2 Stormwater Storage and Use

2 Stormwater Storage and Use 2. Rainwater Storage Systems Figure 1. Rainwater tank components. (Coombes and Kuzcera 2001.) Background At the lot scale, rainwater storage tank systems are an effective way of capturing stormwater for non-potable use (such as garden watering, toilet flushing, in washing machines and for car washing) and therefore helping to conserve scheme drinking water supplies. There are increasingly innovative designs which hold rainwater in a range of forms including bags, fences, walls and roof gutters, with these systems designed for different uses, volumes and aesthetic requirements. Slimline plastic or metal tanks are increasingly popular in urban areas due to the unobtrusive form of the tanks adjacent to or beneath buildings. Rainwater storage systems can also be applied at a larger scale, for example high volume underground storage systems applied to capture runoff from paved or parking areas or the roofs of buildings. There are a number of new below-ground storage propriety devices that are designed for both roof water catchments and impervious surface catchments, such as hardstand areas. The storage systems are modular and their volumes can be anything from 5 kL to 5 ML. These devices have different filtration systems over the inlet for primary treatment and some have a secondary filter, depending on the quality of the runoff and the desired water use. A pump system returns the collected water to provide a non-potable supply for domestic, commercial, industrial and municipal buildings, such as sports centres and community halls. Collected stormwater could be used for irrigation, vehicle washing, art/water features, toilet flushing, cooling water systems and many other applications that currently use high grade potable water. Stormwater used for cooling water systems might have to be treated to a higher standard because nutrients and heavy metals in the water may cause slime formation and microbial growth, while suspended solids could cause blockages and fouling. In high density urban areas, there is limited space for outdoor gardens, so roof water and stormwater-fed roof gardens or courtyard gardens are an attractive option for outdoor living space. In Western Australia, Stormwater Management Manual for Western Australia: Structural Controls 47 these systems are a great opportunity to provide a water sensitive garden in urban environments. Plant species that require irrigation throughout the dry season will not be suitable due to the need for top up water. Urban space designers can also use harvested stormwater as a feature in the landscape. However, emphasis should be placed on the ephemeral nature of our environment, so these features should be aesthetically pleasing with and without water. Use of groundwater and scheme water to top up water features over summer is not considered water wise or beneficial to the environment. Rainwater tanks have primarily been used to provide an alternative water supply source and reduce scheme water consumption. However, in areas of limited infiltration (high water table, clay soils) they provide additional benefits of reducing catchment runoff (peak flow and volumes) from smaller rain events, which assists the post-development catchment to replicate its pre-development hydrology. This BMP has good potential in a retrofitting scenario and can be applied in highly impervious built environments that may preclude the installation of other BMPs. The quantity of water that can be detained in these systems will depend on the rainfall (amount and annual and seasonal variability). For large-scale harvesting schemes, the ecological water requirements of the catchment need to be considered in determining the volume of water that can be collected. Unless adequately treated, collected rainwater is not reliably safe to drink due to the possibility of contamination from air-borne chemicals and microorganisms. Department of Health (2003) provides guidelines for urban rainwater collection. There may also be additional State and local government regulations and guidelines for the catchment that will guide the use of roof water and stormwater as a water source. Performance efficiency In urban environments in south-western areas of WA, the Mediterranean climate means that a domestic rainwater storage tank is likely to be dry in the summer months, when garden watering demands are highest. Using rainwater as an effective alternative source for garden watering is therefore considered limited. Substantial reductions in the volume of stormwater runoff and the use of scheme water are achieved when stored rainwater is utilised for indoor use, such as washing machine use and toilet flushing (Figure 2). Use of rainwater for these purposes is supported by the Department of Health (2003). By connecting the rainwater storage system to indoor uses, the water is consistently used, freeing up space in the tank to capture more runoff. Recent research by Fletcher et al. (2006) indicated Figure 2. Elements of a domestic rainwater system that modelling stormwater harvesting in Brisbane (the rainwater treatment train). and Melbourne for three land use scenarios (low, medium and high density) demonstrated that stormwater retention and use on-site could help restore stormwater flows and water quality towards their pre-development level. In a study of eastern states capital cities, Coombes and Kuczera (2003) found that domestic rainwater tanks with capacities of 1 kL to 5 kL provide considerable reductions in scheme water demand and stormwater runoff. The average retention volumes available in rainwater tanks prior to storm events ranged from 0.25–0.7 kL for 1 kL tanks and 2.3–8.4 kL for 10 kL tanks in a study for Brisbane, 4 Stormwater Management Manual for Western Australia: Structural Controls Sydney, Melbourne and Adelaide. Areas with lower annual rainfall had the largest retention volume available in the tank due to internal use emptying the tank and less rainfall to refill the tank. The same study (Coombes and Kuczera 2003) found the annual scheme water savings ranged from 18–55 kL/year for a 1 kL rainwater tank, increasing to 25–144 kL/yr for a 10 kL tank. However, these results should be applied with caution to WA conditions due to considerable differences in rainfall seasonality between the different regions of WA and eastern Australia. The Wungong Urban Water Master Plan District Water Management Strategy (JDA Consultant Hydrologists, CSIRO & GHD 2006) provides local estimates of scheme water savings through use of 3 kL rainwater tanks (integrated with scheme water) in the Armadale region as approximately 25% of in-house water use requirements, equating to 43 kL of annual demand. Cost Costs of rainwater tanks can vary considerably depending on material, design, size and installation requirements. With respect to capital costs, Coombes (2004) reports the cost of small rainwater tanks as ranging from: • $250 (Aquaplate Round) to $340 (Polymer) for a 0.5 kL tank • $530 (Polymer) to $1466 (Aquaplate Slimline) for a 2.4 kL tank • $1460 (Polymer) to $7500 (Concrete) for a 9.0 kL tank Coombes (2004) reports that small household pumps with pressure control cost $340–-$620. Installation costs are highly variable, with full installation of a 4.5 kL above-ground tank ranging from $1300 to $3500 and underground installation an additional $2000. Based on costs sourced from WA manufacturers, GHD (2005) reports an estimated net cost for the installation of a rainwater tank system, including a 3.0 kL tank, installation and plumbing alteration costs, an automated controller and first flush device, as between $1750 and $2250. It is considered likely that reductions in this unit price would result from broad-scale use at a development. GHD (2005) estimates annual maintenance costs as unlikely to exceed an average of $75/year. Economic benefits derived from the use of rainwater storage systems vary with the price of scheme water and the cost of augmenting scheme water supply systems. The magnitude of economic benefits for the community from widespread installation of rainwater tanks is dependent on real interest rates, the value of stormwater savings that result from the use of rainwater tanks and the installation costs of rainwater tank systems (Coombes 2004). Savings include a reduction in the drainage infrastructure that would otherwise be required to manage stormwater runoff and the reduction in provision of scheme water. Economic benefits of rainwater tanks should be considered in conjunction with the environmental benefits of reduced demand on rivers, dams and groundwater for scheme water sources, improved water quality in the receiving environment and reduced erosion and flood damage to receiving environments caused by flashy peak flows. In terms of the unit cost of water, the Guidance on Use of Rainwater Tanks (enHealth Council 2004) reports that the estimated cost of rainwater from domestic tanks ranges from $0.3/kL to $14/kL. Based on GHD 2005 cost estimates and estimated annual scheme water saving of 43 kL for a 3 kL tank (JDA Consultant Hydrologists, CSIRO & GHD 2006), this equates to a unit cost of water of $4.00-4.90/kL, based on a 6% discount rate and 20 year tank life. Stormwater Management Manual for Western Australia: Structural Controls 49 Design considerations The design of a rainwater storage system is dependent on the intended uses of the water and the quality required. There are several factors that contribute to water quality. At the lot, street and precinct scales, the quality of the stormwater will be highly dependant on the type of hardstand area and the use of the surfaces that the runoff flows over. In a lot-scale rainwater supply system, the quality of runoff from the roof depends on roofing materials, the types of materials deposited on the roof and the roof maintenance regime. Storage systems need to consider sediments and organic material as the major contaminants. Physical, chemical and biological processes can improve the quality of the roof water in the storage tank (Coombes & Mitchell 2006).

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