Many people have installed rain barrels to supplement their water use. They are a great way to water lawns and gardens. If you had, to could you drink this water? What sort of treatment would be necessary?
Rain water is inherently pathogen free. There are some bacteria present but they typically aren’t pathogenic. The problem with rain water is with the collection and the storage.
Because most rain water collection systems are not intended for drinking they are not good at preventing contamination.
Let’s look at the first stage of rain water collection. The rain itself. Rain water tends to have very little dissolved in it. Only substances already in the atmosphere are in rain water. These substances are gasses like oxygen, nitrogen, carbon dioxide. Sometimes they are more harmful like carbon monoxide, hydrogen sulfide and ozone. A lot of these gasses will leave the water once it is collected and they are allowed to be released back into the atmosphere. What will usually stay in the water is a lower pH or higher acidity. This can be “acid rain” that dissolves things on contact (this is a problem in many areas), but can also refer to water with a pH less than 7 (neutral) and may never do any significant damage, which is much more common. Basically, because of the low levels of dissolved substances and the low pH, rain water is “hungry” to dissolve substances. Meaning it is ready to absorb contaminants from everything it touches.
Many systems funnel water that falls on the roof of a building into a holding tank or a cistern. Many like this system (link) use a 55gal barrel. These systems are great for non drinking uses. The problem of this style is that the rain falls on the roof first and the roof is the first point of contamination. When was the last time you were on your roof? How clean was it? Were there any bird droppings? All it takes is for some of the water to come into contact with fecal matter for the whole reserve to become contaminated with e-coli.
Then there is the roofing material. Most roofs are covered with waterproof shingles. Many of these shingles are petroleum based, and often contain other anti-rot chemical treatments. All the chemicals in the products that make up your roof, can leach into the water that lands on your roof. Even though they may be in low concentrations, you are better off not drinking any of these chemicals.
Because most places have 55 gallon barrels, (if you don’t and want one instructions are here). I will talk about some modifications that can be done to improve the quality of your collected rain water. Let’s start with the roof. To keep the water clean roofs need to be coated to prevent contamination. In Bermuda, where all drinking water is from rain water (some desalination, but it is expensive) they have slate roofs which are sealed with water grade sealant.
At home this could be done two ways. One, laying fiberglass down on your roof, which is very permanent and expensive. The other is to cover your roof with plastic sheeting. The goal is a clean, waterproof barrier between the water and the roof. Once you have that,you funnel your water into the barrel. The construction in the link provided is adequate, I would only change one thing. I would add a cover over the screen, so that only debris from the roof will clog the screen.
Two suggestions I would also install in my system. The first is to use HDPE (high density polyethylene) barrels or one designed for drinking water. Lower quality plastics will leach more toxins into the water. The other is to cut away any trees to discourage animals from climbing on your roof. Utility lines should have a barrier of some kind too, like a smooth metal plate around the connection so rodents can’t climb on to the house. Remember, preventing contamination is the best thing you can do for this source if you intend on drinking it. Disinfecting the whole system with sodium hypochlorite (bleach) after construction/repairs is a good idea. That way you know the system didn’t cause any contamination. Cleaning the screen and removing any floating debris in the tank is recommended daily maintenance. A net for an aquarium is a good size for a 55 gallon barrel rain water collection system.
Now, how should this water be treated before drinking? If you have been good at keeping debris and other contaminants out of the water, then very little treatment is required. In many cases, disinfection through boiling or bleach or water tablets is sufficient. If you failed to keep the water clean then treat it the way I recommended in previous articles (there are many)