Storing rainwater

Specify a tank that is suitable for the purpose – whether the rainwater is to be used for drinking or only for gardening – and ensure that rainwater is stored in ways that minimise risk of contamination.

On this page:

• Tank size
• Tank materials
• Filters
• Backflow prevention
• Maintenance

Once rainwater has been harvested, it must be stored in a tank for use. Tanks are available in different sizes – from small barrels for gardening water to tanks that are large enough to cover all of a building’s water needs.

The Building Code requires, among other things, that water supplies must be protected from contamination. This means that the tank must be designed to minimise the risk that rainwater will become contaminated. See water supply for general Code requirements for water supply, and mains or rainwater for requirements relating to rainwater collection.

To reduce the risk of contamination:

• site the tank, if possible, so that it is shaded from the sun, particularly during the hottest time of the day
• specify tightly fitting covers for all tank inspection ports, insect screens on all vents and openings, and an overflow that can siphon out fine sludge
• specify an intake near the water surface or a floating intake, with optional filter to draw the water from the best area in the tank
• specify a vacuum overflow that clears debris from the bottom of the tank.

Tank materials

If rainwater is being harvested for human consumption, tank materials must comply with AS/NZS 4020 Testing of products for use in contact with drinking water.

Suitable materials for water storage tanks include galvanised steel, fiberglass, plastics such as polypropylene, concrete, and ferro-cement.

Tank materials should not transmit light, as light will encourage organic growth.
When specifying materials, while the first consideration is health and safety, you may also want to take account of sustainability considerations such as embodied energy. See material use for more.

Filtration systems

Filtration systems may be:

• point-of-use (attached to a tap or plumbed in with a dedicated faucet), or
• point-of-entry (centrally installed system to treat all water).

Types of filters include:

• mesh filters of various sizes to remove different types of particles
• carbon filters
• reverse osmosis filter
• UV sterilisers to kill bacteria.

Ideally, a filtration system should include a number of different types of filters.

Backflow prevention

As we noted in mains or rainwater, any rainwater system that is connected to a mains water supply must be designed to minimise the risk of contamination of that mains water supply.
Acceptable Solution G12/AS1 provides that there must be no likelihood of cross-connection between a private water supply system (such as a rainwater system) and mains water supply. This can be achieved by using using an air gap or a backflow prevention device such as a double non-return valve. See backflow prevention for details.

Maintenance

Rainwater systems should be maintained following a programme that includes:

• three monthly inspection – clear gutters, leaf guards and first flush diverters; clean and replace filters as necessary; trim back trees overhanging the roof
• annual inspection – clean tank to remove accumulated sediment; sludge may be removed by siphoning or pumping without requiring the tank to be emptied.

The manufacturer’s recommendations for tank and filter maintenance should be followed, and if necessary tank water should be treated to  remove organic materials if necessary.