The rainy season in the tropics provides an abundance of rainwater.  And if youʻre like me, you try to save every drop that falls in storage tanks, buckets, garbage cans, whatever you can find to store it.  But for how long can you keep that rainwater in a tank?

You can keep rainwater in a tank for an indefinite amount of time, as long as you take precautions to prevent algae growth and contaminants. The water must be blocked from the UV light so that algae does not form and the occasional addition of bleach will keep pathogens and pests away.

Whether you are saving rainwater for personal consumption or for use in your garden, in both cases, you need to take precautions to ensure the integrity and cleanliness of the storage container and collection system.

Straight rainwater might seem pure, but the moment it hits your roof or flows through a pipe it has the potential to become contaminated and unfit to drink.

Related: One of the best ways to know if the water in your tanks is still good is to test it. Home test kits that you can get off of Amazon make this job really easy. Hereʻs a link to the test kits I use to ensure my water is still good inside my tanks.

17 in 1 Premium Drinking Water Test Kit

How Long Can I Keep Rainwater For Drinking?

You can keep rainwater for drinking anywhere from 7 days to 30 years or more. Yes, that’s a massive range, and I’ll tell you why: it all depends on how you store and treat your rainwater. 

But, please do not assume rainwater is potable or safe to consume without the proper treatment measures. You can drink rainwater, yes – after you have filtered it and treated it, but that’s another topic, more clearly outlined in this post covering 3 Ways To Make Rainwater Drinkable.

While some folks swear their emergency water stores are safe for forever, I grimace a little at that idea. It’s actually not the water that will ‘expire’, per se, but rather the container it is in. 

Everything has an expiration date, even if the product itself (like honey) doesn’t expire – the plastic, fiberglass or metal container may break down over years and years and leach things like chemicals or rust into the water. 

That’s why I won’t say the water is safe for an indefinite amount of time – the water didn’t change, but the material of the container may have.

The proper preventative, treatment and maintenance measures can make a huge difference in the shelf life of your rainwater, and I’ll describe them below. 

How Long Can I Keep Rainwater For Plants?

You can keep rainwater for plants almost indefinitely — if you treat it and take the proper precautions, just like for drinking water.

Rainwater is what plants naturally use, so you might think there is no need to take measures to properly store it. However, if rainwater grows algae or becomes stagnant, you should not use it for your plants because it could cause them to grow poorly, encourage disease or might even kill them.

Important Factors For Storing Rainwater

The most important factors to keep in mind for storing rainwater long-term are collection, storage and treatment.

Rainwater Collection 

How you collect your rainwater plays a big role here. If you collect water through your roof, via your gutters, then the biggest questions are if your roof is made of the proper material (which is a painted corrugated metal roof) and you have appropriate filters, leaf screens and a ‘first flush’ to eliminate debris and dust from the roof.

If you do, then you’ve already helped to stretch the life of your rainwater for both drinking or in your garden.

If your roofing is made of the wrong material, you rarely clean your gutters and don’t have a first flush, then your rainwater may only last for 3 days because it already is contaminated with dust, dead bugs, animal excrement and other items.

Now, I know some folks send their rainwater directly to their gardens, that’s fine. As long as there is no storage where the water sits for a few days, and no pipes or tanks to clean (that can allow for algae or bacterial growth), then you’re not ‘collecting’ the rainwater, you’re just directing it to your garden.

Rainwater Storage

Once rainwater has been filtered (and the first flush diverted), the next question is how you store the rainwater. This means what type of tanks or rain barrels you use and where you set them up.

If you want to learn more about installing a rainwater harvesting system of your own, this post goes step by step into exactly what it entails. Iʻm sure youʻll find it to be very worthwhile.

Keep Stored Rainwater Out of the Sun

Sunlight encourages algae growth, and this is probably the biggest concern for rainwater storage, whether you are drinking it or using in your garden. Algae can wreak havoc on your plants and soil, stunting growth or even killing young plants. 

You do not want to use rainwater that has had contact with algae on your garden, and you definitely do not want to drink water that has algae growing in it!

Your rain barrels or tanks should be kept out of direct sunlight, either in the shade, covered up, stored underground or within a structure to prevent exposure to the sun. Anything kept outdoors (even in the shade) should be entirely opaque.

Treating Rainwater

A small amount of chlorine added to rainwater will prevent algae and other bacterial growth. A ¼ of a teaspoon of bleach for every gallon of rainwater is generally considered safe for plants.

For drinking water, the water must start off as clean as possible.  After running through a set of pre filters that take out sediment, you can treat rainwater with chlorine, iodine, UV light, Quantum Disinfection, or boil it (as well as many others). Learn more about what Quantum Disinfection is in this post.

These methods address bacteria, parasites, and germs found in water, but unfortunately, none of them will remove chemicals from contaminants on your roof, or higher acid levels found in acid rain. This is where a first flush and prefilters for your rainwater collection system is essential.

Other contaminants

Also, look out for anything entering your tanks, such as mosquitoes or small animals, like frogs. Ensure your barrels or tanks are properly sealed, have secure lids and screens. If even the smallest insect enters (like a tiny mosquito) they will lay eggs in the water and contaminate it.

Start off right

Always start off with cleaned, treated tanks. Make sure your system is dry and airtight. If you’re using plastic or metal tanks, wipe everything down with bleach before you begin storing rainwater.

5 Ways To Help My Rainwater Last Longer

Here are five simple ways to help your rainwater last longer:

  1. Paint your rain barrels black.

Remember that sunlight promotes algae growth, so if you think your rain barrels might be letting in any amount of sunlight, paint the outside black to reinforce their opacity.

  1. Add oil to rainwater that is exposed to mosquitoes.

If you’re using something like an Intex Swimming pool to store rainwater, you may be facing some challenges with finding a secure way to cover it that keeps out mosquitoes. 

A quick option is to add cooking oil to the water; it will rise to the surface and discourage mosquitoes from laying their eggs in the water. The water will remain entirely safe to use on plants.

You could also get a cover for that Intex Pool.  They do a great job at keeping the water clear and the algae out.

  1. Treat your rainwater 

Treat Rainwater with ¼ of a teaspoon of chlorine to every gallon of water after a heavy rainfall. After 24 hours, the water will be treated and germs and parasites (like giardia) will have been eliminated. The chlorine will have evaporated, too.

Use safe, approved chlorine tablets, NOT industrial, swimming-pool grade chlorine for this.

  1. Wash your rain barrels
    You should wash rain barrels used for non potable water every 6 months to a year. Washing between periods of heavy rain will allow you to have the barrel refilled in no time.

If you use rain barrels for drinking water collection, try to clean them thoroughly on a regular basis, maybe every other month and check them every other week.

To clean the barrels, I like to use a pressure washer, stick that nozzle through the bunge hole and spray the inside of the barrel, empty it out, reattach the bunghole cover and it should be good to go.

  1. Check your rainwater collection system every month.
    Be proactive about maintaining your system; check filters, screens, gutters and pipes on a monthly basis. This can help ensure that debris, pesky mosquitoes or others do not contaminate your rainwater to begin with.

Rainwater is so useful, but you need to make the proper precautions to safely store it. If you keep your tanks in a clean, dry environment out of the sun, and regularly treat your water, you can save it for decades without a problem.

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