Water
Designing homes to use water efficiently, reduce costs and cope with more frequent droughts or storms.
Minimising contamination of collected water
Find out about:
- contamination sources
- leaf screens and guards
- diverters
- tank design
- filtration systems
- preventing contamination of the piped water supply.
Contamination sources
Water from roofs may be contaminated by:
- decaying vegetable matter such as leaves, petals and pollen
- faecal matter from birds, possums and rats
- dead insects, birds and animals
- particulates from solid fuel flues
- pollution
- chemical spray drift.
Reduce the risk of contamination by:
- siting the tank, if possible, so that it is shaded from the sun, particularly during the hottest time of the day
- specifying leaf guards over the gutters
- specifying leaf screens on rainwater heads
- specifying a first flush diverter to prevent the first 20–25 litres of water, which may be heavily contaminated, from entering the tank
- specifying tightly fitting covers for all tank inspection ports
- specifying insect screens on all vents and openings
- landscaping so there are no trees overhanging the roof to reduce leaves and bird droppings
- locating the TV aerial away from the collection roof to avoid bird droppings falling onto the roof
- ensuring that the flue from a solid fuel burner is located so that soot and other discharges are carried clear of the collection area.
- specifying a floating intake, with optional filter to draw the water from the best area in the tank
- specifying a vacuum overflow that clears debris from the bottom of the tank
- designing self-draining pipework so the system can be flushed out
- advising owners that gutters must be kept clean.
Leaf screens and guards
Leaf screens located on each downpipe keep larger debris out of the rainwater tank.
Diverters
These devices divert the first rain away from the water collection tank, washing dust, leaves and other debris off the roof before water is collected.
One form of diverter has a float that rises as the rainwater flows in. When the floater reaches the top of the diverter pipe, it seals it off allowing the rainwater to flow into the tank.
Generally, the more water that is diverted, the better the quality of the collected water. To calculate the amount of water to divert, consider the level of the pollutants on the roof.
- for minimal pollution, allow 20 litres/100 m2 (0.2 litre/m2) of roof area.
- for heavy pollution, allow 50 litres/100 m2 (0.5 litre/m2) of roof area.
Tank design
Good tank design can enhance water quality by including:
- an intake near the water surface
- an overflow that can siphon out fine sludge
- tightly fitting inspection/opening covers.
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.
Preventing contamination
NZBC G12/AS1 3.0 Protection of potable water requires that a private water supply system is installed so there is no likelihood of cross-connection between the system and the mains water supply.
This will apply where rainwater is used for household purposes and a mains supply is used as back-up. An air gap is the most cost-effective and reliable way to avoid potential backflow contamination of mains supply water by the collected rainwater.
Prevent the possibility of backflow from the private supply into the mains piped supply by:
- a floating weight to operate a valve, used in conjunction with an air gap for backflow prevention, or
- a double check valve installation.
- Mains water supply top-up valve and air gap backflow prevention
If a piped supply is used to top-up the rainwater storage tank, a simple commercially available floating switch will ensure that top-up water is added only when the level in the tank is low. A float valve should not be used because it will add piped water whenever there is any draw-off.
- Double check valve installation to prevent backflow contamination of a potable water supply
If there is any direct connection between mains supply water and a rainwater collection system, then a means of preventing backflow, such as a double check valve, must be designed into the system.