontoret (2007) and Malmqvist et al.
(2006). Analysing the risk of component failure and the output from the risk matrix
the biggest difference comes not on the probability, but rather from the consequence.
Where in network based systems, component failure leads to problems on a wide
area and with multiple houses, the dedicated tank collection system bears poises
consequences only to a single household. Limitation to indicators comes from risk
of component failure (53) direct overlapping with maintenance need for water utility
(52). As introduced in theory chapter according to Matos et al., 2003a this is not an
optimal situation and results in pronounced influence of component failures for the
total score. Overlapping is however contained inside a single sustainability criteria
of Technical function (5). Challenge with evaluating non-existent systems based
on literature data was in great part concentrated to technical function. Data on
operational systems is scarce and mostly available from northern German vacuum
systems by Rohde (2016), Otterpohl, Albold, and Oldenburg (1999), and Sievers
et al. (2016).

4.6 Multi-Criteria-Analysis and scenarios

Overall result from weighted scores summarised for each criteria is presented in figure
4.1. In overall comparison alternative 3 vacuum system gains highest scoring after
weighting procedure. Where more conventional sewer technologies gained higher
marks for socio-cultural criteria, the unconventional vacuum system excels in the
chosen environmental indicators. High score from MCA for the reference alternative
0, tank collection system is considered noteworthy result highlighting functional
robustness of simple technical system as alternative.

46



4. Results and discussion

Figure 4.1: MCA results diagram

Explaining and supporting the high score of alternative 3 from weighting perspec-
tive is supported by reference to previously presented figure 3.21, which shows the
strongest 28% weight given to Environmental criterion. This benefits vacuum system
as it is the highest ranked alternative in environmental criterion before weighting.
Results from overall MCA analysis are however subject to debate as they are based
on weighting gained from one project group and narrow take size. Even within a
project group influence from personal and professional preferences affect scoring and
are likely to cause alteration in results depending on which persons participate in
the weighting procedure.
The results from the scenario 1 (even-weight), presented in figure 4.2) however sup-
port findings as only internal changes in results are detected while the order of
alternatives remains the same. By weighting all subsystems and indicators evenly a
clear difference is seen as this effect especially economic and environmental subsys-
tem index scores. It is to be noted that even in scenario 1 a singular indicator gains
a high overall weight in the case of criteria 3 (Health and hygiene), where only 1
indicator presents the criteria. The effect from indicator selection and distribution
hereby affects the end-result considerably.
Scenario analysis by applying electric truck BE on transportation in alternative 0,
provides an interesting result: outcome of MCA by changing input values for only
alternative 0 (tank collection) for indicators 12 (Opex), and 21 (Primary energy
consumption). Research by Sen, Ercan, and Tatari (2017) and Zhao et al. (2016)
suggests that electric truck transport may provide an environmentally sustainable
solution only if energy production is done in a sustainable way. In the scenario 2
analysis results, presented in figure 4.3, alternative 0 benefits greatly in Environ-
mental criteria through improvement from result in indicator 21 (Primary energy
consumption). However electric truck transport provides very little benefit over
indicator 12) operation cost, due to massive volume of transport and costs com-

47



4. Results and discussion

Figure 4.2: MCA scenario 1, even-weight results diagram

Figure 4.3: MCA scenario 2, electric-truck results diagram

pared to other alternatives. Results portray substantial potential with electric truck
transport but are limited to primary energy use. Secondary effects and especially
environmental effect from carbon dioxide emissions should be considered carefully
if alternative 0 is considered.

4.7 Method evaluation and recommendation

Considering the overall applicability of this study, a major limitation stems from the
delimitation and defined system boundary (Lindholm and Nordeide, 2000). Treat-
ment of blackwater sewage and transport of effluent to agricultural reuse are out-
side the scope of this study. Results show that produced blackwater volumes differ
greatly between alternatives which implies a difference exists as well outside the
system boundary (ie. treatment and transport). The hypothesised effect on overall

48



4. Results and discussion

sustainability outside the system boundary is larger energy need for maintaining
constant process temperature, and energy use for transportation, due to larger vol-
umes of blackwater observed for alternatives 0 to 2. Further research on applying
low flush technologies to reduce BW volume, and difference in treatment for the
mentioned alternatives is needed.
Another limitation on accuracy of the study results is thought to arise from the
chosen method and indicators. Suggested tools for evaluating sustainability criteria
according to chosen framework in chapter 2.4.1 as defined by Hellström, Jeppsson,
and Kärrman (2000) include for environmental criteria are: life-cycle assessment,
computer-based modelling, material-flow analysis, and exergy analysis. Author
agrees fully that mentioned tools would are recommended to gain a more in-depth
view of sustainability but were considered too complex for the scope of this study.
The aim and objectives of this study are considered to be satisfactorily fulfilled by
a sustainability analysis of more holistic nature, as presented in this report.
Contradictory to contents and description of the chosen framework technology and
user aspects of system alternatives were considered in more detail over organisa-
tional aspect that was by most part neglected. Reasoning behind this was both the
recognised research gap, highlighting lack of technical know-how on source separat-
ing systems. Considering organisation aspect as integral part of this sustainability
study might influence the result, lifting conservative technology alternatives that
organisations are commonly used to operate. This would however be counterpro-
ductive as resistance of implementing new technology from an organisation could
possibly undermine and limit further research and application of new rising technolo-
gies in pilot stages. Education of organisation, careful design, and correct technical
solutions required for all alternatives despite selected technology are seen as best
remedies to guarantee a functional sewer system and overcome prejudice.
Major problematic over the course of this study was applying industry standard
indicators, such as performance assessment indicators CARE-S presented in section
2.4.2, on source separated sewers as these are targeted for a) Existent, operational
systems and b) Conventional sewer systems. Another encountered problematic was
found with applying existent data as garbage grinders and organic material are
commonly in many pilot projects mixed with blackwater and possible cause for
crustation and blockages. For peri-urban area such as Munga where no grinders are
implemented the effects of this fraction should be neglected, but strictly relevant
data was scarce.
Author highlights the need of further research and data collection on the effect of
grinder