Stormwater from rooftops can be captured for use in buildings and landscape areas, reducing the requirements for mains water supply.
Runoff from ground level surfaces can also be captured, and often contain entrained nutrients which can be beneficial for irrigation of landscape areas.
Rainwater poses little health risk when applied to non-potable uses such as landscape irrigation or direction to toilets and laundry. Additional treatment such as redirection of first flush, cartridge filtration, or UV disinfection, is generally required when rainwater is to be used as a potable supply. In general, well-maintained rain storage facilities outside of heavily urbanised catchments are likely to only contain traces of nutrients and no contaminants in harmful concentrations. However, care should be taken for stormwater collection from treated timber, metals (copper and zinc), bitumen roofs, or beside air conditioning facilities. In all instances confirmation should be sought from the Council and health authorities prior to potable use of stormwater.
Rainwater from roofs can go to above or below ground storage facilities, but there is a preference for gravity fed situations or solar pumps to header tanks to reduce energy use. Raintanks come in many materials and finishes, with slim-line designs able to be placed partially underground or to fit underneath the eaves of buildings. Construction should also consider stress loading of a full tank and the necessity for maintenance access.
Collection of surface water flows from streets, yards, etc. usually requires pre-treatment prior to storage, usually in an underground facility. There are multiple systems on the market for underground stormwater collection, which balance the load bearing requirements of above ground uses with optimal water storage capacity. These systems may be applied to single lots or as combined systems to control peak flows and/or recharge groundwater in localised areas of permeable soils.
Harvesting and re-use of greywater and blackwater may also contribute to a holistic watercyle approach. However, this is not discussed as part of this guideline document.
Challenges and solutions
The table below describes some of the common issues relating to capture and re-use of stormwater. In all circumstances there is a potential planning or design solution, which must be balanced against other objectives for the project and the maintenance requirements of the approach.
Raintanks are perceived as having high capital and life cycle costs
Raintanks may be the only feasible stormwater management response in a retrofit situation, where space is limited. Capture and re-use may also be justified by providing resilience to water supply.
Raintanks are most appropriate if there are significant cost savings from diverting from watermain connections, and consequently reduced water rates.
Communal systems for multiple units may provide for economies of scale, or underground storage systems can be investigated which usually have an increased service life and therefore a reduced life cycle cost.
Detention capacity may be reduced by multiple storms which also generally occur during low demand periods
Raintanks can be designed to have storage volumes for re-use, and a separate detention volume with a controlled discharge rate.
Superposition of peak flows may result from the combined overflow from raintanks in heavy or cumulative rainfall events
A controlled release point should generally be directed to vegetated landscape areas with enhanced infiltration capacity.
Purchasers of a property may not view raintanks as desirable, and instead view them as a liability
Raintanks should be designed for reduced maintenance and for multiple benefits, such as passive heating and cooling, and overflow to landscape irrigation.
A manual describing raintank function and maintenance requirements and expected life of the device could be provided to the owners. Access to the tank should be provided for the ease of maintenance and/or replacement.
Information on the benefits of raintanks to the owner and to the environment should be provided to educate the purchaser on the benefits of raintanks.
Tanks may be considered unsightly by potential purchasers
Water storage can be incorporated into architecture to integrate with or enhance the building design and structure. Water tanks can also be placed behind buildings or under decks with the use of a pump system.
Raintanks require maintenance to ensure their long-term function. Raintanks should be checked every two years. Inspections should include: checking that pumps are still operational; checking if drainage connections to the tank are still functioning; and checking that connections to internal plumbing are operational, including backflow prevention valves. Any issues identified during the inspection should be the responsibility of the owner to fix.
Correctly maintained raintank systems have a design life of approximately 20 years. If maintenance is not performed, lifespan will reduce depending on whether pumps are used in the design and the potential for clogging of the tank inlets and outlets.