Sydney’s new Ian Thorpe Aquatic Centre harvests rainwater for internal uses.

Guenter Hauber-Davidson explores how to conceive, design and implement commercial projects in hospitals, office buildings, parks, shopping centres and sports facilities.

Australians often lament that “we live on the driest continent on Earth”, but overlook the fact that while dam catchments are in the driest locations, most of us live in areas with annual rainfalls of up to 1,200mm. This is not dry. In fact, rainwater is a safe and economical alternative water source.

Australians have responded to the water crisis by adopting residential rainwater tanks, aided by a range of government rebate schemes. Yet one option often overlooked is commercial rainwater harvesting from large roof areas. Rather than the 100–200m2 of roof on a typical home, commercial projects can capture from 1,000–10,000m2 and more. Add in stormwater and you get collection areas of 50,000m2 and beyond.

Some 30 per cent of Australia’s urban water consumption is non-residential. At least a quarter of that could be reduced through water efficiency measures, with some eight per cent of remaining demand supplied by commercially viable rainwater harvesting schemes – even under current low water prices and price distortions.

Domestic and commercial uses for harvested rainwater are similar, except there are two major additional supply points in the commercial sector: cooling towers and process water. Wash water is a third category that can represent a large demand.

In warehouses it may even be possible to connect rainwater directly to the mains supply, making it the ‘default’ water supply for an entire building, with a new line run for the few truly potable demands such as in kitchenettes. With appropriate labelling and information, such a scheme can be approved by the authorities.

Large rainwater harvesting schemes are – or should be – of interest to hospitals, works depots, shopping centres, tertiary institutions, military bases, prisons, sports facilities and parks and gardens, especially when neighbourhood schemes are considered where water is shared across boundaries.

The key parameters
All too often, people opt for excessive storage volumes without fully appreciating the background for installing a rainwater tank in the urban environment. This is not ‘out bush’ where the tank is the only source of water.

The role of a rainwater tank here is to supplement the reticulated water supply, not replace it. Water from the tanks should therefore be used for all appropriate purposes and when the tank is empty one can switch back to the reticulated water supply.

Authorities should show greater support and flexibility by better acknowledging the role rainwater tanks can assume in an integrated urban water supply network, one where the large central systems work hand in hand with thousands of mini-dams.
Users should be encouraged to use their rainwater for sensible demands as opposed to ‘saving’ it for otherwise restricted uses, such as irrigation. This would maximise the tank’s water savings potential.

An examination of several large rainwater harvesting schemes in various phases of design, development, implementation and operation shows the key to maximise water savings from such schemes is to make them work hard; an empty tank is a good tank.

One system showed that using it as part of an integrated urban supply scheme could triple water savings, as long as the water authority permitted the use of scheme water for otherwise restricted uses when the tank was empty.

The ideal demand is high throughout the year and indep-endent of rainfall, but slightly higher in summer than in winter to coincide with the rainfall pattern. Along the Eastern seaboard, water demands from cooling towers match the bill perfectly, with one hospital studied utilising 86 per cent of the total rainfall collected despite a relatively small storage volume of 35 litres/m2. The demand on it is so high that when the next rain falls, the tank is empty ready to be filled again. Hence, some of the most viable commercial rainwater harvesting systems in the urban context are those traditionally dismissed as “with our demand, we would empty the tank in two days”.

A rule of thumb design figure is to size tanks so that about 50mm of rain would fill them. Water savings are about two-thirds of the rainfall amount. Another interesting figure is that in Sydney, for a well designed system, about 10kL of water savings can be expected per kL of tankage installed.

Based on this rule, the indicative value of rainwater can be determined (see Fact File). In areas close to the coast in Sydney, 1,000m2 of roof area can replace more than $2,000 worth of water a year, while in Adelaide a combination of low rainfall, low water supply and fixed sewerage charges creates a yield of $400 only.

Approvals and rainwater quality
As with domestic rainwater, the water quality typically meets Australian Drinking Water Guidelines for most parameters tested, even for inner city catchments next to busy roads.

However, many councils, health authorities and professional associations remain confused about what is permissible. Recently, a case was reported where an authority would approve rainwater for pool top up in an outdoor pool (presumably because they could not stop the natural top up anyway) but would not approve it for an indoor pool.

At a hospital scheme, where rainwater was meant to be supplied to the cooling towers, the operator wanted to void all performance guarantees regarding legionella unless there was a separate filtration system on the rainwater supply capable of destroying legionella.

Similarly, while rainwater supply to the hot water system is generally accepted by health authorities for the domestic market, companies require tenacity and goodwill to obtain all the necessary approvals in a commercial environment.

Low energy rainwater, collected where the rain falls and people live and work can present a viable cost-effective alternative to energy intensive desalination plants or large recycling and pumping schemes.

This is an edited version of a paper to ‘Water Efficiency 2008. Guenter Hauber-Davidson is MD of Water Conservation Group. Contact at guenterhd@watergroup.com.au