Rainwater Harvesting and Treatment: Comparison
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Rainwater harvesting is an ancient practice currently used for flood and drought risk mitigation. It is a well-known solution with different levels of advanced technology associated with it.  This study is aimed at reviewing the state of the art with regards to rainwater harvesting, treatment, and management. It focuses on the environmental and social benefits of rainwater harvesting and links them to the Sustainable Development Goals. The review identifies characteristics of laws and regulations that encourage this practice and their current limitations. It presents methodologies to design a rainwater harvesting system, describes the influence of design variables, and the impact of temporal and spatial scales on the system’s performance. The manuscript also analyzes the most advanced technologies for rainwater treatment, providing insights into various processes by discussing diverse physiochemical and biological technology options that are in the early stages of development. Finally, it introduces trends and perspectives which serve to increase rainwater

harvesting, water reuse, and effective management.

  • rainwater harvesting
  • climate change
  • rainwater harvesting systems
  • rainwater treatment
  • low impact development
  • sustainable urban drainage systems
  • water supply
  • rainwater reuse
  • mitigation
  • resilience

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. The interest in this practice is widespread in all continents, 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).

2. 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] 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]. 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] 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] 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] 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] 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 respapearchr, 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] and have already proposed a framework to appraise investments in urban water management systems under uncertainty. Following on from this woresearchk, Manocha and Babovic [15] 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]. 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]. Sefton et al.  [18] 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.

3. Quality of First-Flush Roof Runoff and Harvested Rainwater

3. Conclusions

Rainwater is considered a clean commodity, and its treatment methods have received significant interest in recent years. For domestic and industrial applications, the main concern is its quality characteristics. Compared to surface water and groundwater, rainwater has a nearly neutral pH, no hardness, and no presence of any disinfection by-products [19]. However, the physical, chemical and microbiological characteristics of first-flush roof runoff and harvested rainwater is highly affected by the catchment characteristics, storage material properties, and environmental conditions [20][21]. For instance, Despins et al. [22] reported that the rainwater collected on the catchment surfaces comprised of steel material adsorbs less atmospheric particulates than asphalt material and delivers higher-quality rainwater. They also reported that the pH of rainwater stored in plastic reservoirs tends to be marginally acidic. In contrast, the rainwater stored in concrete containers is slightly basic in nature. Regarding chemical quality, the presence of total organic carbon (TOC), nitrate (NO3−), and sulfate (SO42−) is likely in rainwater due to the excrement of birds and rodents, lichens, and other depositions on the surfaces from which the runoff occurs. Compared to rooftops with concrete tiles, clay tiles, and galvanized steel material, roofs with wooden shingles are reported to promote the growth of lichens and mosses due to their relatively high porosity which subsequently increases the TOC, NO3−, and SO42− levels of the rainwater [23]. Concerning microbiological quality, the rainwater harvested from roofs covered with galvanized steel sheets was reported to have the lowest number of bacteria and adenosine triphosphate content compared to runoff from roofs covered with concrete tiles, ceramic tiles, and epoxy resin [24]. Additionally, Zdeb et al. [24] emphasized the effects of environmental conditions, particularly temperature, on the microbiological quality of the rainwater. They reported that the rainwater collected in the autumn tends to have the best microbial quality while water collected during summer has the worst.
Most studies which focus on the quality of the first-flush roof runoff and harvested rainwater suggest that the collected rainwater quality needs to be of higher quality for direct use [25] due to the microbiological quality risks owing to the presence of coliforms [26] and potential human pathogens such as Legionella and adenovirus [27].Thus, disinfection is necessary before rainwater use. Furthermore, the prevalence of heavy metals and other inorganic ions in rainwater due to fossil fuel combustion, and the likely presence of pesticide residues and fertilizers due to agricultural activities, [28] create a demand for advanced treatment strategies.

4. Rainwater Treatment State of the Art

Generally, rainwater is poor in biodegradability, so physicochemical treatment is a suitable option for improving rainwater quality for domestic and industrial use. There are many physicochemical treatment options available and selection is entirely determined by the required quality of the effluent and the recommended use of the treated water [29][30]. The treatment options generally proposed for rainwater can be divided into two broad categories: disinfection and filtration. In addition, recently proposed biological treatment options for rainwater treatment are also briefly discussed below.

4.1. Disinfection

Due to the likely presence of pathogens, drinking untreated harvested rainwater could impact human health. Numerous techniques have been proposed to make the harvested rainwater meet potable water standards. Out of the many options, disinfection has received the greatest attention [31]. Chlorination is the most widespread disinfection technique adopted in rainwater treatment systems, mainly due to its affordability [32]. Nonetheless, chlorination has several challenges, such as inconvenience in storing chemicals, taste and odor problems, and failure to eliminate microorganisms such as Cryptosporidium parvum and Giardia lamblia cysts [33]. Furthermore, to properly use chlorination, the dose and chlorine demand must be calculated by performing tests that might not be feasible at the household level. Although not as cost-effective, ultraviolet (UV) disinfection is also suggested as a potential alternative to chlorination for improving the microbiological quality of rainwater [34]. UV disinfection alters the DNA/RNA composition of pathogens and effectively destroys protozoa such as Cryptosporidium and Giardia. The advantage of UV disinfection over chlorination is that it does not generate any disinfection by-products. However, unlike chlorine treatment, UV disinfection produces no residual effect. Hence, recontamination of the treated rainwater can occur shortly after UV treatment if kept in a storage device [35].
Though old-fashioned, one of the most effective disinfection methods is raising the water temperature to 50–70 °C. At this temperature, the heat will either kill the pathogens or inactivate them [25]. Raising of temperature can be achieved either by boiling [36] or solar disinfection (SODIS) [37]. In addition to inducing heat, the portion of solar radiation (UV-A, 315–400 nm, visible violet, and blue light in the range of 400–490 nm) work synergistically in inducing microbicidal effects when using the SODIS technique [38]. The effectiveness of SODIS techniques in inactivating pathogens and other microorganisms and their ability to improve the microbiological quality of rainwater has been verified by several researchers [39][40]. The main disadvantage of the SODIS technique is that it does not offer any residual disinfection.

4.2. Filtration

Although pathogen removal is necessary to enhance the suitability of rainwater for domestic consumption, the effectiveness of disinfection techniques is influenced by the physical characteristics of rainwater, specifically, turbidity. Several filtration methods have been suggested, such as slow sand filtration, dual media filtration (sand, coal, and gravel), and membrane filtration [33]. Apart from removing the suspended solids from rainwater, filtration techniques induce other benefits, such as significantly improving physiochemical and microbiological characteristics, removing odor and taste problems, decreasing turbidity, and lowering the dose of chemicals for disinfection [41][42]. Below is an overview of the different filtration techniques recommended for rainwater treatment.
Ceramic filters are cone-shaped filters manufactured from locally acquired clay and are low-cost treatment options for rainwater [43]. During manufacture, the clay is mixed with rice husks and water to induce porosity and is painted with silver nitrate to reduce microbial growth. These filters have been reported to be effective in removing E. coli and other bacteria from rainwater [44]. However, these filters cannot treat large quantities of rainwater.
Compared to ceramic filters, the slow sand filter consists of sand and supporting gravel beds. The advantages of a slow sand filter includes low capital cost, straightforward design and construction, and low operational cost. The slow sand filters can be easily scaled up for small or medium towns or large villages. However, the large area requirement for large-scale applications makes them less attractive. It has been reported that slow sand filters are very effective in removing heavy metals, protozoa, E. coli, and bacteria from rainwater [45]. Furthermore, due to their ability to reduce turbidity to a significant extent (~95%), a slow sand filter also enhances the efficiency of disinfection techniques. Nonetheless, a slow sand filter may not function efficiently with a highly turbid water supply and is inefficient in odor removal [46]. Another commonly adopted filtration technique employs granular activated carbon (GAC) media instead of sand. These filters primarily work based on the principle of adsorption and are commonly used to remove natural organic matter, odor, and unpleasant taste [47]. Although there are reports of GAC filters removing turbidity, E. coli, and total coliform [48], they are not considered effective in removing bacteria and viruses from rainwater [44], and turbidity can significantly reduce their lifespan. Dual media filters combine sand and GAC media and are commonly applied to remove E. coli from water. They are more effective than slow sand filters and have a longer lifespan. However, their performance is reported to vary with environmental conditions [49]. Furthermore, dual media filters are reported to be less efficient in removing turbidity than slow sand filters. Hence, additional coagulation–flocculation treatment may be required for treating highly turbid waters [50].
Traditional membrane filtration techniques include reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF), and are categorized based on pore diameter [51]. Out of the four membrane techniques, MF is traditionally the most utilized in rainwater treatment [52]. However, single MF units can only eliminate a fraction of large-diameter organics and pathogens, so they are not very effective as a rainwater treatment technique [53]. Thus, Shiguang et al. [54] recommend membrane surface modifications or combined use with other water treatments for effectively improving the performance of MF techniques in rainwater treatment. Nonetheless, considering the current advancements in MF techniques, there are ample reasons to believe that MF techniques have a future use for rainwater treatment [55].
Compared to MF techniques, UF techniques have higher filtration capacity and more significant potential for rainwater treatment. UF membranes also repel more macromolecular substances than MF membranes [52]. Therefore, these membranes can remove colloidal and/or suspended solids and pathogens in rainwater and yield demineralized effluent [56]. Compared to RO techniques, the production of demineralized effluent is less energy intensive and more cost-effective. Due to this reason, they are more environmentally friendly. However, in order to become a more attractive rainwater treatment technique, UF membranes must improve the filtration of greasy matter, increase the removal of heavy metals, and reduce the risk of membrane fouling [57]. Therefore, UF membrane technology still needs improvement in order to be used for practical applications in rainwater treatment.
NF membranes have properties in between UF and RO membranes, so utilizing NF techniques for rainwater treatment makes it viable to produce high-quality potable water [58]. In a recent study, Köse-Mutlu [59] has shown that when applied for rainwater treatment, NF techniques can achieve up to 99% natural organic matter removal and >99% SO42− removal. This proves the capability of NF membranes to remove organic and inorganic ions from water and effectively produce demineralized effluent. The evidence from another study [60] showed that NF membranes could reduce the opportunistic pathogens load (from 23.4% to 7.77%) and ensure the biosafety of the treated effluent. Yu et al. [60] also reported that the rainwater treated using NF techniques shows a lower disinfection by-product formation, making it safer for chlorine treatment. However, the effects of manufactured emerging contaminants were not considered in their study. Hence, further exploration is required to substantiate the claims of beneficial NF techniques for rainwater treatment in the literature.
RO techniques have been extensively researched in the field of rainwater treatment. The most well-known example is Singapore, which uses RO technology for rainwater treatment [61]. The RO membranes can effectively remove dissolved and colloid solids and opportunistic pathogens in rainwater by up to 99.9% [60]. Though RO techniques can produce high-quality effluent, it comes with considerable disadvantages. RO membranes demand frequent maintenance, and its absence can easily cause membrane fouling, leading to higher operating pressure, flux decline, and shorter membrane life [62]. Future research is expected to develop innovative membrane materials, enhance filtration efficiency, lower energy use, and alleviate membrane fouling [63][64][65][66].

4.3. Biological Treatment Options

Recently, biological treatment methods that facilitate the reduction in persistent organisms and the nonselective removal of microbial contaminants have gained attention. Among them, biological treatments employing predatory bacteria and bacteriophages have received more attention. Bdellovibrio and like organisms are a group of Gram-negative bacteria identified as probable “live antibiotics” because of their ability to prey on and lower the concentration of primarily Gram-negative bacteria in co-culture experiments [67]. Waso et al. [68] applied Bdellovibrio bacteriovorus as a pretreatment to SODIS and solar photocatalysis for treating synthetic rainwater spiked with pathogens (Klebsiella pneumonia and Enterococcus faecium). The results showed that the pretreatment with Bdellovibrio bacteriovorus could effectively enhance the disinfection of Gram-negative bacteria in particular, such as Klebsiella and Enterococcus. However, the efficiency of predatory bacteria in disinfecting rainwater samples that contain mixed bacterial communities is yet to be investigated. In addition, the real-world applications of combining biological treatment constituting predatory bacteria with physical treatment methods are yet to be validated.
Bacteriophages, viruses that infect and lyse bacteria [69], have also been investigated for the targeted removal of pathogens from aquatic systems [30]. However, studies have reported that bacterial species may develop resistance to bacteriophages over time [70]. Hence, this must be addressed in order to apply bacteriophage in microbiological quality control of water samples successfully. Recently, Al-Jassim et al. [71] and Reyneke et al. [72] integrated bacteriophage treatment with SODIS to treat water samples. Results from the studies indicated the effectiveness of bacteriophage treatment. However, the efficiency of the bacteriophages for water treatment was only analyzed in small-scale experiments. The real-world functionality of bacteriophages in rainwater treatment is yet to be studied.
In addition to the above, bioretention is another popular rainwater management technique often employed in urban environments to deal with water quality issues. Bioretention systems consist of the vegetation at the top, followed by a substrate (growth media), drainage module, and an underdrain. Vijayaraghavan et al. [73] reported that although the advantages of bioretention systems for rainwater treatment are attractive from the environmental sustainability viewpoint, more concrete research studies are needed to ensure actual knowledge of the performance of these systems over an extended period of operation in the field.

This research provides an overview of recent developments and trends in the field of rainwater harvesting. It shows that the advantages of rainwater harvesting (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 research 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 gravity-driven membrane (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.

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