at SciVerse ScienceDirect

Journal of Environmental Management 104 (2012) 51e61

Contents lists available

Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

A compatibility-based procedure designed to generate potential sanitation systemalternatives

Max Maurer*, Ahmed Bufardi, Elizabeth Tilley, Christian Zurbrügg, Bernhard TrufferEawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, 8600 Dübendorf, Switzerland

a r t i c l e i n f o

Article history:Received 11 July 2011Received in revised form21 February 2012Accepted 10 March 2012Available online 5 April 2012

Keywords:Alternative generationCompatibility matrixMulti-criteria decision analysisMCDASanitation system

* Corresponding author. Tel.: þ41 44 823 5386; faxE-mail addresses: max.maurer@eawag.ch (M.

eawag.ch (A. Bufardi), elizabeth.tilley@eawag.ch (E.eawag.ch (C. Zurbrügg), bernhard.truffer@eawag.ch (B

0301-4797/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.jenvman.2012.03.023

a b s t r a c t

Regarding multi-criteria decision analysis (MCDA), the problem of generating alternatives has notreceived the attention it deserves. Most research is currently devoted to the problem of alternativeselection, where it is assumed that a set of appropriate alternatives is already given. This paper addressesthe generation of potential alternatives in the domain of sanitation systems planning and decision-making. A compatibility assessment procedure is proposed to determine the set of technically feasibleor potential sanitation system alternatives. This is based on a clear definition of such an alternativecontaining sub-processes that include a user interface, storage, conveyance treatment and reuse/disposal. A newly developed compatibility matrix is applied to identify incompatibilities between theoptions of the sub-processes.

A potential sanitation system alternative is therefore defined by the absence of two-by-two incom-patibility between all its options.

The compatibility assessment acts as a first filter on the set of sanitation system alternatives toeliminate those that are inoperable before the feasibility assessment. The objective of both steps is toobtain a set of alternatives that are of reasonable and manageable size fromwhich the final solution maybe selected.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

1.1. Motivation

At the beginning of the year 2000, over 25% of the developingworld’s urban population lacked adequate sanitation (WHO/UNICEF, 2005). Millennium Development Goal 7, Target 10 is tohalve the proportion of people without sustainable access to basicsanitation by 2015 (WHO/UNICEF, 2004). To fulfill this goal, largenumbers of new sanitation systems must be implemented in manydifferent parts of the world.

A sanitation system (SanSys) can be understood as a set oftechnologies which, in combination, treat human excreta from thepoint of generation to a final point of reuse or disposal. A SanSysincludes an extensive range of possible combinations of technicalcomponents, including means of storage, conveyance andtreatment.

: þ41 44 823 53 89.Maurer), ahmed.bufardi@

Tilley), christian.zurbruegg@. Truffer).

All rights reserved.

In a modern understanding, the sanitation infrastructure musttake into account not only the technical aspects of the ‘solution’, butalso its economic, social, organizational, institutional and envi-ronmental aspects (Schertenleib, 2005). Evaluating an appropriatesanitation system is therefore a classical multi-criteria decisionanalysis (MCDA) problem. This is well recognized in the literatureand numerous publications describe how to solve a multi-criteriaanalysis problem once it has been structured. However, there arefew methods to aid analysts and decision-makers in systematicallygenerating alternatives at the structuring phase; methods appro-priate for usewith sanitation are evenmore rare. This is the focus ofthis paper.

1.2. Application of MCDA to sanitation systems in the literature

The literature that deals with particular issues relating toselecting alternative sanitation systems can be structured accord-ing to the following groupings:

� Application of a specific MCDA method to the selection ofa SanSys alternative from a given set of alternatives such as theAnalytic Hierarchy Process (AHP) (Ellis and Tang, 1991; Tang

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e6152

and Ellis, 1994; Tang et al., 1997), and Hierarchy Grey RelationalAnalysis combining AHP and Grey Relational Analysis (GRA)(Zeng et al., 2007);

� Selection of a SanSys for a certain type of context such asdeveloping countries (Ellis and Tang, 1991; Loetscher andKeller, 2002), disaster-relief situations (Fenner et al., 2007),and wastewater treatment plants for small communities(Comas et al., 2004; García et al., 2001; Machado et al., 2007);

� Consideration of a partial aspect of the problem: sustainableindicators for SanSys (Ashley and Hopkinson, 2002; Balkemaet al., 2002; Lundin et al., 1999; Lindholm and Nordeide, 2000;Muga and Mihelcic, 2008), design and planning of SanSys(Castro et al., 2007; Chen and Beck, 1997; Hopkins et al., 2001).

All these approaches share a focus on the selection procedureand lack a systematic approach for generating system alternatives.In their review of multiple-criteria analysis for water resourceplanning andmanagement, Hajkowicz and Collins (2007) noted theexistence of a wide range of algorithms to solve a multi-criteriaanalysis problem once it has been structured. However, there arefew methods to aid analysts and decision-makers in choosingalternatives and criteria at the structuring phase. The authorsmentioned the generation of alternatives and selection of criteria asbeing among the main future research needs to be investigated inthe domain of multiple-criteria analysis as applied to waterresource planning and management.

This need for systematic support in the structuring phase isintensified by the rapid development of wastewater treatmenttechnologies. Novel approaches, such as ecological sanitation(Werner et al., 2009), urine source separation (Larsen et al., 2009),decentralized wastewater treatment (Larsen et al., 2010; Pinkhamet al., 2004; Green and Ho, 2005), water recovery concepts(Hochstrat et al., 2008), ‘terra preta’ production (Factura et al., 2010)are emerging in the scientific literature and in practice. It willbecome increasingly difficult to ‘intuitively’ generate an exhaustiveset of SanSys alternatives in the structuring phase of an MCDAprocess. A systematic and aided procedure is needed that will stayflexible enough to include new concepts and future technologicalcomponents.

Systematically generating system alternatives from a set ofcomponents leads us to the problem of assessing the compatibilityof a set of components. A clear-cut definition of the concept ofcompatibility does not exist in the literature. One explanation forthis is that compatibility is used so widely and in such a variety ofcontexts that its meaning and use can vary from one context toanother. Indeed, compatibility can be considered as a polymorphicconcept that cannot be defined by a classical definition, i.e. as a setof necessary and sufficient features that are universally valid. Themeaning of a polymorphic (non-classical) concept should beunderstood within a particular context, i.e. in relation to somepurpose or intended use, and seen from a certain perspective(Compton and Jansen, 1989). For example, compatibility is under-stood in the sense of similarity in disciplines such as psychology(Cross and Sudkamp, 2002), whereas it refers to the notions ofcomplementarity and matching in disciplines such as engineering.

There are many works relating to compatibility assessment inthe domain of product design. Akarte and Ravi (2007) investigatethe compatibility of cast product design with casting processes andcasting producers/suppliers by using a number of performancecriteria. Di Marco et al. (1994) consider the compatibility of productdesign with an end-of-life intent such as reuse and recycling usinga six-level linguistic scale (very compatible, compatible, ...),whereas Singhal (1978) used a scale with only two levels(compatibility and incompatibility) for compatibility assessment inproduction.

Hohenegger et al. (2007a,b) provide both crisp and fuzzycompatibility structures to measure compatibility relationshipsbetween options of different product development activities.Shankar (1993) considers three types of compatibility inthe early phases of new product development: material e processcompatibility, process e architecture compatibility andarchitecture e material compatibility, and indicates that the inves-tigation of these compatibilities is helpful in selecting appropriatearchitectures, materials and processes. Singhal (1978) proposesa compatibility matrix based approach to evaluate compatibilityrelationships between alternatives belonging to different produc-tion subsystems in a perspective designed to support integratedproduction decisions. Singhal and Singhal (1996) determine thefeasible designs of a system, i.e. designs composed of compatiblealternatives of the different subsystems and show that in general thenumber of feasible designs decreases exponentially with increasingnumbers of incompatible pairs of alternatives. In a later paper(Singhal and Singhal, 2002), the authors consider the technologicaland marketing compatibilities in design and product developmentin order to identify feasible product designs at various stages ofproduct development, and investigate the production/marketinginterface. In Singhal et al. (1988), the authors propose an algorithmto generate compatible combinations of alternatives froma completed compatibility matrix. Yu et al. (1993) consider thecompatibility between aluminum product design andmanufacturing processes on the basis of product specifications inorder to select the most appropriate manufacturing process.

For evaluating compatibilities between options, thesepublications either rely on the subjective judgment of expertsand specialists or consider performance criteria, which areunsuitable for feasibility assessment. The use of such criteriaonly allows relative compatibility assessment, i.e. the assess-ment of the level of compatibility of an alternative relative toother alternatives. An ‘autonomous’ compatibility assessmentprocedure that evaluates an alternative independently andwithout any reference to other alternatives is, to our knowledge,lacking in the literature.

1.3. Goal of the paper and outline

For the ranking and selection of suitable sanitation systems,MCDA is frequently applied in the literature due to the need toconsider multiple monetary and non-monetary criteria simulta-neously. Once the problem has been structured, a wide range ofalgorithms is available to solve a multi-criteria analysis problem.However, there are few methods to aid analysts and decision-makers in choosing alternatives and criteria in the structuringphase (Hajkowicz and Collins, 2007).

The determination of a set of appropriate alternatives involvesthreemain steps (see also Fig. 1): (i) the identification of all possiblealternatives, (ii) the identification of potential alternatives amongall possible alternatives by testing the compatibility of theelements, and (iii) the determination of feasible alternatives amongpotential alternatives. The objective is to obtain a set of alternativesof reasonable and manageable size from which the final solutioncan be selected.

In this paper, we develop the first two steps: a compatibility-based procedure to generate potential sanitation system alterna-tives. We present a definition of such an alternative (Section 2) andthe structural relationships between the (sub-) processes (Section3). The compatibility relationship between two options defined inthis paper refers to the possibility of them coexisting in the samesanitation system alternative (Section 4.2). We develop a compati-bility matrix from which the potential sanitation system alterna-tives can be systematically generated (Section 4.3) before showing

Set of all possible SanSys alternatives

Determination of compatible SanSys alternatives

Set of potential SanSys alternatives

Set of all possible SanSys alternatives

Determination of compatible SanSys alternatives

Set of potential SanSys alternatives

Determination of feasible SanSys alternativesGeneration of the entire alternative space

Set of all possible SanSys alternativesDefinition of processes and technological options

: Input to/output of process step

: Process step

:

:

:

:

:

:

Determination of compatible SanSys alternatives

Set of potential SanSys alternatives

Feasibility analysis

Set of appropriate SanSys alternatives

Determination of compatible SanSys alternativesMCDA….

Step 1

Step 2

Step 3F

oc

us

o

f th

is

pa

pe

r

Fig. 1. Main steps of the process for determining the set of potential SanSys alterna-tives, and their inputs and outputs. This paper focuses on the first two steps; the thirdstep can be done with the help of sanitation experts or with a systematic approachdescribed in (Bufardi et al., in press).

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e61 53

the padding of the matrix in section 5 and drawing some conclu-sions in Section 6.

A potential SanSys alternative is defined as one which providestechnically functional treatment of human waste from origin toreuse or disposal regardless of the demands and boundary condi-tions that an application case has. Therefore, the third additionalscreening step should identify the alternatives feasible for a specificapplication case in order to further reduce the number of alterna-tives. We use the term ‘appropriate’ for these, meaning that theelements of a SanSys alternative (i.e. its functions) are compatibleand that it is a feasible alternative for the given application case.The feasibility analysis can be performedwith the help of sanitationexperts familiar with the application case. A systematic approach tosupport this step is described in detail in (Bufardi et al., in press). Itis based on a set of generic attributes that include generic proper-ties of sanitation systems. The comparison of the profile of eachpotential SanSys alternative against the profile of the applicationcase allows the set of appropriate SanSys alternatives to beobtained from which the final solution can be selected.

2. Problem characterization

2.1. Sanitation system (SanSys)

We define a sanitation system (SanSys) as a set of technologieswhich, in combination, treat human excreta from the point ofgeneration to a final point of reuse or disposal.

This is in line with a World Health Organization study group(WHO, 2007) that defined sanitation as ‘themeans of collecting anddisposing of excreta and community liquid waste in a hygienic wayso as not to endanger the health of individuals and the communityas a whole’. Options which allow for the disposal of human wastewithout any kind of treatment (e.g. open defecation or hanginglatrine) are excluded from this definition.

The factors underlying the complexity of the SanSys alternativeselection problem can be summarized as follows:

� A SanSys involves several interrelated and non-sequentialprocesses (user interface, onsite collection storage and treat-ment, conveyance/emptying and transport, (semi-) centralizedtreatment, reuse/disposal); each with multiple technologicaloptions (for example, the user interface may be a dry toilet,a urine diverting dry toilet, a pour-flush toilet, a cistern flushtoilet, or a flush toilet with urine separation);

� A SanSys may handle different wastes such as human waste(urine, faeces), greywater and stormwater; each with differentcharacteristics (composition and concentration of constituents);

� A large number of potential solutions may exist for a givenSanSys application;

Hence, to efficiently select a SanSys alternative, all these issuesshould be taken into account. At present, we are not aware of anytool or methodology that allows this to be done. The choice iscurrently tackled rather simplistically by using a top-downapproach (Zurbrügg et al., 2009): experts decide on the best San-Sys, the local authority is put in charge of its implementation andthe target population is told how to use it andwhat to pay. This typeof supply-driven sanitation solution has led to frequent failures(Jurga, 2009) and is one of the reasons why theworldwide situationhas not progressed as anticipated. Household-led choices whichtake into account factors such as cost, maintenance, etc. depend onunderstanding what alternatives are technically feasible first (Lüthiet al., 2011). Stakeholder decisions are only as good as the alter-natives presented: Here we present a way to systematically createthese alternatives.

2.2. Approach

In MCDA terminology, a potential alternative is one that can beimplemented, or simply one that deserves to be included in thedecision-making process under consideration. This paper addressesthe issue of identifying potential SanSys alternatives among allpossible SanSys alternatives in the case of a human settlementwhere no or inadequate SanSys exists. For cases where an alreadyelaborate SanSys is in place, the suggested options presented inFig. 2 (and Fig. 4AeC) are too coarse and need to be refined. Thedetermination of a set of potential alternatives is crucial for thesuccess of the decision-making process, since the final solution ofthe problem should be selected from this set.

The determination of a set of appropriate alternatives involvesthree main steps (shown in Fig. 1): (i) the generation of all possiblecombination of SanSys elements, termed possible alternatives orthe full alternative space based on a clear definition of theprocesses, sub-processes and technical options of the SanSys, (ii)the identification of the potential alternatives with compatibleelements among all possible alternatives, and (iii) the determina-tion of feasible alternatives among potential alternatives. Theobjective is to end up with a set of alternatives of reasonable andmanageable size from which to select the final solution. The sug-gested systematic approach is flexible and open to novel techno-logical elements and approaches.

From the large space of all possible combinations, a compati-bility assessment approach is used to identify the alternatives withcompatible elements, i.e. to eliminate dysfunctional SanSys alter-natives and identify alternatives that could function. This is donewith a compatibility matrix that defines the relationship betweenthe elements.

3. Identifying the SanSys alternative space (step 1)

The definition of the SanSys alternative is crucial for this firststep and allows the SanSys alternative space to be identified. This

Fig. 2. Processes of the SanSys and lists of technological options for (sub-) processes. The thick arrows indicate the relationships between the processes. A SanSys can bedecomposed into processes; processes can be decomposed into sub-processes and different options exist for each sub-process. This is not a definitive list of options since wastetreatment technologies are evolving all the time, which means that new technology options may be introduced and some existing ones may be abandoned.

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e6154

definition is based on the decomposition of the SanSys intoprocesses/sub-processes/options and the dependencies betweenthem.

A SanSys can be decomposed into processes; processes can bedecomposed into sub-processes and different options exist for eachsub-process (Fig. 2). For example, the ‘onsite collection, storage andtreatment’ process (Fig. 2, middle right) is decomposed into threesub-processes, namely ‘dry onsite collection, storage and treat-ment’, ‘wet onsite collection, storage and treatment’ and ‘urineonsite collection, storage and treatment’. The ‘urine onsite collec-tion, storage and treatment’ sub-process has a single option,

Fig. 3. General representation of a SanSys (sub-) process in terms of functions, inputs,outputs, auxiliary materials and by-products.

namely ‘urine storage tank/container’. It is noteworthy that the userinterface is the only SanSys process that is not decomposed intosub-processes and to which the options are directly assigned.

For the definition of the five SanSys processes, we generallyfollow those given in Tilley et al. (2008):

� ‘User interface’ describes theway inwhich the user accesses theSanSys, generally the type of toilet. Greywater and stormwaterdo not originate at the user interface, but may be treated alongwith the products that do originate there. It is worth notingthat water availability is a key factor in the selection of anappropriate technological option for the ‘user interface’process;

� ‘Onsite collection, storage and treatment’ describes the ways ofcollecting, storing, and sometimes treating the productsgenerated at the ‘user interface’. The treatment provided bythese technologies is often a function of storage, and is usuallypassive. Thus, the products ‘treated’ by these technologiesoften require subsequent treatment before use or disposal;

� ‘Conveyance/emptying and transport’ describes the transport ofproducts from one process to another. Although products mayneed to be transferred in various ways between processes, thelongest and most important gap is between ‘onsite collection,storage and treatment’ and ‘(semi-) centralized treatment’stages; thus, for simplicity, conveyance is limited to trans-porting products at this point.

� ‘(Semi-) centralized treatment’ refers to treatment technologiesthat are generally appropriate for large user groups (i.e.

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e61 55

multiple households). The operation, maintenance, and energyrequirements for technologies within this process are moreintensive.

� ‘Reuse/disposal’ refers to the methods by which products areultimately returned to the environment, as either usefulresources or reduced-risk materials. Furthermore, products canalso be cycled back into a system (e.g. the use of treatedgreywater for flushing).

Fig. 2 shows these five SanSys processes and the thick linesindicate the dependencies between them. It should be noted that:(i) the processes are not necessarily sequential, and (ii) there existdependency relationships between different processes. Forexample, in some cases urine and faeces can be reused just after‘onsite collection, storage and treatment’ without necessarilypassing through ‘(semi-) centralized treatment’. ‘Onsite collection,storage and treatment’, and ‘(semi-) centralized treatment’ aremutually dependent in that at least one of them should be applied.Also, if ‘(semi-) centralized treatment’ is considered, then‘conveyance/emptying and transport’ should necessarily beconsidered. In the case of the Arborloo (which is a special case ofa single pit), it is covered with soil when it is full and the contentsare not removed, which means that the ‘conveyance/emptying andtransport’ process does not apply in such a case.

Fig. 4. A: Part 1 of the compatibility matrix. The small sketch in the upper left corner indicmatrix. The small sketch in the upper left corner indicates the location of this section withinleft corner indicates the location of this section within the entire matrix.

For some (sub-) processes which may not be present in someSanSys alternatives, we introduce a fictive option, namely theempty option denoted as ‘B’. The selection of this option in a SanSysalternative means that the corresponding (sub-) process is notapplied in that alternative. For example, in the case of systemswhich do not divert and separate urine (i.e. it is mixed with faeces),‘B’ is assigned to the sub-process ‘urine onsite collection, storageand treatment’ of the ‘onsite collection, storage and treatment’process, which means that there is no onsite collection and storageof urine, since it is mixed with faeces and possibly flushing water.

This leads us to the definition of a SanSys alternative:

A SanSys alternative is comprised of an option for the ‘user inter-face’ and exactly one option for each sub-process of the other fourprocesses, and is written as

0@I;

0@

SWU

1A;

0@

TMC

1A;

0@

RFZ

1A;

0@

LED

1A1A

where the letters correspond to a specific option of the sub-processin Fig. 2 or Fig. 4(A, B, C).

An example of a SanSys alternative is the ‘composting chamberonsite urine diverting dry toilet’ defined as:

ates the location of this section within the entire matrix. B: Part 2 of the compatibilitythe entire matrix. C: Part 3 of the compatibility matrix. The small sketch in the upper

0@I2;

0@

S5BU1

1A;

0@

BBB

1A;

0@

BBB

1A;

0@

L1BD1

1A1A

¼0@Urine diverting dry toilet;

0@

Composting chamberBUrine storage tank

1A;

0@

BBB

1A;

0@

BBB

1A;

0@

Irrigation=fertilisationBLand application

1A1A

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e6156

In this example, urine diverting dry toilet is the option assigned tothe ‘user interface’ process. For the ‘dry onsite collection, storageand treatment’ sub-process, the considered option is the compost-ing chamber. Because there is no liquid part to treat, the corre-sponding option of ‘wet onsite collection, storage and treatment’ isB. For urine, the corresponding option of the ‘urine onsite collec-tion, storage and treatment’ sub-process is the urine storage tank.The ‘conveyance/emptying and transport’ and ‘(semi-) centralizedtreatment’ processes are not applied, which is why B is associatedto all their sub-processes. For the ‘reuse/disposal’ process, there isno effluent to handle, which is whyB is the option associated to thecorresponding sub-process. Land application (compost reuse) is theoption associated to the ‘solids/faecal sludge reuse/disposal’ sub-process and irrigation/fertilization (urine reuse) is the option asso-ciated to the ‘urine reuse/disposal’ sub-process.

The variety of options available for each (sub-) process makesthe number of all possible combinations very large, but most

Fig. 4. (cont

importantly, finite. For example, on the basis of the set of optionsshown in Fig. 2, there are 61,585,920 SanSys alternatives. Thecompatibility assessment technique, presented in the next sections,considerably reduces the number of alternatives for furtherconsideration through the elimination of non-operable SanSyscombinations.

4. Compatibility assessment procedure (step 2)

4.1. Compatibility assessment procedure description

In MCDA terminology, a potential alternative designates analternative that can be implemented, or simply an alternative thatdeserves to be included in the decision-making problem underconsideration. There are no rules or guidelines for defining potentialalternatives. For the caseof theSanSys alternative selectionproblem,we consider as potential alternatives those that are compatible.

inued).

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e61 57

Following the definition of Singhal (1978):

Compatibility is the absence of incompatibility. Two options areincompatible if there is a violation of one or more of the basic lawsof nature or the technical capabilities of the system.

A compatible combination of options for (sub-) processes isa combination in which no incompatibility arises between any twooptions from the combination. Incompatibility between twooptions indicates an irreparable contradiction at least in the short-term horizon where the decision-making process takes place.

Another indispensable aspect for the compatibility evaluation oftechnological options is the notion of products. Here, products referto human wastes and their derivatives. Hence, the compatibilitybetween two options is defined in the sense that when the twooptions are put together, they allow the corresponding products tobe handled.

A compatible combination is a potential solution since thecompatibility ensures that the objective (i.e. the waste(s) is/aretreated) can be met at least from the functional viewpoint withouttaking into account any kind of constraints induced by the appli-cation environment under consideration.

4.2. Definition of compatible SanSys alternative

An input entering a (sub-) process undergoes one or more of thefollowing functions: mixing, separation, transfer, and transformation(Fig. 3). Each SanSys (sub-) process has a number of inputs and

Fig. 4. (cont

a number of outputs. The outputs are the results of the applicationof the function(s) of the (sub-) process to the inputs. For example,the ‘dry toilet’ option of the ‘user interface’ process has ‘faeces’,‘urine’ and possibly e kept to a minimum e ‘anal cleansing water’as inputs, ‘mixing’ as function and ‘excreta’ as outputs.

Depending on the direction of waste handling, two types ofcompatibility relationships are considered: forward compatibilityand backward compatibility. Since only one option is selected fromthe ‘user interface’ process and from each sub-process of the otherprocesses, the compatibility is only measured between options thatbelong to (sub-) processes that are different.

Definition of forward compatibility. An option O1 of a SanSys(sub-) process is forward compatible with an option O2 of anotherSanSys (sub-) process if, and only if, there exists at least one output ofO1 that can be an input of O2 designed to undergo one or more O2

0

functions.For example, ‘urine diverting dry toilet’ is forward compatible

with ‘urine storage tank/container’, since urine, which is an outputof ‘urine diverting dry toilet’, is also an input of ‘urine storage tank/container’.

Definition of backward compatibility. An option O1 of a SanSys(sub-) process is backward compatible with an option O2 of anotherSanSys (sub-) process if, and only if, O2 is forward compatible with O1.

For example, ‘urine storage tank/container’ is backward compat-ible with ‘urine diverting dry toilet’ simply because ‘urine divertingdry toilet’ is forward compatible with ‘urine storage tank/container’.

inued).

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e6158

Forward and backward compatibility refer to adjacent compat-ibility where the products (i.e. wastes and the derivatives) canmove directly from one of the two options to the other withoutpassing through one or more intermediate options.

Definition of compatibility. An option X of a SanSys (sub-) processis compatible with an option Y of another SanSys (sub-) process if andonly if there exists a set of options O1, O2, ..., On and a set of inputs/outputs ix/ox, i1/o1, i2/o2, ..., in/on, iY/oY, such that:

� X is forward/backward compatible with O1, Oi is forward/back-ward compatible with Oiþ1, i ¼ 1, ..., n�1, and On is forward/backward compatible with Y;

� ix ¼ o1 or ox ¼ i1; ij ¼ ojþ1 or oj ¼ ijþ1, j ¼ 1, ..., n�1; iY ¼ on oroY ¼ in;

� ox results from ix; oj results from ij, j ¼ 1, ..., n; oY results from iY.

Following graph/network terminology, an option of the ‘userinterface’ process corresponds to a source and an option of a ‘reuse/disposal’ sub-processes corresponds to a sink. Hence, only theoutputs are considered for options of the ‘user interface’ process,and only the inputs are considered for ‘reuse/disposal’ sub-processes. For these two types of options, the condition ‘outputresults from input’ is not applied.

For example, ‘urine storage tank/container’ is compatible with‘composting chamber’ because ‘urine storage tank/container’ isbackward compatible with ‘urine diverting dry toilet’ with urine asthe input of ‘urine storage tank/container’ and the output of ‘urinediverting dry toilet’. Equally, ‘urine diverting dry toilet’ is forwardcompatible with ‘composting chamber’with faeces as the output of‘urine diverting dry toilet’ and the input of ‘composting chamber’.

The idea of compatibility between two options refers to thepossibility for the two options to coexist in a same SanSys. If anoption O1 is forward or backward compatible with an option O2,then O1 is obviously compatible with O2 and vice-versa. Forinstance, ‘Urine storage tank/container’ and ‘composting chamber’can coexist in a ‘urine diverting dry SanSys’.

This leads us to the formal

definition of a potential SanSys alternative. A potential SanSysalternative is defined by the absence of two-by-two incompatibilityamong all its sub-processes’ options.

For applications where no SanSys infrastructure yet exists, asconsidered in this paper, the compatibility relationship betweenoptions belonging to the same sub-process is meaningless sinceonly one option is selected from the user interface process and fromeach sub-process of the four other processes. However, this may notbe the case for applications relating to the improvement/adaptationof sanitary systems.

4.3. Compatibility relationship definition

The core element in the procedure of compatibility assessmentof SanSys alternatives is the compatibility matrix which is used toevaluate the compatibility relationship between options fromSanSys (sub-) processes which are different.

Definition compatibility relation. If X and Y are two options oftwo different SanSys (sub-) processes, then C(X,Y) is defined as follows:

C(X,Y) ¼ 1, if the two options are fully compatible;C(X,Y) ¼ 0.5, if the two options are not fully compatible and notfully incompatible;C(X,Y) ¼ 0, if the two options are fully incompatible;C(X,Y) ¼ NA, if the sub-process to which the second option belongsis not applicable to the first option. Only the empty option B from

the sub-process to which the second option belongs is compatiblewith the first option.

Two options are considered as being not fully compatible andnot fully incompatible either if putting them together necessitatesadditional measures to allow them to fit together or because theavailable information is insufficient to enable a definitive judgmentabout whether or not the two options are compatible. If additionalmeasures are needed, we can talk about weak compatibility, andSanSys alternatives incorporating one or more pairs of options withweak compatibility and no pair of incompatible options can beconsidered as potential SanSys alternatives with low priority untilfurther investigations are carried out to determine whether therequired additional measures can be realized or not.

In the case where the compatibility between two options isevaluated as being NA (not applicable), then any compatiblecombination including the first option will not include any optionfrom the sub-process to which the second option belongs, exceptthe empty option ‘B‘.

Since the compatibility relationship C is symmetrical (for proofssee Appendix), only the upper part of the matrix situated to theright of the diagonal needs to be completed. However, thecompatibility relationship is generally not transitive and the wholeupper part of the matrix should therefore be completed. If therelationship were to be transitive, then only the consecutive blocksof the matrix which are adjacent to the diagonal blocks would needto be evaluated.

5. Results

5.1. Padding the compatibility matrix

For the options in Fig. 2 a compatibility matrix was padded,based on the extensive experience of the authors. The objective ofFig. 2 is not to present an exhaustive list of options for each (sub-)process but rather to provide a wide range of options that might beof importance for different application cases. The most importantissue here is the way in which these options are used for paddingthe compatibility matrix and generating the potential SanSysalternatives. It is difficult to establish definitive lists of options sincewaste treatment technologies are evolving all the time, whichmeans that new technology options may be introduced and someexisting ones may be abandoned.

The compatibility assessment is based on the compatibilitydefinition given in the previous sub-section and represents theauthors’ expert knowledge about sanitation systems. Since thecompatibility matrix is very large, it is broken up into three sepa-rate figures (Fig. 4A, B and C).

5.2. Generating compatible SanSys alternatives

The obvious approach to generate potential SanSys alternatives isto generate the entire alternative space as outlined in Section 3. Thenevery combinationwithin an alternative is tested with the aid of thecompatibility matrix. Alternatives containing incompatibilities arediscarded. The remaining alternatives contain the potential sanita-tion systems. As indicated previously, this approach requires thegeneration of a large number of alternatives. However, because theprocedure can be easily automated, this shouldnot be of anyconcern.

An alternative that simplifies and speeds up the calculations isto represent the SanSys options in a tree form. The tree represen-tation allows a large number of incompatible combinations to beeliminated each time two incompatible options are encountered.Indeed, if two options are incompatible, then any combination

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e61 59

containing the two options and any options from the subsequentsub-processes is incompatible and hence eliminated.

With the padded matrix in Fig. 4, the presented methodologyproduces the following results:

Step 1 e Identifying the entire SanSys alternative space: Thereare 324 combinations for the dry, 1056 combination for the ‘wet’and 36 for the urine stream. Multiplied with the 5 possible userinterfaces this yields in total of 6105850920 possible SanSysalternatives.Step 2 e Compatibility assessment: Using a simple R code (R,2008) 4521 fully compatible and 2933 weakly compatible San-Sys alternatives can be identified, corresponding to 0.0073% and0.0048% of the entire SanSys alternative space.

6. Conclusion

We presented a novel compatibility assessment procedure tosystematically determine the set of potential SanSys alternativesamong all possible ones. It aims at providing a systematic tool forthe structuring phase of a MCDA process, one of the missingfeatures in the literature as identified by Hajkowicz and Collins(2007). The procedure involves two main steps (shown in Fig. 1):(i) the generation of all possible combinations of SanSys options,termed possible alternatives or the full alternative space based ona clear definition of the processes, sub-processes and technicaloptions of the SanSys, (ii) the identification of the potential alter-natives with compatible elements among all possible alternatives.

The procedure acts as a first filter to eliminate inoperable SanSysalternatives. The constraints exerted by the application environ-ment on the SanSys alternatives are considered at the next stage ofthe SanSys alternative selection process (feasibility assessment).Non-functional SanSys alternatives have no chance of beingselected as final solutions to the SanSys alternative selectionproblem since they fail to fulfill the basic function of a SanSys,which is the handling of wastes. Therefore, particular attentionwasgiven to the definition of an alternative SanSys in order to identifyall possible such alternatives.

A potential SanSys alternative is then defined as a SanSysalternative in which the options are two-by-two compatible. Theconcept of compatibility defined here refers to the possibility forSanSys options to coexist in the same SanSys. A compatibilitymatrix is used to establish the compatibility relationships betweenthe options of SanSys (sub-) processes. An important characteristicof the compatibility matrix is that it is not dependent on the cor-responding application case, which makes the knowledge itcontains reusable for similar SanSys application cases.

The compatibility assessment procedures found in the litera-ture, which determine the compatibility relationships through theuse of performance criteria, provide only a relative compatibilityassessment of alternatives with respect to each other. In contrast,the compatibility assessment procedure proposed in this paperprovides objective and absolute compatibility relationships.Indeed, these compatibility relationships are determined on thebasis of tangible characteristics of the SanSys options in terms offunctions, inputs and outputs, and the inputs/output links betweenthem. Also, our compatibility assessment procedure allows it to bedetermined in an absolute way whether or not a given SanSysalternative represents a compatible combination of constitutingoptions without any reference to other SanSys alternatives.

The suggested methodology is very capable of systematicallyproducing a large variety of SanSys alternatives. With the followingcompatibility procedure the number of alternatives is very effec-tively reduced; in our example by almost 4 log-units. Nevertheless,the remaining number of potential alternatives is still too high for

the direct use in a MCDA. Reducing the number of alternatives evenfurther cannot be done without knowledge about the applicationcase. A possible next step would involve a set of generic attributesfor the sanitation system. Important attributes could include:economic (e.g. annual cost), demographical (e.g. type of settle-ment), institutional (e.g. compatibility with legal dischargerequirements), socio-cultural (e.g. religious constraints), technicaland site specific (e.g. soil permeability) ones. The comparison of theattribute profile of each potential SanSys alternative against theprofile of the application case allows obtaining the set of appro-priate SanSys alternatives among which the final solution can beselected. The feasibility analysis, together with a set of attributesare being prepared in an accompanying second paper (Bufardi et al.,in press).

The SanSys model presented in this paper can be extended infour different ways: (i) including other processes such as convey-ance/transport from the (semi-) centralized treatment plant to thereuse/disposal destination, (ii) including other waste streams suchas greywater and stormwater, (iii) considering the organizationaland management aspects as being part of the SanSys alternativesand (iv) adapting to the situation of extending an existing SanSys, inwhich case the compatibility of the new SanSys options with theexisting sanitation infrastructure should also be taken into account.It is noteworthy that the logic behind the model remains the sameand only the size/complexity of the system is increased.

The padding of the compatibility matrix was done based on thewell-founded expertise of the authors. However, this subjectivitycould be reduced by using a larger pool of experts and by analyzingdifferent of combinations of sub-processes and options fromsuccessful and failed sanitation systems. Methodological questionsabout the elicitation technique or the handling of the uncertaintywould have to be solved.

Waste other than human waste can either be treated separatelyor be combined with human waste. If it is treated separately,a dedicated compatibility matrix should be considered for it, and ifit is combined with human waste, the compatibility matrix may bedifferent from the case where only human waste is considered.However, the structure and the logic underlying the compatibilityassessment approach are unaffected by the consideration of one ormore wastes other than human waste.

The compatibility assessment procedure presented here can beeasily adapted to complex systems other than SanSys, whichcomprise several components/processes withmultiple options. Thecompatibility relationship between options should be carefullydefined on the basis of the functional characteristics of the systemunder consideration. The compatibility assessment can also beapplied to options belonging to different systems.

Acknowledgments

We thank Andreas Scheidegger for implementing the code in R.

Appendix. Proof of symmetrical compatibility relationship

The compatibility relationship between two options issymmetrical but not necessarily transient.

Let us recall that a binary relation R on a set A is:

symmetrical 5 c x, y ˛ A, x R y 0 y R x,

transitive 5 c x, y, z ˛ A, x R y and y R z 0 x R z.

Let C denote the compatibility relation induced by ‘definitioncompatibility’ and C(X,Y) the value of the compatibility relationshipbetween option X and option Y.

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e6160

Proposition (Symmetry) The compatibility relation C issymmetrical.

ProofIf it is assumed that an option X is compatible with an option Y,

then, from the ‘definition compatibility’, it follows that there existsa set of options O1, O2, ..., On, and a set of inputs/outputs ix/ox, i1/o1,i2/o2, ..., in/on, iY/oY, such that:

� X is forward/backward compatible with O1, Oi is forward/backward compatible with Oiþ1, i ¼ 1, ..., n�1, and On isforward/backward compatible with Y.

� ix ¼ o1 or ox ¼ i1; ij ¼ ojþ1 or oj ¼ ijþ1, j ¼ 1, ..., n�1; iY ¼ on oroY ¼ in

� ox results from ix; oj results from ij, j¼ 1, ..., n; oY results from iY.

For each of the pairs (Y, On), (Oi, Oi�1), i ¼ n, ..., 2, and (O1, X), ifthe second option of the pair is forward compatible with the firstoption of the pair, then the first option is backward compatible withthe second option, and if the second option of the pair is backwardcompatible with the first option of the pair, then the first option isforward compatible with the second option.

If O0i ¼ On�iþ1, i0i ¼ in�iþ1, and o0i ¼ on�iþ1, i ¼ 1, ..., n, then there

exists a set of options O01, O0

2, ..., O0n and a set of inputs/outputs ix/

ox, i01/o01, i02/o02, ..., i0n/o0n, iY/oY, such that:

� Y is forward/backward compatible with O01, O0

i is forward/backward compatible with O0

iþ1, i ¼ 1, ..., n�1, and O0n is

forward/backward compatible with X� iY ¼ o0

1 or oY ¼ i01, i0j ¼ o0jþ1 or o0j ¼ i0jþ1, j ¼ 1, ..., n�1, and

ix ¼ o0n or ox ¼ i0n� oY results from iY, o0j results from i0j, j ¼ 1, ..., n, and ox resultsfrom ix.

According to the ‘definition compatibility’, Y is compatiblewith X.Consequently, C is symmetrical.

Remark (Transitivity) The compatibility relation C is nottransitive.

Proof (Counter example)Composting chamber is forward compatible with human powered

emptying and transport, human powered emptying and transport isbackward compatible with septic tank, and septic tank is backwardcompatible with cistern flush toilet but composting chamber isincompatible with cistern flush toilet.

Appendix. Implementation

The implemented code can be accessed on the web: www.eawag.ch/wmaurer/wisdom.

References

Akarte, M.M., Ravi, B., 2007. Casting product-Process-Producer compatibility eval-uation and improvement. International Journal of Production Research 45,4917e4936.

Ashley, R., Hopkinson, P., 2002. Sewer systems and performance Indicators e intothe 21st Century. Urban Water 4, 123e135.

Balkema, A.J., Preisig, H.A., Otterpohl, R., Lambert, F.J.D., 2002. Indicators for theSustainability assessment of wastewater treatment systems. Urban Water 4,153e161.

Bufardi, A., Tilley, E., Truffer, B., Zurbrügg, C., Maurer, M., in preparation. Anattributes-based procedure to screen for feasible sanitation system alternatives.

Castro, P.M., Matos, H.A., Novais, A.Q., 2007. An efficient heuristic procedure for theoptimal design of wastewater treatment systems. Resources, Conservation andRecycling 50, 158e185.

Chen, J., Beck, M.B., 1997. Towards designing sustainable urban wastewater infra-structures: a screening analysis. Water Science and Technology 35, 99e112.

Comas, J., Alemany, J., Poch, M., Torrens, A., Salgot, M., Bou, J., 2004. Development ofa knowledge-based decision support system for identifying adequate

wastewater treatment for small communities. Water Science and Technology48, 393e400.

Compton, P., Jansen, R., 1989. A Philosophical basis for knowledge acquisition. In:Boose, J., Gaines, B., Ganascia, J.G. (Eds.), Third European Workshop onKnowledge Acquisition for Knowledge-Based Systems, pp. 75e89.

Cross, V.V., Sudkamp, T.A., 2002. Similarity and Compatibility in Fuzzy Set Theory:Assessment and Applications. Physica-Verlag, New York.

Di Marco, P., Eubanks, C.F., Ishii, K., 1994. Compatibility analysis of product designfor recyclability and reuse. In: Proceedings ASME International Computers inEngineering Conference and Exhibition, pp. 105e112.

Ellis, K.V., Tang, S.L., 1991. Wastewater treatment optimization model for developingworld. I: model development. Journal of Environmental Engineering 117,501e517.

Factura, H., Bettendorf, T., Buzie, C., Pieplow, H., Reckin, J., Otterpohl, R., 2010. TerraPreta sanitation: re-discovered from an ancient Amazonian civilisation e

integrating sanitation, bio-waste management and agriculture. Water Scienceand Technology 61 (10), 2673e2679.

Fenner, R.A., Guthrie, P.M., Piano, E., 2007. Process selection for sanitation systemsand wastewater treatment in Refugee Camps during disaster-relief situations.Water and Environment Journal 21, 252e264.

García, J., Mujeriego, R., Obis, J.M., Bou, J., 2001. Wastewater treatment for smallcommunities in Catalonia (Mediterranean Region). Water Policy 3, 341e350.

Green, W., Ho, G., 2005. Small scale sanitation technologies. Water Science andTechnology 51 (10), 29e38.

Hajkowicz, S., Collins, K., 2007. A review of multiple criteria analysis for waterresource planning and management. Water Resources Management 21,1553e1566.

Hochstrat, R., Wintgens, T., Melin, T., Jeffrey, P.J., 2008. Towards a European waterrecycling policy. Water Practice & Technology 3 (2).

Hohenegger, J., Bufardi, A., Xirouchakis, P., 2007a. A new concept of compatibilitystructure in new product development. Advanced Engineering Informatics 21,101e116.

Hohenegger, J., Bufardi, A., Xirouchakis, P., 2007b. Fuzzy compatibility structures innew product development. Advanced Engineering Informatics 21, 41e54.

Hopkins, L.N., Lant, P.A., Newell, R.B., 2001. Increasing flexibility in the design ofwastewater treatment processes. Water Environment Research 73 (4),486e493.

Jurga, I., 7e8 December, 2009. Workshop Report e Erdos Eco-Town Project: LessonsLearned and Ways Forward. Stockholm Environment Institute and University ofScience and Technology Beijing, Beijing.

Larsen, T.A., Alder, A.C., Eggen, R.I.L., Maurer, M., Lienert, J., 2009. Source separationand decentralization: will we see a paradigm shift in wastewater treatment?Environmental Science and Technology 43, 6121e6125.

Larsen, T.A., Binz, C., Maurer, M., Störmer, E., Truffer, B., 2010. On-site treatmenttechnology versus sewer based wastewater treatment: an interdisciplinaryapproach. Submitted to Environmental Science and Technology.

Lindholm, O.G., Nordeide, T., 2000. Relevance of some criteria for sustainability ina project for disconnecting of Storm Runoff. Environmental Impact AssessmentReview 20, 413e423.

Loetscher, T., Keller, J., 2002. A decision support system for selecting sanitationsystems indevelopingcountries. Socio-economicPlanningSciences36, 267e290.

Lundin, M., Molander, S., Morrison, G.M., 1999. A set of indicators for the assessmentof temporal variations in the sustainability of sanitary systems. Water Scienceand Technology 39, 235e242.

Lüthi, C., Morel, A., Tilley, E., Ulrich, L., 2011. Community-Led Urban EnvironmentalSanitation Planning: CLUES. Guideline published by Eawag-Sandec, WSSCC andUN-HABITAT. Download on: http://www.eawag.ch/forschung/sandec/publikationen/sesp/dl/clues_guid.pdf (accessed: 13.12.11).

Machado, A.P., Urbano, L., Brito, A.G., Janknecht, P., Salas, J.J., Nogueira, R., 2007. Lifecycle assessment of wastewater treatment options for small and decentralizedcommunities. Water Science and Technology 56, 15e22.

Muga, H.E., Mihelcic, J.R., 2008. Sustainability of wastewater treatment technolo-gies. Journal of Environmental Management 88, 437e447.

Pinkham, R.D., Hurley, E., Watkins, K., Lovins, A.B., Magliaro, J., Etnier, C., Nelson, V.,2004. Valuing Decentralized Wastewater Technologies: a Catalog of Benefits,Costs, and Economic Analysis Techniques. Rocky Mountain Institute, Snowmass,CO, 81654, USA.

R, 2008. A Language and environment for Statistical Computing. Version 2.14.1(22. December 2011), free download on. http://www.R-project.org (accessed:05. 01. 12).

Schertenleib, R., 2005. From conventional to advanced environmental sanitation.Water Science and Technology 51 (10), 7e14.

Shankar, S.R., 1993. A Generalized Methodology for Manufacturability Evaluation,Ph.D. thesis, Texas A&M University, Texas, 1993.

Singhal, J., Singhal, K., 1996. The number of feasible designs in a compatibilitymatrix. European Journal of Operational Research 94, 186e193.

Singhal, J., Singhal, K., 2002. Supply chain and compatibility among components inproduct design. Journal of Operations Management 20, 289e302.

Singhal, J., Singhal, K., Weeks, J.K., 1988. Long range process design and compati-bility among operations. Management Science 34, 619e632.

Singhal, K., 1978. Integrating production decisions. International Journal ofProduction Research 16, 383e393.

Tang, S.L., Ellis, K.V., 1994. Wastewater treatment optimization model for devel-oping world. II: model testing. Journal of Environmental Engineering 120,610e624.

M. Maurer et al. / Journal of Environmental Management 104 (2012) 51e61 61

Tang, S.L., Wong, C.L., Ellis, K.V., 1997. An optimization model for theselection of wastewater and sludge treatment alternatives. Journal of theChartered Institution of Water and Environmental Management 11,14e23.

Tilley, E., Lüthi, C., Morel, A., Zurbrugg, C., Schertenleib, R., 2008. Compendium ofSanitation Systems and Technologies. Swiss Federal Institute of Aquatic Scienceand Technology (Eawag), Dübendorf, Switzerland.

Werner, C., Panesar, A., Rud, S.B., Olt, C.U., 2009. Ecological sanitation: principles,technologies and project examples for sustainable wastewater and excretamanagement. Desalination 248 (1e3), 392e401.

WHO, UNICEF, 2004. Meeting the MDG Drinking Water and Sanitation Target. In:A Mid-Term Assessment of Progress. World Health Organisation, Geneva,Switzerland.

WHO/UNICEF, 2005. JMP e Joint Monitoring Programme for Water Supply andSanitation. In: Water for Life: Making It Happen. World Health Organisation,Geneva, Switzerland.

World Health Organization (WHO), 2007. Technology for Water Supply and Sani-tation in Developing Countries. Report of a WHO Study Group, Geneva. WHOTechnical Report Series, No. 742.

Yu, Y.-C., Lotfi, S., Ishii, K., Trageser, A., 1993. Process selection for the design ofaluminum components. Advances in Engineering Software 18, 177e186.

Zeng, G., Jiang, R., Huang, G., Xu, M., Li, J., 2007. Optimization of wastewatertreatment alternative selection by Hierarchy Grey relational analysis. Journal ofEnvironmental Management 82, 250e259.

Zurbrügg, C., Bufardi, A., Tilley, T., Maurer, M., Truffer, B., 2009. Decision-making forSanitation Systems. Sandec News. No. 10, 20e21.