Making your Rainwater Pure and Safe for Drinking

Rainwater harvesting can be as simple as putting a barrel outside your house to catch water, or as complex as having rainwater tanks, roof washers and an elaborate filtration and purification system. The reason for purification of rainwater for potable use (i.e. indoor use) is that, even if water travels through the atmosphere straight to a clean tank, our atmosphere is no longer as unpolluted as it once was, hence the water cannot be used off-the-tank for drinking, cooking or bathing. 

Once the water reaches the ground, it also absorbs minerals, heavy metals, faecal contaminants and other substances that make it hard and more unsafe for drinking. However, through simple filtration systems, you can enjoy the benefits of using soft and unpolluted rainwater in your home. 

Step 1: Minimize contamination 

Reducing the chances of exposure to contaminants is key if you intend to use your rainwater in the kitchen. Keep your rain water tanks clean – using opaque materials like coloured plastic, metal or wood will prevent light transmission and discourage bacterial and algal proliferation in the tank. Ensure that the top of your downspouts are fitted with fine mesh to prevent bugs, leaves and other debris from getting into the tank. 

Keep the tank tightly sealed to prevent mosquitoes from breeding in your stored water. You can use a diverter to redirect the first rainwater flush outside the tank, since it carries the highest number of contaminants. Installing a roof-washer can serve as the diverter – you can get commercially designed washers for $100-600 from the Australian Company SafeRain. 

Step 2: Water purification

Even with the above precautions, your rainwater is still not safe for kitchen use and must be purified to kill microbes and other contaminants. The following are some methods you can easily use in your home:

• Boiling – this is the best way to kill off disease-causing microbes in water. The water should be heated to a rolling boil and maintained in this state for at least one minute before allowing it to cool down. In high-altitude areas, keep the water boiling for at least three minutes since the boiling point of water will be lower. You can add very little salt to improve its taste for drinking. 
• Chemical disinfection – if you cannot boil for any reason, WHO recommends filtration prior to chemical treatment using any of the many chlorine- or iodine-based chemical purifiers available in the market. Filtration is as simple as passing water through a cotton piece of cloth before treatment, getting rid of any debris. However, chemical treatment may not eliminate all parasites, nor is it suitable for pregnant women or those with iodine hypersensitivity. 
• Water filters – commercial water filtration systems can be fitted in your kitchen sink and connected to the pump from your rainwater tank. There are many different types, so look for those whose filter sizes are less than 1 micron, as these will be effective against most microbes. You may have to add iodine after purification, since most systems do not exclude viruses because they are much smaller than bacteria and other common pathogens.