To gain insight in the achievable sustainability of wastewater treatment systems, we are developing a model based decision support tool that enables the comparison of a large variety of systems using a multi disciplinary set of sustainability criteria. This paper describes the selection of sustainability criteria, the method of multi criteria analysis and the outlines of the decision support tool.

The efficiency of technological systems such as wastewater treatment plants is usually assessed through mass and energy balances. ‘Environmental efficiency’ is expressed in resources use and emissions. To indicate sustainability additional criteria reflecting economic and social cultural aspects have to be incorporated.

In literature various lists of sustainability criteria can be found (Lundin 1999, Mels 1999, Otterpohl 1997). The differences originate from the various goals and scopes of the researches as well as the various sustainability principles on which the selection of criteria is based. In developing a decision support tool we aim to include a comprehensive list of sustainability criteria to provide the decision maker with a complete overview of the various aspects of the technology. Realising that a decision maker can easily ignore a criteria if not essential while adding criteria may be difficult, we included some hard-to-quantify-indicators qualitatively.

Figure 1: Interaction between technology and environment.

To define the list of sustainability criteria we used the representation of technology-environment-interaction as shown in Figure 1. The figure shows that a demand of the end user is translated into functional criteria to be fulfilled by a technology. In order to function, the technology uses resources out of its environment and affects the environment through emissions. For instance, raw materials are extracted from the physical environment and pollution is emitted to this environment. Additionally, capital and labour may be taken from the economic environment, to which benefits return. Social-cultural environments are influenced through legislation, organisation and acceptance, at the same time a change of behaviour may be initiated by the technology.

Sustainable technology is technology that does not threat the quantity and quality (for instance diversity) of the resources. Based on Figure 1, we defined the following list of sustainability criteria:

• functional criteria including performance (removal of BOD, TS, TSS, N, P, FC and heavy metals), adaptability, durability, flexibility, maintenance, and reliability,
• economic criteria such as affordability, cost, cost effectiveness, and labour intensity,
• environmental criteria covering resource use (energy, nutrients, organic matter, water, and land area) and emissions (in water and sludge: BOD, TS, TSS, N, P, FC, heavy metals, and gaseous emissions such as CO₂, CH₄, NH₃, NO_x).
• social-cultural criteria such as institutional requirements, acceptance, expertise, participation, and stimulation of sustainable behaviour.

When comparing technologies on sustainability, all criteria defined need to be considered in the decision making process. This means that these criteria need to be normalized and weighted such that they can be captured in one single objective.

An example of frequently used method based on multiple criteria is Life Cycle Assessment (LCA), a standardised method to evaluate the environmental impacts of products or services from cradle to grave. LCA is a structured method basically consisting of 3 phases, namely (1) the goal and scope definition, (2) the life cycle inventory (data collection, mass and energy balances), and (3) the impact assessment (classification of emissions in environmental impact categories, normalisation and weighting of these categories).

The first 2 phases are straightforward and frequently applied in environmental assessments of wastewater treatment systems (Bengtsson 1997, Emmerson 1995). The third step, however, includes some subjectivity, as there is no full consensus on the environmental impact categories (global warming potential is a generally accepted category, however aquatic toxicity is a more difficult one, as it comprises summarising very diverse data for example data on chronic and acute toxicity). Furthermore, choices made in normalisation and weighting are more political than scientific. Therefore, many researchers present the results of the first two steps of LCA, leaving the third step to the decision maker. However, if all results are presented in one graph this may tempt the decision maker to use these non-normalised and non-weighted figures straight away.

In developing a decision support tool we include the possibility for a normalisation and weighting step, however factors for normalisation and weighting will have to be set by the decision maker. Our interest is to gain insight in the sensitivity of optimal solutions with respect to different normalisation and weighting factors.

Our decision support tool uses models of a large variety of wastewater treatment systems to comparison these systems. We developed models of wastewater treatment unit operations that can be combined to complete wastewater treatment systems. As very many combinations are possible we are able to build models of numerous systems.

The decision support tool is set up in three parts: (1) data storage, (2) model, and (3) optimisation. All data is stored in a spreadsheet, for instance inputs like water use, contaminants added to the water, and local conditions as prices and climate. This spreadsheet provides decision makers with an overview of data used for the calculation with the possibility to adapt the data to their local situation. The spreadsheet is linked with the model in Matlab Simulink.

All calculations are performed in the Matlab Simulink model, from water use inside the household up to wastewater treatment and discharge, including all sustainability criteria. The qualitative sustainability criteria, for instance on cultural acceptance, are indicated relevant or not. If indicated relevant a remark is added to inform the decision maker on this aspect.

Figure 2: Outline model in Matlab / Simulink 
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The model outline shown in Figure 2 shows a superstructure, an aggregated structure of all possible water systems. Choices on the type of toilet, transport and treatment as well as choices on directions of streams can be defined in the input part of the model. Once choices are made the model simplifies, as non-selected parts do not exist for this particular calculation. For instance, choosing to mix all wastewater and treat it in one central wastewater treatment plant eliminates the yellow and grey water transport and treatment options.

At present we are working on an optimisation routine to automatically select sustainable wastewater treatment systems. By means of integer programming an optimal set of choices will be selected based on an objective function defined as weighted sum of the sustainability criteria.

In the course of summer 2000, case studies will be executed to test the decision support tool in the field. The project will be finalised in spring 2001.

• Bengtsson M. et.al. (1997), Life Cycle Assessment of Wastewater systems, case studies on conventional treatment, urine sorting and liquid composting in three Swedish municipalities, Technical Environmental Planning, report 1997:9, Göteborg Sweden.
• Emmerson R.H.C. et.al (1995), The life-cycle analysis of small-scale sewage-treatment processes, J.CIWEM, 9 (June), 317-325.
• Lundin M. et. al. (1999), A set of indicators for the assessment of temporal variations in sustainability of sanitary systems, Wat. Sci. Tech., vol. 39, no. 5, pp 235-242.
• Mels AR, Nieuwenhuijzen AF et.al. (1999), Sustainability criteria as a tool in the development of new sewage treatment methods, Wat. Sci. Tech., Vol.39, no.5, pp.243-250.
• Otterpohl Ralf et.al (1997), Sustainable water and waste management in urban areas, Wat. Sci. and Tech., vol. 35, no.9, pp.121-134.

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