Rainwater tanks often provide a reliable and affordable water supply source in rural and remote areas where piped water supply systems are unfeasible due to economic considerations. However, over recent decades there has been an increase in the adoption of rainwater harvesting as part of the water supply source mix in modern cities. The uptake of rainwater harvesting has been influenced by the rise of ecological sustainable development as a mainstream practice. Rainwater harvesting is now implemented as part of an integrated urban water management approach to alleviate pressure on traditional water supply sources due to increased demand, driven by the rapid growth of urbanised populations. While examples of rainwater harvesting in human settlements can be found since ancient times, there are still gaps in understanding the role that it can play in modern cities. This paper reviews current international experiences with rainwater harvesting, particularly examining the drivers for their adoption in different urban contexts and the impediments faced for greater mainstream adoption. The paper then reviews the current state of research associated with understanding the value of rainwater harvesting in modern cities, which include impacts on reducing mains water demand, public health risks, energy implications, environmental impacts, and cost-effectiveness.
Rainwater systems have been part of the earliest examples of urbanisation. This is partly because densification of human settlements is not possible without a reliable water supply and some form of organised sanitation. There are recorded examples of rainwater harvesting in the ancient civilisations of the Middle East (Evenari et al. 1971; AbdelKhaleq & Alhaj Ahmed 2007). However, as the population of cities grew in the modern era, the main approach to water supply became sourcing water from catchments outside of cities, which is then supplied via large-scale reticulated networks. These centrally managed and operated systems have usually provided safe and reliable services in developed countries. However, in the last decade there has been a renewed interest in developed countries in using rainwater tanks for augmenting urban water supply. In Australia, the number of household rainwater tanks in cities more than doubled over the period 1994–2010, with more than one million households in Australian cities now having a rainwater tank (ABS 2010).
The fundamentals of rainwater system design have not changed significantly over the centuries, in that runoff from an impervious area with a high runoff coefficient is channelled by gravity into large impermeable storage devices. In modern settlements, building roofs provide the impervious runoff area, whilst collection is via roof gutters and down pipes to storage tanks. By adding an electric pump, water can be supplied to any elevation in the dwelling at a flow rate and pressure acceptable for most domestic uses.
While the focus of this chapter is on the experiences of rainwater tanks in cities of the developed world, it is worth noting that in developing countries, rainwater can supply an ‘improved’ drinking water source. Rainwater systems can be particularly useful in conditions where surface water is contaminated by faecal pathogens, and groundwater is either not readily available or contaminated by pathogens or chemicals, such as naturally occurring arsenic in Bangladesh (Chakraborti et al. 2010).
In developed countries, rainwater tanks were historically used as an interim measure before reticulated potable water became available, except of course in rural areas where mains water supply was (and still is) limited to the central parts of towns. For example, in Australia, where local groundwater sources are often limited in extent, accessibility or quality and surface streams are strongly ephemeral, the use of rainwater tanks has allowed human settlement in rural areas, and more recently peri-urban areas that are without access to reticulated water. Consequently, Australia has significant uptake of rainwater tank systems in the community relative to other developed countries (Ward 2010). We suggest that such high adoption of rainwater tanks in urban areas is unique to Australia and we will explore some of the reasons for this phenomenon.
This paper summarises a range of rainwater research issues. The ubiquity of rainwater tanks in urban areas raises a number of issues, which include the integration of rainwater tanks with existing centralised water supply systems; the impact on reducing demand for mains water; long-term reliability and yield; energy implications; supply cost relative to other supply and demand options; water quality considerations and public health risk; the suitability of different types and scale of urban development; the need for improved planning and design guidelines; community preferences and the need for changes to management models for urban water systems.
The Australian experience with rainwater tank systems
In the late 1990s, ecologically sustainable urban development emerged as a strong aspirational paradigm in Australia. Self-sufficiency in water supply was part of the sustainability agenda, with a number of well-published projects that demonstrated the successful use of rainwater as an alternative water source. Examples include the Sustainable House in Sydney (Mobbs 2010); Payne Road development in Brisbane (Gardner et al. 2006); Currumbin Ecovillage on the Gold Coast (Hood et al. 2010); and Fig Tree Place in Newcastle (Coombes et al. 2000). Analysis of these exemplar developments showed that rainwater tank systems, with back-up supply from mains water, could provide a number of benefits, in particular large reductions in mains water use. The uptake of rainwater tanks as a non-potable water source received significant impetus for more mainstream adoption during the extended drought experienced in many Australian cities during the 2000s. The extended drought saw the water catchment of capital cities fall to unprecedented low levels, with the real possibility of cities running out of water unless radical steps were taken (Apostolidis et al. 2011). It was this factor that ‘focused the minds’ of the water planners in all Australian states and lead to an explosion of investment in alternative water supply sources (seawater desalination, reuse of highly treated wastewater, stormwater harvesting and reuse) and rainwater tanks (Marlow et al. 2013). In Melbourne, around $6.7 billion was invested in water supply augmentation, which was primarily a 150 GL capacity desalination plant (Grant et al. 2013). While in South East Queensland (SEQ), around $9 billion was invested in the ‘water grid’, which linked water supply sources across the region and invested in increased dam capacity, a desalination plant and recycled wastewater treatment plants (Brown et al. 2011).
In Australian cities, the uptake of rainwater tanks was encouraged both by financial rebates for installation and changes to building codes, which encouraged households to install a secondary alternative water source for non-potable demands such as toilet flushing and garden irrigation (NSW Planning and Infrastructure 2004; Queensland Department of Infrastructure and Planning 2008). Also, rainwater tanks have been promoted under water sensitive urban design and integrated urban water management as an approach for reducing the impact of urban stormwater on the natural environment (Sharma et al. 2012, 2013). Rainwater tank systems continue to be encouraged in Australia as a supplementary water source through financial incentives and regulations, and their roles in securing the long-term water supply–demand balance in Australian cities has been embedded in strategic planning documents (e.g. Queensland Water Commission 2010).
International experiences with rainwater tank systems
This paper presents research on rainwater tanks from the perspective of the Australian experience. However, it is acknowledged that there is important research and insights from international experiences with rainwater tanks. The international experiences with rainwater tank systems for urban water supply can largely be partitioned into developed and developing countries. The following presents a review of international literature on rainwater tanks.
For many countries in Africa and developing countries of South East Asia, access to a safe drinking water supply is poor, especially in rural areas, which leads to increased rates of illness (Mwenge Kahinda et al. 2007; Baguma et al. 2010). In addition, the source of the water supply is often remote from village settlements and travel time and effort for its collection is very onerous (Baguma et al. 2010). Although household rainwater systems often do not meet microbiological standards for potable water (Ahmed et al. 2011), it is possible that a well-designed and managed rainwater system can provide an improved drinking water source in rural villages with unsanitary conditions where ground and surface water sources are likely to be contaminated with faecal matter (Mwenge Kahinda et al. 2007). Consequently, it is not surprising that non-government organisations operating in Africa strongly support the installation of local rainwater harvesting systems as part of the Water and Sanitation for Health program (e.g. RAIN 2012).
In developed countries of Europe and Asia, the interest in rainwater harvesting systems is driven by concerns for stormwater discharge into combined sewers (i.e. collection of both sewage and stormwater flows), local flooding control, and lastly water supply (Herrmann & Schmida 1999). In Germany, for example, the rainwater industry is well established with over 1.6 million installations (4% of households) (Nolde 2007). The major motivation by government and water companies is to reduce the high capital costs of increasing the discharge capacity of combined sewers. Hence, rainwater tanks are part of the system to retain stormwater runoff on site for subsequent local infiltration (Herrmann & Schmida 2000). The ‘disconnection of impervious areas’ from combined sewers can lead to direct government subsidy and/or a reduction in household sewage charges from water companies (Herrmann & Schmida 2000). However, the individual household economics are not very attractive, and the high incidence of voluntary installation of rainwater harvesting systems in new private dwellings in Germany (over 65%) is driven by two main factors: (i) peoples’ motivation to be environmentally friendly (by reducing stormwater into streams); and (ii) the concept of self-reliance (Schuetze 2013). Rainwater tanks in new dwellings can be connected to toilets, gardens, external uses and laundry and, of course, local infiltration systems. Apparently local authorities and water companies are often not supportive of facilities to use rainwater in lieu of mains water because of reduced water sales that would otherwise pay for the centralised infrastructure (Schuetze 2013).
In France, rainwater tank adoption is low, due in part to French law preventing rainwater use for clothes laundry, showering and drinking (Vialle et al. 2011) presumably because of concerns for public health. This contrasts markedly with a European Union Directive that puts a priority on urban water savings, including rainwater harvesting and reuse in buildings (Palla et al. 2011).
In the United Kingdom, there are similar government ‘policy’ documents that encourage rainwater harvesting in private and public dwellings (DEFRA 2008; Ward 2010) as well as providing technical documentation on its implementation (e.g. UK Environmental Agency 2008). As with Germany, there is concern with stormwater discharge into streams, but mainly because of local flooding issues (Farnsworth 2012). Hence, rainwater tanks in individual and communal dwellings are seen as part of implementing a sustainable urban drainage system (Farnsworth 2012). As there is neither subsidy nor water company rebate for individuals installing rainwater harvesting systems, new installations are about 6,000 per year (Ward 2010; Farnsworth 2012) with a strong focus on schools.
In other countries of Northern Europe such as Sweden, the concept of ecologically-sustainable development is motivating the installation of rainwater harvesting systems in large (1,100 dwellings) community housing complexes (Villarreal & Dixon 2005).
RAINWATER TANK MODELLING AND POTABLE WATER SAVINGS DUE TO RAINWATER SUBSTITUTION
Modelling of rainwater tank systems
To predict the long term rainwater supply from rainwater systems, various modelling tools have been developed. Due to the apparent hydrological simplicity of raintank systems, many types of spreadsheet models (Vieritz et al. 2006, 2007; Imteaz et al. 2011a) as well as models covering the total water cycle concept have been created (Mitchell et al. 2001; Coombes 2003; Mitchell & Diaper 2005). These models use historical rainfall data, connected roof area, a rainfall loss factor, storage tank volume and rainwater demand. The rainfall data requirements significantly vary among these models, ranging from daily to 6-minute time-steps (Vieritz et al. 2007).
Daily or sub-daily time-steps are used for the continuous simulation of rainwater tank models using historical rainfall and end use data. Coombes et al. (2002) and Villarreal & Dixon (2005) have used sub-daily time-steps for continuous simulation. The sub-daily time-step allows for more accurate representation of both the diurnal variation in household consumption and hydraulic/hydrological responses of the rain catchment system connected to the tank. However, accurately predicting the diurnal variation in consumption is difficult and the availability of reliable pluviograph rainfall data is limited, therefore, the adoption of sub-daily time steps in modelling rainwater tank performance can be difficult (Jenkins 2007).
Jenkins (2007) developed the ‘RainTank’ model to simulate the collection and use of rainwater from a rainwater tank. The model uses local daily rainfall and household consumption data and conducts a continuous simulation of rainfall and runoff from the connected roof area catchment. The model is coupled with a daily water consumption tool for water stored in the raintank. The model allows options for raintank top-up, and different uses of rainwater within the house. The model is not available in the public domain.
Vieritz et al. (2015) demonstrated how each of the parameters in a generalised model of rainwater tank behaviour affected the simulated results by comparing simulated results of reliability of supply from rainwater systems with monitoring studies. This found that modelling results were sensitive to assumptions around demand such as household occupancy. Vieritz et al. (2015) also demonstrated that the common practice of spatial lumping, where the modelling of an ‘average’ rainwater system is up-scaled to represent a region, often results in modelling results overestimating the yield from a rainwater system (Vieritz et al. 2015).
The optimisation of rainwater tank storage size based on volumetric reliability is also present in some models (Mitchell et al. 2001). Attempts are also being made to optimise the rainwater tank size based on life cycle cost estimates considering payback periods, expected water savings, capital and operational cost (Imteaz et al. 2011b). Okoye et al. (2015) presented a rainwater tank optimisation model which takes into account the site specific data such as the rainfall profile, the roof area of the building, the water consumption per capita and the number of residents. The model applies linear programming optimisation techniques to decide on the size of the rainwater storage tank to build, such that the net present value of the total tank construction costs and freshwater purchase costs is minimised. Some of the commonly used rainwater tank models in Australia are described briefly below.
Aquacycle is a daily water balance model developed on the concept of an urban water cycle (rainwater, mains water, stormwater and wastewater) operating at allotment, cluster and development scales. The primary purpose of the model is to allow what-if scenario modelling of mains and alternative water supply systems. The model produces daily, monthly and annual estimates of water demand, potable water, rainwater, stormwater and wastewater usages, including stormwater and wastewater yields for selected alternative resource reuse strategies (Mitchell et al. 2001). The model is also capable of optimising raintank size in kilolitres (kL) based on the input parameters, e.g. annual rainfall, rainfall pattern, connected roof area, end usage and occupancy rates. Further information about the model can be accessed at: http://184.108.40.206/Tools/Aquacycle.
Urban volume and quality model
Urban volume and quality (UVQ) was developed using the Aquacycle model as a starting point. It is also a daily water balance model developed on the concept of an urban water cycle operating at allotment, cluster and development scale. As the name indicates, UVQ quantifies both water and contaminant balances, enabling the user to track flow paths and contaminant concentrations through the urban water cycle. User defined percentage reductions in contaminants can be specified as input data, based on the proposed treatment processes. As such, no process decay algorithms are used in the model. The model does not optimise the rainwater tank size (Mitchell & Diaper 2005). For further information see: http://www.clw.csiro.au/products/uvq/download.html.
The probabilistic urban rainwater and wastewater reuse simulator (PURRS) model is based on an urban water cycle concept designed to simulate water flows for allotment and cluster scale developments. The rain falling on the household roof is routed through a first flush device to a rainwater tank. The rainwater tank can be topped up with mains water, and the rainwater can be supplied to hot water systems, toilets and external usage. Six-minute rainfall data are used as input data along with other household data. It uses behavioural and climate dependent water demand algorithms. The model provides output on daily water use from mains water, total demand, tank water supply, tank overflow, wastewater reuse, wastewater flow and stormwater runoff (Coombes 2003).
The model uses a daily time-step water balance approach for allotment scale application, mainly designed for application by urban planners. The tank model has an MS EXCEL front end and embedded FORTRAN executable file for computational efficiency. The model simulates the rainwater captured by an urban roof using runoff coefficients. It also has the functionality to plug in a first flush diverter and automatic mains water top-up, or supplementary water periodically brought in by truck. The rainwater demand is defined by the user for both internal and irrigation applications using a sophisticated menu option. The model was designed with the aim of assessing the long-term (e.g. 100 years) ability of the rainwater tank to meet the water demand of the urban allotment, and provides statistical reports on the degree of self-sufficiency and the frequency and amounts of top-up requirements (Vieritz et al. 2006, 2007). The effect of varying the connected roof area, tank volume and end use demands on rainwater supply can be readily explored for a given climate data set. The model is available from email@example.com.
Assessment of potable water savings due to rainwater substitution
The modelling of rainwater tanks can predict the amount of rainwater available for the specified household end uses. In urban areas, rainwater is generally permitted for use in non-potable applications including toilet flushing, cold water to washing machines and garden watering. The amount of rainwater used by households is numerically equal to the potable water savings. The assessment of modelling results with the measured household potable water savings is essential to gauge the accuracy of these modelling tools. Validation of the potable water savings is also essential to avoid pitfalls in the long-term water planning for a city or a region. Hence, modelling predictions alone should not be used for strategic water planning purposes. Various approaches were developed to confirm potable water savings due to rainwater substitution, and applied using SEQ, Australia, household data. Although the results of these approaches were valid only for the SEQ area, the method should be applicable wherever rainwater is used for potable water substitution. The following section summarises these assessment approaches, with the results of these assessments shown in Table 1.
|Study .||Qld Dev. Code (WBM 2006) .||Beal et al. (2012) .||Chong et al. (2011) .||Umapathi et al. (2013) .|
|Average household demand (kL/hh/y)||314||163||145||153|
|Rainwater use range (kL/hh/yr)||52–91||20–95||25–89||1–69 for 20 homes 17–69 for 19 homes|
|Average rainwater use (kL/hh/yr)||70||50||58||40|
|Year of study||Modelled||2008||2009, 2010||2012|
|Study .||Qld Dev. Code (WBM 2006) .||Beal et al. (2012) .||Chong et al. (2011) .||Umapathi et al. (2013) .|
|Average household demand (kL/hh/y)||314||163||145||153|
|Rainwater use range (kL/hh/yr)||52–91||20–95||25–89||1–69 for 20 homes 17–69 for 19 homes|
|Average rainwater use (kL/hh/yr)||70||50||58||40|
|Year of study||Modelled||2008||2009, 2010||2012|
Beal et al. (2012) developed a method to assess potable water savings from mandated tanks (compulsory rain tank installation at new households due to local development code). The approach investigated the average water yield from internally-plumbed mandated rainwater tanks using a pair-wise comparison of mains water consumption data for homes with and without rainwater tanks, but in similar geographic areas. Statistical analysis methods were used for randomly selecting homes for the comparison. They applied this approach in SEQ using 2008 water billing data for nearly 1,200 residential properties with/without rainwater tanks. Socioeconomic data on the individual homes were not available.
Chong et al. (2011) assessed the potable water savings from rainwater tanks by comparing mains water consumption data from households with rainwater tanks against the regional average residential water consumption for the same period. They conducted an average potable water savings analysis using main water consumption of 690 residential properties with rainwater tanks for year 2009 and 2010 in SEQ. The householders were specifically recruited for the study and provided socio-economic information such as household occupancy. This, in turn, allowed adjustment of measured household water consumption to that of the average regional occupancy. Recruitment also allowed for longitudinal water savings studies, as well as other types of research projects requiring householder permission and participation.
Potable water savings can also be estimated using real-time monitoring and measurement of rainwater usage at residential properties over a long period of time. Such an approach, although very cost intensive, provides more accurate but possibly less precise information when compared with a large number of paired householders. Umapathi et al. (2013) presented a detailed method for instrumentation and monitoring of household rainwater tanks for measuring rainwater use. They described the potable water savings from 20 monitored homes in SEQ for a 12-month period in 2012. The participating homes were recruited from the 690 householders participating in the Chong et al. (2011) study.
The above approaches were developed to test the SEQ Water Strategy and Queensland Development Code provisions (WBM 2006) that roof-water harvesting using a rainwater tank will provide an average water supply of 70 kilolitres per household per year (kL/hh/y). The outcome of these studies is shown in Table 1. The assessment shows that the average potable water savings per household were less than the expected values, however still significant on the basis of overall water consumption.
The increased uptake of rainwater tanks and their inclusion in policies have renewed interest in the composition of rainwater, including its microbiological and chemical quality (Magyar et al. 2007). Rainwater quality may be affected by the roof and storage tank materials, overhanging trees and the standard of maintenance of the rainwater tank system (Rodrigo et al. 2010). Lee et al. (2012) investigated the quality of harvested rainwater on the basis of roofing materials in Korea. Of the wooden, concrete, clay tiles and galvanised steel roofing materials investigated, galvanised steel was found to be most suitable for rainwater harvesting, probably due to ultraviolet light and the high daily surface temperature acting as disinfection agents.
Huston et al. (2012) conducted a study to identify the contributors of heavy metals and ionic contaminants in rainwater tanks in Brisbane, Australia. They identified four source factors influencing the bulk deposition at the bottom of the tank, which included crustal matter/sea salt, car exhaust/road dust, industrial dust and aged sea salt/secondary aerosols. These factors contributed 65% of the total contaminants on average. They also identified six collection system factors which included plumbing, building material, galvanising, roofing, steel and lead flashing/paint. These factors contribute nearly 35% of the contaminants. The concentration of Al, Zn, Fe, Mn and Cu in excess of Australian Drinking Water aesthetic guidelines was also reported in their study.
Magyar et al. (2007) reported that the concentration of lead exceeded Australian Drinking Water Guidelines (ADWG) values in five of the nine tanks investigated in metropolitan Melbourne, Australia. O'Connor et al. (2009) investigated the water quality in 52 tanks across the Melbourne metropolitan area, and identified relationships between lead concentration in the tank water, tank sediments and various environmental variables. In 14 of the 52 tanks, lead concentrations exceeded ADWG health guidelines and lead flashing, prevailing winds, proximity to roads and commercial zones had statistically significant relationships with lead concentration in rainwater. However, no single factor was identified as the major cause. Huston et al. (2012) also indicated the lead concentration exceeded the ADWG health guidelines of 10 μg/L in 15% of the samples, mainly due to the contact of rainwater with lead flashing used in roof construction. Lead has the most potential to impact seriously on human health, particularly on infants and children (Goyer 1993).
A global meta-analysis by Magyar & Ladson (2015) of studies into chemical water quality in rainwater tanks found that in 32 cases metals were found to cause a water quality issue, and in 31 of these cases lead was the issue. The findings from this analysis indicated that for Australian cities about 22% of rainwater tanks could be expected to exceed the ADWG in terms of lead concentration. A study of water quality in Alaskan rainwater catchments found that 10% of the rainwater catchments analysed showed copper and lead concentrations above the maximum contaminant level according to the EPA's National Primary Drinking Water Regulations (Hart & White 2006). Water quality testing of 42 rainwater systems in the Palestinian Territories found that nearly all of the samples analysed were within WHO guideline values for physiochemical parameters, however nearly all rainwater storages analysed had microbiological contamination (Daoud et al. 2011).
The microbial quality of rainwater depends on a number of factors. It has been reported that weather has a significant role in the microbial composition of roof-harvested rainwater. For example, Evans et al. (2006) found airborne microorganisms potentially contributed a significant bacterial load to roof water, with the magnitude influenced by wind velocities and wind direction. Yaziz et al. (1989) found that the contamination of rainwater measured by total coliform concentration increased with longer dry periods between rainfall events, increasing rainfall intensity, and type of roofing material.
Based on the outcome of a study of 30 households in Northern Kentucky, USA, high bacterial load was attributed to the roof catchment and not the subsequent storage of the rainwater (Lye 2002). Ahmed et al. (2010) studied 214 rainwater samples from 82 rainwater tanks in SEQ, Australia, for the presence of zoonotic bacterial and protozoal pathogens using binary and quantitative polymerase chain reaction (PCR). Of the tested 214 samples, 10.7, 9.8, 5.6, and 0.4% were positive for the Salmonella invA, Giardia lamblia, Legionella pneumophila mip, and Campylobacter jejuni mapA genes, respectively. They also conducted quantitative microbial risk assessment (QMRA) analysis to quantify the risk of infection associated with the exposure to potential pathogens from roof-harvested rainwater. In another study of 24 household rainwater tanks in SEQ, 63% of rainwater tanks contained Escherichia coli exceeding the limit of ADWG (Ahmed et al. 2012a). Ahmed et al. (2012b) concluded that there was a probable link between bird and possum faecal contamination and the presence of clinically significant E. coli strains containing toxic genes in rainwater.
Commonly used indicators of faecal contamination such as faecal coliforms and E. coli measurements may be inadequate for identifying microbial risks associated with consumption of rainwater (Lye 2002; Ahmed et al. 2011) due to their poor correlation with pathogens. The microbial assessment should involve analysis of actual pathogenic species, and not just the faecal indicators. The application of sensitive molecular typing methods and QMRA using data on the reported incidents of microbial pathogens in rainwater can be valuable tools to assess overall health risk with rainwater use (Ahmed et al. 2011). PCR technology allows rapid, specific, sensitive detection of pathogens in source waters that would otherwise be difficult using traditional methods (Ahmed et al. 2009). However, issues of organism viability using this technique remain, leading to a conservative estimate of health risk when quantitative PCR methods are used (Ahmed et al. 2011). Ahmed & Toze (2015) have further reviewed the literature on the microbial quality of roof-captured rainwater and provided insight on the potential health risks associated with the consumption of untreated tank water.
The chemical and microbiological quality of rainwater harvested from roof collection systems vary significantly and depend on a number of factors. Various studies have found chemical and microbiological qualities of rainwater exceeding drinking water guidelines. Chemical contamination appears not to be a health issue except for lead from lead flashing. However, pathogens are more ambiguous; E. coli is a ubiquitous contaminant, but there is recent concern some strains may be pathogenic or at least containing toxic genes. In addition, there are zoonotic pathogens including Giardia and Salmonella that occur with a frequency and concentration that are likely to pose a health risk when assessed using the QMRA technique. However, epidemiological studies of children in South Australia could identify no adverse health link between ingestion of rainwater and gastroenteritis (Heyworth et al. 2006).
Based on the research undertaken into rainwater quality, it can be seen that care should be taken in using rainwater for typical fit-for-purpose end uses, and proper treatment of rainwater would be required if used for drinking purposes.
Guidelines for rainwater use
The widespread adoption of rainwater tanks as an alternative water source has been encouraged to reduce mains water use and to provide environmental benefits such as reduced stormwater flows. However, the increased use of rainwater tanks in urban areas does raise the need for guidelines that manage the risks associated with their use. In Australian cities, nearly 26% of households are using rainwater as a water source (ABS 2010). Most households with a rainwater tank are using it for non-potable uses such as garden watering and, to a lesser extent, toilet flushing. However, a survey of Australian households did show that around 2% of surveyed households in capital cities used rainwater as the main source of drinking water. Outside of capital cities, 22% of surveyed households reported rainwater as their main source of drinking water (ABS 2010). The quality of rainwater for human use (potable or non-potable) is not regulated by any standard that is internationally recognised (Birks et al. 2004). Therefore, many countries and regions have developed local guidelines for local needs (Schuetze 2013). Guidelines for rainwater use need to consider the water quality requirements of the intended use of the rainwater and the likely microbiological and chemical risks posed. The preceding section has detailed the rainwater quality, while this section assesses the guidelines that are available to manage risks associated with rainwater quality.
In Australia, there is a national guidance document, Guidance on the Use of Rainwater, which does a systematic analysis of the potential hazards and risks associated with rainwater use (EnHealth Council 2010). This document identifies that, while the health risks associated with drinking rainwater from a properly maintained tank are low in most parts of Australia, the microbial and chemical quality of rainwater is likely to be less than a mains water supply (EnHealth Council 2010). Therefore, state guidelines recommend drinking from a mains water supply where it is available (NSW Health 2007). While the potable use of rainwater is possible with disinfection treatment, this section focuses on the guidelines for non-potable use of rainwater. Where the rainwater collected in a tank is intended for ingestion, then drinking water guidelines, such as the World Health Organization (2011), should be applied.
The Australian EnHealth Council (2010) identified that there are practical preventative measures that can be used to minimise most of the health and aesthetic hazards associated with rainwater collected in tanks. These preventative measures include maintenance practices and regular monitoring. The maintenance practices may include keeping intact insect screens on the tanks, cleaning the area around the tank to prevent establishment of a breeding ground for mosquitos, regular cleaning of gutters and trimming of overhanging trees to discourage possums. In Australia, there is a range of other guidelines and standards that provide comprehensive technical advice on all aspects of rainwater tank design, installation and maintenance. These include a handbook for rainwater tank design and installation (Standards Australia 2008), which includes guidance on maintenance tasks for rainwater tanks, and the frequency with which these tasks should be carried out. Rainwater tank system components need to comply with relevant standards, but the level of specification varies among the different components (WSAA 2002). Pipes generally need to show compliance and quality assurance, but some non-polymer rainwater tanks can be sold without documentation that demonstrates quality assurance. There is also guidance on the installation and approval requirements for each state by the peak Australian body representing plumbers (MPMSAA 2008).
In the United States, there is a lack of uniform guidance for the use of rainwater that has resulted in different guidelines for use and treatment among state and local governments (Kloss 2008). This confusion and lack of specific guidance on the use of rainwater has meant that some jurisdictions have based requirements for rainwater on guidelines used for recycled water sources, which are likely to result in more stringent requirements (Kloss 2008). Also, there are significant differences across jurisdictions on the guidelines for rainwater use and treatment. For example, in Texas, rainwater is promoted for all uses including potable as long as appropriate treatment is carried out, while in Portland, rainwater is only recommended for non-potable applications (Kloss 2008). Kloss (2008) recommended that to move towards national guidelines on rainwater use there is a need to consider the likely harvested rainwater quality and yield with the demand requirements of targeted end uses. Focussing rainwater use on irrigation and selected indoor non-potable uses may be the most appropriate, as it can significantly reduce mains water demand while requiring minimal or very little treatment. When rainwater systems are plumbed for indoor non-potable applications, the primary concerns of guidelines are to avoid the risks of cross-contamination with drinking water supply and to reduce human contact with pathogens that may be present in harvested rainwater.
Fewtrell & Kay (2007) found that in the United Kingdom there are no regulations on the microbial quality required for the non-potable use of rainwater. Furthermore, where water supplied from rainwater tanks is used solely for non-potable purposes, there is often no requirement for water quality monitoring. Fewtrell & Kay (2007) considered that adaption of other microbial water guidelines to non-potable use of rainwater was unlikely to be successful. Rather, they argue that the development of a standard for the microbial quality of rainwater tank water should take a health impact assessment approach, and that microbial concentrations are set at a level to protect human health, based on the intended uses.
COST EFFECTIVENESS OF RAINWATER TANKS
Selection of alternative water supply sources such as rainwater tanks needs a consideration of their cost effectiveness relative to a centralised mains water supply. The cost effectiveness of a rainwater tank will be a product of the whole of life cost and the water yield delivered over time (Marsden Jacobs Associates 2007). Cost effectiveness can be considered from either the perspective of an individual householder or that of the whole of the community (Marsden Jacobs Associates 2007). The whole of community cost considers not only the costs and benefits borne by the households but also accounts for savings on capital and operating expenses for public water supply and stormwater systems (Marsden Jacobs Associates 2007).
The direct capital costs for rainwater tanks are: storage tank, tank installation and fittings, concrete slab or tank stand, household plumbing and a pump. The ongoing operating costs for a household are: energy costs for pumping, and maintenance of the tank and pump (Marsden Jacobs Associates 2007). Tam et al. (2010) undertook an analysis of the installation costs for a rainwater tank in Australian cities. Their study found that, for a 5 kL tank plumbed for indoor and outdoor uses, installation cost was approximately $3,900 for the Gold Coast. In most Australian cities, there are rebates available to offset the costs of a household installing a rainwater tank. The amount of rebate varies among jurisdictions, and often with the size of the tank and the connected end uses. In Victoria, a $1,350 rebate is available for a rainwater tank 4 kL or greater that is plumbed to the toilet or laundry (Department of Environment & Primary Industries 2012).
As the cost effectiveness of rainwater tanks is a function of both yield and whole of life costs, there is a need to consider factors that influence yield. The efficiency of a rainwater tank system is influenced by tank size, roof catchment area, and the amount and frequency of the water demand (Tam et al. 2010). Of course the local rainfall temporal pattern and amount influences the yield from a rainwater tank. The analysis by Marsden Jacobs Associates (2007) found that roof catchment area had the greatest impact on yield followed by annual rainfall and tank size. This highlights the need to consider both the optimal configuration of rainwater systems, and the local climate in maximising cost-effective investment in rainwater systems.
The lifecycle costs and benefits for a household can be calculated using a discounted cash flow analysis based on the estimated costs (initial and ongoing), yield, and the avoided costs (Marsden Jacob Associates 2007). The avoided costs from the householder perspective will relate mostly to the impact of rainwater tanks on reducing water utility bills. A discounted cash flow analysis, such as net present value analysis, applies a discount rate to determine the present value of costs and benefits over the life of the investment. If a householder is considering investing in a rainwater tank system, but the present value of life cycle costs exceeds the present value of life cycle benefits, it may be necessary to offer a rebate to cover the shortfall (Marsden Jacobs Associates 2007).
Marsden Jacob Associates (2007) compared the levelised costs (net costs divided by the expected water yield) of rainwater tank systems with the cost of mains water. The actual costs of rainwater tanks vary considerably depending upon how the systems is considered (catchment area, tank size and intended uses), the location, and also the household size (used to estimate demand). This analysis found that in Australian cities, the levelised rainwater costs were between two and 11 times the cost of mains water. The higher levelised costs were associated with relatively small catchment areas (50 m2), and cities that have an extended seasonal dry period (Adelaide and Perth). The analysis also challenged the popular belief that plumbing the rainwater tank for indoor use is more cost effective than just being used for outdoor use, due to the increased demand. The modelling showed that when the rainwater system was installed only for outdoor uses, thereby removing the need for the in-house plumbing and mains back-up switching device, the levelised cost was significantly reduced. For example, for a rainwater tank system in Brisbane with a 5 kL tank and 200 m2 collection area, the levelised costs when plumbed for both indoor and outdoor uses was $2.29. But, when installed only for outdoor use, the levelised cost halved to $1.25 (Marsden Jacob Associates 2007).
The economic cost benefit ratio may be more favourable where the rainwater tanks can be used to avoid or defer investment in stormwater infrastructure. Based on studies, Coombes et al. (2000) found that a water sensitive design approach that included rainwater tanks could be 25% more cost-effective than a conventional approach to providing stormwater infrastructure. Coombes & Kuczera (2003) found that investment in rainwater tanks provided greater economic benefits to the community than investment in traditional water supply and stormwater management options. Marsden Jacob Associates (2007) made the point that the savings from deferring, downsizing or avoiding investment in traditional infrastructure due to rainwater tanks is only likely to be realised in greenfield areas prior to the construction of water supply and stormwater infrastructure.
Based on a monitoring study of rainwater tank installations at 52 homes in Sydney, Australia, which included monitoring flows from the rainwater tank and pumping energy demand, Ferguson (2011) found that the typical levelised costs for rainwater installations is likely to range from $4 to $8/kL. Hall (2013) investigated the cost-effectiveness of rainwater tanks in SEQ, Australia. The rainwater tank cost for the scenario modelled was an average levelised cost of $9.22/kL, with 95% confidence limits of $6.73 and $12.77/kL. The variation in yield, pump and tank life, and maintenance had the largest effect on the variation in the cost effectiveness. The results were also sensitive to discount rate assumptions (Hall 2013). As with all models, the outcomes of economic studies are strongly dependent on the input assumptions. But the results indicate that the supply costs of household rainwater systems in SEQ cannot compete economically with other alternative water supply sources, which is consistent with Figure 2.
A study in Southern Brazil compared the cost effectiveness of using rainwater systems and/or greywater systems to provide potable water savings (Ghisi & Ferreira 2007). This study showed that greywater systems were marginally more cost effective than rainwater systems, as they provided greater reliability of supply to meet toilet-flushing demand.
Cluster scale or communal rainwater systems collect roof water through a collection system discharging to a cluster scale tank and then water is supplied back through a reticulated system to the households. Gurung et al. (2012) undertook an economic analysis that compared individual rainwater tank systems with cluster scale rainwater systems. The analysis also explored the economics of scale with cluster scale rainwater systems to determine the optimal housing scale (assuming flat topography and 20 houses per hectare). This study found the optimal scale was between 200 and 300 connected houses. This provided economies of scale for communal infrastructure, such as storage tanks, whilst beyond this number of households, increasing pipe costs resulted in dis-economies of scale. Individual rainwater tanks were found to be more cost-effective than optimal scale cluster systems due to the lower costs for distribution pipe network. If a 0.5% slope were assumed, then costs between individual tanks and cluster scale systems were comparable due to lower pipe trenching costs in sloping terrain (Gurung et al. 2012; Gurung & Sharma 2014). However, communal rainwater tank systems have other benefits such as greater overall supply reliability, and better water quality because of better system management including disinfection.
INDIRECT COSTS AND BENEFIT OF RAINWATER TANKS (EXTERNALITIES)
Outcomes from rainwater tanks that are not captured in the direct costs and benefits to the affected parties can be considered an externality. Generally, the parties that bear the costs or receive the benefits from a rainwater tank are the householder and the water utility, where the latter may be directly impacted by reduced demand for mains water and/or offer a rebate for rainwater tank adoption. More specifically, an externality is any cost (or benefit) that relates to rainwater tanks, but which is not incurred by the utility or the householder. Hence, the externality is not adequately captured by the market price to reflect the full cost (or benefit) of rainwater tanks. There can be positive as well as negative externalities from rainwater tanks. This section discusses some of the externalities that are associated with the adoption of rainwater tanks as an alternative local water source. One of the most challenging aspects of evaluating the costs and benefits of alternative water policy options is the quantification of such externalities.
Impact on stormwater flows
A study by Parkinson et al. (2005) investigated the impacts of domestic water conservation on urban sewerage systems in cities with combined sewer overflows (CSOs) and concluded that rainwater harvesting was the most beneficial water conservation method. The justification for this was that rainwater tanks could assist in reducing in sewage discharge from CSOs during high rainfall events (Parkinson et al. 2005). This finding is supported by Vaes & Berlamont (2001) who showed, based on modelling, that rainwater storage systems can significantly reduce rainfall to combined sewer systems, including reduced peak flow if installed on a sufficiently large scale. However, there is a need to account for storage levels at the start of the rainfall event to accurately estimate the impact of rainwater storages on downstream flows (Vaes & Berlamont 2001).
In Australian cities (which do not have combined sewer systems), the objectives of rainwater systems are primarily the augmentation of potable water supplies and to some extent restoration of pre-development flow regimes to the receiving waters (Burns et al. 2012). The impact of rainwater tanks on the latter is not yet well understood or quantified. Modelling undertaken by Burns et al. (2012) found that rainwater tanks can assist in restoring catchment retention capacity closer to that of the natural conditions. However, this will depend upon the match between roof size, storage volume and demand profile. A report to the National Water Commission considered the impact of rainwater tanks on the size of stormwater drains. The report demonstrated that rainwater tanks, under certain conditions, can reduce peak flows to stormwater drains by up to 50% (Marsden Jacob Associates 2007). This means that stormwater drain capacity could be reduced in new developments. However, there will be a limit to size reduction since drains are typically designed to manage peak flow events (Marsden Jacob Associates 2007). Generally speaking, smaller roof areas are likely to divert a larger proportion of the run-off than larger ones, since there is less total run-off generated on smaller roofs and their companion tanks are less likely to be full. Furthermore, the best reduction in peak stormwater flows was found when a large tank was connected to a small roof. Significant reduction to stormwater flow may justify smaller drains as well as reduced costs associated with detention and treatment of stormwater for nutrient removal (Marsden Jacob Associates 2007).
Australian researchers modelled the impact of rainwater harvesting and greywater reuse on stormwater flows in a 13,890 home estate located in a 2,990 ha catchment in Canberra, Australia (Sharma et al. 2008). A reduction in stormwater flow of up to 20% was estimated for an 85% uptake of rainwater tanks combined with a 15% uptake of grey water reuse. This study demonstrated that a high uptake rate of rainwater tank systems was essential to achieve significant reductions to stormwater flows.
The discharge of urban stormwater runoff to receiving waters impacts on the ecological health of streams and other waterways (Christopher et al. 2009). In Melbourne, Australia, a link has been established between heavy rainfall events and nutrient accumulation in Port Phillip Bay (Melbourne Water 1996). Christopher et al. (2009) argued that if stream ecosystems are to be restored or protected, there is the need to reduce the hydraulic connectivity between impervious surfaces in urban areas and streams for small, frequent rainfall events. Water harvesting by rainwater tanks will help reduce peak flow events and therefore reduce total nutrient loads entering receiving bodies of water. Nitrogen reduction has been shown to be significant when rainwater tanks are installed (Sharma et al. 2008). Khastagir & Jayasuriya (2010) modelled the impact of rainwater tanks on runoff quality. They found that a 3 kL rainwater tank used for toilet flushing, laundry and garden watering could reduce nitrogen loads by 81%.
In the case of combined sewer systems (carrying wastewater and stormwater flows), the use of stormwater settling tanks (SSTs) as retention basins was found to have a beneficial effect on the performance of a wastewater treatment plant in terms of the quality of the water discharged to receiving waters. A simulation investigated several strategies to improve treatment performance such as SST. Significant reductions in chemical oxygen demand (COD) (13%), organic N (28%), and total N (15%) were reported with the use of SSTs (Langeveld and Van der Graaf 2001). As SSTs are effectively acting as rainwater tanks by capturing stormwater during peak flow events, this result is an indication that rainwater tanks installed as part of water conservation initiatives would have a beneficial impact on COD and N discharge at the treatment plant in the case of combined sewer systems.
Impact on centralised water systems and water quality
Widespread rainwater harvesting and use practices can also affect the mains water demand and quality. Grandet et al. (2010) studied the effect of rainwater harvesting on centralised urban water systems in Northern France. The study identified that rainwater use resulted in a permanent decrease in mains water demand, leading to an increase in water age and hence quality deterioration in the distribution system. Water age was generally affected when rainwater supplied more than 30% of the overall water demand. However, the rainwater supply systems might be profitable for the community if rainwater use allowed the deferment of new water mains infrastructure. Lucas et al. (2010) investigated the impact of diurnal water use patterns, demand management and rainwater tanks on water supply network design. The study suggested that the rainwater tanks with mains water trickle top-up produced diurnal ‘mains water’ use patterns different to ‘household’ water use patterns, and when simulated correctly, significantly reduced peak hour ‘mains water’ demand. This outcome impacts upon water supply network design criteria and provided opportunities to offset water infrastructure costs.
ENERGY CONSUMPTION IN RAINWATER TANK SYSTEMS
There is widespread adoption of distributed rainwater harvesting across the world, yet limited research has been conducted on the energy use with rainwater supply to dwellings, particularly when it comes to the expanded uptake of rainwater tanks in urban areas (Retamal et al. 2009). Australian studies conducted since 2006 are the most extensive sources of information currently available on this topic. In the last decade, studies conducted in Australia have measured the in-situ energy consumption for rainwater pumping to households across diverse climatic conditions, geographical locations and development settings (Gardner et al. 2006; Beal et al. 2008; Hauber-Davidson et al. 2009; Retamal et al. 2009; Hood et al. 2010; Ferguson & Amaro 2012; Umapathi et al. 2012), and evaluated the energy breakdown and the various systemic factors that influence it (Cunio & Sproul 2009; Retamal et al. 2009; Hauber-Davidson & Shortt 2011; Tjandraatmadja et al. 2012a). The energy footprint of rainwater supply has been compared with the energy footprints of traditional and alternative water sources supplies (Kenway 2008; Retamal et al. 2009; Hall et al. 2011).
Whilst traditional rainwater tanks consisted typically of stand-alone tanks with direct supply of rainwater to dwellings through a pump, modern systems in urban areas include options such as mains water top-up or switch systems to ensure that water supply is uninterrupted. In addition, rainwater end uses for urban dwellings are typically restricted to non-potable applications such as filling of toilet cisterns, laundry cold water supply, outdoor uses and, in some cases, supply to hot water systems (Tjandraatmadja et al. 2012a, 2013). The implications of such factors on energy consumption are described in the following section.
Factors impacting energy usage
The energy associated with household-scale rainwater pumping is a function of the pump characteristics, the system set-up and the household water-use patterns (Beal et al. 2008; Retamal et al. 2009; Hauber-Davidson & Wetherall 2010; Tjandraatmadja et al. 2013). The energy can be expressed as the ‘total energy’ consumed over a period of time for pumping (e.g. kWh per month or year) or as ‘specific energy’, i.e. the energy required to pump a set amount of water (e.g. kWh per kL of rainwater).
Retamal et al. (2009) estimated that the losses in pumping energy attributed to motor losses and pump losses comprised 36% of energy consumption in a typical domestic rainwater system. Motor and pump losses are a function of the design and manufacture of individual pumps’ mechanical and electrical components. For example, the smaller the (space) tolerances between the casing and the impeller, the more efficient will be the pump (Retamal et al. 2009; Hauber-Davidson & Shortt 2011).
The power consumed by the pump when connected to a power supply, whether active or not, is also a function of the design of electric circuits and the circuitry components of the individual pump. Retamal et al. (2009) estimated 22% of energy loss was through stand-by power consumption, whilst Hauber-Davidson & Shortt (2011) have shown that this value could vary from negligible to 29% of the energy consumed, depending on the active pumping time.
Rainwater supply system configurations can include mains back-up systems to allow continuous water supply, filters for improving water quality, UV disinfection for potable end uses, and pressure vessels aimed at reducing pump start-ups.
The adoption of automatic mains back-up switches over trickle top-up systems has been shown to reduce energy usage. Umapathi et al. (2012) monitored 20 houses in SEQ, Australia, for 7 months. The mean energy values varied with system set-up and were higher for trickle top-up (float valve) systems at 43.4 kWh/dwelling compared to 33.2 kWh/dwelling for automatic switches, as all the top-up volume of water needs to be pumped from a tank fitted with a trickle top-up system. They also reported monitoring outcomes for 20 homes based on 12 months' data (Umapathi et al. 2013).
When filters are incorporated, periodic maintenance and exchange of filters is required as, with time, sediment accumulation in filters can increase the resistance to flow in the line, increasing the pump duty (Retamal et al. 2009; Ferguson 2011; Hauber-Davidson & Shortt 2011). Pressure vessels, on the other hand, can reduce the energy for rainwater supply as they can reduce the number of pump start-ups per day (which has a high transient energy consumption), and pump operating time when sized correctly (Ferguson 2011; Tjandraatmadja et al. 2012b, 2013).
Appliance water needs and pump flow rates
Mismatch of rainwater pump flow rate to the end use water requirements is a very important factor responsible for the majority of pumping energy losses in a system (Retamal et al. 2009; Cunio & Sproul 2009; Ferguson 2011; Hauber-Davidson & Shortt 2011; Tjandraatmadja et al. 2012a, 2013). Retamal et al. (2009) estimated as much as 34% of the total energy losses could be attributed to this factor. Figure 4 also shows that using a pump of lower motor capacity will reduce the energy consumed for rainwater supply, and yet achieves the same service requirements. Consequently, oversized pumps which are not matched to the dwelling end use service needs will result in inefficient operation and unnecessarily high energy consumption, Figure 4.
Monitoring of energy usage in household rainwater supply
The in-situ energy for rainwater supply (kWh per kilolitre of rainwater supplied) at individual dwellings in Australia has been reported to range from 0.6 to 11.6 kWh/kL based on eight major studies conducted across the country (Gardner et al. 2006; Beal et al. 2008; Retamal et al. 2009; Hauber-Davidson & Wetherall 2010; Ferguson & Amaro 2012; Umapathi et al. 2012). However, the median energy consumption of rainwater supply across most studies ranged between 1.4 and 2.0 kWh/kL.
In net terms, the overall energy use is a function of the rainwater usage patterns in a dwelling, the frequency of use (which is related to the number of occupants) and the type of end uses. Retamal et al. (2009) verified an energy footprint from 0.9 to 2.3 kWh/kL for eight households equipped with various rainwater pump and mains switch systems, different numbers of occupants, and diverse rainwater end uses. The household with the highest water use required 0.9 kWh/kL, whilst the household with the lowest water use, and water efficient appliances, required the most energy at 2.3 kWh/kL. However, the total energy (kWh per month) consumed by the latter system was lower (Retamal et al. 2009). Hence, energy assessment requires both an understanding of the energy for water supply in kWh/kL (the Specific Energy of the system) and the total energy use over a period of time, which has units of kWh per dwelling.
For example, Ferguson (2011), who monitored 52 houses in Sydney over 12 months, verified a median energy use of 62 kWh/year/dwelling and a pump-specific energy of 1.48 kWh/kL. In addition, in SEQ, Umapathi et al. (2012) verified a median of 41.6 kWh/dwelling and 1.65 kWh/kL for 20 houses over 7 months, and Hood et al. (2010) verified a median of 1.4 kWh/kL for 40 dwellings at Currumbin Ecovillage, where rainwater supplied all end uses.
The large variability observed between dwellings is a reflection of the wide range of rainwater system configurations and characteristics of the different individual households that use rainwater. It is also an indication that the energy requirements for rainwater pumping can be optimised for dwellings through appropriate system design and configuration set-up.
The cost associated with the energy use for pumping was estimated to be on average A$15.00 per year per dwelling for say 50 kL of rainwater. However, typical levelised costs for rainwater installations were estimated to be $4 to $8/kL of rainwater (Ferguson 2011; Ferguson & Amaro 2012).
Small pumps used for rainwater supply are typically much less efficient than large pumps used for bulk water supply (Retamal et al. 2009). In major Australian capital cities, the centralised supply and reticulation from dams and reservoirs typically consume between 0.09 and 1.85 kWh/kL of water supplied for Melbourne and Adelaide, respectively, subject to topography (gravity dominates Melbourne's supply) and pumping distances (distance dominates Adelaide's supply) (Beal et al. 2008). This places the median energy for distributed rainwater supply of 1.5–1.8 kWh/kL within the upper range of centralised gravity supply. However, when compared to other alternative sources such as recycled water or desalinated water, which use on average 2.8 kWh/kL (Knights et al. 2007) and 3.5 kWh/kL (Apostolidis 2010), respectively, rainwater supply is the least energy intensive option.
However, research to date has shown that such energy consumption values for rainwater supply are achieved using the most common current technology and system set-ups adopted by the market. Energy requirements for rainwater delivery can be reduced even further given that energy optimisation in rainwater supply has been largely unexplored because of the lack of understanding of the rainwater pumping set-up.
Reduction of the energy use per kilolitre of water supplied can be achieved by either increasing the flow rate of water supply (more end uses supplied by rainwater), selection of a pump that matches the specific end use needs of a dwelling, and the adoption of suitable ancillaries, e.g. automatic mains top-up switches, pressure vessels, and possibly header tanks in the ceiling. Thus, increased access to information and guidance on rainwater system set-up to optimise water and energy savings would seem to be a priority for a society interested in reducing both its demand for potable water and its carbon footprint.
Rainwater harvesting is not a new concept, and rainwater tanks continue to provide a reliable and relatively safe water supply in rural and remote areas where piped water supply systems are not feasible due to economic considerations. In urban areas, rainwater tanks are now being implemented under integrated urban water-management concepts to provide a substitute for mains water for non-potable household uses. This substitution concept is based on fit-for-purpose uses, where the water quality of the supply source is matched to the requirements of the end use. In Australian cities, there has been a rapid uptake of rainwater tanks as a supplementary water source to help address the increase in demand for freshwater resources due to rapid population growth and urbanisation, and water scarcity due to drought. The uptake of rainwater tanks in urban areas has been encouraged through subsidy provisions and regulations as part of the cities’ strategic planning response for water supply demand to cope with population growth and climate variability. In other countries, such as Germany and Korea, there has been a greater interest in the role of rainwater tanks in reducing flows to combined sewer systems.
There has been worldwide research into developing modelling tools to simulate rainwater tank water supply potential based on long-term climate data, tank size, connected roof area and end use demands. Less widespread have been validation studies to confirm the modelling predictions in the real urban world. Basically, two methods are available: (i) paired statistical studies using a large household population base with/without tanks; and (ii) detailed instrumentation studies on a small household population base. The studies have indicated that a mandated, internally-plumbed rainwater tank can save around 50 kL/hh/year of mains water in SEQ, Australia.
Various studies have highlighted that the chemical and microbiological quality of rainwater harvested from roof collection systems varies significantly and depends on a number of factors including roofing material, proximal land uses, and maintenance practices. It has been found that the chemical and microbiological qualities of rainwater can exceed drinking water guidelines. Chemical contamination appears not to be a health issue except for lead from lead flashing. However, pathogens are more ambiguous; E. coli. is a more concerning contaminant. There are recent concerns that some strains may be pathogenic or may contain toxic genes. In addition, there are zoonotic pathogens including Giardia and Salmonella that occur with a frequency and concentration that are likely to pose a health risk when assessed using the QMRA technique. Care should be taken in using rainwater at a household based on the fit-for-purpose concept and proper treatment of rainwater would be required if used for potable applications. However, adopting a regular maintenance regime for the rainwater tank system is essential to avoid water quality issues. The quality of rainwater for non-potable human use is not regulated by any standard that is internationally recognised. Therefore, many countries and regions have developed local guidelines for rainwater harvesting and use. Guidelines for rainwater use need to consider the water quality requirements of the intended use, and the likely microbiological and chemical risks posed.
The cost-effectiveness of rainwater tanks is a function of water yield and whole-of-life costs. The yield of a rainwater tank system is influenced by tank size, roof catchment area, the planned uses of rainwater, and the local rainfall pattern. Various studies have found that roof catchment area had the greater impact on yield, followed by annual rainfall and tank size. This highlights the need to consider both the optimal configuration of rainwater systems and the local climate for maximising cost-effective investment in rainwater systems. There are some significant socio-economic benefits for the general public with the uptake of rainwater tanks. Water harvesting by rainwater tanks will help reduce peak flow events of the stormwater flows and can reduce total nutrient loads entering receiving bodies of water. However, the collection and use of roof rainfall can result in a permanent decrease in mains water demand, leading to an increase in water age in the distribution system, which may have an adverse impact on water quality. Also, the levelised cost of rainwater water supply is higher than centralised water supply options, even after externalities are taken into account.
The energy consumption of a household rainwater pumping system is a function of the pump characteristics, the system set-up, and the household water use patterns. Mismatch of rainwater pump flow rate to the end use water flow requirements is an important factor responsible for the majority of pumping energy losses in a system. Water efficient appliances and fittings, such as toilet cisterns, washing machines and taps in Australia, typically require flow rates much less than 15 L/min for operation, whilst pumps used for rainwater supply typically operate most efficiently at flow rates of 15–20 L/min. Consequently, pumps which are not matched to the dwelling end use service needs will result in inefficient operation and unnecessarily high energy consumption.
Properly designed, operated and maintained rainwater tank systems can provide a valuable additional water source to supplement centralised mains water supply sources and to reduce demands on fresh water resources in urban areas, and can augment water supply systems in rural and remote areas.
This review has identified the knowledge gaps in current practice that need to be addressed for greater uptake of rainwater systems in different development contexts and to address different objectives, such as mains water savings or reduced discharge of runoff to the environment and/or sewers. There is a need for improved models, guidelines and regulations to guide the design of rainwater systems that are cost effective, maximise yield and minimise risks to the public and the environment. A particular gap is the lack of studies that validate the actual performance of rainwater systems in the field in different contexts. These monitoring studies can also be used to inform better risk management practices to protect public health and maintenance guidelines, to ensure effective long-term operation of rainwater systems. There is the potential to explore alternative configurations of rainwater systems, such as cluster-scale systems, which might provide a cost effective option for rainwater harvesting in more densely populated areas. Rainwater harvesting can play an important role in modern cities, not only in addressing primary objectives such as reducing demand for potable water or inflow to sewers, but also moving towards the objectives of a water-sensitive city where water services are planned to maintain and enhance the local environment.