How to capture rainwater and drink it safely
Drinking rainwater is probably safe if you are out in a rain shower and open your mouth to the sky. And in many parts of the world, particularly the tropics, people routinely capture rainwater and store it in cisterns for later household use – including drinking. But whenever the rainwater has come into contact with a possible source of contaminants, or is held for more than a few minutes in a non-sterile environment, you need to purify it.
In general, if you live somewhere with a treated municipal water supply, you are better off drinking the tap water. Drinking rainwater is unlikely to save you much money, since most people struggle to drink more than a half gallon a day, a tiny fraction of the US per capita usage of 100 gallons a day. But if you’re unhappy with the taste of your tap water, or concerned about contaminants or excess chlorine in the tap water, you may be tempted to capture your own rainwater and drink that.
Let’s look at the three main sources of contaminants for drinking rainwater, so that we can identify how to capture, store and treat the water in a way that makes drinking rainwater safe. The three main sources of contaimination are:
- Airborne contamination
- Contamination during collection
- Contamination in storage
First, let’s dispel the notion that there is such a thing, in nature at least, as pure water. When you are drinking rainwater you are drinking a mixture of water, solids, oils, and dissolved chemicals. Rain drops form in clouds when droplets of water vapor condense around a droplet of oil, or a microscopic piece of salt, dust or soot. As the rain falls, depending on the quality of the surrounding air, it picks up chemicals and particulate in the air. When you are drinking rainwater, you are drinking dozens of different compounds beyond simple H2O.
In a Minnesota study, 16 samples of rainwater were taken by students in January/February, and fluoride, chloride, sulfate, sodium, potassium, magnesium, and calcium ions were found in every one of the samples, while nitrates were present in 11 of the samples. In another study involving 51 samples, chloride was found in all studies, while a portion of the samples contained each of the following ions: sulfate (49/51), nitrate (43/51), phosphate (7/51), and nitrite (5/51). Rainwater that falls near sea water tends to have higher concentrations of chloride ions and sodium chloride (table salt), because the water vapor it came from typically originated from sea water. Rainwater that falls far inland from sea water has lower concentrations of salt.
Rainwater is also typically much more acidic than ground water. ‘Pure’ water has a neutral pH value of 7, while lake water typically has a pH between 6.5 and 8.5, and rainwater often has a pH of 5.6 (or less in areas with acid rain). Remember, this scale is logarithmic, so rainwater at 5.6 is nearly ten times more acidic than the lake with a pH of 6.5, and almost 1,000 times as acidic as a lake at 8.5!
If you live in a polluted area, or downwind of a major industrial area or source of airborne pollution, drinking rainwater amounts to drinking a share of that pollution. Rainwater that then falls to the ground, sinks through the soil and into groundwater, may, under poor conditions, become contaminated by chemicals such as agricultural fertilizers and pesticides, petrochemicals from roads, and countless other contaminants, and if your ground water is that bad, then drinking rainwater is probably a safer bet. But in most relatively clean areas, the process of rainwater filtering through the soil removes most of the airborne contaminants, and properly treated ground water should be safe to drink.
Contamination during collection
For most people, the easiest way to collect rainwater is to capture the water that drains off a roof. It takes a large surface area to collect enough rainwater for household use. Just think of the typical weather forecast, which on a really wet day might call for 20 mm or 3/4 inch of rain. If you stick a margarine tub that’s 5 inches wide out in a clearing with no overhanging trees or buildings, that 5 inch wide tub will have about a cup of water in it when that 20 mm or 3/4 inch rainfall is over. You’d need a good rainy day and about a dozen margarine tubs to get the 3 quarts of water a typical adult male should be drinking in a day. Why not use a roof instead? The roof over my house covers a horizontal area of about 750 square feet or 70 square meters. That 20 mm or 3/4 inch rainfall will produce roughly 1,400 liters, or 350 gallons, of rainwater going down my downspouts (or over the edges of the eavestrough if the rain falls really hard!).
But remember, the rainwater falling on your shingles is not the same as the rainwater coming out the bottom of the downspout. Along the way, that water picks up bits of organic matter from tree pollen, tree flowers, leaves, dead or even living insects, bird droppings, insect droppings, and that gross rotting sludge you clean out of your eavestroughs every fall. It also picks up inorganic matter such as bits of shingle grit, some of the petrochemicals used to hold the shingle material together, some of the aluminum oxide that forms over time in aluminum eaves troughs and downspouts, and the various pollutants that tend to deposit themselves on your roof over time. So while it may be relatively safe to open your mouth to the falling rain, it’s probably not a great idea to dip a ladel into the rain barrel under your downspout and drink that.
Contamination in storage
Finally, if you’re not careful about how you store it, drinking rainwater can be a source of biopathogens, especially if you weren’t careful in the first place about removing the sources of contamination during collection. For microbes to flourish, they need a source of food, they may need a source of oxygen (in the case of aerobic microorganisms), and of course they need some microbes to start the reproduction. That means that a dirty storage container will lead to more contamination than a clean one, that a sealed storage container will likely have less contamination than an open one, and that a dirty source of water will lead to more biological contamination than one where where the organic compounds (the ‘food’) has been filtered out.
How should I treat rainwater so it’s safe to drink
If you plan on drinking rainwater, you need to find a way to remove all three sources of contamination – airborne, collection system, and storage system sources. I suggest the following guidelines to reduce your risk and improve the health outcome of drinking rainwater – although, as I said before, in most cases it is safer to just drink your municipal tap water.
Let it rain before you start gathering: Contaminants, both in the air and on the rooftop or other collection surface you use to gather rainwater, tend to accumulate during dry spells. Naturally, these contaminants are much more concentrated in the first few minutes of a rainfall, than an hour later when the rain is still drumming down. So give the rain a chance to flush out the contaminants before you start gathering it.
Keep your collection system clean: If you’re gathering rainwater from a roof, keep the roof clear of debris and obvious sources of contamination. Make sure you don’t have birds roosting in dormer eaves above the rest of your roof, or racoons living in your chimney. Clean out the eavestroughs in the spring (after tree flowers have dropped), in the summer (if you live in an area with maples, clear out the maple keys), and in the fall (after the trees are bare). Make sure the eavestroughs are properly angled so that there are no stagnant pools of water, and keep them clear of shingle debris. And if the roof is getting long in the tooth, consider replacing it, as the older the roofing material, the more likely it is to contaminate the rainwater.
Filter out solids: If you’re only gathering enough rainwater to meet your own drinking needs, you can do the filtering manually without too much trouble. Start by filtering with a wire mesh filter to remove the larger solids. Then pass it through a coffee filter – either a disposable fiber filter or a reusable metal filter – and store it in a large carboy such as a 3- or 5-gallon spring water or home brewing carboy.
Clean the final storage container: Clean the inside of the container you will be storing the water in – at the least, with soapy water and a good rinse. Even better, you can use a home brewing cleaner such as B-Brite cleanser – a tiny bit of this dissolved in a pint of water does an excellent job of cleaning and sterilizing even a 5 gallon carboy.
Use activated charcoal: For the final filtering step, place a paper coffee filter in a coffee funnel and put a small amount of granulated activated charcoal in the filter. Set the container of filtered rainwater on a table or chair above the height of the filter, and use a thin length of plastic tubing to syphon water from the filtered water source into the final storage container. The activated charcoal helps remove both chemical and organic contaminants and will significantly improve the taste of the water.
Seal the container and store in a dark area: Seal the container so you can keep out oxygen, along with any other contaminants that might work their way into an unsealed container; and store the container in the dark, to prevent algae from growing on the walls of the container. You might think a dark-glassed or opaque container is better since it would prevent algae from growing – but it also prevents you from seeing what’s inside the container.
Drink promptly: The longer you wait between gathering and drinking rainwater, the bigger the risk of microbial growth. If you’ve followed all the above steps, the risk of getting sick from drinking rainwater is pretty slight, but the odds are still higher than drinking tap water from a municipal source.
In the end, is it worth it?
If you’re a conspiracy theorist, and think the government is putting drugs in our water to control us, or if your municipal water source is known to be contaminated, or if you don’t have a clean water supply and rainwater is your only choice for drinking water, the above steps will help improve the quality of rainwater and reduce the risks. But it sure seems like a lot more work than it’s worth, assuming you’re hooked up to the municipal supply.
Still, some people take pride in being self sufficient. An uncle of mine has been making his own drinking water following the above process, for decades. He gathers the water from his bungalow, filters it in a multiple step process, and drinks liters of it every day. He never drinks tap water, at least not in his own house. As to whether it has been good for his health is anyone’s guess. He certainly thinks so, and maybe that’s all that drinking rainwater has to do to provide a health benefit – maybe just believing you’re drinking the right stuff is enough for whatever water you drink to be the right stuff.