The Potential of RainWater Harvesting Systems in Europe – Current State of Art and Future Perspectives

RWH systems have a long history, described in many works (Fewkes 2012; Angelakis 2016; Yannopoulos et al. 2017). They were utilized less in the 20th century due to better supply technologies, but they are presently becoming more popular as a water source worldwide. Despite advancements in technology, providing clean drinking water remains a challenge worldwide due to increasing demand and depleting traditional sources (Lieste et al. 2007; Schets et al. 2010; Santos et al. 2020). These negative impacts are caused by unsustainable human activity, population increase, urbanization, and climate change (Che-Ani et al. 2009). Water supply systems often face long-distance water distribution issues (from water intake to the users). On hot days, when demand is high, the water supply will face one of their biggest challenges (Rak et al. 2021). In the event of waterworks failure or inefficient water supply wells, RWH systems provide an alternative source of water (Marlow et al. 2013), and sometimes may be even the principal source (de Sá Silva et al. 2022). RWH can reduce water stress and be an adaptation strategy to climate change (Hassell 2005; Yannopoulos et al. 2019).

RWH is not confined to water-scarce areas. Rainwater is free and decreases water companies’ costs. Energy used to collect, purify, distribute, and store water accounts for most of these expenditures, e.g. in Germany it accounts for 1.71 kWh/m³, and it is estimated that water supply systems use 7% of global electricity (Wakeel et al. 2016). Alternative water sources reduce tap water and energy use. However, RWH systems use electricity, therefore energy consumption is a key factor in assessing their profitability (de Sá Silva et al. 2022).

Due to reduced tap water consumption, RWH extend the lifespan of centralized water distribution infrastructure (Lopes et al. 2017; Viola et al. 2023) and can extend house devices life due to the purity of rainwater (Che-Ani et al. 2009). Many implementation manuals and design guidelines define local RWH system design configurations, including German (DIN 1989), British (British Standards Institute 2013a, b), Italian (UNI 2012), and French industrial standards (AFNOR 2011). RWH systems are cheap and easy to maintain. Users benefit from their modularity, which allows extension, reconfiguration, and transfer (Che-Ani et al. 2009), as well as from reduction of tap water bills, e.g.in a typical Slovak household it accounts for €225/year (Pavolová et al. 2019). RWH systems improve the state economy by reducing tap water use and water purification expenses (Vargas-Parra et al. 2013). Reduced environmental pollution, heat waves, and fine dust (Dallman et al. 2021; Nandi and Gonela 2022; Jin et al. 2023), reduction of new investments in aging or expanding urban water infrastructure, and expanding water infrastructure are other benefits (Lopes et al. 2017).

Rainwater should be also seen as a hazard that must be controlled. Urban floods, erosion, landslides, and unexpected surges undermine infrastructure due to severe rainfall. Climate change affecting air temperature and precipitation patterns, and increases flood frequency (Faragò et al. 2019; Hofman-Caris et al. 2019; Bieroza et al. 2021). Urban drainage systems typically cannot absorb climate-induced rainwater streams (Hussain et al. 2022; Kumar et al. 2022), but can be relieved through rainwater collection, which makes RWH a strategy for minimizing the flood risk and surface water quality degradation (Palla et al. 2017; Stephan and Stephan 2017). New research has examined the application of RWH systems to reduce soil erosion (Jiang et al. 2013), urban runoff volumes and peaks (Malinowski et al. 2015), urban drainage system pressure and electricity demand (de Sá Silva et al. 2022).

Proper rainwater management reduces dry conditions and boosts resilience. Technical, economic, and social innovations are needed (Musz-Pomorska et al. 2020). RWH systems improve water use and energy efficiency (water-energy nexus), and bring environmental and social advantages (de Sá Silva et al. 2022). However, these systems need government actions to make them financially attractive (Wanjiru and Xia 2018) and numerous European governments begun legal or subsidy schemes to promote RWH, similarly as the EU (Campisano et al. 2013). RWH implementation and technology selection depend heavily on local rules, and varies between European countries (Campisano et al. 2017; Musz-Pomorska et al. 2020). Scientific literature also shows growing interest in RWH systems - in 2017, 57% of publications about rainwater collection were published in Europe (Morote et al. 2020).

Germany is one of the leaders in the RWH systems use and research. The concept of using rainwater appeared in 1970–1975 due to management issues with water supply and sewage systems (Fewkes 2012). It evolved in the 1980s with the focus on low impact development technologies (García Soler et al. 2018). In the 1990s, the conceptualization of combining several decentralized rainwater management techniques appeared, which is now the approach being a mandatory element of integrated storm water management (DWA 2016). Rainwater must be kept on site whenever possible under German water law (Fletcher et al. 2015). RWH systems became popular because urban drainage fees are based on impermeable property surfaces - disconnecting roofs from the sewage system is profitable (Herrmann and Schmida 2000). Around 75,000 RWH systems are installed in Germany each year, and 2/3 of newly built homes have them. Property owners only need to notify water supply companies about construction, no authorization is needed. Bremen subsidize RWH installation at 1/3 of the cost of residential RWH. Thuringia offers residential property program loans up to EUR 5,000. Saarland and Schleswig-Holstein both provide subsidies (Schuetze 2013).

For climate change adaptation, the Netherlands has set policy goals to increase urban water retention capacity. The first Water Plan (2001), Water City 2035 (2005), the transformation to the second integrated Water Plan (2007), the Rotterdam Climate Proof Program (2008), the Rotterdam Adaptation Strategy (2013), the Resilience Program (2014), and the Water Sensitive Rotterdam Program (2015) are programs related to the water management vision in Rotterdam. Rotterdam Water City 2035, a non-official policy approach produced by urban planners and urban water experts, is noteworthy (de Graaf and der Brugge 2010).

Great Britain also protects water resources. The 2008 Future Water Program (DEFRA 2008) aimed to cut tap water use to 130 L per person per day by 2030. It is achievable when using rainwater for flushing toilets or watering plants. About 7,500 residential and commercial RWH systems were installed in 2011 (Fewkes 2012). There is no regulation in the UK for them, but there is a code of practice (BS 8515:2009) for system design, installation, maintenance, water quality, and risk management (Ministry of Housing 2009).

RWH systems are not well-developed in other European countries. Italian researchers studied rainfall efficiency for rainwater use (Paciarotti, C.; Ciarapica, F.E.; Giacchetta; Liuzzo et al. 2016). The main obstacles to RWH system development in Italy were consumer reluctance and lack of design guidelines. Several Spanish towns mandate RWH system installation for new buildings with gardens (Domènech and Saurí 2011; Campisano et al. 2017). France imposed legal regulations in 2008 (now withdrawn) and a tax relief (Article 49 of the Water Act of December 30, 2006). RWH was used in sustainable urban development systems in a former big industrial and port area in southern Stockholm (Campisano et al. 2017). In Flanders (Belgium), new buildings with roofs over 100 m² must have RWH systems (De Gouvello et al. 2014). RWH is also becoming popular in Austria (Knoll et al. 2021), Switzerland, Denmark, and Poland (Carollo et al. 2023; Viola et al. 2023), where the main factor in RWH system development is the price of tap water (Campisano et al. 2017).

Climate change threatens water supply worldwide by changing meteorological variables (precipitation patterns, air temperature, humidity), affecting extreme weather events and heatwaves. RainWater Harvesting has been studied in susceptible places to improve community resilience to unexpected weather and climate change affecting water availability. The intricate interactions of air circulation, precipitation patterns, and climatic fluctuations affect European RWH systems. Exposure projections show substantial hazard scenarios, especially those associated with rising temperatures and geographical patterns largely regulated by local climate (Forzieri et al. 2016). European heatwaves and precipitation patterns are affected by the NAO and atmospheric blockage. The NAO phase index, in particular, has been associated with changes in precipitation distribution, e.g. with NAO positive, unrelated events leading to increased precipitation and cold air in the southern part of Europe, resulting in high temperatures contracting to the northern part of Europe (Li et al. 2020).Variations in regional air circulation across Europe during the Last Glacial Maximum affected precipitation patterns in Southern, Central, and Eastern Europe (Ludwig et al. 1955). Long-term variations in seasonal rainfall patterns in Europe have been connected to dynamics, highlighting the complex interaction between meteorological dynamics and precipitation patterns (Hoffmann and Spekat 2021). From the 1960s to 1990, synoptic weather patterns decreased anticyclonic and increased cyclonic, decreasing solar irradiance variability in Northern Europe (Parding et al. 2016). The need to understand and anticipate atmospheric circulation and precipitation patterns is also underscored by uncertainties in estimating future atmospheric river changes and their effects on heavy precipitation across Europe (Gao et al. 2016). Northern Europe has seen a 10–40% rise in rainfall, while Southern Europe has seen a 20% decline. This could lead to a decrease in drinking water availability in Southern Europe, affecting water resources (Senent-Aparicio et al. 2017; Dezsi et al. 2018; Gwoździej-Mazur et al. 2022).

Understanding how climate change affects RWH in Central European households, is crucial. Summerville and Sultana (Summerville and Sultana 2019) discovered that household RWH in 46 European cities can save 20–100% of non-potable water, depending on climate zones. Rainwater quality is affected by shifting European precipitation patterns. The health dangers assessment related to rainwater obtained from different rooftops has emphasized the need for a rigorous inquiry to determine the true effects of continuous rainwater consumption and alternative solutions (Hamilton et al. 2019; Osayemwenre and Osibote 2021; Juiani et al. 2023). These studies reveal the practical ramifications of RWH systems under varied meteorological circumstances and their efficiency in regulating precipitation outside Europe, making this topic global.

RWH systems collect rainwater generated from roofs, terraces, and courtyards during rainfall events and store it in reservoirs to meet water demands for various uses (Zhang et al. 2019; Teston et al. 2022; Jegnie et al. 2023). Households use 10% of EU water for hygiene, flushing toilets, washing, cooking, watering gardens, and other cleaning activities (EEA 2017). The majority of tap water (62%) is used for hygiene and toilet flushing. Detailed water use structure in selected European countries is given in Online Resource 1. Water consumption purposes can be divided into potable and non-potable uses. In the first scenario, water must meet strict quality standards because it has contact with humans and may harm them. Potable uses include drinking, cooking, and hygiene. Due to no human contact, the water may be lower quality in the second situation. These include toilet flushing, laundry, cleaning, car washing, and garden watering (Antunes et al. 2020).

Rainwater replaces tap water for non-potable and potable uses worldwide (Campisano et al. 2017; Zhang et al. 2019). Depending on meteorological circumstances, RWH systems in residences can save 12–100% tap water. RWH is mostly pushed in wealthy nations like Belgium, France, and Germany to supplement non-potable uses including toilet flushing, clothes washing, outside washes, and irrigation (Silva et al. 2015). Still, using rainwater just for toilet flushing can cut tap water use by one-third for an average family (Nolde 2007; Campisano et al. 2013). Rainwater is eagerly used for home applications due to its often better properties in terms of composition compared to hard tap water (Sklenárová 2011):

In certain nations, rainwater is only used for non-potable purposes due to industrial air pollution and severe water quality laws, e.g. according to The Netherlands’ water regulations (Schets et al. 2010) household rainwater can only be used as gray water for flushing toilets (a consequence of hundreds of people getting sick from low-quality home water due to drinking water and rainwater networks cross-connection). The standards warn of the danger of infection from gray water exposure – it should be less than one infection per 10,000 persons yearly. No worldwide standard governs potable or non-potable rainwater quality. In Germany, the DIN 1989 standard allows rainwater to be used without restrictions for non-potable purposes. Simultaneously, the German Drinking Water Act requires using tap water for washing (Schuetze 2013). The DL 23/95 in Portugal limits the use of non-tap water to pavement washing, irrigation, firefighting, and nonfood-related industrial production. Rainwater cannot be used for drinking or personal hygiene in France (Vialle et al. 2015).

Rainwater is used for outdoor purposes (Sample and Liu 2014; Zhang et al. 2019): watering lawns and gardens, composting, washing vehicles and equipment, fire protection, outdoor ponds, swimming pools, cleaning the exterior of buildings. Plants develop better when watered with rainwater (as being chlorine-free and salt-free). Plants grow faster by developing larger root system and become less sensitive to droughts by reducing salt accumulation in the soil (rainwater “moves” salts away from the root zone) (Che-Ani et al. 2009; Salehi-Lisar and Bakhshayeshan-Agdam 2016). Although there is limited study on using rainwater for urban agriculture, it can be used to grow food in the city (Hume et al. 2022). In Antalya, where half of Turkey’s greenhouses are located, RWH systems may cover plant needs (Ertop et al. 2023). Recently, green roofs and rain gardens have gained popularity due to improving urban microclimate. They require water for maintenance and using harvested rainwater saves water and cools the urban heat island (Fewkes 2012). The research on rainfall-runoff relation between green roof and RWH system in Paris has been investigated (Xie et al. 2023).

Rainwater is frequently collected in office buildings, which utilize approximately 50% of the water for flushing toilets (Griggs, J.C., Shouler, M.C.; Hall 1997). RWH systems for flushing toilets has high water savings efficiency, varying between 0.6 and 100% (depending on rainfall) (Ward et al. 2012; Yan et al. 2018). Research results proof that commercial buildings can meet water demand with rainwater. The literature discusses using RWH in educational buildings, where the highest potential for potable water savings was 53.2% (Teston et al. 2022), and in hospitals, where 20–30% water savings are estimated, but these few studies were conducted in a semi-arid region (Fulton 2018). Another article discusses the RWH system designed for a sewage treatment plant near Mirandela (Portugal) to wash vehicles, equipment, clean concrete and asphalt surfaces, and water greenery (Sanches Fernandes et al. 2015). Similar research was undertaken in the South of Portugal, where a substantial potential for the feasibility of RW systems in retail stores has been proven (Ferreira et al. 2023).

Airports may require as much water as medium and big towns (equal to a city of 30,000–34,000 people). Nearly 65% of airport water demand does not require tap water quality: air cooling, irrigation, aircraft and hard surface washing, fire protection, and toilet flushing. The rainwater replacement rates depend on the purpose of usage (Blokker et al. 2011): terminals and offices had the highest factor of 0.7 due to toilet flushing’s large contribution of water use; hangars and cargo buildings amounts to 0.5, and hotels to 0.1 (the lowest value) due to utilizing water for hygiene and cooking. Airports may collect more rainwater than households due to their huge roofs (Jaiyeola 2017). Among European airports with RWH systems, following can be listed: Frankfurt International, Orly and Charles de Gaulle, Brussels International, Heathrow, Zurich. Only a few studies have examined RWH’s potential at airports. RWH system was considered for Manchester Airport, for non-potable uses including road sweepers and fire training (as of 2007) (Carvalho et al. 2013). Rainwater might replace 58% of Schiphol airport’s non-potable water demand while being stored in five empty jet fuel silos (Kuller et al. 2017), but the long payback period (30-year) makes it uneconomical.

Rainwater quality criteria vary by use. If consumed, it must fulfill drinking water standards. WHO, EPA, and EU have issued such guidelines. WHO established drinking water quality standards (WHO 2022) and regulates physicochemical and microbiological factors to ensure that water is safe to drink. These inspired other nations to pass WHO-based laws. However, EPA prepares reports on water contaminants. To preserve water resources from agricultural pollution, the EU has passed significant laws. The Drinking Water Directive (DWD) of Council Directive 98/83/EC (and its modification EU 2015/1787) was the basis for state-level regulation in Europe. When water distribution systems serve more than 50 people or produce more than 10 m³/day, water quality analysis is required. However, the regulation does not apply to families and small water treatment plants. The option of creating water from different sources is also ignored. The Drinking Water Directive warns consumers of the dangers of contaminated water and ensures clean drinking water (Morales-Figueroa et al. 2023). Regulation of the Minister of Health of December 7, 2017, on the quality of water meant for human consumption sets Polish quality standards. The document specifies physicochemical and microbiological parameters, and also sampling technique and frequency. The document controls ion concentration, pH, conductivity, color, and turbidity. For microbiological parameters, regulate E. coli, coliform bacteria, Pseudomonas aeruginosa, and Legionella.

The WHO Joint Monitoring Program calls rainfall an “improved” water source and stresses the need to prevent fecal bacteria contamination. The program covers small and large drinking water sources. However, the law should also cover other drinking water sources. Water quality and safety must be updated. Proposal EC COM 753/2017 on the quality of water intended for human consumption, EC Best Environmental Management Practice in the Tourism Sector, Harvesting Rainwater for Domestic Uses: An Information Guide (Environment Agency), European Commission, 83/1998 EC DWD also cover rainwater use and drinking water quality.