Water use, sustainability, and efficiency by choosing quality systems and materials, and providing environmentally friendly solutions.
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 types and sizes
- Tank materials
- Filtration systems
- Backflow prevention
- Building consent requirements
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 buildings water needs.
If the tank is supplying water for use in the house, make sure that it has a gauge that shows the water level at any time.
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 calmed inlet to reduce disturbance of the sediment layer during high rainfall events
- specify a vacuum overflow that clears debris from the bottom of the tank.
Tank types and sizes
The tank selected should provide adequate supply for the purpose, whether that is irrigation, toilet flushing or all household use, and should be appropriate for the location. There are many types and sizes of rainwater tanks available in New Zealand, designed for different needs. They include:
- conventional above-ground cylindrical tanks, as shown in the drawings on this page. These generally have the highest volume, with single tanks holding up to 30,000 litres
- narrow above-ground tanks for smaller urban sites
- tanks designed for installation under decking
- below-ground tanks. They generally hold smaller volumes of water than big cylindrical tanks and have higher initial costs.
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, fibreglass, 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 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.
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.
Building consent requirements
In many cases adding a water tank to a property will not require building consent. Exemptions to consent are set out in Schedule 1 in the Building Act 2004. Whether or not consent is required chiefly depends on the size of the tank and how high above the ground it will be. For example, a tank of up to 35,000 litres supported directly by ground does not need building consent; but if a tank is 4 metres above the ground it can only hold 500 litres without needing a consent. Flexible water storage bladders are also exempt provided they meet certain requirements.
You can find more details in the MBIE publication Building work that does not require a building consent.
Update: 20 September 2021