Rain harvesting - Using rainwater around the home
Friday, September 22, 2006
In New Zealand, rainwater is a primary water supply for many rural and small town residents. Our larger communities have become accustomed to a centralised water supply but there is a good argument that all households should catch and use rainwater for part of their water needs.
Household water services are charged on a user-pays basis to cover the cost of supply and disposal. If you are connected to a reticulated system the installation of a tank may do little to reduce your water rates - part of the council charge for a reticulated supply is a fixed charge regardless of whether you use it. Costs are also worked out on a discharge basis, because waste pipes are needed to remove used water regardless of its source of supply.
Rain harvesting has advantages for all, however, and many city councils now actively encourage its adoption by offering rebates on tank installations. Capturing and using rainwater lowers the volume of stormwater flowing through an urban environment, reduces the likelihood of local surface flooding and reduces the overloading of underground sewage systems in sudden downpours. Not only does tank collection conserve a valuable resource, it reduces the overall need for a centralised water supply with its treatment and pumping stations, thereby lowering greenhouse gas emissions.
Health Issues
Health authorities are keen to stress that tank water should be ‘appropriately treated’ if it is to be used for drinking. Tanked water supplies can be easily contaminated by agricultural spray drift, bird and animal droppings, nasties (dead birds, rodents and the like.), lead contaminants in dust and lead roof paints, lead, copper and zinc contaminants from roofing materials and flashings, and the general debris that finds its way into roof gutters. But if sensible precautions are taken and the system is maintained in good repair, it can deliver excellent potable water that is free of the chlorine and fluoride often found in reticulated town supplies.
If you intend to drink water collected off a roof, check that the roofing and guttering material is sound, that there are no lead flashings, lead paints, or treated timber shingles. Gutters should be regularly cleaned and the tank occasionally de-sludged.
Asbestos roofs should never be water-blasted to make them cleaner for water collection (or to prepare them for a coat of paint). Water-blasting dislodges millions of microscopic asbestos fibres which, when dry, become a serious airborne health hazard. There are many asbestos-cement water pipes in use throughout New Zealand and a significant percentage of tap water contains large numbers of asbestos fibres. There is no clear evidence, however, to suggest asbestos fibres are harmful in drinking water.
Potable water should at all times look, smell and taste healthy. Tank water can be filtered or purified (chemical or UV disinfection) before use but if there is any question over its quality, a sample should be laboratory tested (ask your council or local public health service for an approved laboratory).
Rain-harvesting options
There are several straightforward rain harvesting options but if you plan to do anything more than water your garden, a building consent is generally required. This is because the tanked water supply connects into the existing household plumbing system. (The use of a registered plumber to carry out the plumbing work is a requirement of the building consent.)
Tanks larger than 6,000 litres may also require resource consent, depending on their proximity to boundaries.
- Option 1: Water for the garden - no building consent required
- Option 2: Water for the garden, laundry and toilet cistern, without mains backup - building consent required
- Option 3: Water for the garden, laundry and toilet cistern, with mains back up supply - building consent and an approved (i.e. not just a non-return valve) backflow prevention device required on the mains inlet. (This is to stop water being siphoned out of a self-contained system and back into the mains supply, where it could possibly contaminate supplies to other households.)
Tanks
Storage tanks should be opaque, with smooth internal walls and a tight-fitting cover to keep insect, bird and animal pests out. They should be situated in a cool or shady spot to discourage algal growth.
Precast concrete, corrugated galvanised steel and plastic tanks are common. The ecological implications of tanks manufactured from all three materials are arguably equal.
Concrete is durable and keeps water cool but is a heavyweight material, which makes it difficult to install in some locations. Tanks can be precast or built insitu to almost any size or shape.
Steel is lightweight and cheap but comparatively short-lived; it rusts in time.
Newer, plastic tanks are made from polyethylene - a non-sustainable material that is, nevertheless, often marketed as ‘environmentally friendly’ because it can be ‘recycled’. It doesn’t rot or corrode and is lightweight, making it very easy to install in tricky locations. It can be repaired by heat welding if damaged. Its formulation also resists algal growth, through some means other than the addition of fungicides.
Tank size
Tank size is best determined by the frequency of rainfall, the intended water use and the size of the roof collection area. Whether or not you have mains back up is also critical.
An average-sized roof (150m2) in a location with an average annual rainfall of around 1000 mm will catch enough water to provide 150,000 litres per year (150m2 x 1000 mm = 150,000 litres). A 10% allowance for spillage and evaporation still leaves enough water to meet the reasonable needs of most households.
A tank as small as 200 litres will support much of the average home’s toilet water use if rain occurs every few days (200 litres/5 litres = 40 flushes).
A tank of 4500-9000 litres will provide storage for 50-75% of the water needed to supply the combined average needs of a garden, toilet and laundry.
Tank placement
Tanks can be sited on the ground, under the ground or on a raised platform. Tanks can sometimes be sited in such a way that they deliver sufficient pressure on a gravity-feed basis. Often, a pump is needed to either pressurise the system or to feed a small header tank. (The pump can be run by a solar panel.)
Rainwater collection systems are either wet or dry. A wet system does not allow the delivery pipes to drain out and should therefore be fitted with screens at each end to ensure mosquitoes can’t enter and breed. A dry system allows the delivery pipes to completely empty of water after rain.
Screens and leaf guards are a good idea in that they prevent unwanted material finding its way into the tank. So too is a first flow diverter that diverts the first flush off a roof, the water most likely to contain contaminants, into a separate chamber which drains before the next rain. Tanks should be drained and cleaned out every couple of years.
Gutters and downpipes
Gutters and downpipes should be installed with sufficient slope to deliver the water and then drain dry - mosquitoes will breed in the small residues of water left in the open system.
Gutters, even when kept free of overhanging vegetation, need to be inspected and cleaned regularly (don’t forget to divert the dirty water away from the tank!).
Top Tips for Water
- Install your own rainwater tank.
- Use a dual flush cistern; adjust the float.
- Put a brick in older toilet cisterns.
- Add an aerator or low-flow showerhead to the shower.
- Choose water efficient appliances.
- Purify drinking water.
Integrated water management