Drinking rainwater is generally perceived as being a healthier alternative to drinking town supply water, but often it isn’t, so if you are thinking about collecting rainwater for your house supply it pays to know a little bit about the nature of rainwater and the best way of collecting and storing it in its purest form.

The Nature of Rainwater

Drinking rainwater in any quantity before the invention of leak-proof containers was virtually impossible. People drank from water sources such as streams and springs. This water is quite different from rainwater. Rainwater is slightly acidic due to dissolved carbon dioxide and (occasionally) nitrogen, whereas spring water has permeated through the earth, absorbing minerals and being magnetised by the earth’s magnetic field, which gives it a slightly alkaline nature. Acid water is generally best for external use, being very good for skin and hair care, whereas alkaline water is the healthiest for drinking. You may want to test your tank supply. If the water is slightly acid, then you can always use an extra treatment process to “activate” the water by adding minerals to the water that you drink.

Collecting and Storing Rainwater

Where does rainwater come from? Well from the sky of course, and generally in New Zealand our skies are clear enough for us not to have to worry about contamination from acid rain. But our fondness for living near the coast may mean at times wind-driven salt spray is mixed in with the lashing rain. Also it is good to remember that the rain doesn’t fall from the sky into your cup – it is collected on a roof, travels through pipes, sits for a while in a tank, and travels through more pipes before eventually coming out at the tap. Along this journey are many opportunities for turning your “pure” water into something less than desirable:

• During dry spells, a roof can collect leaves, dust, dirt, salt, guano and other deposits which are rinsed off during the first 10 minutes or so of a rain shower. There are now commercially produced, simple “diverters” which ensure the first run-off from the roof doesn’t get into your water tank.

You should consider the material the roof is made of as it can leach unwanted chemicals into your water supply. One of the best and most readily available materials is colour-coated metal roofing, although the coating may eventually start flaking off many years down the line. Clay tiles and slate are excellent if they are not radioactive (you need to get them tested). Some materials are fine after an initial period of leaching, these include cement tiles and naturally durable timber shingles. Materials that are not recommended include bitumen impregnated products, synthetic rubber, treated timber, uncoated galvanised roofing and asbestos cement.

• You may wish to consider a seperate tank for drinking water.

  If your water is going to sit for a while in a water tank, the potential for being contaminated by the material for the tank is higher. The best materials for water tanks are stainless steel (you may wish to consider a separate tank for drinking water), concrete (if it has not been made with additives for fast curing, is unpainted, and has been finished its initial leaching), and food-grade polyethylene. A tank should be sited so that the water is away from light and heat, to inhibit the growth of bacteria.

• The filtering, purifying and sterilising of your rainwater is important for a healthy water supply. A filter removes solids from the water, which include sediment, rust, asbestos, algae and cysts. A purifier removes dissolved products from the water, which in your tank may include pesticides, sulphur and heavy metals. A steriliser is used to kill bacteria (although not all bacteria are harmful). This can be done without chemicals, by using ultra-violet light.

If you wish to know more about available products for rainwater systems as well as about other building materials, the Building Biology and Ecology Institute has an Eco Products and Services List, available for $20 hardcopy, $15 email copy.