From Technical Feasibility to Governance Integration: Developing an Evaluation Matrix for Greywater Reuse in Urban Residential Areas

Abstract

Greywater reuse presents a promising strategy for reducing potable water demand and supporting the irrigation of urban green infrastructure, yet its implementation in early planning phases remains limited by fragmented regulations, data gaps, and the absence of practical decision support tools. This study develops a comprehensive evaluation matrix based on Multi-Criteria Decision Analysis (MCDA) to assess the feasibility of greywater reuse in residential district development. The framework integrates eight domains (legal, technical, infrastructural, ecological, economic, and social factors) and is complemented by automated supporting worksheets for water balance, ecological indicators, and economic parameters. Application of the matrix to two contrasting residential case studies demonstrated its diagnostic value: the new-build district in Dortmund showed a high reuse potential, strongly influenced by favourable infrastructure conditions and ecological indicators, whereas the existing building in Weimar yielded a moderate potential due to infrastructural constraints and lower greywater availability. Sensitivity analyses further revealed that local water tariffs, intended-use scenarios, and stakeholder weightings substantially affect outcomes. Overall, the results show that the matrix supports transparent early-stage decision-making, identifies critical bottlenecks, and strengthens governance-oriented integration of greywater reuse in sustainable urban development.

1. Introduction

Urban areas are increasingly facing challenges associated with water scarcity, climate change, and population growth [1,2,3,4,5,6,7,8]. These dynamics place considerable pressure on conventional freshwater resources, while the ecological and social significance of urban green spaces continues to grow [9]. Green infrastructure contributes to climate regulation, biodiversity conservation, and human well-being, but its sustainable irrigation remains a major challenge in residential district development [10,11]. Conventional irrigation practices, which predominantly rely on potable water, are not ecologically or economically viable in the long term.

Greywater reuse has emerged as a promising approach to reduce freshwater demand and support the irrigation of urban vegetation [12,13,14,15,16,17,18]. Greywater, defined as lightly polluted wastewater from showers, sinks, and washing machines [19], is generated in large quantities and can be treated to meet non-potable standards [20,21]. While numerous pilot projects and studies have highlighted its ecological and economic potential [8,22,23,24,25,26,27,28,29,30], implementation in residential district planning remains limited [22]. A key obstacle is the absence of systematic, transparent tools that enable planners and developers to evaluate the feasibility and benefits of greywater reuse in specific contexts [31,32,33,34,35].

To address this gap, this study develops an MCDA-based evaluation matrix tailored to early-stage decision-making in residential district development. The matrix consolidates legal, technical, ecological, economic, and social dimensions into a transparent structure that supports practitioners in determining whether greywater reuse is feasible and under which conditions it may be beneficial. Three automated supporting worksheets—water balance, ecological indicators, and economic parameters—provide practical guidance for calculating greywater availability, contextualizing climate and hydrological stress, and estimating financial conditions. The framework is applied to two contrasting case studies to illustrate its usefulness as an early-stage screening tool.

2. Background and Literature Review

2.1. MCDA Tools for Water Reuse

Multi-Criteria Decision Analysis (MCDA) is one of the foundational frameworks used in the assessment of greywater reuse options. It provides a structured approach for balancing environmental, social, technical, and economic criteria within a transparent decision-making process. In the context of greywater reuse for irrigation, MCDA supports policymakers and planners in evaluating trade-offs between factors such as greywater quality, crop or landscape suitability, cost efficiency, user acceptance, and long-term sustainability. Recent studies demonstrate its effectiveness in identifying optimal reuse configurations in both agricultural and urban water-management settings [36,37,38]. By quantifying trade-offs and integrating stakeholder preferences through weighting, MCDA enables evidence-based decisions that enhance resource efficiency while minimizing health and environmental risks.

Beyond these direct MCDA applications, recent systematic reviews of decision support tools in water reuse offer additional methodological insights. The review by Sampaio et al. [39] shows that most existing tools focus on optimizing treatment technologies, modelling microbiological risks (QMRA), or conducting LCA/LCC-based sustainability assessments. Such tools typically require detailed technical input data and are designed for advanced planning stages rather than early feasibility assessments. The review also highlights that MCDA applications in water reuse often emphasize technological or environmental dimensions, while frameworks that integrate legal, infrastructural, ecological, economic, and social factors remain limited. These findings contextualize the need for practitioner-oriented, multi-domain evaluation tools and underline the relevance of the framework developed in this study, which is explicitly designed for early planning phases with limited data availability.

Complementary perspectives arise from water-footprint (WF) assessment frameworks, which also employ structured, criteria- or indicator-based matrices to evaluate water use efficiency, environmental impacts, and sustainability performance. For example, the review published in the AUTEX Research Journal (24(1): 20240004) [40] provides a comprehensive overview of WF evaluation tools and illustrates how multi-criteria structures are used to assess water consumption, quality-related impacts, and environmental trade-offs. These WF-oriented approaches share conceptual similarities with the evaluation matrix proposed here, as both integrate environmental, technical, and governance-related criteria within a transparent decision support structure. Incorporating insights from WF literature therefore reinforces the methodological grounding of this study and situates the framework within a broader family of water-assessment tools.

2.2. Greywater Reuse in Urban Context

To enable the reuse of greywater, it is essential to consider the applicable legal and regulatory frameworks. In most jurisdictions, treated greywater is classified as service or process water, which may be applied in various contexts including private households, municipal facilities, and commercial settings [17]. Typical uses include toilet flushing and laundry, but also irrigation of gardens, cemeteries, sports facilities, horticultural operations, and agricultural land [22,41,42,43]. In addition, service water may serve functions such as vehicle cleaning, fire-fighting, cooling, or sewer flushing [44,45].

Practical experience demonstrates that greywater reuse has been successfully implemented in a variety of settings, including office buildings, student and senior residences, hotels, multi-family housing, residential developments, camping sites, and municipal street-cleaning operations [22,46,47,48,49,50,51,52].

2.3. Legal Situation in Germany

In parallel, a comprehensive literature review about regulatory frameworks was examined. A key reference was [53], which summarizes regulatory approaches across several countries. In total, 27 relevant guidelines and regulations were identified and added to the database.

In many countries, the absence of a legal framework for greywater management impedes the development and implementation of reuse systems [22]; likewise, in Germany, no overarching legal framework for greywater reuse currently exists. Instead, a complex system of binding laws, ordinances, and technical guidelines must be observed. While laws and ordinances are legally enforceable, norms and technical standards (such as DIN, DWA, or VDI guidelines) represent generally accepted rules of engineering practice and may gain binding force when explicitly referenced in legal texts. When treated greywater is applied as irrigation water, potential impacts on soils, plants, and consumers must be carefully assessed. This requires compliance with a broad range of environmental and health protection laws. Figure 1 illustrates the regulatory instruments relevant to greywater reuse, particularly in the context of irrigation [54,55].

2.4. Treatment Requirements for Greywater as Irrigation Water in Germany

For the use of greywater as irrigation water, there are more advisory guidelines compared to other application purposes. Most of these guidelines focus on the microbiological quality of the water, as shown in

Table 1. Overview of guidelines for greywater reuse by application [55].

Application for Use as	Regulation	Microbiological Parameters	Physico-Chemical Parameters
Irrigation water	EU Regulation 2020/741Minimum requirements for water reuse [56]:	x	x
	DIN 19650 Hygienic aspects of irrigation water [57]	x
	DIN 19684-10 Soil properties—Chemical laboratory tests—Part 10: Investigation and assessment of water in irrigation measures [58]		x
Irrigation water and utility water	DIN EN 16941-2 On-site non-potable water supply systems—Part 2: Systems for the use of treated greywater [59]	x	x
	DWA-M 277 Guidelines for the design of plants for the treatment and use of greywater and greywater streams [60]	x	x
Bathing water	EU-Richtlinie 2006/7/EG 2006 Quality of bathing water and its management and repeal of Directive 76/160/EEC [61]	x

.

At present, only the standard DIN EN 16941-2 [59] explicitly addresses the use of greywater for irrigation and as service water. This standard applies to both irrigation and service water, defining microbiological as well as physico-chemical parameters. For service water applications, it additionally distinguishes between spray applications and non-spray applications. Due to the potential for aerosol formation, stricter requirements are imposed on spray applications, including parameters for Legionella. Furthermore, for service water, separate requirements are defined for laundry use and toilet flushing. A direct comparison of the various guidelines for greywater reuse is often difficult, since different parameters are considered.

To ensure the safe use of greywater for both humans and the environment, appropriate protective measures should be derived from the results of a risk assessment for the respective application and use scenario (for a detailed overview, please refer to the Supplementary Material, Table S1).

The literature review highlights that greywater reuse is shaped by a broad set of interdependent factors, including regulatory requirements, technical and infrastructural conditions, greywater quantity and quality, ecological and climatic context, and economic or social considerations. While MCDA frameworks have been applied in water reuse and related fields, existing tools typically address single dimensions—such as treatment optimization, risk modelling, or sustainability assessment—and rely on detailed datasets more suited to advanced planning stages. Complementary insights from water-footprint assessment further show the value of structured, multi-criteria approaches for integrating technical, environmental, and governance-related criteria. Taken together, the literature indicates a clear need for a comprehensive yet practical decision support instrument that enables feasibility assessments in early planning phases.

Against this background, the following section outlines the methodology used to develop the evaluation matrix, including the derivation of criteria, the structuring of categories, and the integration of supporting worksheets for water balance, ecological indicators, and economic parameters.

3. Methodology for Developing Evaluation Framework

MCDA comprises a set of decision support approaches designed to address multi-objective problems [62]. Its application has expanded considerably within the environmental sciences over the past decades [36,37,63,64,65,66].

Gregory et al. [67] identifies the initial phases of MCDA as (1) clarifying the decision context and (2) defining objectives and attributes. For the further development of the evaluation framework of this study, the process according to Lück and Nyga [68] was followed. The subsequent steps included (3) Structuring the objectives and attributes, (4) Weighting, (5) Scoring, (6) Integration of supporting worksheets and (7) Consolidation and discussion of results. This process is structured as depicted in Figure 2.

4. Results

In the following sections, the results based on the methodology (Figure 2) are presented in detail, beginning with the formulation of the objectives and the definition of the criteria for greywater reuse. The subsequent discussion elaborates on the scoring system and the role of weighting in refining the decision-making process. Furthermore, the research process revealed the necessity of including supporting worksheets to capture supplementary calculations, thereby ensuring transparency and traceability of the evaluation.

Together, these results provide both a methodological foundation and a practical decision support tool for advancing the integration of greywater reuse in sustainable urban development.

4.1. Clarifying the Decision Context

The results of this research aimed to develop a comprehensive evaluation matrix for real estate developers and urban planners addressing the irrigation of urban green spaces and the integration of greywater reuse. This evaluation matrix is intended to serve as a valuable tool for professionals in urban planning to implement sustainable and efficient practices in the management of water resources in urban green areas. With the evaluation matrix, potential developers are enabled to assess whether, and to what extent, their neighbourhood is suitable for greywater reuse. The evaluation tool has been designed for straightforward application through a simple scoring system, thereby allowing for a rapid preliminary assessment of potential. In addition, the framework allows for the integration of weighting, providing the option to place greater emphasis on specific criteria in line with stakeholder preferences and thereby enabling a more differentiated and context-sensitive evaluation.

4.2. Identifying Objectives and Attributes

This step involved a comprehensive review of studies to identify objectives and attributes for evaluating greywater utilization for irrigation of urban green spaces. These investigations focused on technical, economic, and legal aspects to establish a holistic approach for implementing sustainable water reuse practices.

The objectives of this review were as follows: (1) to review the legal frameworks in Germany to ensure compliance with water quality standards, greywater disposal regulations, and relevant building codes; (2) to assess existing and new building infrastructures with respect to storage, transport, and distribution capacities to ensure seamless integration; (3) to assess greywater availability, which depends on the type of building or neighbourhood use and the number of users. Greywater availability is further differentiated by source categories, since volumes may vary across sub streams; (4) to determine the required amount of greywater to be treated, derived from the intended use, because water demand varies according to specific uses. Fundamentally, only as much greywater should be treated as is approximately required; (5) to ensure that treated greywater meets required quality standards by evaluating various treatment technologies—including conventional processes such as filtration and disinfection, as well as advanced membrane-based systems (e.g., electrodialysis, forward osmosis, and reverse osmosis [69,70,71,72,73]) and novel nature-based treatment approaches integrated into building-scale green façade systems [74,75] regarding both efficiency and applicability in residential contexts. In this context, we examined additional factors to optimize the integration of greywater in residential areas. These included (a) assessing water demand for urban green spaces to enable targeted and efficient allocation and (b) monitoring greywater quality with parameters suitable for AI-based processing and advanced indicators for continuous, adaptive oversight; (6) to assess the economic feasibility of greywater reuse by analyzing installation costs, potential freshwater savings, and associated environmental benefits, in order to derive actionable recommendations for households and commercial stakeholders; (7) to address social acceptance within the community as a critical factor for the successful implementation of greywater reuse. Beyond technical, economic, and regulatory considerations, the long-term viability of such systems depends on the willingness of residents to adopt and support them.

Together, these review steps provided a comprehensive understanding of the key factors that shape the feasibility of greywater reuse in urban settings and informed the development of the evaluation framework.

4.3. Structuring of Objectives and Attributes

Technical feasibility, economic viability, legal and regulatory compliance, ecological effects, and social acceptance were identified as decisive influencing factors within the literature review [76]. These categories provided the foundation for structuring the evaluation matrix. Each overarching categories of the evaluation matrix were subdivided into several sub-criteria that enable a detailed assessment of greywater reuse potential. Based on the literature review, the criteria presented in Figure 3 were identified. These are explained in detail in Section 4.6.

4.4. Weighting

Infrastructure projects are generally associated with multiple objectives that they are intended to serve. From the perspective of decision-makers, these sub-objectives may vary in urgency and importance. This can be expressed through the assignment of weights to the sub-objectives [77,78].

Weighting is often determined through surveys, for example, using methods based on the Analytical Hierarchy Process (AHP) [68,79]. In this study, the weighting of the main objectives by stakeholders (e.g., through pairwise comparison using AHP) could be incorporated into the model.

The category 1 “Legal Frameworks” was not included in the weighting, as it represents a knock-out criterion: if legal requirements cannot be fulfilled, the implementation of greywater reuse is not feasible.

4.5. Scoring

For the evaluation process, a five-point Likert scale [80,81] was applied to ensure a standardized and transparent assessment of the defined criteria. The scale ranges from 1 to 5, where a score of 1 represents very low or no potential, and a score of 5 represents very high potential for greywater reuse. Intermediate values (2–4) allow for a nuanced differentiation of cases that do not fall at the extremes, thereby capturing gradual variations in suitability. The use of a Likert scale facilitates both comparability across criteria and the aggregation of results in the evaluation matrix. Furthermore, the scale is intuitive for practitioners, enabling its application not only by scientific experts but also by developers and planners with limited prior experience in greywater management.

Where additional data collection is necessary, supporting worksheets (see Section 4.7) are provided to assist users in completing the evaluation. Additionally, the criteria catalog provides detailed explanations, required data sources, and references to regulations. In this way, the tool allows both developers and planners to carry out systematic evaluations, ensuring comparability across projects

4.6. Detailed Description of Influencing Factors in the Evaluation Matrix

The evaluation matrix was designed to assess greywater reuse potential in residential districts in a transparent and replicable way. It combines the legal framework and further seven overarching categories and 26 criteria. Each category and its associated criteria are presented in detail in the following sections.

4.6.1. Category 1: Legal Aspects

The review of the legal and regulatory landscape shows that, although greywater can be used in various private, municipal, and commercial applications, Germany lacks a unified legal framework. Instead, a fragmented system of laws, ordinances, and technical standards applies, depending on the intended use. A comparison of guidelines indicates that most regulatory focus lies on microbiological quality, while physicochemical parameters are addressed less consistently. Among the existing standards, DIN EN 16941-2 [59] is the only one that explicitly regulates greywater reuse for irrigation and service water, specifying both microbiological and physicochemical requirements and distinguishing between applications with and without aerosol formation. EU Regulation 2020/741 [56] additionally defines minimum quality and monitoring requirements yet offers no detailed provisions for trace contaminants such as pharmaceuticals or surfactants. Overall, the analysis reveals multiple overlapping frameworks with varying levels of specificity, creating both opportunities and uncertainties for the practical implementation of greywater reuse in irrigation.

In Germany, the following legal and regulatory frameworks must be considered [82]:

• Water Resources Act [83];
• Wastewater Ordinance [84];
• Groundwater Ordinance [85];
• Surface Water Ordinance [86];
• Regulation on Water Reuse [56]’
• Federal Soil Protection Ordinance [87];
• Fertilizer Ordinance [88];
• Exemption from the Obligation to Connect to and Use Public Wastewater Systems;
• Exemption from the Obligation to Connect to and Use Public Drinking Water Systems

If greywater reuse is not legally permissible, it constitutes a primary exclusion criterion. A legal review is therefore essential and can only be carried out once the intended use has been defined. Although the initial application of the matrix assumes legal compliance, the legal framework is presented first to underline its overriding importance. Given the complexity of the regulatory landscape, an up-to-date expert assessment is strongly recommended.

4.6.2. Category 2: Building and Infrastructure

The feasibility of greywater reuse is strongly influenced by the condition of buildings, the availability of infrastructure for service water, and the space required for treatment systems. This category therefore comprises criteria that assess both existing conditions and future adaptability. All criteria in Category 2 must be assessed directly by the developer through their own estimation.

Criterion 2.1: Condition of Buildings: Whether greywater reuse is feasible for the developer depends largely on the condition of the existing building and its piping system. In existing buildings, the installation of an additional wastewater pipeline is required. A complete renewal of the piping system may be sensible if the building is undergoing major renovation or full refurbishment.

Alternatively, state-of-the-art solutions from science and research can be considered, such as the use of a wastewater diverter or the retrofitting of an existing piping system by inserting a second pipe [89].

The evaluation options for this criterion cover both new buildings or districts as well as existing structures. For existing buildings, the degree of renovation required plays an important role. This is reflected in the scoring categories, ranging from 2 (building with minor renovation needs) to 4 (complete renewal of the piping system required). Mixed forms combining new and existing buildings cannot be evaluated within this criteria catalog (see

Table 2. Criteria of Category 2 with Scoring Options [82].

Criterion	Assessment Options
2.1 The considered building or neighbourhood is…	1 = an already renovated existing building or a newly completed building
	2 = a building requiring minor renovation
	3 = a building requiring major renovation
	4 = a building where renewal of the piping system is necessary anyway
	5 = a new building in planning
2.2 Existing infrastructure for service water (non-potable water) use…	1 = not available and cannot be added
	2 = not available and can only be added under difficult conditions
	3 = not available, but can be added without problems
	4 = available and in moderate condition
	5 = available and in good condition, or a new building is being planned, so the service water network can be easily integrated
2.3 Are there areas available where a greywater treatment system could be installed? Guideline value: 2 m2 per m3 of treatment capacity (ARIS)	1 = There is no possibility of installing a system, neither in the basement nor in outdoor areas
	2 = Space in the basement or outdoor area is limited but installation is possible
	3 = Space in the basement or outdoor area is of moderate size
	4 = Sufficient space is available outdoors
	5 = Sufficient space is available in the basement

).

Criterion 2.2: Infrastructure for Service Water: If the treated greywater is to be used as service water, an additional pipeline system is required alongside the potable water network. In some existing buildings, a non-potable water network may already exist due to previous rainwater use. This is considered in the evaluation. In certain cases, rainwater may in the future be infiltrated rather than used as service water in the building, or it may be supplemented with treated greywater.

In such scenarios, the existing non-potable water network can be modified and, if necessary, refurbished, significantly reducing effort and investment costs for the developer. It is also essential to assess whether a second piping system can be installed at all, given potential spatial, technical, or structural limitations. These aspects are reflected in the evaluation categories of this criterion.

Criterion 2.3: Available Space for Treatment: Depending on the chosen treatment technology, the required footprint for the installation can vary. For non-nature-based solutions in single buildings, an appropriately sized basement room is ideal, as it is better protected from external influences. Otherwise, a separate building or container for the installation must be constructed outside.

Nature-based systems require sufficient outdoor space, which must also be secured against external influences (e.g., fencing). In the evaluation categories, the availability of a sufficiently sized basement room is ranked more favourably than outdoor space, due to its advantages. The gradation is then determined by the specific spatial conditions (see Table 2).

A first estimate of the required area can be made using a benchmark of 2 m² per m³ of treatment volume, as specified by ARIS (Wernau, Germany) [90] a for a fluidized bed reactor. This serves only as a rough guideline, since actual space requirements vary depending on the chosen treatment system.

4.6.3. Category 3: Greywater Availability

Greywater availability depends on the type of use of the building or district as well as the number of users. In the criteria catalogue, the number of users is represented in terms of inhabitants (IH). This figure must either be estimated by the developer or derived from existing planning documents and then translated into the evaluation categories. The scoring system is designed such that the potential for greywater reuse increases with the number of inhabitants. For greywater treatment systems serving more than 50 IH, the highest score of 5 is assigned (see

Table 3. Criteria of Category 3 with Scoring Options [82].

Criterion	Assessment Options
3.1 Showers/Bathtubs	1 = 1–5 inhabitants (IH)
3.2 Washbasins	2 = 6–15 inhabitants (IH)
3.3 Kitchen sinks	3 = 16–30 inhabitants (IH)
3.4 Dishwashers	4 = 31–50 inhabitants (IH)
3.5 Washing machines	5 = more than 50 inhabitants (IH)

).

Greywater availability is further divided into criteria according to the different sources of greywater (Criterion 3.1 Showers/Bathtubs, Criterion 3.2 Washbasins, Criterion 3.3 Kitchen Sinks, Criterion 3.4 Dishwashers, Criterion 3.5 Washing Machines). This distinction is necessary because the volume of greywater varies depending on the specific sub-stream. In addition, the subdivision of individual greywater sub-streams is directly relevant for Criterion 5: Greywater Quality, where the source of greywater plays a decisive role in determining its suitability for reuse.

For assessing the criteria of category 3, the assessor should refer to average quantities of greywater by source (according to DWA-M 277 [60]).

4.6.4. Category 4: Intended Use

The intended use determines the volume of greywater that must be treated. Since water demand varies depending on the intended application, the different types of non-potable water use are divided into subcategories. Hence, the intended use of treated greywater can be evaluated under the following criteria: Criterion 4.1 service water for toilet flushing, Criterion 4.2 service water for washing machines, Criterion 4.3 service water for household cleaning, and Criterion 4.4 irrigation water for surrounding green areas and trees.

Like greywater availability, the required volume of non-potable water is assessed based on the number of inhabitants. For irrigation of green infrastructure, the evaluation is based on the number of trees and the square metres of green space to be irrigated. Average demand for service water is based on DWA-M 277 [60], while average irrigation demand for trees and green areas is derived from published literature and empirical data. The scoring categories correspond to those used for greywater availability, with higher scores assigned as the number of users increases (see

Table 4. Criteria of Category 4 with Scoring Options [82].

Criterion	Assessment Options
4.1 Use as service water for toilet flushing	1 = 1–5 inhabitants/trees/m2
4.2 Use as service water for washing machines	2 = 6–15 inhabitants/trees/m2
4.3 Use as service water for household cleaning	3 = 16–30 inhabitants/trees/m2
4.4 Use as irrigation water for surrounding green areas and trees	4 = 31–50 inhabitants/trees/m2
	5 = more than 50 inhabitants/trees/m2

). In principle, only as much greywater should be treated as is approximately needed for the intended use.

The client (building owner) assesses the intended use by specifying for how many inhabitants (IH), trees, or square metres of green area the treated greywater is to be used.

4.6.5. Category 5: Greywater Quality

Greywater quality depends on the chosen greywater sub-stream. Based on the criterion 5.1 pollutant load, the appropriate treatment technology is selected in a later planning phase. Which sub-streams must be used depends on the balance between greywater availability and the intended reuse. Ideally, it is sufficient to collect and treat lightly contaminated greywater from sanitary facilities. If more treated greywater is required, additional sub-streams must be included, which increases the pollutant load of the greywater.

With the help of Supporting worksheet 1 (Water Balance, see Section 4.7.1), it can be determined whether the available greywater is sufficient for the desired use and whether treatment of only the lightly contaminated sub-stream is adequate. Following this assessment, the developer can evaluate the expected pollutant load (criterion 5.1) of the greywater to be reused. The evaluation distinguishes between very high, high, medium, low, and very low levels of contamination (see

Table 5. Criteria of Category 5 with Scoring Options [82].

Criterion	Assessment Options
5.1 Pollutant Load	1 = very high contamination (e.g., greywater solely from washing machines and kitchens)
	2 = high contamination (e.g., predominantly greywater from washing machines and kitchens)
	3 = medium contamination (e.g., mixed greywater from all sub streams)
	4 = low contamination (e.g., from sanitary areas and washing machines or kitchens)
	5 = very low contamination (e.g., from sanitary areas)

). The detailed composition of the greywater is described in the Supplementary Material (see Table S2).

4.6.6. Category 6: Ecology

In order to assess the environmental conditions relevant for greywater reuse, three climate- and hydrology-related criteria were defined (see

Table 6. Criteria of Category 6 with Scoring Options [82].

Criterion	Assessment Options
6.1 The amount of summer precipitation has decreased on average over the past 5 years compared to the reference period by…	1 = > −5%
	2 = −5% to −15%
	3 = −15% to −25%
	4 = −25% to −40%
	5 = < −40%
6.2 The percentage of lowered groundwater levels in the area is…	1 = < 5%
	2 = 5–10%
	3 = 10–20%
	4 = 20–30%
	5 = > 30%
6.3 In the potential application area, soil moisture has been so low in the past 5 years that the Drought Monitor has recorded a … during the month with the most severe drought	1 = unusual dryness
	2 = moderate drought
	3 = severe drought
	4 = extreme drought
	5 = exceptional drought

). These indicators reflect current challenges posed by climate change, droughts, and declining groundwater resources in Germany. Together, these criteria provide a structured basis for integrating climate variability and water resource stress into the evaluation matrix.

Criterion 6.1: Deviation of Summer Precipitation: In recent years, a noticeable shift in precipitation patterns has occurred, with rainfall increasingly concentrated in autumn and winter rather than in summer [91]. Maps published by the German Weather Service [91] show the percentage deviation in precipitation compared to the reference period 1971–2000. These maps can be used to determine the changes in precipitation in the study area.

Because precipitation varies significantly from year to year, the last five years should be considered. To evaluate the individual years and calculate an average, Supporting worksheet 2 can be used (see for details Section 4.7.2). This supporting worksheet includes DWD maps for the years 2018–2022. Where newer maps are required, a DWD web link [91] with the necessary settings is provided. The evaluation scale is based on the DWD map from 2020, which reflects mixed deviations in precipitation.

Criterion 6.2: Percentage of Declining Groundwater Levels in the Area: The proportion of declining groundwater levels is also an important indicator of the environmental situation in the study area. With the aid of the overview map available on the CORRECTIV [92] website and Supporting worksheet 2, the developer can determine the percentage of measurement points where groundwater levels have decreased.

Criterion 6.3: Soil Moisture in the Last Five Years (Assessment of Drought): In recent years, Germany has experienced increasingly frequent droughts [93]. The UFZ Drought Monitor [94] provides maps that indicate the extent of drought conditions for each month of the year, allowing the developer to estimate drought severity in the study area for the past five years. For each year, the month with the most severe drought in the area must be selected by the developer.

This process is relatively demanding; therefore, Supporting worksheet 2 (see for details Section 4.7.2) provides a table for recording interim values. The scaling of the evaluation categories corresponds directly to the classification used by the UFZ Drought Monitor [94].

4.6.7. Category 7: Economy

In addition to environmental aspects, economic conditions play a decisive role in evaluating the potential for greywater reuse. To capture these factors, six criteria were defined that reflect both external cost structures and the financial capacity of developers (see

Table 7. Criteria of Category 7 with Scoring Options [82].

Criterion	Assessment Options
7.1 The difference in drinking water charges compared to the national average currently amounts to …	1 = ≤ 0.00
	2 = 0.01–0.10
	3 = 0.11–0.20
	4 = 0.21–0.30
	5 = > 0.30
7.2 Drinking water costs have increased between 2014 and 2022 (i.e., over the last 9 years) by …	1 = < 0%
	2 = 0–5%
	3 = 5–10%
	4 = 10–15%
	5 = > 15%
7.3 The wastewater charge amounts to … €/m3	1 = < 1.30
	2 = 1.30–2.30
	3 = 2.30–3.30
	4 = 3.30–4.30
	5 = > 4.30
7.4 Wastewater costs have increased in the last 9 years by …	1 = < 0%
	2 = 0–5%
	3 = 5–10%
	4 = 10–15%
	5 = > 15%
7.5 I am … to invest in new technologies with a longer payback period	1 = not willing
	2 = rather not willing
	3 = undecided
	4 = rather willing
	5 = willing
7.6 I am … to invest in a technology with ongoing monthly operating costs	1 = not willing
	2 = rather not willing
	3 = undecided
	4 = rather willing
	5 = willing

). Together, these indicators provide a comprehensive framework for linking cost dynamics and investment readiness with the feasibility of greywater reuse.

Criterion 7.1: Difference in Drinking Water Tariff: This criterion assesses the deviation of the local drinking water tariff compared to the national average. This provides an indication of whether greywater reuse has high or low potential. In 2022, the national average drinking water tariff in Germany was €1.83/m³ [95].

Ideally, the developer should evaluate this criterion based on their contract with the local drinking water supplier. If no specific values are available, Supporting worksheet 3: Economy (see for details Section 4.7.3) provides an approximate assessment by federal state, based on data from the Federal Statistical Office [95]. A link to the DESTATIS database is also included for access to more recent figures.

The scoring scale is based on calculated tariff differences in Supporting worksheet 3. Assuming an even distribution of 16 federal states across five categories, each scoring level corresponds to about 3.2 states. A difference of more than €0.30/m³ places a state in the highest category, while lower differences are distributed evenly down to a positive deviation of less than €0/m³.

Criterion 7.2: Percentage Increase in Drinking Water Costs in Recent Years: Unlike Criterion 7.1, this criterion evaluates the increase in drinking water tariffs over the past nine years. Again, local data should be used where possible. Otherwise, Supporting worksheet 3 provides data on cost increases per federal state. The classification of categories follows the same scheme as Criterion 7.1.

Criterion 7.3: Level of Wastewater Charges: Wastewater charges vary significantly depending on the regional wastewater association. Because of the high variability, no national reference values are available, so the absolute wastewater charge must be assessed.

The scoring scale is based on a linear interpolation of prices per cubic metre derived from the 2023 Wastewater Fee Ranking [96]. For a four-person household, annual wastewater charges range from €245.17 in Worms (equivalent to €1.34/m³) to €985.15 in Mönchengladbach (equivalent to €5.40/m³). The intermediate scoring levels in the matrix are calculated by linear interpolation between these extremes.

Criterion 7.4: Percentage Increase in Wastewater Charges in Recent Years: The percentage increase in wastewater charges must be determined from the actual fee statement of the local wastewater association. Due to the lack of consistent national data, the scoring scale for this criterion follows the same classification scheme as Criterion 7.2 (drinking water costs).

Criteria 7.5 and 7.6: Willingness to Invest and Cover Operating Costs: These criteria assess the willingness of the developer to invest in technologies with possibly longer amortization periods and to bear ongoing operating costs. The level of investment costs depends strongly on the treatment capacity, existing building structures, and the chosen treatment system. Data must be obtained directly from manufacturers if there is interest in greywater reuse. Operating costs depend strongly on the chosen treatment system. Data must be obtained directly from manufacturers if there is interest in greywater reuse.

Because estimating actual investment and operating costs is highly complex—depending on system size, greywater quality, intended use, local conditions, and required treatment processes—the criteria are formulated in qualitative rather than quantitative terms. Scoring is thus based on the developer’s self-assessment of their readiness to allocate financial resources for investment and long-term operation.

4.6.8. Category 8: Social Aspects

Social factors are central to the successful implementation of greywater reuse. The acceptance and engagement of residents determine not only the feasibility of system integration but also its long-term sustainability. Four criteria were therefore defined: Criterion 8.1 addresses residents’ acceptance of greywater reuse, while Criterion 8.2 considers their willingness to engage with the technology in everyday life. Criterion 8.3 examines the implications for tenants’ utility costs, and Criterion 8.4 highlights the importance of residents’ well-being in relation to the maintenance of green spaces. Together, these indicators emphasize the role of social acceptance in shaping practical and user-oriented solutions for greywater reuse (see

Table 8. Criteria of Category 8 with Scoring Options [82].

Criterion	Assessment Options
8.1 The residents’ acceptance of greywater reuse is …	1 = not given
	2 = rather not given
	3 = neutral
	4 = present
	5 = fully present
8.2 The residents’ willingness to engage with the technology is …	1 = not given
	2 = rather not given
	3 = neutral
	4 = present
	5 = fully present
8.3 The additional costs borne by residents in recent years, based on drinking and wastewater charges, were …	1 = minimal
	2 = rather low
	3 = average
	4 = slightly increased
	5 = high
8.4 Assessment of plants and green areas in relation to water shortage without additional irrigation, leading to a decline in tenant well-being	1 = Green areas remain vital due to natural water balance
	2 = Individual dry patches occur
	3 = Increasingly dry patches occur
	4 = The majority of the area is dried out
	5 = Almost all green areas and hedges are dried out

).

Criterion 8.1: Resident Acceptance of Greywater Reuse: Resident acceptance is a crucial factor, as the occupants are the end-users of the treated greywater and may come into direct contact with it. The assessment of this criterion can be based on the developer’s judgement or on surveys conducted among the affected residents.

Criterion 8.2: Willingness to Engage with the Technology: Ideally, tenants should be willing to engage with the technology of greywater treatment and understand its connection to their everyday practices. The developer can evaluate this willingness based either on personal assessment or by conducting surveys among residents.

Criterion 8.3: Level of Tenants’ Utility Costs: A positive social aspect of greywater reuse is the potential reduction in tenants’ utility costs. This criterion requires the developer to indicate whether operating costs (driven by drinking water and wastewater fees) have so far been passed on to tenants only minimally or to a significant degree.

Criterion 8.4: Well-being of Residents Due to Dried-Out Green Spaces: The well-being of residents can be enhanced by the presence of adequately maintained green spaces in their living environment. If these spaces dry out during summer, well-being may be negatively affected. This criterion therefore evaluates the extent to which green infrastructure would deteriorate without artificial irrigation.

Since no quantitative benchmark data could be identified for Category 8, the evaluation is conducted qualitatively, based on descriptive assessment rather than numerical values.

4.7. Supporting Worksheets to the Criteria Catalog

The supporting worksheets were developed to further simplify the application of the evaluation matrix. The following sections provide a detailed explanation of the three supporting worksheets on water balance (1), ecology (2), and economy (3).

4.7.1. Supporting Worksheet 1: Water Balance

The first supporting worksheet automates the water balance required to determine the necessary greywater sub-streams. Developers only need to enter the number of IH for each sub-stream and the intended uses. The daily greywater volume and the required amount of treated greywater are then calculated automatically based on reference values from DWA-M 277. Irrigation demand for green areas and trees is estimated using example data from a model neighbourhood [97]. At the end, the tool checks whether the generated greywater volume meets or exceeds overall demand, including a separate verification for lightly contaminated greywater (see

Table 9. Example of automatic calculation and verification of the water balance for 53 inhabitants [55].

3. Greywater Availability
Greywater sub-stream	Reference value (DWA-M 277)	Inhabitants (IH)	ltr/day
3.1 Showers/bathtubs	45.0 ltr/(IH x d)	53	2385
3.2 Handwash basins	12.5 ltr/(IH x d)	53	663
Subtotal lightly polluted Greywater			3048
3.3 Kitchen sinks	7.5 ltr/(IH x d)	53	398
3.4 Dishwashers	7.5 ltr/(IH x d)	0	0
3.5 Washing machines	12.5 ltr/(IH x d)	53	663
Subtotal heavily polluted Greywater			1060
Total Greywater availability			4108
4. Developer’s Reuse Intention
Type of use	Reference value (DWA-M 277)	Inhabitants (IH)	ltr/day
4.1 As service water for toilet flushing	33.0 ltr/(IH x d)	53	1749
4.2 As service water for washing machine	15.0 ltr/(IH x d)	0	0
4.3 As service water for household cleaning	7.0 ltr/(IH x d)	0	0
Subtotal service water			3048
4.4 As irrigation water for trees	20.0 ltr/(tree x d)	17	340
4.4 As irrigation water for green areas	7.5 ltr/(m2 green area x d)	1560	1092
Subtotal irrigation water			1432
Total usage demand			3181

).

4.7.2. Supporting Worksheet 2: Ecology

The second supporting worksheet can be applied to all three ecological criteria (6.1 to 6.3).

Criterion 6.1: Deviation of Summer Precipitation: For the deviation in summer precipitation over the past five years, the percentage deviation must be read from the DWD maps [91]. The supporting worksheet provides a table (

Table 10. Development of summer precipitation amounts compared to the reference period [82].

	Year	1	2	3	4	5	Transfer
		>−5%	−5 to −15%	−15 to −25%	−5 to −40%	<−40%
6.1 The precipitation amount in summer has, on average over the last 5 years, decreased compared to the reference period by … Maps from DWD (see [91])	1					x	5
	2	x					1
	3			x			3
	4					x	5
	5					x	5
	Average						3.8

) for recording the intermediate results for each year. In addition, the average rating is automatically calculated and displayed in the corresponding evaluation colour, which then needs to be transferred into the criteria catalog.

On supporting worksheet 2, the DWD maps for the years 2018 to 2022 are already integrated [91]. To maintain clarity, the maps are presented as small figures (see Figure 4).

When the cursor is placed below a respective map in the Excel sheet, the image enlarges, allowing the percentage deviation to be conveniently read for the specific location. In addition, the maps can also be accessed via an attached link.

Criterion 6.2: Percentage of Declining Groundwater Levels in the Area: Criterion 6.2 can also be evaluated with the support of supporting worksheet 2. Using the overview map provided by CORRECTIV [92], groundwater monitoring points in the affected area can be displayed. By clicking on the respective area with the cursor, more detailed information on the monitoring stations is revealed.

The number of groundwater monitoring stations in the respective categories—strongly declining, slightly declining, no significant trend, slightly increasing, and strongly increasing—can then be transferred into supporting worksheet 2. The percentage share of declining monitoring stations is automatically calculated, and the corresponding evaluation color is displayed in the background. As in the previous case, a direct link to the online tool is included in supporting worksheet 2 (see

Table 11. Automatic calculation of the shares of slightly and strongly declining groundwater levels [55].

Criterion	Assessment Options	Number
6.2 The groundwater levels have … decreased. In the right-hand column, the number of groundwater measurement stations can be entered as needed. The percentage share is calculated automatically. Online tool from Correctiv (see [92])	Strongly declining	0
	Slightly declining	1
	No strong trend	3
	Slightly rising	0
	Strongly rising	0
	Total	4
	Share of declining groundwater levels	25%

).

Criterion 6.3: Soil Moisture in the Last Five Years (Drought Assessment): The procedure for determining soil moisture follows the same approach as for Criterion 6.1. When retrieving the maps via the provided link, one map must be selected per year from the twelve available, identifying the month in which the assessed area experienced the driest conditions. Figure 5 illustrates this procedure for the years 2018 to 2022 as an example.

The severity of drought for each year can then be recorded in the overview table (see

Table 12. Table for recording intermediate values from the Drought Monitor, assessment marked with “x” [82].

	Year	1	2	3	4	5	Transfer
		Unusually dry	Moderate	Severe	Extreme	Exceptional
6.3 The soil moisture in the potential application area was so low in the last 5 years that the Drought Monitor for the total soil column indicated a drought in the month with the highest drought level … Drought categories are indicated according to UFZ (see [93])	1					x	5
	2				x		4
	3					x	5
	4					x	5
	5					x	5
	Average						4.8

). As with the other criteria, the average value is automatically calculated.

4.7.3. Supporting Worksheet 3: Economy

In supporting worksheet 3, the developer is not required to provide any additional information, as this sheet serves solely as supplementary reference material. It contains the average drinking water prices per federal state for the years 2014 and 2022, as well as the deviation of these prices from the national average and the percentage increase or decrease in drinking water tariffs over the past nine years (see

Table 13. Compilation of drinking water charges and price development [55], based on data [96].

	Year		Criterion 7.1		Criterion 7.2
Federal State	2022	2014	Deviation of the drinking water charge from the national average by federal states in 2022	Rating number	Percentage increase/decrease in the drinking water charge from 2014 to 2022	Rating number
Germany	1.83	1.69	0.00		8.3	…
Baden-Württemberg	2.33	2.0	0.50	5	14.2	4
Bavaria	1.78	1.48	−0.05	1	20.3	5
Berlin	1.81	1.81	−0.02	1	0.0	1
Brandenburg	1.57	1.53	−0.26	1	2.6	2
Bremen	2.44	1.98	0.61	5	23.2	5
Hamburg	1.93	1.77	0.10	2	9.0	3
Hesse	2.16	1.97	0.33	5	9.6	3
Mecklenburg-Vorpommern	1.60	1.61	−0.23	1	−0.6	1
Lower Saxony	1.43	1.22	−0.40	1	17.2	5
North Rhine-Westphalia	1.64	1.65	−0.19	1	−0.6	1
Rhineland-Palatinate	1.82	1.70	−0.01	1	7.1	3
Saarland	2.00	1.88	0.17	3	6.4	3
Saxony	2.01	1.94	0.18	3	3.6	2
Saxony-Anhalt	1.75	1.73	−0.08	1	1.2	2
Schleswig-Holstein	1.57	1.44	−0.26	1	9.0	3
Thuringia	2.08	2.00	0.25	4	4.0	2

). These values can be used as supporting information for the evaluation of Criteria 7.1 and 7.2. However, actual data from local water suppliers should be prioritized whenever available.

4.8. Overview of Results

This study delivers a practical, scientifically grounded evaluation matrix to assess the potential for integrating greywater reuse into the irrigation of urban green spaces. The matrix operationalizes decision-making across eight domains: a knock-out legal category (whose non-compliance precludes implementation) and seven weighted categories covering buildings and infrastructure, greywater availability, intended use, greywater quality, ecological conditions, economic factors, and social aspects. The structure supports transparent, replicable appraisal by planners and developers.

A weighting option aligns the matrix with stakeholder priorities (e.g., via AHP-based pairwise comparison), while keeping Category 1 (Legal) outside the weighting as an exclusion criterion. Criterion-level scores are aggregated using a five-point Likert scale (1–5), where 1 denotes no or very low potential and 5 denotes very high potential; intermediate values capture gradations in suitability. To ensure usability, the tool employs a checkbox format and is accompanied by a criteria catalog specifying definitions, data needs, and normative references, thereby enhancing consistency and transparency across projects.

Three supporting worksheets streamline application: (1) an automated water balance to match greywater sub streams with intended uses; (2) an ecology sheet to compile five-year summaries of summer precipitation deviations, groundwater-level trends, and drought severity; and (3) an economy sheet providing reference tariffs and tariff trends (while prioritizing local utility data). Collectively, the matrix, weighting option, scoring scheme, and supporting worksheets furnish a coherent results package that supports rapid screening, stakeholder-sensitive prioritization, and traceable documentation for the integration of greywater reuse in sustainable urban development.

4.9. Case Studies

To clarify how the matrix is applied in practice, the following short example outlines the typical workflow for planners:

(1)

Collect basic site information: A planner gathers readily available data such as IH, building condition, existing piping, available space for treatment, local water tariffs, size of green areas, and intended reuse options.

(2)

Determine greywater availability and demand: Using Supporting worksheet 1, the planner enters population equivalents for each substream. The tool automatically calculates daily greywater availability and compares it with the required demand (e.g., toilet flushing and irrigation).

(3)

Assess each criterion category

(4)

Apply weighting (optional): If a developer or municipality provides stakeholder preferences, the planner incorporates weighting through the AHP-based scoring option. If not, the matrix can be used in unweighted form as a neutral baseline.

(5)

Calculate the final evaluation: Scores are aggregated, and the planner obtains an overall potential rating (e.g., “moderate,” “high”). The matrix also highlights which category most strongly influences feasibility (e.g., limited space, low greywater volumes, high irrigation demand, economic constraints).

This approach was applied in the following two cases studies.

4.9.1. Case Study 1: Micro-Apartments in Bergmannsgrün (Dortmund, Huckarde)

The evaluation matrix was applied to a planned new build comprising ~53 micro-apartments within the Bergmannsgrün model district, a showcase project for the IGA 2027 emphasizing climate mitigation/adaptation, energy, and sustainable materials. Based on the developer’s reuse intention, treated greywater is primarily designated for irrigating surrounding green spaces (≈2740 m² and 17 trees), with toilet flushing considered as a stabilizing year-round demand. Assuming one resident per unit, showers/handbasins/kitchen sinks and shared laundry yield high greywater availability (>50 IH per sub stream). Buildings and infrastructure score highly due to new-build planning and dual piping, while available plant space is rated moderate given basement/outdoor uncertainties. A water-balance supporting worksheet indicates that lightly polluted sub streams alone do not entirely meet peak demand; adding the kitchen sub stream raises expected pollutant load (Category 4) but secures supply. Ecological indicators yield a strong ecology score (≈4.3). Economic factors score low under state-level tariff proxies, though sensitivity tests show that using local tariff data (including base fees) can materially improve the “Investor” scenario. Social criteria are favourable given the district’s sustainability profile and ongoing resident engagement. Unweighted, the case returns an overall potential of ~3.8 (high), and scenario analyses (see

Table 14. Overview of results across the four scenarios in Dortmund [55].

Category	Sustainability	Investor	User Benefit	Mixed Interests	Without Weighting
2. Buildings and infrastructure	15.0%	20.0%	12.5%	20.0%	14.3%
3. Greywater availability	15.0%	5.0%	5.0%	20.0%	14.3%
4. Developer’s Reuse Intention	15.0%	10.0%	20.0%	10.0%	14.3%
5. Greywater quality	5.0%	5.0%	10.0%	12.5%	14.3%
6. Ecological Factors	35.0%	5.0%	10.0%	12.5%	14.3%
7. Economic Factors	5.0%	40.0%	10.0%	15.0%	14.3%
8. Social Aspects	10.0%	15.0%	32.5%	10.0%	14.3%
Overall result	4.0	3.4	3.6	3.9	3.8
Deviation compared to without weighting	0.2	−0.4	−0.2	0.1	0.0

and

Table 15. Excerpt from the evaluation scale for results.

From	To	Potential Assessment
2.5	<3.5	Potential is moderate
3.5	<4.5	Potential is high

) (“Sustainability,” “Investor,” “User Well-being,” “Mixed Interests”) demonstrate that outcomes are particularly sensitive to economic inputs and to how seasonal irrigation demand is complemented by year-round uses (e.g., WC flushing). For the fully completed evaluation matrix for Case Study 1, please refer to the Supplementary Material, Table S3.

4.9.2. Case Study 2: Reference Building in Weimar

To contextualize results, the matrix was transferred to an existing six-storey apartment block (pre-renovation baseline) in Weimar where mixed greywater from 11 units is already collected in a constrained basement. Under conservative assumptions (partial retrofitting complexity, limited space, no pre-existing service-water network), Buildings and infrastructure score lower than in the Dortmund case; greywater availability reflects ~22 residents with mixed appliance penetration. Intended use prioritizes WC flushing and laundry; pollutant load is favourable (primarily sanitary substreams), yielding a high greywater-quality score (5). Ecology is derived from DWD [91]/UFZ [93]/CORRECTIV [92] sources for Weimar (mid-to-high scores across the three indicators). Economic scoring relies on state-level references (with local wastewater charges ~€1.75/m³ incl. base fee), and social criteria are assessed qualitatively. Across all four weighting scenarios, the Weimar object exhibits a moderate reuse potential, lower than Bergmannsgrün, with a wider spread among category scores (strong in quality, weak in intended use). Comparative sensitivity analysis (see

Table 16. Overview of results across the four scenarios in Weimar [55].

Scenario	Sustainability	Investor	User Benefit	Mixed Interests	Without Weighting
Overall result	3.2	2.8	2.9	3.1	3.1
Deviation from unweighted result	0.1	−0.3	−0.2	0.0	0.0

) indicates that (i) category divergences amplify weighting effects, (ii) robust local cost data (rather than state averages) are essential to avoid biassing economic outcomes, and (iii) refining the intended-use criterion to account for the ratio of demand to availability and intra-annual demand variability (e.g., irrigation seasonality) would improve discriminatory power and practical relevance of the matrix. For the fully completed evaluation matrix for Case Study 2, please refer to the Supplementary Material, Table S4.

5. Discussion

This study proposes and tests a transparent, MCDA-based evaluation matrix to appraise the potential for integrating greywater reuse into the irrigation of urban green spaces in residential developments. By operationalizing eight domains—one legal knock-out category and seven assessable categories, the matrix translates a diffuse, multi-dimensional decision problem into a structured, traceable appraisal. The two case studies demonstrate that the framework is applicable to both new-build and existing contexts, supports early-stage screening, and highlights the trade-offs inherent in technical integration, seasonal demand, and stakeholder priorities.

A central insight is the primacy of legal permissibility. Treating the regulatory context as a knock-out category is appropriate because non-compliance makes any subsequent optimization moot. At the same time, the fragmented and evolving nature of standards implies that legal feasibility is time- and use-specific. Consequently, the requirement for an up-to-date, expert legal review should be retained as a procedural safeguard, and the matrix should be read as conditional on current compliance for the specified application (e.g., irrigation with/without aerosol formation).

A further limitation arises from the fact that several criteria—particularly in the building/infrastructure, economic, and social categories—require self-assessment by developers or urban planners. Such inputs may introduce subjectivity or bias, for example, through optimistic assumptions, incomplete information, or strategic over- or underestimation of site conditions. While the matrix is designed to remain usable in early planning phases, when many parameters are not yet fixed, this reliance on self-reported information can affect the consistency and comparability of results.

To mitigate these risks, the criteria catalogue provides structured guidance, definitions, and examples to promote more standardized scoring. Wherever possible, users are encouraged to rely on verifiable data sources, such as building documentation, legal reviews, measured consumption values, tariff tables from utility providers, or official climatic datasets. Cross-checking self-assessed entries against such objective information can substantially reduce bias. Furthermore, the evaluation can be strengthened through stakeholder validation—for instance, by involving municipal authorities, planners, operators, or resident representatives in reviewing the scoring of key categories. In multi-stakeholder settings, consensus scoring or facilitated workshops (e.g., AHP-based pairwise comparison sessions) can provide additional transparency and reproducibility. Taken together, these measures do not eliminate subjectivity but help ensure that self-assessed inputs remain as consistent, transparent, and traceable as possible.

The weighting mechanism proves valuable for embedding stakeholder preferences (e.g., via AHP pairwise comparisons) but also reveals sensitivity of outcomes to value judgements and data inputs. In Dortmund (Bergmannsgrün), scenario analyses (“Sustainability,” “Investor,” “User Well-being,” “Mixed Interests”) show that results shift notably when economic criteria are emphasized and when local tariffs (including base fees) replace state averages. In Weimar, a wider dispersion among category scores amplifies the effect of weight changes. These findings underscore the importance of (i) eliciting weights through a transparent participatory process, (ii) reporting robustness checks (e.g., one-way sensitivity, weight-swing analysis, or Monte-Carlo simulation over plausible weight intervals), and (iii) prioritizing project-specific data over proxy values wherever feasible.

Methodologically, the five-point Likert scoring enables consistent aggregation and comparability across diverse criteria while remaining accessible to practitioners. Nonetheless, the case studies highlight two refinements that would improve discriminative power and practical relevance. First, for intended use, scoring should incorporate the ratio of demand to availability and explicitly account for intra-annual variability (e.g., irrigation seasonality), acknowledging that biological treatment systems cannot be cycled off without cost or risk. Second, for availability, kitchen sinks and dishwashers could be combined into a single “kitchen” sub stream to better reflect compensating behaviours (absence of a dishwasher typically increases sink usage). These targeted adjustments would reduce structural bias and better align the matrix with operational realities.

While the three supporting worksheets (water balance, ecological indicators, and economic parameters) increase transparency and usability, they also rely on assumptions and data sources with inherent limitations that must be acknowledged. At the same time, they play an instrumental role in ensuring transparency and reproducibility. The automated water-balance worksheet reduces calculation errors and clarifies the implications of adding (or avoiding) higher-load substreams for meeting demand. The ecology supporting worksheet operationalizes climate- and hydrology-relevant evidence (summer precipitation anomalies, groundwater trends, drought severity) over a five-year window, supporting site-adaptive interpretation rather than single-year snapshots. The economy supporting worksheet is helpful for first-pass screening, but the Dortmund analysis shows that relying on regional averages can misclassify projects; thus, local utility tariffs and fee structures should be obtained early and documented in the annexes to prevent systematic bias.

(1) Water Balance—Generalizability of Reference Values: The water-balance supporting worksheet uses standardized reference consumption values from DWA-M 277 to estimate greywater availability and service-water demand. These values provide a consistent baseline but cannot fully represent the variability in user behaviour, building typologies, appliance efficiencies, or seasonal occupancy. Deviations can be particularly pronounced in atypical residential configurations (e.g., micro-apartments, senior housing, student residences). Therefore, the water-balance supporting worksheet should be interpreted as an early-stage screening instrument rather than a substitute for detailed water-use monitoring or engineering design.

(2) Ecological Indicators: The ecological supporting worksheet draws on datasets from the German Weather Service (DWD) [91], the UFZ Drought Monitor [93] and Correctiv’s groundwater trend maps [92]. These sources vary in spatial resolution from roughly 1 km² grid cells to coarser regional layers and in temporal granularity (monthly, seasonal, or multi-year averages). As a result, the supporting worksheets cannot capture microclimatic effects, localized groundwater dynamics, or fine-scale soil conditions. The five-year aggregation window improves robustness, yet extreme years or emerging climate trends may still influence results. Thus, ecological scores should be regarded as indicators of broader climatic stress rather than precise local measurements.

(3) Economic Parameters are drawn primarily from federal-state averages. Although these provide a uniform, accessible basis, significant differences exist between municipal utilities, particularly regarding base fees, tariff structures, or special charges. Economic trajectories also reflect political decisions, infrastructure investments, and regulatory changes. As demonstrated in the Dortmund case, reliance on regional averages may misclassify local conditions. Consequently, planners should replace reference values with site-specific tariff and fee data whenever possible.

Uncertainty and Transferability: Due to these factors, all three supporting worksheets should be understood as context-dependent aids rather than universally generalizable tools. Their outputs inevitably contain uncertainties related to data resolution, standardized reference values, and regional-to-local discrepancies. These uncertainties do not diminish the usefulness of the matrix as a transparent and reproducible early-stage assessment tool, but they underscore the need to complement the supporting worksheets with detailed, site-specific information as projects progress toward design and implementation.

From a planning perspective, the matrix has three practical implications. First, it enables early design moves that stabilize system performance—e.g., pairing seasonal irrigation with year-round uses (toilet flushing) to maintain steady loading, protect process stability, and right-size storage. Second, it makes explicit the quality–technology linkage: maximizing lightly polluted substreams can reduce treatment stringency, energy use, and O&M costs, while still meeting non-potable standards. Third, the social dimension—resident acceptance and engagement—is not merely contextual but a determinant of long-term viability; structured communication and feedback loops should therefore be embedded in project governance and reflected in weighting where appropriate.

Several limitations merit attention. The current scoring does not yet integrate health risk assessment (e.g., Quantitative Microbial Risk Assessment (QMRA)) or life-cycle environmental impacts (LCA) (e.g., energy use, chemical consumption, and sludge management) of treatment options; future work should therefore couple the matrix with streamlined LCA and QMRA modules. Uncertainty is handled implicitly through sensitivity of weights and scenario testing; extending the approach with probabilistic ranges for key inputs (demand factors, tariff trajectories, drought frequency) would strengthen decision confidence. Finally, although the framework is developed for Germany, its structure is transferable; however, legal and quality requirements are jurisdiction-specific and must be re-parameterized accordingly.

Beyond meeting legal requirements, the successful implementation of greywater reuse systems depends on broader governance arrangements that shape coordination, accountability, and long-term system performance. Greywater reuse intersects with multiple institutional domains like urban planning, water utilities, environmental protection agencies, housing associations. An effective integration requires clear allocation of responsibilities across these actors. Governance considerations therefore extend to the establishment of planning procedures, approval pathways, and operational mandates, as well as mechanisms for coordinating greywater provision with irrigation management and building operation.

In addition, governance integration involves developing transparent data-sharing arrangements (e.g., on water consumption, system performance, ecological conditions), creating monitoring and reporting routines, and clarifying how system maintenance and cost recovery are organized. Participatory elements are equally important: engaging residents, municipal departments, and local utilities can strengthen acceptance, identify concerns early, and support co-creation of locally appropriate solutions. Such governance processes complement the technical and economic assessments captured in the matrix by ensuring that implementation pathways are institutionally realistic and socially anchored. Integrating governance considerations into the evaluation framework therefore enhances its practical relevance, linking regulatory compliance with the organizational, procedural, and participatory conditions necessary for long-term system success.

In sum, the proposed evaluation matrix advances decision support for greywater reuse by combining legal feasibility, technical/infrastructural fit, environmental context, economic viability, and social acceptance within a single, usable instrument. The case studies confirm its diagnostic value and reveal where refinements—demand/availability ratios, seasonality handling, consolidated sub streams, stronger local data—can enhance accuracy. With these enhancements and the addition of explicit risk and life-cycle modules, the matrix can provide a robust basis for evidence-based, stakeholder-sensitive planning of greywater reuse in ecologically oriented urban development.

The present study has several limitations that should be acknowledged. First, the evaluation matrix is designed as an early stage screening tool, meaning that it simplifies complex planning and engineering processes and cannot replace detailed technical design, hydraulic modelling, or site-specific water quality assessments. Second, the matrix relies partly on standardized reference values (e.g., DWA-M 277 [60]) and self-assessment by planners or real estate developers, which may introduce bias and may not fully capture local behavioural or infrastructural variability. Third, the ecological and economic supporting worksheets draw on regional datasets whose spatial and temporal resolution may not reflect micro-scale conditions or rapid changes in tariffs, groundwater trends, or climatic extremes. Fourth, the framework is tailored to the German regulatory and institutional context, limiting its immediate transferability without adapting legal and governance elements to other jurisdictions. Finally, the matrix does not include a quantitative risk assessment (e.g., QMRA) or a life-cycle perspective on environmental and economic performance; integrating such modules would strengthen future applications. Despite these constraints, the matrix provides a transparent, structured, and practical foundation for evaluating greywater reuse potential at the neighbourhood scale.

6. Conclusions

This study developed and applied an MCDA-based evaluation matrix to assess the feasibility of integrating greywater reuse into the irrigation of urban green spaces. The main results can be summarized as follows:

(1)

Comprehensive and practical decision support structure: The matrix consolidates eight domains—legal, infrastructural, availability, intended use, quality, ecology, economy, and social factors—into a transparent and replicable early-stage screening tool that can be used by planners and real estate developers.

(2)

Legal compliance as a non-negotiable prerequisite: Treating the legal framework as a knock-out criterion proved essential, as regulatory permissibility determines whether greywater reuse can be pursued at all.

(3)

Automated supporting worksheets enhance accuracy and transparency: The water-balance, ecological, and economic supporting worksheets reduced calculation errors, helped interpret climatic and hydrological constraints over a five-year window, and highlighted the importance of obtaining local—not regional—economic data.

(4)

Case studies demonstrate feasibility and discriminative power: Application to a new-build neighbourhood (Dortmund) and an existing multi-storey building (Weimar) showed that the matrix differentiates clearly between site conditions, identifies bottlenecks, and supports early planning decisions.

(5)

Weighting and local data strongly influence outcomes: Scenario analyses confirmed that category weights (e.g., investor vs. sustainability perspectives) and the specificity of tariff data can meaningfully shift results, underscoring the need for transparent weighting processes and up-to-date local input values.

(6)

Greywater availability and intended use require careful balancing: The case studies highlight that lightly polluted sub streams often cannot fully cover irrigation peaks, and combining seasonal irrigation with year-round uses (e.g., toilet flushing) improves system stability and sizing decisions.

(7)

Governance and acceptance are critical for implementation: Beyond regulatory compliance, successful integration depends on institutional coordination, data-sharing, monitoring responsibilities, and resident acceptance—components that complement the technical and economic assessment encoded in the matrix.

Overall, the evaluation matrix provides practitioners and policymakers with a structured, practical, and adaptable decision support instrument that strengthens transparency and supports evidence-based planning of greywater reuse in urban residential areas.

7. Outlook

Future work should strengthen and expand the presented framework along five complementary directions.

First, risk and sustainability analytics: The evaluation matrix should be coupled with quantitative risk and sustainability assessments, such as simplified Quantitative Microbial Risk Assessment (QMRA) for health protection and Life Cycle Assessment (LCA) or Techno-Economic Analysis (TEA) for environmental and economic performance. This would allow planners and policymakers to assess long-term trade-offs between safety, cost, and ecological benefit, thereby enhancing evidence-based decision-making.

Second, data quality and uncertainty management: To improve robustness, future applications should incorporate site-specific data instead of regional averages, supported by explicit treatment of uncertainty (e.g., sensitivity analyses, confidence intervals, or probabilistic scoring). Moreover, dynamic weighting mechanisms could be introduced to adapt to shifting stakeholder priorities and evolving socio-environmental conditions.

Third, operational refinement: The intended-use criteria should be refined to capture the ratio of greywater availability to demand and account for seasonal variability in irrigation needs. Future versions of the matrix could also consolidate substreams with compensating behaviour (e.g., kitchen sinks and dishwashers) and include indicators for storage capacity, system automation, and operational resilience under fluctuating climatic and demographic conditions.

Fourth, governance and institutional integration: Governance remains a critical enabler for mainstreaming greywater reuse [98,99]. Future research should explore mechanisms to embed the evaluation matrix within urban planning, permitting, and water management frameworks. This includes clarifying institutional responsibilities between water utilities, developers, and municipalities; identifying legal and administrative barriers; and developing standardized approval procedures. In addition, establishing monitoring and reporting obligations, as well as transparent data-sharing protocols among stakeholders, would enhance accountability and trust. The inclusion of participatory governance processes—such as stakeholder consultations, co-design workshops, and multi-level policy dialogues—can ensure that local knowledge and public preferences are systematically reflected in decision outcomes.

Fifth, digitalization and participatory decision support: Linking the evaluation matrix to digital tools such as GIS platforms, online dashboards, or digital twins could facilitate real-time scenario analysis and enable data-driven decision-making. Integrating AI-based monitoring of water quality and system performance would further enhance adaptive management. In parallel, participatory MCDA tools, such as the Analytic Hierarchy Process (AHP) combined with structured stakeholder workshops, could operationalize co-decision processes and strengthen legitimacy.

Beyond the developments outlined above, several further directions could enhance the scope and applicability of the evaluation matrix. First, future work should explore dynamic greywater modelling approaches that incorporate temporal variability in water generation, storage, and irrigation demand. Coupling such models with the matrix could improve sizing decisions and better represent seasonal fluctuations. Second, integrating long-term monitoring and empirical validation of greywater systems would allow calibration of reference values, verification of pollutant-load assumptions, and refinement of ecological indicators under real operating conditions. Third, deeper investigation into multi-level governance arrangements—including inter-agency coordination, operator responsibilities, financial mechanisms, and public engagement—would advance understanding of how greywater reuse can be institutionalized in urban planning processes. Fourth, extending the matrix with digital support tools, such as GIS-based modules, semi-automated data import, or scenario-based dashboards, could improve usability for practitioners and support decision-making under uncertainty. Fifth, future studies should explore integration with urban water and climate models, allowing planners to assess how district-scale reuse interacts with broader hydrological systems, stormwater management, and climate resilience strategies. Finally, comparative cross-city or cross-country applications of the matrix would help evaluate its transferability, identify which components require localization, and support the development of an internationally adaptable version of the framework.

Overall, integrating governance, data quality, and digital innovation with the technical and environmental dimensions of the framework will transform the evaluation matrix from a static assessment tool into a dynamic, institutionalized decision support system. This evolution will help ensure that greywater reuse is not only technically feasible and economically viable but also socially accepted, transparently governed, and effectively embedded in sustainable urban development strategies.