Application of a decision support tool for industrial and agricultural water reuse solutions in international case studies

Treated wastewater is expected to constitute an essential part of the urban water cycle as an additional water resource in water-scarce or densely populated regions in the future. As decisions on the implementation of water recycling measures should always consider local conditions, the project ‘MULTI-ReUse: Modular treatment and monitoring for wastewater reuse’ has developed a comprehensive sustainability assessment tool, designed to support decision-makers in examining the technical feasibility, economic viability, ecological compatibility and social acceptance of alternative service water supply solutions at local level. This article describes the structure of this sustainability assessment tool and its underlying multi-criteria assessment approach based on 23 evaluation criteria. Already in the development phase, the tool was tested in a German and a Namibian case study. Both case studies are presented with a special focus on the technologies used and the results of the analysis with the sustainability assessment tool. Case study testing proved that the tool is applicable in various environmental and societal settings with widely differing climatic conditions, limited resource availability, for varying feed water qualities and water quality requirements. The comprehensive, straightforward assessment approach enabled the local users to identify the most sustainable supply system or strategy for their decision case.


INTRODUCTION
In many countries, the effluent of wastewater treatment plants (WWTPs) is discharged into receiving waters without further use. In the long run, it is expected that treated wastewater will, as an additional water resource in waterscarce or densely populated regions, constitute an essential part of the urban water cycle, also in Germany. Although Currently, global annual freshwater withdrawal amounts to 4,000 km 3 per year and about 75% of the water is used for agricultural production (WWAP ), wherein an overwhelming proportion is used for irrigation of arable fields.
The need for irrigation is the highest in water-scarce regions.
Consequently, in these regions, a large fraction of treated or untreated wastewater is used for irrigation. For example, in Israel, 86% of wastewater is reused providing about 50% of the national irrigation water demand (Tal ). To meet the urgent need for additional irrigation water in countries of the European Union (EU), the European Commission adopted new rules to stimulate and facilitate water reuse in the EU for agricultural irrigation in May 2020 (European Commission ). So far, irrigation has played only a marginal role in Germany, comprising about 2.2% of the agricultural area (Statistisches Bundesamt ). Due to very rigid legal regulations, treated wastewater is used in a few exceptional cases only. However, facing a series of dry years and a predicted increase of dry seasons in the next decades due to climate change, there is increasing pressure on these water resources.
In contrast, water recycling in industry and commerce already plays a major role in several sectors in Germany.
The process industry is a major water user and an important solution provider for innovative products, technologies and services that enable more sustainable water management.
Technologies and concepts for water reuse promote the economical use of water resources, make companies independent from freshwater resources and thus bring clear locational advantages, both in Germany and internationally (competition for water use, quantitative and qualitative water shortage/stress). Some process industries, which require large quantities of water, have been recycling or reusing water for a long time. In the paper industry, loop separation and counterflow control as well as extensive mechanical circuit water purification are state of the art (Lyko ). The steel industry also circulates large quantities of water, especially for cooling and gas scrubbing, e.g. circuits between 500 and 5,000 m 3 /h in blast furnaces or between 500 and 3,000 m 3 /h in steelworks (Track & Kozariszczuk ). The aim is to reduce freshwater and wastewater costs, but also to recover resources or energy, for example.
The aim of the transdisciplinary research project 'MULTI-ReUse: Modular treatment and monitoring for wastewater reuse' was the development of advanced modular treatment process technologies to provide service water that is ideally suitable for industrial and agricultural purposes which do not require water of a quality suitable for drinking. Process technologies, for instance new reverse osmosis (RO) membranes and ultrafiltration (UF), were developed to process different qualities and quantities of treated wastewater to provide water that is fit for purpose and suitable to substitute the use of other drinking water resources at an economically competitive level (Schramm & Zimmermann ).
In order to examine advantages and disadvantages of the service water supply using MULTI-ReUse technologies with the current water supply concept, a comprehensive sustainability assessment tool was developed and applied in national and international case studies. In the subsequent sections, the methodological approach and the structure of this sustainability assessment tool are described.
Furthermore, the applicability of the tool to assess the advantages of water reuse solutions compared with existing supply systems in an industrial and an agricultural context is demonstrated in a German and a Namibian case study.

Multi-criteria evaluation approaches
In order to select a scientifically sound and practicable assessment methodology for the multi-criteria decision support tool, widely used multi-criteria analysis (MCA) approaches and procedures for the inclusion of quantitative and qualitative data were examined for their applicability and suitability for the comparative assessment of different water reuse solutions in national and international contexts. For the adaptation within a decision support tool, a large part of these procedures had to be excluded due to the one-dimensionality of the assessment, an undue degree of complexity of the assessments (e.g. goal, aspiration or reference level models and outranking methods) or the expectation of insufficient data availability for further use within the intended evaluation tool.
Based on these considerations, value measurement models were found to be best suitable for the assessment purposes of the MULTI-ReUse tool. Value measurement models are based on the idea that a numerical use value is Based on this evaluation approach, sustainability assessments for two case studies were carried out based on different data sources. For the German case study, technical

Case study Nordenham, Germany
In North Germany, the water company Oldenburgisch-Ost-   system (option 1-A, Figure 1). In order to ensure the selection of the most sustainable solution to cope with the local challenges identified for today as well as in long-term planning, the assessment is performed from a current point of view (year 2020) and with a view to the future (year 2030).
In both temporal scenarios, option 1-A covers the service water supply for industrial customers in the City of Nordenham with drinking water from the municipal water-

Case study Outapi, Namibia
In the northern Namibian town of Outapi, a water reuse system for domestic wastewater of approximately 1,000 inhabitants has been operating successfully for more than 6 years. The system feeds a

Tool description
Based on the considerations on different multi-criteria evaluation approaches and the selection of a suitable A process scheme of the MULTI-ReUse sustainability assessment tool is depicted in Figure 6.
Assessment resultscase study Nordenham, Germany The application of the decision support tool for the assessment of the northern German case study confirms that currently the existing water supply system is preferable from a technical, social and economic perspective (Figure 7).
For example, the specific energy requirement for the provision of process water from water reuse (option 2-B) is approximately 1 kWh/m 3 , whereas the total specific energy demand for drinking water supply (option 1-A) is only 0.7 kWh/m 3 on average (Sattig ). The calculated specific costs for the process water supply are thus about 10% higher than the costs for a process water supply from drinking water resources.
However, as an increase in water abstraction or an installation of additional extraction wells is restricted by current water rights for this location, water reuse should still be considered as an alternative to drinking water supply in the future. It is expected that water reuse will reduce the pressure on scarce regional groundwater resources in the long run by meeting the increased industrial water needs with appropriate water quality at acceptable prices. Other contributions to the protection of ecosystems, that can already be recognized today, include a reduced land requirement, a potential improvement in the discharge quality with regard to the annual loads (especially regarding the discharge of trace organic contaminants) when dosing the powdered activated carbon and a reduction in the entry of pathogenic germs into the surface water by membrane installations (Figure 8). Significantly lower chlorine and salt contents of the reuse water also enable multiple water recirculation within industrial processes, so that the  consumption of water resources for production can even be reduced. In addition, option 1-B also allows greater flexibility in terms of quantity and quality requirements for customers, and it makes a greater contribution to local value creation and increases environmental awareness. Furthermore, the use of the MULTI-ReUse approach (option 1-B) leads to a minimized use of cleaning chemicals (e.g. flocculants and precipitants) in the various process steps.
Assessment resultscase study Outapi, Namibia The sustainability assessment of the three options in the Namibian case study shows that option 2-A is the most sustainable system for water reuse, followed by option 2-B and option 2-C ( Figure 9). This is mainly due to the good proves to be the most sustainable option due to its ability to innovation leadership and its lower opportunity costs (follow-up costs due to a system failure). In contrast, option 2-C has the comparatively lowest specific annual costs. All three options considered show a comparable contribution to local value creation.

DISCUSSION
One of the key objectives of the MULTI-ReUse project was to develop a sustainability assessment tool to evaluate different water recycling solutions against the current system configuration in order to identify the most sustainable water supply system for the future. This represents a major challenge since current water supply systems regularly developed historically, are economically written off and optimized in a way that they can operate efficiently in large scale implementation. In contrast, innovative water reuse technologies are usually making use of different water treatment technologies and are currently implemented on much smaller scales. Thus, it is a really challenging task to find common system boundaries and comparable water supply systems. The MULTI-ReUse assessment tool helps the user to identify the advantages and disadvantages of different system configurations. However, a mandatory requirement for its application is the existence of basic concepts for water reuse which have to be developed and made comparable by the tool user individually.
Another challenge in meeting this objective was to identify a suitable multi-criteria assessment methodology that is comprehensive enough to deal with the diverging objectives attached to such decision cases and at the same time offers the highest level of flexibility to be applicable in various contexts. As described, the development of the MULTI-ReUse decision support tool was based on a focused review of existing multi-criteria approaches for water reuse technologies and a structured compilation and review of criteria lists, including experts from different disciplines served to ensure a common understanding of terminology and consideration of contradicting stakeholders' viewpoints of water reuse technology implementation. However, it must be stated that the MCA approach used allows only for a rather superficial assessment of the considered solution.
Nevertheless, the practice partners involved in the development process of the MULTI-ReUse tool, attached great importance to define such rather simple but holistic approach as this enables them to provide key information relevant in an early planning phase of water reuse projects, as a more detailed assessment of the solutions for implementation planning has to be carried out in-house using specific modeling and calculation approach. Thus, creating a consistent approach for detailed assessments that is useful for all Figure 10 | Spider diagram of the assessment results for the case study Outapi with regard to environmental criteria (scale: normalized non-dimensional utility value between 0 (minimum) and 1 (maximum)).
types of users is very difficult to find and might not be appropriate.
The limitations of the MCA approach used in the tool are principally based on data requirements and data availability at such an early phase of the planning process which might force the user to incorporate expert judgements in the weighting of individual indicators required for aggregation along the evaluation and in the evaluation of (especially qualitative) indicators themselves, which could potentially result in uncertainty and inconsistency if the assumptions made are not adequately documented. This is, however, a common issue of MCA approaches. Depending on available data for a given case study, this uncertainty can be reduced through the higher employment of measured or modeled data, keeping the use of assumptions to a minimum.
Furthermore, social science approaches can be used to reduce the impact of subjective rating within the MCA approach.
A subsequent challenge was to transfer this assessment approach into a tool, providing transparent and applicable decision support to its different user groups. The application of the decision support tool for the sustainability assessment of the two case studies provided its developers with key insights about advantages as well as limitations that were necessary to modify the prototype of the tool and transform it into the practice-oriented tool. The involvement of hypothetical end users in the evaluation of the application tests has helped to reveal lack of understanding, gaps and inconsistencies.
As the implementation of water reuse technologies is occasionally hampered by risk concerns and lack of social acceptance, these factors should be considered in decisionmaking in an early phase of planning. Further development requirements for the tool exist, in particular with regard to a stronger incorporation of risk management approaches requiring additional data. However, by focusing on localscale evaluations of comparable technical solutions, the tool provides accessible, lean and yield results that are directly relatable and actionable for stakeholders and decision-makers.

CONCLUSION
The application of the developed decision support tool in two international case studies proved that the tool is applicable in various environmental and societal settings with widely differing climatic conditions, limited resource availability, for varying feed water qualities and water quality requirements. Due to its user-friendly design, a transparent valuation approach as well as the clear and comprehensible presentation of results, the local users became more aware of the strengths and weaknesses of the considered option and were able to identify the most sustainable supply system or strategy for their decision case. However, due to its simplified semi-quantitative approach, the assessment tool is designed and more suitable for the application in an early planning phase. Thus, the decision support tool can be a good starting point to foster in-house and local discussion on the implementation of water reuse solutions.
As the approach requires information input from various sources, it is well suited for collaborative decision-making.
In order to make it also suitable as a decision-support tool in the following implementation phases, future research should focus on connecting models and tools to the MULTI-ReUse decision support tool that allow for a more detailed and reliable assessment of preselected water supply options.