Rainwater Treatment Options

I’m always amazed at how good the quality of the water is that we catch in our barrels. Usually, not much treatment needs to be done. Rainwater is great quality water, but like all water, some precautions need to be taken when using it. This is especially true if you want to drink it, grow food with it, or bring it into your home. Water responds very well to treatment. Almost any problem can be remedied with today’s array of water treatment options.

Water Treatment Options:

  • First Flush: A roof can collect dust, pollen, bird and insect dropping, and whatever else might be blowing in the wind. Allowing the rain to wash your roof, before harvesting the water, is one of the best things you can do to get good quality water. There are first flush devices that attach to your downspout, or you can manually divert the water to the barrel once enough rain has fallen.
  • Barrel Lids: Always close your rain barrels. This will prevent mosquitoes breeding in them. It will also stop insects, frogs, and lizards falling it them. Visually inspect the inside of the rain barrel before closing. Use a flashlight if necessary.
  • Leaf Diverters, Filter Socks and Baskets: All these devices prevent debris getting into your rain barrel, and work very well.
  • Filtration: pumping your water through a common house filter can be a good way to remove debris. These filters are usually available and most hardware stores.
  • Chlorination: Chlorination of water to 1ppm free chlorine is enough to kill almost all pathogens, harmful to humans, within an hour. This does not apply to cryptosporidium, which needs much longer contact time. More on that later. Make sure the pH of the water is less than 7.5. At a higher pH, Chlorine becomes less effective. Make sure the water you are chlorinating is clean, clear and free of debris, otherwise you will need a lot more chlorine to get to 1ppm. You can check free chlorine levels with commonly available free chlorine tests. You can also use and ORP Meter when chlorinating to check the oxidative reductive potential. An ORP of 600-700 mV is enough to destroy most pathogens dangerous to humans within an hour. Use 5% regular unscented bleach (sodium hypochlorite) at 2ml per 25 gallons to get 1ppm free chlorine. Use 0.6 grams dry 70% calcuim hypochlorite per 100 gallons to get 1ppm. You can chlorinate up to 4ppm if you feel it is necessary. Make sure the container being used is clean and suitable for chlorination. Dechlorination can done by agitating the water with an aerator. It is easier to dechlorinate small amounts of water than large amounts. Chlorine levels should be zero if you want to add fish. NSF/ANSI Standard 60 Chlorine is high purity and rated to treat drinking water.
  • Cyst Filters: a good way to deal with cryptosporidium. Here is a good article from the CDC. Cryptosporidium has been detected in rainwater, but it is not that common.
  • Reverse Osmosis (RO): Removes all contaminates, chemical and microbial, from water, but it can be expensive. Your filters should last longer filtering rainwater compared to tap water due to the low dissolved solids in rainwater. RO systems produce wastewater. Usually about 1 part RO water to 2 parts wastewater. Some units are 1 to 1.
  • Water Testing: Testing in one way to find out exactly what is in your water. It can be expensive. Here is a water testing kit, or you can find a lab that will test the water for you.

Articles on Rainwater Collection and Treatment:

How we treat our rainwater – The roof is always given a good first flush. Next, the water is diverted to the barrel, preferably when there is heavy rainfall. This limits the time the water contacts the roof. Usually, the water is already very good quality. The insides of the barrels are inspected then closed up soon after the rain ends. I usually chlorinate the water to 1 ppm free chlorine, and let it sit overnight. This helps get rid of pathogens harmful to humans, and algae that might start growing when the nutrients are added in a reservoir or aquarium. Chlorine kills fish. Be sure to completely dechlorinate your water before it is used with fish. You can do this by aerating the water or use a dechlorinating product for aquariums. Chlorine can affect plant growth. You can dechlorinate water with an aquarium aerator or by letting the water circulate with a pump. It is easier to chlorinate and dechlorinate small batches of water compared to large batches. I usually don’t treat for cryptosporidium. I don’t drink the rainwater. It is only used for growing plants. I make sure the water doesn’t wet the parts of the plant I’m going to eat, and I wash all produce. No treatment is done for outdoor nonedible container plants.