Each time it rains, water runs into roof gutters then enters the stormwater system and flows into the River Derwent.
Much of this water could be captured and stored, then used around the home. This would reduce the amount of treated water being used for purposes other than drinking, as well as the amount of stormwater entering the river.
Some ways roof water could be reused on residential properties include:
- watering the garden
- washing the car
- in the laundry, bathroom or toilet
- as additional fire protection.
Rainwater storage systems
Rainwater tanks are available in many different sizes, styles, colours, shapes and materials. The main types of rainwater storage systems are:
- rainwater tanks on the ground
- elevated rainwater tanks (the main advantage of having elevated storage is that it may avoid pumping)
- specially designed rain storage gutters.
When thinking about installing a rainwater storage system, make sure you consider the following matters.
Water collected on private property is considered a private water supply and is the responsibility of the property owner. Any water used for drinking must meet the National Health and Medical Research Council's guidelines.
Contamination from organic matter
- Remove any foliage overhanging the roof to decrease leaf litter, bird and possum droppings and other animal contamination.
- Cover all guttering with a screen to prevent leaves entering or accumulating in gutters.
- Install a first-flush trap to collect the initial roof runoff from each rainfall, which may contain a build-up of dust.
- Install a suitable trap or filter prior to the tank inlet to prevent contaminants entering the tank.
- Ensure all traps and filters are regularly operated and maintained.
Contamination from materials
- Do not drink tank water if you have unpainted lead flashings on your roof, unsealed lead screw washers on a steel roof, lead-based solders, or if lead-based seal paints have been used on the roof.
- Any materials in contact with water to be used for drinking must comply with AS/NZS4020 (Australian/New Zealand Standard: Testing of products for use in contact with drinking water).
- Avoid corrosion caused by dissimilar metals, e.g. don't use stainless steel screws on steel or on aluminium pre-painted roofing materials.
- Do not use copper roofing or guttering materials upstream of aluminium, or galvanised or Zincalume® steel products.
- A concrete or soldered galvanised tank should be lined with an approved tank liner/coating if the water is for drinking (compliant with AS/NZ4020).
- Do not use galvanised gutters in combination with materials such as Zincalume® or Colourbond® steel or terracotta tiles, as this can lead to accelerated corrosion of guttering.
- Any chemically treated wood (e.g. treated pine) in contact with water to be used for drinking should be painted to prevent chemicals leaching into the water.
Design and installation
When estimating the size of the tank you need, think about what you want to use if for and the monthly usage. Below are some average volumes of monthly water use:
- Garden – 17 000 litres
- Laundry – 4200 litres
- Toilet – 3400 litres.
In Hobart, on a long-term average basis, rainfall is approximately evenly distributed over the year. Annual rainfall can be roughly worked out as possible annual supply (L) = roof area (m2) x annual average rainfall (mm) x 0.9. NB: annual average rainfall varies between 600 mm and 1000 mm across Hobart.
Tanks are available in pre-made sizes ranging from about 220 L to around 25 000 L. For an indication of size, a round tank storing 220 litres will have a diameter of 0.75 metres whereas a large tank capable of storing 25 000 litres will have a diameter of approximately 3.5 metres, depending on the height of the tank. Underground tanks are also available although electric pumps may be required to get the water back to house or garden level. In this case, a design is required from a registered professional engineer.
When you buy your rain storage system, make sure you select a suitably-sized tank for your needs. You can check the Yellow Pages to find local suppliers, and talk to them about what you want from your tank.
Ensure a registered plumber installs the tank or storage gutter and associated plumbing. The system should have an inlet above the top water level, a visible air gap complying with plumbing regulations, a means to scour and clean out accumulated sediment and an outlet positioned above the maximum level of sediment. The plumber must install a flow restrictor on any potable supply line to a tank.
All pipe materials conveying water from a storage system must be labelled 'rainwater' where above ground, as should the tank itself.
If the pipes are below ground they should be covered with ID tape (as required by AS/NZ 1345 – Identification of the contents of pipes, conduits and ducts).
Connecting with our infrastructure
Where the tank or storage gutter is topped up from mains water, backflow prevention must be installed to protect the public water supply (as required by AS/NZ 3500 – National plumbing and drainage set). The overflow from the tank must be fed to the stormwater system or an alternative system as approved by our design engineer. We will install a water meter, at our cost, on properties installing water storage. These meters will be used to provide information for statistical purposes only.
The installation of any water storage will require a plumbing permit. In addition, any installation on properties in the Sullivans Cove Planning Scheme areas require a planning permit application and, if a building is classified as heritage or lies within a heritage zone, you also need to submit a heritage application. If a tank has a capacity greater than 35,000 litres, or if it is on a stand higher than 1.2 metres from the natural ground level, you need a building permit.
All planning and plumbing regulations must be complied with. Property owners should consult with a City Development Appraisal Planner by telephoning 03 6238 2715 to discuss individual requirements.
If all precautions are taken to avoid contamination, a rainwater collection and storage system should need little maintenance. However, if material has accumulated in the tank or if contamination is suspected, the tank can be dosed with chlorine tablets (according to manufacturer's instructions) and cleaned out. Water samples may also be taken to a laboratory for testing if any uncertainties remain. It is a good idea to test samples after contamination has occurred and the problem has been fixed to ensure the water is of an acceptable quality. Samples should be tested for total plate count, total coliforms and E Coli. If materials contamination is suspected, samples should also be tested for metals, e.g. lead, zinc, copper.
For further information please contact:
Development Appraisal Planner – 03 6238 2715
Project and Development Inspector – 03 6238 2967
See the Federal Government's Department of Health’s information on use of rainwater tanks.