Water
Designing homes to use water efficiently, reduce costs and cope with more frequent droughts or storms.
Rainwater Systems: Minimising Contamination
As well as choosing appropriate materials, contamination can be minimised by fitting devices such as first flush diverters and leaf guards, filtration systems as well as through good maintenance. Preventing contamination of piped water supplies is also crucial.
Minimising contamination by prevention
Water from roofs can be contaminated by:
- decaying vegetable matter such as leaves and petals and pollen (this can make the water more acidic)
- faecal matter from birds, possums and rats
- dead insects, birds and animals
- particulates from solid fuel flues, pollution
- in rural areas chemical spray drift.
Studies of rainwater systems in New Zealand indicate that over 50% of existing rainwater tanks are contaminated by unacceptable levels of bacterial organisms and 30% showed heavy faecal contamination. The risk of contamination should be reduced by:
- advising owners that gutters must be kept clean
- if possible siting the tank so that it is shaded from the sun, particularly at the hottest time of the day
- specifying leaf guards over the gutters
- specifying leaf diverters on rainwater heads
- detailing first flush diverters to prevent the first 20 – 25 litres of water that may be heavily contaminated from entering the tank
- specifying tight lids for all tank inspection ports
- specifying insect screens on all vents and other openings however small
- designing the landscaping so there are no trees overhanging the roof – to reduce leaves and bird droppings
- not locating the TV aerial on the collection roof (to avoid bird droppings)
- making sure that flues from solid fuel burners are positioned 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 helps to clear debris from the bottom of the tank
- designing pipework that is self-draining so that it can flushed out.
- Leaf screen fitted to downpipe
Leaf screens located on each downpipe help to keep larger pieces of debris out of the rainwater tank.
- Principle of first-flush diverter
These devices should divert the first 20 to 25 litres of rain away from the water collection tank – this first amount of rain washes dust, leaves etc off the roof.
Diverters often have a float which rises as the first rainwater runs in. Once the diversion pipe is full, the float seals it off and the rest of the rainwater runs into the tank.
- Storage tank features that help to deliver clean water
Leaf guards and a first flush diverter help reduce contamination before rainwater reaches the tank, but good tank design can also enhance water quality.
A water intake near the surface and an overflow which siphons out fine sludge are useful features.
Minimising contamination by good maintenance
Studies have shown that few owners maintain their rainwater tanks adequately. Owners must be informed of their responsibility and they should be provided with scheduled maintenance procedures. Maintenance will include:
- regular three monthly inspection and cleaning, as necessary, of gutters, leaf guards, and first flush diverters
- yearly inspection of tanks and cleaning of accumulated sediments from the bottom of the tank (if necessary sludge can be removed by siphoning or pumping without emptying the tank)
- cleaning or replacing filters (if any)
- following the manufacturer’s requirements for tanks, filters etc
- treating the tank water to remove organic materials.
Preventing contamination of the piped water supply
NZBC G12/AS1 3.0 Protection of potable water requires the water supply system to be installed so that there is no likelihood of cross-connection between a potable water supply system connected to a water main, and any other source including a private water supply. This is to prevent the possibility of backflow from the private supply into the public piped supply. This requirement will apply where rainwater is used for household purposes and a piped supply is used as a back-up.
- 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. This arrangement requires only one system of pipes to the fixtures within the house as all water is sourced via the tank.
- Principle of air gap used for backflow prevention
Designing an air gap into the system is the most cost-effective and reliable way to avoid potential backflow contamination of mains supply water by the collected rainwater.
- 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.