Summary
Reuse of stormwater and roofwater can provide substantial benefits to the community including reduced:
Strategies for the capture and reuse of stormwater and roofwater are discussed in the context of government policies and regulations, Australian Standards and public health requirements in this article.
Introduction
The urban water cycle starts with water extracted from streams and aquifers, stored in reservoirs and then processed to potable quality before delivery through an extensive pipe system to consumers. Some of this water is then used to transport wastes through a network of sewers to treatment plants which discharge effluent into receiving waters such as rivers, lakes and oceans. Rainfall falling on the consumer’s allotment contributes to the urban catchment’s stormwater, and is collected by an extensive drainage system for disposal into receiving waters.
At the allotment all three components of the urban water cycle meet with water consumed and storm and wastewater discharged. Source control through management of the cycle at this level offers the opportunity to provide benefits for the consumer and the environment. The philosophy of source control is to minimise cost-effectively the consumption of mains water and the production of storm and wastewater. Source control can be implemented through
Retention of roof rainwater (rainwater tanks),
Stormwater detention
On-site treatment of greywater (laundry, bathroom and kitchen) and blackwater (toilet),
Use of water efficient appliances and practices, and On-site infiltration.
Less than 4% of urban water consumption is used for drinking. However, all mains water supply is treated to potable quality [Mitchell et al., 1997]. Considerable scope exists for the strategic reuse of stormwater (and rainwater) for second quality uses (including toilet flushing, outdoor, laundry and hot water).
The traditional urban drainage paradigm involving use of more and bigger capacity pipes to discharge stormwater runoff as quickly as possible results in costly solutions and adverse environmental impacts.
The use of source control measures can result in cost savings of 30% to 80% over traditional stormwater drainage measures. In Germany rainwater tanks are subsidised and are used to supply water for toilet flushing and irrigation to avoid the development of new water resources [Schilling and Mantoglou,1999]. The strategic reuse of stormwater (and rainwater) has the potential to provide significant benefits across the entire urban water cycle.
Reuse of Roofwater and Stormwater at the Allotment Scale
About 75% of impervious surfaces in an urban catchment are in the allotment and 70% of those surfaces are roofs. Clearly the allotment is a major contributor to flooding and water quality problems in urban catchments. However traditional design practices are dominated by street drainage and end of pipe measures ignoring stormwater mitigation opportunities on allotments. An excuse often given for discharging roofwater directly to street gutters is that it is relatively clean. However, “clean” roofwater discharged directly to the street gutter can acquire considerable kinetic energy, which can act within the catchment to erode soils and carry contaminants to waterways.
Capture and reuse of roof water and stormwater is an effective stormwater management method that provides an additional benefit of mains water use reduction.
A distinction must be made between the reuse of roofwater(rainfall that is directly collected as the roof runoff from buildings) and the reuse of stormwater(rainfall that is collected after it runs off urban areas such as paved and vegetated surfaces). The quality of roofwater is typically better than stormwater allowing a wider variety of reuse opportunities.
Reuse of Stormwater
Stormwater runoff from roofs, paved and garden areas can be captured in underground tanks, ponds or infiltration systems for active or passive reuse. An ancient example of integrated water supply is the capture of roofwater in an above ground tank for drinking and cooking uses. Overflow from the above ground tank and stormwater runoff from paved and grassed surfaces was captured in a pond or underground tank [Pacey and Cullis, 1991]. Stormwater from the pond or underground tank is used to supply all other water uses.
Ancient stormwater management practices involving the capture and reuse of as much stormwater as possible are the antithesis of modern practice, as described for example in Australian Rainfall and Runoff [IEAust, 1987], that encourages rapid discharge of stormwater to the environment. It is ironic that sustainable stormwater management practice has rediscovered ancient practices. There are many strategies for reuse of stormwater at the allotment scale, including:
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Direct roof water and stormwater to gardens or lawns rather than the street drainage system
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Capture overflow from rainwater tank and stormwater in ponds and reuse for outdoor and toilet uses
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Capture overflow from rainwater tank and stormwater in underground tanks and reuse for outdoor and toilet uses,
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Direct roofwater and stormwater to a gravel filled infiltration trench. A shallow gravel layer adjacent to or under a garden area will provide passive irrigation to the area [Argue et al., 1998], and
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Direct roof water and stormwater to water sensitive gardens that may include ponds, swales, contour banks, infiltration measures and mulching [van Gelderen, 1998] (DES 19).
Knowledge of the climate, terrain, soil type, geology and the receiving water environment is important to the design process. The designer should carefully consider the issue of sediment management, particularly during the construction phase of the development.
Strategies for Reuse of Stormwater at the Subdivision Scale
At the subdivision scale sustainable stormwater management includes conveyance controls such as grass swales, water sensitive road design and natural waterways; and storage methods that include detention basins, infiltration basins, constructed wetlands and aquifer recharge. These storage methods offer opportunities for stormwater reuse for irrigation of parklands, sporting fields and for cluster housing groups. There are many different methods for stormwater reuse including:
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Capture of stormwater in urban lakes for outdoor reuse
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Capture of stormwater in urban lakes or cluster scale tanks for outdoor and toilet reuse
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Aquifer storage and recovery
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Constructed wetlands
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Water harvesting
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Industrial reuse.
 
Urban lakes are usually constructed lakes within the urban area that are used to capture, store and treat stormwater for outdoor reuse on gardens and lawns. The lakes also improve urban amenity and provide habitats for flora and fauna. Stormwater can also be captured in urban lakes or housing cluster scale tanks for reuse in toilets and gardens in households.
Aquifer recharge is the capture and treatment of stormwater for injection or discharge to a suitable aquifer. Stormwater can be captured in urban lakes, wetlands, dry basins or gravel trenches and allowed to percolate to an aquifer or can be injected via a bore into an aquifer.
The stormwater is stored in the aquifer for subsequent reuse to meet outdoor water demand at a later date. Successful examples of this technique include the, the New Brompton Estate and the Mawson Lakes development. Constructed wetlands are similar to urban lakes except they also contain selected grasses and aquatic reed beds designed to improve stormwater quality. Stormwater stored in constructed wetlands can be reused for outdoor purposes.
Water harvesting involves the capture and storage of stormwater during periods of considerable stormwater runoff or streamflow. The stormwater runoff or streamflow is directed to an offline urban lake or wetland for subsequent reuse for outdoor purposes. Stormwater captured in urban lakes, wetlands or by aquifer recharge and storage can also be reused for industrial purposes such as cooling, boiler and process water, and for wash down purposes.
Infrastructure Cost Savings
Stormwater or roofwater reuse can provide substantial cost savings for the construction of stormwater water infrastructure in new developments. The Figtree Place development in NSW provided a 1% cost saving ($960 per dwelling) in stormwater infrastructure. Research has found that roofwater reuse in a new development would reduce the need for stormwater pipes and end of pipe water quality devices resulting in a 3% cost saving (including the cost to install rainwater tanks).
The reuse of stormwater or roofwater can also have significant impact on the provision of water supply headworks and distribution infrastructure. Research shows that the introduction of rainwater tanks to supply domestic toilet, hot water and outdoor uses will significantly defer (38 – 100 years) the need to construct new dams in the Sydney, Lower Hunter and Central Coast regions of NSW.
It was also found that the use of rainwater tanks with mains water trickle top can reduce annual maximum daily peak demands by up 40% for domestic dwellings. This can reduce the cost of water distribution (pipes) infrastructure. Unfortunately these infrastructure cost savings can only be realised if approval authorities accept that stormwater and roofwater reuse provides water supply and stormwater management benefits thereby reducing the requirement for centralised infrastructure.
The pipe system recipes derived from Australian Rainfall and Runoff [IEAust, 1987] and pipe discharge based models dominate local government assessment of stormwater management solutions. The recipe or models with discharge philosophies rather than storage philosophies cannot provide reliable guidance for approval authorities.
Evaluating the impact of stormwater or roofwater reuse on the urban water cycle is an extremely complex task. Yet the historical evaluation of such impacts has been dominated by ‘back of the envelope’ calculations, the use of untested assumptions and institutional constraint. There are many ‘classic’ untested assumptions about roofwater reuse. A common argument used to claim that rainwater tanks do not provide stormwater management benefits is that the tank will have no storage available prior to a storm event. Monitoring and analysis by the University of Newcastle finds this assumption to be incorrect. Coombes et al., [2001b] found that rainwater tanks used to supply toilet, hot water and outdoor uses will have 42% of their capacity available for roofwater retention prior to a 100 year ARI storm and will reduce peak stormwater discharges by about 80% for the one year ARI storm event in the Parramatta region of NSW.
Fortunately new models and design methods for stormwater and roofwater reuse technologies are being developed by the Australian research industry. The Aquacycle model (available from the CRC for Catchment Hydrology) allows the designer to understand daily water balances.
The allotment water balance model (currently being beta tested by Brisbane City Council) operates at small time steps allowing understanding of the impact of stormwater or roofwater reuse on water supply and stormwater infrastructure. The WUFS (Water Urban Flow Simulator] model is for design of traditional pipe and water sensitive approaches for subdivisions or catchments.
Retrofitting Opportunities and Economic Implications
Retrofitting of sustainable stormwater management elements to developed areas can be difficult and appear to be expensive. However these measures also present opportunities for catchment repair in urban areas subject to environmental stress and loss of serviceability from aging or overloaded infrastructure. The urban allotment presents the most promising opportunity for installation of stormwater and roof water reuse technologies in developed areas. The small-scale nature of source control solutions allows relative ease of installation. It is far easier to install a rainwater tank with an area of 2 – 6 m2 in a number of allotments than to construct an urban pond with an area of 200 – 2000 m2 in a fully developed catchment.
Installation of stormwater and roof water reuse elements cannot replace the need for urban water cycle infrastructure in a fully developed urban catchment. However, it can substantially reduce the load on water cycle (stormwater, wastewater and water supply) infrastructure. As a result, the service life of water cycle infrastructure (pipes, treatment plants and dams) can be substantially increased resulting in significant long-term savings. The current short-term nature of economic analysis results in the illusion that retrofitting opportunities are expensive. Lifecycle analysis of urban water cycle infrastructure with retrofitting of stormwater and roofwater reuse measures reveals large economic and environmental savings to the community.
Conclusion
The benefits of source control approaches such as stormwater and roof water capture and reuse arise from reduced demand on water supply and stormwater infrastructure. Rainwater tanks contribute significantly to these benefits. Water levels in rainwater tanks used to supply domestic inhouse and outdoor uses are constantly drawn down. This ensures that the tank regularly has storage capacity available to accept roof runoff resulting in reduced mains water use and stormwater discharge.
Extracted from an article by Peter Coombes and George Kuczera, Department of Civil, Surveying and Environmental Engineering, University of Newcastle.
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