1. Introduction

Water is strongly related to human health, socio-economic prosperity, food production,and the environment. The water–food–energy nexus identifies this natural resource as fundamental for life on Earth. Despite this, millions of people in developing countries still do not have access to enough clean water to satisfy basic needs. The sixth Sustainable Development Goal (SDG) of the United Nations Agenda 2030, Clean Water and Sanitation, states that more than 733 million people still live in countries with high and critical levels of water stress [1]. The world population is growing (especially in developing countries), and by 2050 about 64% of people are expected to live in cities. This will cause an increase in water demand, which has already quadrupled in the 20th century [2]. Moreover, climate change is intensifying extreme events all around the world. This occurs not only in countries traditionally affected by water scarcity but also in regions usually characterized by high availability of water resources, which are often misused or wasted. The World Meteorological Organization (WMO) states that droughts have risen by 29% since 2000, and 2.3 billion people suffered from water stress in 2022, forecasting that droughts may affect over three-quarters of the world population by 2050 [3]. At the same time, in the past two decades, 163 annual floods were recorded, and 223 large-scale floods occurred in 2021 alone [4]. Rainwater harvesting (RWH) and reuse is an ancient water supply practice; examples of systems date from the Neolithic period [5]. The development of civilizations often benefited from the storage of rainwater and its planned use over time. RWH is still utilized as the primary source of water supply for millions in developing countries [6]. However, even in developed countries, rainwater harvesting and reuse are increasingly encouraged by regulations and laws, representing a sustainable solution for improving water supply resilience.
RWH and reuse belong to a set of water management techniques known as best management practice (BMP), which is also named low-impact development (LID) solutions or sustainable drainage systems (SuDS) depending on the country. The interest in this practice is evident by the sharp increase in the number of documents obtained through a Scopus search with the keyword “rainwater harvesting”, which shows no signs of diminishing (Figure 1). The interest in this practice is widespread in all continents (Figure 2), with a high number of contributions from the United States, China, and India. These data confirm global interest in this topic and in efforts towards making cities and communities resilient to challenges posed by climate change. The top journals publishing papers on RWH are Water (MDPI), Agricultural Water Management (Science Direct), Journal of Cleaner Production (Science Direct), Water Resources Management (Springer), Resources Conservation and Recycling (Science Direct), Water Science and Technology Water Supply (IWA Publishing), Science of the Total Environment (Elsevier), Sustainability (MDPI), and Physics and Chemistry of the Earth (Science Direct). This manuscript aims to review papers on RWH and treatment and identify the most important findings and progress in this field of research. Section 2, “Rainwater Harvesting”, introduces the multiple advantages of this practice and its technical, social, and financial limitations (Section 2.1) and links them to the SDGs, providing new insights into the utility of these systems. Section 2.2 presents key characteristics of regulations, laws, and design manuals which can help or hinder widescale RWH implementation. Section 3, “Rainwater Harvesting Systems”, describes modern RWH systems focusing on domestic usage. It reviews state-of-the-art methods for their design and modeling. It also examines the performance of these systems depending on different variables as well as their spatial and temporal scales, and the aim of the RWH system. Section 4, “Rainwater Treatment”, presents current and emerging technologies to
achieve set water quality standards for the different reuse of rainwater.

5. Trends and Perspectives

Although the benefits of RWH have been well documented, their implementation is somewhat sporadic [7]. Recent trends have focused on addressing this by demonstrating the multi-purpose nature of RWH in terms of its environmental, financial, and social benefits. Quinn et al. [8^[1]] address this by suggesting a framework incorporating water supply and stormwater management metrics that provide a robust characterization of performance during significant rainfall events and on a longer-term basis. However, currently, this framework does not contain any of the societal and economic benefits which are more difficult to quantify. The design of these systems can also act as a barrier to implementation; for example, in the UK, these systems are designed to manage runoff from 1 in 100-year design storms, which results in recommendations for large, costly tanks [9^[2]]. Stovin et al. [9] apply their framework to design, illustrating that a balance between size and stormwater management performance can be achieved by designing for the retention of rainfall events with smaller return periods. Recently, larger-scale modelling has been applied to demonstrate the utility of advanced technology, such as real-time control (RTC), to RWH systems. For example, Xu et al. [10^[3]] illustrate the benefits of RTC of RWH on reducing erosion and restoring the pre-development conditions in sensitive receiving waters and suggest that investments in RTC technology would appear to be more promising than investments in increasing RWH detention volume. Campisano et al. [7] highlighted that financial viability is a significant barrier to implementation. As such, LCA has been adopted to examine the environmental and economic costs of RWH. It is challenging to compare different LCAs due to the assumptions made when creating them and their sensitivity to geographical parameters. Leong et al. [11^[4]] compared decentralized RWH, greywater recycling, and hybrid rainwater–greywater systems and found RWH to be the optimal option, as it had the second highest mains water savings, lowest environmental impact scores relative to mains water in seven categories (i.e., acidification, eutrophication, freshwater ecotoxicity, global warming, human toxicity, photochemical ozone creation, and water stress index), and is the first system to become financially attractive at USD 2.00/m3 . Ghimire et al. [12^[5]] found similar results with their RWH system outperforming the mains water system in all categories except ozone depletion, although they did not examine cost. Van Dijk et al. [13^[6]] apply a different approach to illustrating the financial benefits of RWH; they demonstrate that well-designed and implemented rooftop RWH systems can meet multiple infrastructure development needs of a city with reduced public expenditure as compared to centralized systems, and that RWH is a viable, profitable climate change adaptation strategy. One of the key challenges when planning infrastructure is the uncertainty regarding future climate scenarios. As discussed throughout this paper, RWH is a viable option to mitigate the impacts of both drought and floods; however novel approaches to value this flexibility are needed. Deng et al. [14^[7]] and have already proposed a framework to appraise investments in urban water management systems under uncertainty. Following on from this work, Manocha and Babovic [15^[8]] add a cost–benefit analysis to decision-making approaches focusing on uncertainty, which provides additional insights to policymakers. The decentralized nature of many RWH systems offers a unique opportunity for communities to be actively involved in water management, which has been shown to yield multiple benefits [16^[9]]. RWH is often used as part of a systematic catchment-based approach to stormwater management, where multiple SuDS are used to holistically manage surface water runoff. For example, the sponge city initiative in China has championed this approach and investigations into the optimal placement of systems to manage urban flooding is ongoing [17^[10]]. Sefton et al. [1^[11]8] suggest there are transformative advantages to a more community-oriented approach to flood resilience by including participatory RWH management, particularly the move towards a process of mutual learning and two-way communication.

6. Conclusions

This paper provides an overview of recent developments and trends in the field of rainwater harvesting. It shows that the advantages of RWH are often understated and there is potential to link this practice to all of the SDGs; however, the limitations of the systems and current research is acknowledged. Regulations are of great importance in ensuring the widespread implementation of this systems and in the future, they should not only include technical and environmental guidance but also economic and social supports. Similarly, trends in the advancement of RWH research towards more multidimensional benefit analysis are shown. Modern RWH systems include several components and high-level technologies. The design of the tank is crucial to satisfy different water needs and the variability of stormwater patterns must be considered. Multi-objective design of tanks is needed to increase the reliability and the efficiency of the systems to meet different goals. In addition, this paper examined the existing state of the art in rainwater treatment. The characteristics of different physicochemical treatment options in rainwater treatment, specifically, disinfection and filtration, with emphasis on membrane technologies, were summarized. The recent developments in biological treatment options for rainwater treatment were also analyzed. The research on GDM techniques and the process of treating rainwater with various physicochemical and biological technology combinations are still under analysis. The current advancements in the state of the art prove that future prospective treatment techniques are worth looking forward to.

This entry is adapted from the peer-reviewed paper 10.3390/w15081518