The use of high-quality potable water for non-essential activities results in substantial water waste. Therefore, treating and utilizing rainwater and greywater which contain low concentrations of organic substances and microbes as alternative domestic water resources can enhance the resilience and sustainability of water in urban areas. However, it encounters several challenges. This study was conducted in Banhstadt-Heidelberg, Germany, to estimate domestic water consumption patterns, evaluate the potential of rainwater harvesting from green roofs, and assess the public acceptability of greywater recycling. The primary data were collected from 361 Bahnstadt residents through the questionnaire to estimate water consumption. The non-potable water demand in Bahnstadt was estimated at 228 m3/day for 5,700 Bahnstadt residents. The harvestable rainfall volume was 16,017.58 m3/year. When rainwater supply was compared to water demand, the potential for rooftop rainwater harvesting was 19.25%. This value indicates that the amount of rainwater collected is less than that of non-potable water required per year. To meet the demand, rainwater harvesting will need to be supplemented with treated greywater. However, only 20.78% of respondents are willing to install a greywater recycling system, mostly because of public health concerns. Overall, this study shows the tendencies toward having green roofs with integrated rainwater harvesting and greywater recycling for the possible account of water saving.

  • The potential of rooftop rainwater harvesting in Bahnstadt was evaluated.

  • Approximately 50% of the rainwater that falls on green roofs is feasible to capture.

  • To fulfill non-potable water demands, rainwater harvesting and treated greywater deserve to be integrated.

  • Bahnstadt respondents indicated that public health concerns were the most significant limitation of greywater recycling.

The United Nations predicts that by 2050, 68% of the global population will live in urban areas, up from 55% presently. Cities are major contributors to economic growth, accounting for over 80% of global GDP (United Nations 2022). However, rapid and poorly planned urbanization generates a number of problems. Unsustainable resource consumption patterns and pollution by humans are the primary causes of climate change. Global warming has been one of the major environmental issues facing the world for decades and will remain one of the biggest threats to human existence for many years to come (Musa et al. 2021). Heavy rainfall on the one hand and scarcity of water on the other are two extreme urban threats resulting from climate change. Climate change has also resulted in extreme summer temperatures, leading to significantly higher water demand. As a result of a shift in consumption patterns that has increased the demand for water resources, water stress can occur when the amount of freshwater used exceeds the amount that can be replenished by more than 25%. Severe water stress can have devastating consequences for the environment; access to clean water is a major problem for many countries (Kishore & Lal 2023). In addition, it can hinder social and economic growth. In 2019, the global water stress rate reached 18.6% (United Nations 2022).

In several German cities, increasing investment in traditional infrastructure has been the most common method of mitigating the risks associated with climate extremes. As a result, there has been a shift toward blue-green infrastructure (BGI), a major change that recognizes the importance of incorporating urban hydrology into urban water management (Ramboll 2016). Green roofs are one type of BGI that can capture rainfall and reduce urban flooding. Green roofs can also be paired with rainwater harvesting systems to collect excess rainwater for reuse in household activities such as toilet flushing and garden irrigation. These water reuse techniques are capable of conserving tap water that meets drinking-water standards in Germany. The use of high-quality potable water for unnecessary activities results in significant water resource waste. However, when rainwater volumes are low, greywater recycling and rainwater reuse should be combined to increase the amount of household water reused. The nearly continuous production of greywater from daily household activities ensures a steady supply of reclaimed water during periods of insufficient rainwater.

This study was conducted in 2023 in the residential areas of Banhstadt, Heidelberg, one of the largest urban development projects in Germany. The study was aimed at water reuse in green roof buildings, the feasibility of rainwater harvesting, and greywater recycling.

Study area

The study area was located in the Bahnstadt district, Heidelberg, Baden-Württemberg, Germany (Figure 1). Bahnstadt is geographically located between 49°24ʹ10″N latitude and 8°39ʹ58″E longitude, with a total area of 116 hectares. Bahnstadt is one of Germany's largest urban development projects and the world's largest passive house community (Stadt Heidelberg 2022). This study will focus on the finished and occupied residential buildings in Bahnstadt.
Figure 1

Geographic location of the study area: Bahnstadt, Heidelberg, Germany (adopted from Stadt Heidelberg 2022).

Figure 1

Geographic location of the study area: Bahnstadt, Heidelberg, Germany (adopted from Stadt Heidelberg 2022).

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Data collection

The primary data were collected to obtain comprehensive information from residents of the study area by using a survey questionnaire. The target group of respondents was Heidelberg-Bahnstadt inhabitants. The current population size (N) of Bahnstadt is approximately 5,700 people (Stadt Heidelberg 2022). Therefore, the required sample size (S) is 361 people, referring to the Morgan table for determining the sample size that is representative of a given population (Krejcie & Morgan 1970). The measured value in this sample size corresponds to a 95% confidence level, a 5% margin of error, and a 50% population proportion. The survey has been conducted on-site. The secondary data were collected from databases and literature. Heidelberg's monthly precipitation in millimeters was acquired from the Climate Data Center, Deutscher Wetterdienst (DWD 2022). The catchment area, which is represented by the residential building's roof area in square meters, was measured by virtualizing a Google Earth Pro satellite image.

Data analysis

Estimation of domestic water consumption patterns

Minitab statistical software, version 21.1.0, was utilized to conduct all statistical analyses. Descriptive statistics and a 5-point Likert scale (for qualitative questions and scales) were used to describe the proportions of several independent variables.

Evaluation of rooftop rainwater harvesting potential and determination of the harvestable rainfall yield

The harvestable rainfall yield (YR,t) is the rainwater yield per time step, measured in liters per time step, and it must be calculated using the following equation (McCarton et al. 2021):
formula
(1)
where A is the horizontal projection of the collection area (m2), which is to be drained to the rainwater harvesting system, h is the total rainfall (mm) for a chosen timestep (daily, monthly, or yearly), e is the surface yield coefficient, and η is the hydraulic treatment efficiency coefficient.

Surface yield coefficient (e)

The surface yield coefficient is dependent on the collection surface. Collection surfaces made of different materials have different characteristics regarding the drainage of rainwater. The surface yield coefficient, which is defined as the ratio of water volume that runs off a surface to the volume of rainfall that falls on the surface, has an impact on the volume of harvested rainwater. In Table 1, typical values are provided for various materials (BSI 2024).

Table 1

Surface yield coefficient for different roof collection surfaces (BSI 2024)

Collection surfaceSurface yield coefficient (e)
Pitched smooth surface roof 0.9 
Pitched rough surface roof 0.8 
Flat roof, without gravel 0.8 
Flat roof, with gravel 0.7 
Extensive green roof 0.5 
Intensive green roof 0.3 
Collection surfaceSurface yield coefficient (e)
Pitched smooth surface roof 0.9 
Pitched rough surface roof 0.8 
Flat roof, without gravel 0.8 
Flat roof, with gravel 0.7 
Extensive green roof 0.5 
Intensive green roof 0.3 

Hydraulic treatment efficiency coefficient (η)

The hydraulic treatment efficiency coefficient is the ratio of the outgoing flow of the treated water to the incoming flow of the collected rainwater. The hydraulic treatment efficiency coefficients of a filtration system are in the range of 0.75 to 0.9. If this value is not provided by the manufacturer, 0.9 is normally assumed (BSI 2024).

Determination of the non-potable water demand

The total daily non-potable water demand per household (DN,d) in liters per day (L/d) is calculated using forecasted uses, frequency, and seasonality. The demand varies greatly depending on the region, climate, and the type of building. Demand is also influenced by occupancy levels and socioeconomic status. The non-potable water demand should be calculated as follows (McCarton et al. 2021):
formula
(2)
where Dp,d is the daily per person non-potable water demand in liters per person per day (LPD), and n is the number of persons in the connected buildings.

Determination of the annual rainwater harvesting supply coefficient

The following equation can be used to figure out the annual rainwater harvesting supply coefficient (S) from the annual rainwater yield and the annual non-potable demand (McCarton et al. 2021):
formula
(3)
where YR,a is the annual harvestable rainfall yield, and DN,a is the annual non-potable water demand.

Evaluation of greywater recycling potential

The evaluation of greywater recycling potential in this research focused on aspects of public acceptance and perceptions. The 5-point Likert scale was used to convert the opinions of respondents into a mean of satisfaction, with responses ranging from strongly agree to agree, neutral, disagree, and strongly disagree. The two primary questions handed out were participants’ perspectives on greywater treatment solutions for Bahnstadt residential buildings and the challenges to implementing greywater reuse.

Domestic water consumption patterns in Bahnstadt

As shown in Figure 2, the majority of respondents typically take a daily shower for around 10 min. The majority have conventional shower heads and dual-flush commodes. The typical frequency for laundry loads is once per week. Nevertheless, the majority of people did not have gardens and those who did watered them on average once per week. For dishwashing, cooking, and gardening, the water volume was estimated using data from the Federal Association of Energy and Water Industries (BDEW 2022). It is estimated that dishwashing requires 7.6 L/d, cooking and drinking require 5.1 l/d, and cleaning and gardening require 7.6 L/d (Figure 2).
Figure 2

Estimated water demand for each non-potable water activity in Bahnstadt.

Figure 2

Estimated water demand for each non-potable water activity in Bahnstadt.

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In Table 2, the daily household water consumption volume was evaluated. The average household water consumption in Bahnstadt was estimated to be 135 LPD, which is significantly higher than the average household water consumption in Germany in 2021 at approximately 20 liters.

Table 2

The comparison of water consumption in each household activity between Bahnstadt and Germany

Household activitiesHousehold water consumption (LPD)
BahnstadtGermanya
Shower 85.00 45.70 
Toilet flushing 21.00 34.30 
Laundry 8.57 15.20 
Dishwashing 7.60 7.60 
Cooking and drinking 5.10 5.10 
Cleaning and gardening 7.60 7.60 
Total 134.87 115.50 
Household activitiesHousehold water consumption (LPD)
BahnstadtGermanya
Shower 85.00 45.70 
Toilet flushing 21.00 34.30 
Laundry 8.57 15.20 
Dishwashing 7.60 7.60 
Cooking and drinking 5.10 5.10 
Cleaning and gardening 7.60 7.60 
Total 134.87 115.50 

aHousehold water consumption in Germany is derived from the ‘Federal Association of Energy and Water Industries (BDEW) 2022’.

Rooftop rainwater harvesting potential of Bahnstadt residential buildings

The collection area (A) of residential buildings in Bahnstadt is around 49,734 m2. The residential roofs of Bahnstadt are primarily extensive green roofs; therefore, the surface yield coefficient (e) of 0.5 indicates that just approximately 50% of the rainfall that falls on a rooftop can be captured. The hydraulic treatment efficiency coefficient (η) is presumed to be 0.9 due to its unavailability from the manufacturer. The results showed that the average annual rainfall depth in Heidelberg for the 10 years from 2013 to 2022 was 715.70 mm, resulting in an annual harvestable rainfall yield of 16,017.58 m3/year (Table 3).

Table 3

The harvestable rainfall yield for the residential buildings in Bahnstadt

MonthsAverage monthly rainfall depths (mm)Harvestable rainfall yield (liters per month)Harvestable rainfall yield (m3 per month)
hYR,tYR,t
Jan 63.10 1,412,196.93 1,412.20 
Feb 48.20 1,078,730.46 1,078.73 
Mar 41.40 926,544.42 926.54 
Apr 42.00 939,972.60 939.97 
May 75.50 1,689,712.65 1,689.71 
Jun 72.40 1,620,333.72 1,620.33 
Jul 60.00 1,342,818.00 1,342.82 
Aug 55.90 1,251,058.77 1,251.06 
Sep 66.30 1,483,813.89 1,483.81 
Oct 66.10 1,479,337.83 1,479.34 
Nov 61.70 1,380,864.51 1,380.86 
Dec 63.10 1,412,196.93 1,412.20 
  Total 16,017.58 
MonthsAverage monthly rainfall depths (mm)Harvestable rainfall yield (liters per month)Harvestable rainfall yield (m3 per month)
hYR,tYR,t
Jan 63.10 1,412,196.93 1,412.20 
Feb 48.20 1,078,730.46 1,078.73 
Mar 41.40 926,544.42 926.54 
Apr 42.00 939,972.60 939.97 
May 75.50 1,689,712.65 1,689.71 
Jun 72.40 1,620,333.72 1,620.33 
Jul 60.00 1,342,818.00 1,342.82 
Aug 55.90 1,251,058.77 1,251.06 
Sep 66.30 1,483,813.89 1,483.81 
Oct 66.10 1,479,337.83 1,479.34 
Nov 61.70 1,380,864.51 1,380.86 
Dec 63.10 1,412,196.93 1,412.20 
  Total 16,017.58 

In Table 4, the total daily demand for non-potable water was 228 m3/day, which was determined by a projected volume of 40 LPD and the 5,700 inhabitants of Bahnstadt.

Table 4

Non-potable water demand for residential buildings in Bahnstadt

Non-potable water demandValueUnit
Bahnstadt's population 5,700 Inhabitants 
Daily water consumption per capita 40 LPD 
Total daily non-potable water demand (DN,d228 m3/day 
Total monthly non-potable water demand (DN,m7,068 m3/month 
Total annual non-potable water demand (DN,a83,220 m3/year 
Non-potable water demandValueUnit
Bahnstadt's population 5,700 Inhabitants 
Daily water consumption per capita 40 LPD 
Total daily non-potable water demand (DN,d228 m3/day 
Total monthly non-potable water demand (DN,m7,068 m3/month 
Total annual non-potable water demand (DN,a83,220 m3/year 

The monthly demand for non-potable water exceeds the amount of rainfall that can be collected. Negative values in the deficit column indicated that the total annual harvestable rainfall yield was less than the total annual non-potable water demand of 67,202.42 cubic meters per year. The maximum value of the rainwater harvesting supply coefficient (S) was 23.91% in May, and the minimum value was 13.11% in March. In summary, the average annual supply efficiency is 19.25%. Consequently, the quantity of water needed should be supplied from other alternative sources to fulfill the non-potable water demand (Table 5).

Table 5

Annual rainwater harvesting supply coefficient for residential buildings in Bahnstadt

MonthsAverage monthly rainfall depths (mm)Harvestable rainfall yield (m3)Non-potable water demand (m3)Deficit or surplusAnnual rainwater harvesting supply coefficient, S (%)
hYR,mDN,mYR,mDN,m(YR,m/DN,m) × 100
Jan 63.10 1,412.20 7,068 −5,655.80 19.98 
Feb 48.20 1,078.73 6,384 −5,305.27 16.90 
Mar 41.40 926.54 7,068 −6,141.46 13.11 
Apr 42.00 939.97 6,840 −5,900.03 13.74 
May 75.50 1,689.71 7,068 −5,378.29 23.91 
Jun 72.40 1,620.33 6,840 −5,219.67 23.69 
Jul 60.00 1,342.82 7,068 −5,725.18 19.00 
Aug 55.90 1,251.06 7,068 −5,816.94 17.70 
Sep 66.30 1,483.81 6,840 −5,356.19 21.69 
Oct 66.10 1,479.34 7,068 −5,588.66 20.93 
Nov 61.70 1,380.86 6,840 −5,459.14 20.19 
Dec 63.10 1,412.20 7,068 −5,655.80 19.98 
Total 715.70 16,017.58 83,220 −67,202.42 19.25 
Average 59.64     
MonthsAverage monthly rainfall depths (mm)Harvestable rainfall yield (m3)Non-potable water demand (m3)Deficit or surplusAnnual rainwater harvesting supply coefficient, S (%)
hYR,mDN,mYR,mDN,m(YR,m/DN,m) × 100
Jan 63.10 1,412.20 7,068 −5,655.80 19.98 
Feb 48.20 1,078.73 6,384 −5,305.27 16.90 
Mar 41.40 926.54 7,068 −6,141.46 13.11 
Apr 42.00 939.97 6,840 −5,900.03 13.74 
May 75.50 1,689.71 7,068 −5,378.29 23.91 
Jun 72.40 1,620.33 6,840 −5,219.67 23.69 
Jul 60.00 1,342.82 7,068 −5,725.18 19.00 
Aug 55.90 1,251.06 7,068 −5,816.94 17.70 
Sep 66.30 1,483.81 6,840 −5,356.19 21.69 
Oct 66.10 1,479.34 7,068 −5,588.66 20.93 
Nov 61.70 1,380.86 6,840 −5,459.14 20.19 
Dec 63.10 1,412.20 7,068 −5,655.80 19.98 
Total 715.70 16,017.58 83,220 −67,202.42 19.25 
Average 59.64     

Characteristics of rainwater in Heidelberg, Germany

The acceptable concentrations of pH, total dissolved solids, nitrates, sulfate, and zinc in potable water, according to guidelines issued by the World Health Organization (2022), are as follows: 6.50–8.50, 1,000, 50, 500, and 3 mg/L, respectively. As reported by Chowfin et al. (2024), the rainwater quality in Heidelberg from August to September 2023 satisfied the WHO guideline values (Table 6).

Table 6

A comparison between Heidelberg's rainwater quality and WHO drinking-water quality guidelines

ParametersUnitsCharacteristics of rainwater in Heidelberg (Chowfin et al. 2024)
 Mean ± SD
WHO guidelines for drinking-water quality (WHO 2022)
 Limits
pH pH units 7.11 ± 0.08 6.50–8.50 
DO mg/L 1.76 ± 0.57 – 
TDS mg/L 43.9 ± 3.50 1,000 
Conductivity μS/cm 47.56 ± 4.47 – 
Nitrates mg/L 0.59 ± 0.03 50 
Phosphate mg/L 0.12 ± 0.01 – 
Sulfate mg/L 0.00 ± 0.00 500 
Zinc mg/L 0.08 ± 0.01 
ParametersUnitsCharacteristics of rainwater in Heidelberg (Chowfin et al. 2024)
 Mean ± SD
WHO guidelines for drinking-water quality (WHO 2022)
 Limits
pH pH units 7.11 ± 0.08 6.50–8.50 
DO mg/L 1.76 ± 0.57 – 
TDS mg/L 43.9 ± 3.50 1,000 
Conductivity μS/cm 47.56 ± 4.47 – 
Nitrates mg/L 0.59 ± 0.03 50 
Phosphate mg/L 0.12 ± 0.01 – 
Sulfate mg/L 0.00 ± 0.00 500 
Zinc mg/L 0.08 ± 0.01 

DO = dissolved oxygen; TDS = total dissolved solids.

Characteristics of domestic greywater in Germany

Rahman et al. (2023) performed a study in a German household to assess the quality of greywater. In Table 7, the findings indicate that the concentration of Escherichia coli in untreated greywater exceeds the WHO guideline value, which is considered safe for use in restricted irrigation. Greywater used to cultivate crops that are not directly consumed by humans must be treated.

Greywater recycling potential in Bahnstadt in terms of public acceptance

Bahnstadt respondents rated each greywater recycling challenge on a 5-point Likert scale based on their perception (Table 8).

The results of domestic water consumption patterns indicate that residents of Bahnstadt take a shower once a day for up to 10 min with standard showerheads, causing a large amount of water consumption. According to statistics on domestic water consumption, showers consume an average of 8–9 liters per minute (European Environment Agency 2018). It is estimated that the water consumption for showers in Bahnstadt is 85 LPD. On the other hand, due to the use of water-saving toilets by all respondents, Bahnstadt's water consumption for toilet flushing was lower than Germany's average. Similarly, water consumption for laundry was lower than in Germany, as Bahnstadt respondents did laundry only once per week. Therefore, the non-potable water demand in Bahnstadt is estimated at 40 LPD. In a bigger image, the total residential water consumption in Bahnstadt was 134.87 LPD, which is about 20 liters higher than Germany's average (Table 2); however, it was roughly the same as Amsterdam's volume of 133.8 LPD (Waternet 2020). The average household water consumption in Berlin and Hamburg was 119 and 140 LPD, respectively (Statistische Ämter des Bundes und der Länder 2019).

In order to assess the potential of rainwater harvesting, the residential roofs of Bahnstadt are primarily extensive green roofs; therefore, approximately 50% of the rainfall that falls on a rooftop can be captured. The 10-year average annual rainfall depth in Heidelberg from 2013 to 2022 was 715.70 mm, resulting in an annual harvestable rainfall yield of 16,017.58 m3/year. Furthermore, the annual total demand for non-potable water was 83,220 m3/year. The annual rainwater harvesting supply coefficient is equal to 19.25%. This value indicates that the amount of rainwater that can be collected is less than the amount of non-potable water that the Bahnstadt residents demand per year. In the case of a metal roof in Dublin (Ireland), 90% of the rainwater could be collected. The rainwater harvesting potential was 36%, which is significantly higher than the green roof catchment surface (McCarton et al. 2021).

In certain situations, rainwater can serve as a significant domestic water source and is also a valuable component for blending with other water sources in order to mitigate the presence of health-hazardous contaminants. Nevertheless, the quality of rain may decline as a result of collecting, storage, and household use. Microbial concentrations tend to be highest in the first flush of rainwater and subsequently decline as the rainfall continues. During rainy seasons, the level of microbial contamination decreases, especially when catchments receive regular rinses with fresh rainwater. The initial discharge of runoff should not be collected in storage; therefore, an automatic device that diverts the contaminated first flow of rainwater from roof surfaces is required (WHO 2022). In sporadic rainfall events in Heidelberg, the rainwater quality values studied by Chowfin et al. (2024) were within the range of the WHO drinking-water quality guidelines. Nevertheless, it is essential to monitor and purify the rainwater prior to using it for potable purposes. For the Bahnstadt area, where there was not a significant amount of rainfall, it will be necessary to supplement the rainwater harvest with other water sources to meet the demand for non-potable water. According to the BGI in Germany in Stuttgart's impulse project, both slightly polluted greywater and the rainwater runoff from a roof area of temporary workers' apartments are combined and used as water resources for irrigation. Consequently, the continuous production of greywater ensures a steady supply of water for irrigation during periods of low rainfall (Eisenberg et al. 2021).

Greywater generally has a lower concentration of pathogens in comparison to blackwater. Nevertheless, the presence of pathogens in greywater may present a significant hazard to both human health and the environment, such as the risks associated with infectious diseases. Consequently, greywater should be kept away from human contact, and its potential for contaminating potable water sources should be limited (WHO 2006). As shown in Table 7, the concentrations of E. coli in untreated domestic greywater in Germany were higher than the WHO guideline limit. On the other hand, the implementation of constructed wetland roofs resulted in a reduction in both the chemical oxygen demand (COD) and biological oxygen demand (BOD5) values (Rahman et al. 2023). Nevertheless, E. coli advised the implementation of supplementary disinfection treatments in order to comply with secure water reuse criteria.

Table 7

A comparison between the characteristics of greywater in Germany and WHO guidelines for the safe use of greywater

ParametersUnitsCharacteristics of untreated domestic greywater in Germany (Rahman et al. 2023) Mean ± SDWHO guidelines for the safe use of greywater: restricted irrigation (WHO 2006)
Limits
BOD5 mg/L 285 ± 169 – 
COD mg/L 744 ± 653 – 
TSS mg/L 344 ± 687 – 
Total N mg/L 34 ± 16 – 
NH4 mg/L 16 ± 7 – 
Total P mg/L 2.5 ± 2.6 – 
E. coli Numbers 100 mL 2.8 × 106 ± 4.1 × 106 105 
ParametersUnitsCharacteristics of untreated domestic greywater in Germany (Rahman et al. 2023) Mean ± SDWHO guidelines for the safe use of greywater: restricted irrigation (WHO 2006)
Limits
BOD5 mg/L 285 ± 169 – 
COD mg/L 744 ± 653 – 
TSS mg/L 344 ± 687 – 
Total N mg/L 34 ± 16 – 
NH4 mg/L 16 ± 7 – 
Total P mg/L 2.5 ± 2.6 – 
E. coli Numbers 100 mL 2.8 × 106 ± 4.1 × 106 105 

TSS = total suspended solids.

In terms of evaluations of greywater recycling challenges, Bahnstadt respondents overwhelmingly agree that public health concerns are the most significant limitation, followed by investment and operations and maintenance, respectively (Table 8). For water reuse regulations, government commitment, wastewater treatment technology, and public and stakeholder engagement, respondents agree that these factors present an obstacle for greywater recycling, but in lower importance (Table 8). Despite the fact that the public engagement challenge received the lowest rating from Bahnstadt's respondents, reviews revealed that public participation and willingness in water-saving programs is one of the main barriers to wastewater reuse, particularly in terms of general tendencies. Similarly, in a study conducted by Po et al. (2003), it was pointed out that some people dislike using reused water because they believe it is contaminated, and they are afraid of contracting an infection after using it. According to research on the acceptability of recovered greywater in Gothenburg (Sweden), recycled greywater would acquire acceptance easily if the water could be used for washing machines, dishwashers, and toilet flushing (Edström 2022). Thus, social encouragement programs to describe the range of benefits and possible concerns of reused greywater to people, particularly in arid countries, would be highly needed to increase the amount of water reusing as much as possible.

Table 8

Descriptive statistics of the perceptions of respondents regarding the greywater recycling barriers in households (5-point Likert scale; 1 = very disagree, 5: very agree)

VariableTotal countCumNMeanSD
Wastewater treatment technology 361 263 3.52 0.80 
Water reuse limitations and regulations 361 263 4.11 0.43 
Public health concerns 361 263 4.77 0.41 
Investment 361 263 4.57 0.60 
Operations and maintenance 361 263 4.27 0.67 
Government commitment 361 263 3.55 0.80 
Public and stakeholder engagement 361 263 3.43 0.63 
VariableTotal countCumNMeanSD
Wastewater treatment technology 361 263 3.52 0.80 
Water reuse limitations and regulations 361 263 4.11 0.43 
Public health concerns 361 263 4.77 0.41 
Investment 361 263 4.57 0.60 
Operations and maintenance 361 263 4.27 0.67 
Government commitment 361 263 3.55 0.80 
Public and stakeholder engagement 361 263 3.43 0.63 

Note: CumN represents the number of respondents that responded to this question.

In conclusion, rooftop rainwater harvesting techniques could be implemented in residential buildings to conserve household water supplies. Nevertheless, the potential for rainwater harvesting from extensive green roofs was lower compared to that of smooth surface roofs due to rainwater drainage characteristics. With regard to this study, roughly half of the rainwater that falls on a catchment surface could be captured, resulting in a potential harvest of 19.25% from Bahnstadt residences. Additionally, the proposals for combining treated greywater with precipitation to maximize water reuse volume face several hurdles. The majority of residents were concerned about public health issues. Nevertheless, finding the best ways to encourage people to contribute to more water-saving programs according to futuristic water shortage trends would be of help.

This study was supported by the School of Engineering and Architecture at SRH University Heidelberg.

The authors received no financial support for the research and publication of this article.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

British Standards Institution (BSI)
2024
BS EN 16941-1:2024. On-Site Non-Potable Water Systems – Part 1: Systems for the Use of Rainwater
.
BSI Standards Limited
,
London
.
ISBN: 978 0 539 31248 5
.
Chowfin
A.
,
Gluvakovic
N.
&
Gayh
U.
2024
Effect of rainfall on water parameters in recreational lakes in Heidelberg, Germany
.
IgMin Research
2
(
2
),
121
126
.
Deutscher Wetterdienst (DWD)
2022
Monthly Station Observations of Precipitation in mm
.
Climate Data Center
,
Offenbach, Germany
.
Available from: https://cdc.dwd.de/portal/202209231028/mapview (accessed 18 January 2023)
.
Edström
N.
2022
Greywater Recovery in Buildings-Performance Possibilities Actors and Strategies: Case Study of HSB Living Lab, Gothenburg
.
KTH Royal Institute of Technology
. .
Eisenberg
B.
,
Morandi
C.
,
Richter
P.
,
Well
F.
&
Ludwig
F.
2021
The impulse project Stuttgart — stimulating resilient urban development through blue-green infrastructure
. In:
Building Resilience to Natural Hazards in the Context of Climate Change
(Hutter, G., Neubert, M. & Ortlepp, R., eds.). Springer, Berlin, Heidelberg, Dordrecht and New York, pp.
156
171
. https://doi.org/10.1007/978-3-658-33702-5_7.
European Environment Agency
2018
Water Use at Home
. .
Federal Association of Energy and Water Industries (BDEW)
2022
Trinkwasserverwendung im haushalt 2021: Durchschnittswerte bezogen auf die wasserabgabe an haushalte und kleingewerbe [Drinking Water Use in the Household 2021: Average Values Related to the Water Delivery to Households and Small Businesses]. Available from: https://www.umweltbundesamt.de/sites/default/files/medien/384/bilder/dateien/2_abb_trinkwasserverwendung-hh_2022-10-14.xlsx (accessed 23 February 2023)
.
Kishore
T. D.
&
Lal
A.
2023
Coastal groundwater salinization due to saltwater intrusion new insights from a preliminary in-situ assessment
.
International Aquatic Research
15
(
1
),
65
74
.
https://doi.org/10.22034/IAR.2023.1963612.1304
.
Krejcie
R. V.
&
Morgan
D. W.
1970
Determining sample size for research activities
.
Educational and Psychological Measurement
30
,
607
610
.
https://doi.org/10.1177/001316447003000308
.
McCarton
L.
,
O'Hogain
S.
&
Reid
A.
2021
Rainwater harvesting systems
.
The Worth of Water
83
95
.
https://doi.org/10.1007/978-3-030-50605-6_5
.
Musa
N.
,
Low
C. F.
,
Ramli
H.
,
Abd Manaf
M. T.
,
Lee
K. L.
,
Aileen Tan
S. H.
,
Aznan
A. S.
&
Musa
N.
2021
Prominent vulnerability of red hybrid tilapia (Oreochromis spp.) liver to heat stress-induced oxidative damage
.
International Aquatic Research
13
(
2
),
109
118
.
https://doi.org/10.22034/IAR.2021.1924489.1140
.
Po
M.
,
Kaercher
J. D.
&
Nancarrow
B. E.
2003
Literature review of factors influencing public perceptions of water reuse
.
CSIRO Land and Water
.
https://doi.org/10.4225/08/5867f411b42b6
.
Rahman
K. Z.
,
Chen
X.
,
Blumberg
M.
,
Bernhard
K.
,
Müller
R. A.
,
Mackenzie
K.
,
Trabitzsch
R.
&
Moeller
L.
2023
Effect of hydraulic loading rate on treatment performance of a pilot wetland roof treating greywater from a household
.
Water
15
(
9
),
3375
.
https://doi.org/10.3390/w15193375
.
Ramboll
2016
Strengthening Blue-Green Infrastructure in our Cities: Enhancing Blue-Green Infrastructure & Social Performance in High Density Urban Environment
.
Ramboll Liveable Cities Lab
,
Überlingen, Germany
. .
Stadt Heidelberg
2022
Städtebauliche rahmenplanung: Heidelberg Bahnstadt [Urban Framework Planning: Heidelberg Bahnstadt]. Heidelberg, Germany. Available from: https://www.heidelberg.de/site/HD_Satelliten/get/documents_E373201993/heidelberg/Objektdatenbank/Bahnstadt/heidelberg-bahnstadt.de/Pdf/2022_Rahmenplan_Begruendung.pdf (accessed 10 December 2022)
.
Statistische Ämter des Bundes und der Länder
2019
Wasserabgabe der öffentlichen Wasserversorgung [Water Supply from the Public Water Supply]. Available from: https://www.statistikportal.de/de/wasserabgabe-der-oeffentlichen-wasserversorgung.
United Nations
2022
The Sustainable Development Goals Report 2022
.
Department of Economic and Social Affairs (DESA)
,
New York, USA
. .
Waternet
2020
Average Household Water Use Per Day in Amsterdam 2020
.
Amsterdam, Netherlands
. .
WHO
2006
Guidelines for the Safe use of Wastewater, Excreta and Greywater: Volume 1 Policy and Regulatory Aspects
.
Geneva, Switzerland
.
Available from: https://www.who.int/publications/i/item/9241546824 (accessed 22 April 2024)
.
WHO
2022
Guidelines for Drinking-Water Quality: Fourth Edition Incorporating the First and Second Addenda
.
Geneva, Switzerland
.
Available from: https://www.who.int/publications/i/item/9789240045064 (accessed 22 April 2024)
.
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