Rainwater Tanks and Water Security

The practice of rainwater harvesting (rainwater harvesting) can be traced back at least 4000 years, with systems employing cisterns fed with rainwater attached to single households in ancient civilisations such as Rome and Greece. In more recent times, rainwater collection and harvesting have been used largely in the rural areas where the centralised infrastructure was not developed. With the wide adoption of sustainability in the modern era the rainwater harvesting systems have once again made inroads into our cities.

Water security is now becoming a global issue of concern. As countries like Australia continue to experience high population growth and strong economic development that in turn puts increased demand on the water supply. The water supply in these countries has long been relent on the centralised reservoir and distribution networks supplying major metropolitan cities with potable water supply. However, the reliability and capture capacity of these traditional supply sources is stretched to the limit in many areas, leading to a host of new supply options coming into consideration. As such, water security is considered as one of the six key risks in Australia under a changing climate.

One potential solution to bridging supply-demand gaps is the wider use of rainwater tank systems, collecting and distributing water at a decentralised level. rainwater harvesting decentralised system typically collects the runoff from the available roof area then conveys it by gravity into a storage tank with further distribution to the end-uses.

There are various categories of rainwater harvesting systems depending on how they are configured. Some rainwater harvesting systems supply outdoor uses only, while others also supply a range of in-home end-uses. In the latter case - the water supply to the house needs to be pressurised. Therefore, the vast majority of rainwater harvesting systems contain a pump that can extract water from tanks and deliver it under pressure to the house. Traditionally, the single speed pumps were used for outdoor irrigation, however nowadays, with the internal users demand flow rates variations - the more complex pressure vessel setups and variable speed driven pumps are also utilised more often. Most of rainwater harvesting tanks also incorporate a central mains supply switch systems that secures the essential supply is maintained in the dry season.

There are many purported benefits of rainwater harvesting with the predominant benefit being a reduction in urban water demand, reduced water bills for households and reduced demand on mains water supplies. For communities and governments, this can delay the need for large infrastructure investments associated with upkeep of existing and construction of new potable water supply. rainwater harvesting can also assist in raising groundwater levels, an urgent task in many urban locations in Australia. Some of the risks asociated with rainwater harvesting arise from a lack of reliable supply and potentially poor water quality. Both of these issues are typically addressed by the appropriate system setup design and the inclusion of a mains supply backup.

Rainwater Harvesting in Australia

Rainwater harvesting systems have been utilised for generations in rural Australia. Urbanisation and housing developments in the urban areas led to the situation where rainwater harvesting was discouraged for many years with a number of local governments banning rainwater tanks in the 1960s, citing water quality as a prohibitive hazard. A severe drought that ran from 2000 until 2009 affected large portions of south-eastern and south-western Australia leading to critical depletion of freshwater reservoirs. This triggered the introduction of legislation and Government-backed incentives to install rainwater harvesting in urban households. These initiatives were heralded as 'green' and 'sustainable' solutions to the water security crisis while the research available to verify these notions was quite limited at the time.

According to the ABS statistical data for 2007, about 20% of Australian households had some form of rainwater harvesting system installed on their property. The majority of surveyed users of rainwater harvesting systems used fixed speed pumps with potable switch systems with the use of more sophisticated systems (e.g. variable speed pumps) has been somewhat limited due to higher costs.

Indicative costs

Various case studies on rainwater harvesting in the eastern states of Australia indicate that the produced water comes at the costs from $3.70/1m³ to $6.50/1m³ for a 100m² roof area with a 5m³ tank, supplying water for irrigation, laundry and toilet flushing end-uses in a 2.8 person household (Stewart et al, "Performance and economics of internally plumbed rainwater tanks: An Australian perspective.")