Two new studies on the economics of rainwater tanks as an alternative supply option have been conducted by Marsden Jacobs Associates.
One study (March 2007) initiated by the National Water Commission (NWC) analysed the cost-effectiveness of installing a rainwater tank from the perspective of the property owner in different parts of Australia as a substitute for reticulated water supply.
The report highlights that the yield and the cost of rainwater tanks varies significantly based on individual household variables, in particular the size of the roof collection area, tank size, no. of occupants in the house and annual rainfall. A tank’s yield is determined by both the volume and timing of run-off into the tank and the volume and timing of usage.
In every case, the cost per kilolitre of tank water is greater than the price charged by water utilities for reticulated supply. However, for many households with large connected roof areas, particularly in Brisbane, Sydney and Melbourne, the unit cost fell to within 20 per cent of the top tier price of mains water.
The study highlighted that the uptake of rainwater tanks is being driven by many other factors other than direct costs savings for householders. Householders benefit from rainwater tanks through the additional benefits of: offsetting the effects of water restrictions (for example, avoiding the death of non-drought-resistant plants, losses in property value, or reductions in the availability of home grown fruit and vegetables); improving the taste of water in areas of poor water quality; and helping to play a part in addressing water scarcity. The cost of tanks may also be offset by savings in the stormwater system, and by environmental benefits for urban rivers and streams.
A second study (April 2007) by Marsden Jacobs Associates was prepared for the Australian Conservation Foundation, Environment Victoria and the Nature Conservation Council of NSW. This research focused on the impact of a potential targeted rainwater tank roll-out in Sydney, Melbourne and South East Queensland in comparison to other ‘conventional’ water supply options such as new dams and desalination plants. This study compares the yield and levelised cost (i.e. the direct cost per kilolitre supplied) of each supply option.
The study recognizes the broader cost savings of rainwater tanks to the community such as deferred water infrastructure, savings to stormwater infrastructure and environmental externalities such as the cost of greenhouse gas emissions.
The study found that:
Rainwater tanks are cost competitive with dams and desalination plants.
Rainwater tanks are five times more energy efficient than desalination plants (rainwater tanks requiring 1 MWh/ML compared to typical desalination plant of 5 MWh/ML) and twice as energy efficient as the proposed Traveston dam.
If governments deployed rainwater tanks to 5 per cent of households each year in Sydney and South-East Queensland, dams and desalination plants planned for 2010 could be delayed past 2026 (Sydney) and 2019 (South East Queensland).
A preliminary estimate of the cost of a 5 per cent roll out would be approximately $200-$280 million per annum in Sydney; $180-$250 million per annum in Melbourne and $140-$200 million per annum in South East Queensland.
Most Australian houses are suitable for a rainwater tank. In Sydney 65 per cent (or 1.1 million houses), in SEQ 73 per cent (or 900 000 houses) and in Melbourne 72 per cent of existing houses have potential for a rainwater tank.
