Rainwater Harvesting – Design Tips

Tip 1 – Why collect rainwater?

Although the drought has broken, we all know that Australia is a most arid continent and we remain uncertain about the impact of climate change on rainfall patterns. While it is a necessity in areas not serviced by town water, owning your own water supply where mains water is available will afford you a certain degree of independence from water restrictions as well as a water source unpolluted by chemicals – where properly collected and stored. It also helps save money in the long run, in spite of the fact that water bills are usually loaded with a large fixed charge component – which unfortunately offers little incentive to adopt water-saving equipment and practices. Environmentally, rainwater harvesting helps decrease the need for major public infrastructure such as additional dams, interstate water pipes and other desalination plants. It also helps reduce the burden put on stormwater drainage systems. Many states or some agencies provide some sort of rebate system to encourage the uptake of rainwater harvesting in residential environments. All in all, it is indeed a wonder that in a country like Australia, not every house is equipped with its own rainwater supply!

Tip 2 – Where to use your rainwater

Before you start, check with your local Council whether a Planning or Building permit will be required in your area, or if there are any particular guidelines and restrictions you need to know about. As an average across Australian households, 26% of the water is used in the bathroom, 5% in the kitchen, 15% in the laundry, 20% in the toilet and 34% outdoors. The vast majority of these uses do not require potable water although it is wise to consult with your local authorities to determine the untreated rainwater uses that they approve of. Many urban Councils only allow untreated water to be used for garden watering and other outdoor uses, toilet flushing and clothes washing. This doesn’t require any sophisticated filtration system or extensive water testing and still takes care of about two thirds of the water needs of your average Australian household. If you are planning to use the water inside the house, bear in mind that the more water-efficient your washing machine and toilets are, the further your collected water will go. The same applies to taps and shower heads if you are going to use rainwater throughout.

Tip 3 Tank sizing

The size of the tank or tanks you select will be determined by the space you have around or under the house, your roof catchment area, the average rainfall patterns where you live, the projected use of the water collected and your budget. All in all, quite a complex assessment. The simplest system to install is one where the bottom of a small 2000 litre tank is connected to a hose for watering the garden. This will be useful, although limited to the summer months when rain is not plentiful. A mid-size system could have a 7000-15,000 litre storage capacity and where plumbed to the house, will significantly reduce your yearly water use. On the other end of the scale, a 30,000 litre tank can give you quasi-autonomy across a range of climates and roof sizes, and certainly be adequate to take care of a large garden. More accurate determination of tank size for your needs can be found in a document entitled “Guidance on use of rainwater tanks” published in 2004 by enHealth, available online. Note that the smaller tanks costs significantly more per stored litre of water (up to $1) while the larger ones could cost as little to purchase as 10c per litre. All tanks whatever their size should be connected to the stormwater drainage system or other water-authority approved point of discharge to deal with overflow situations. Where the water is used indoors, a pump will be required to supply fixtures and appliances and revert the water supply to town water when tanks are empty. Manufacturers such as Davey and Onga supply such pumps (Rainbank and Waterswitch respectively) for residential applications.    

 Tip 4 – Types of tank

While concrete and fibreglass tanks are available, the most common tank materials used in residential applications nowadays are steel and polyethylene. Steel tanks are lined with a plastic coating to prevent corrosion and are a clear winner in fire-prone areas. Provided that the steel is partly recycled which most often it is, they also have a slightly lower embodied energy. The plastic lining however needs is somewhat delicate and should not be scrubbed when the tank is cleaned or put in contact with any solvent (a small amount of kerosene is sometimes use to kill mosquito larvae in tanks). Polyethylene tanks on the other hand are made using the healthiest variety of plastic and come in an astounding array of sizes, shapes and colours. Round above-ground tanks are the cheapest per collected litre of water, while slimline tanks, underfloor or underdeck bladder have a more complex construction and are dearer. They prove useful where space is limited to the subfloor or the space under the roof eave however. Underground concrete tanks can collect large quantities of water and save space too, but represent an expensive option. Plumbing and earthworks where required are separate costs and will vary greatly from a job to another. Use a professional who specialises in green plumbing wherever you can, and especially if you use several inter-connected tanks and use rainwater inside the house.   

Tip 5 – Preserve water quality

Whilst water in the form of rain is pure where not falling through overly polluted air, collection and storage can affect water quality. It has been shown that tank water remains generally a safe water source, but you will be more confident if system design and maintenance issues have been looked into in order to maximize water purity. Surprisingly, asbestos roof have shown to be safe for water collection but bitumen-coated roofs and roofs painted with lead-based paints (1970s or earlier) will be leaching toxic chemicals in the water.  Roofs will get dirty when it hasn’t rained for a little while and the first 50 litres of rainwater that falls after a dry period in effect washes the roof and is best disposed of, using a so-called “first flush” system. Leaf debris accumulating in gutters and falling into tanks also create organic matter and sludge at the bottom of tanks, which is best avoided using gutter mesh and leaf diverters in treed areas. Some gutters like the Smartflo ones are semi-enclosed and cleverly designed to only admit water. All tanks should be opaque to prevent algae growth and tightly fitted with a lid and mosquito mesh. The main pipe running from the tank to the house can be fitted with a carbon filter to improve water clarity and taste. Gutters should be cleaned twice a year and the filter replaced periodically. Tanks should be inspected every year, especially where water is used for drinking, and cleaned every 2 or 3 years by yourself or a specialist company.