How can I prevent organic scum in rain water barrels?

A few months ago I installed 3 50 gallon rain barrels at my house - one set of two, and a singleton.  We've been very happy with the results, but with the advent of hot humid weather we've gotten a fair bit of organic scum forming on top of the water when the barrels are full.  


Are there additives we should be putting in the water to prevent this?  Or some best practices that we should know about to keep this stuff from forming?  We can scrape it off and hose down the lids every so often but would rather avoid it altogether.

  Topic Around the House/DIY/Gardening Subtopic Gardening Tags rain barrel organic scum recycled water
3 Months 2 Answers 511 views

Christopher Martin

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Answers ( 2 )

  1. David Miller 480 Accepted Answer Community Answer

    Chris, congrats on your rain barrels! They are excellent small scale stormwater best management practices (BMP for short) that any homeowner can use to help the environment and save some water.

    In most cases, the "scum" forming on the water is some variety of alage. While it may be ugly, do not worry it is not harmful. In fact, it may even remove a bit of nitrogen and phosphorous from the water. These are two of the primary pollutants we are worried about.  There are a few steps you can take to minimize the chances of growing more algae.

    1) Keep the rain barrels out of direct sunlight.  Algae requires sunlight to produce energy through photosynthesis. If you are able to keep the rain barrel in the shade, it can reduce algal growth.

    2) Get a dark colored (black) rain barrel. This prevents light from entering the barrel through the sides which leads to increased algal growth. You already bought a lighter color you say? Fear not! You can paint them a darker color, or throw several coats of a lighter color paint which will stop light from reaching the inside. Remember to use a paint that is compatible with plastic.

    3) Reduce the nutrients. The leaves and other detritus that is in your gutters is full of the nitrogen and phosphorous that algae needs to grow. Keep your gutters clean and you will reduce the amount of nutrients that are in your rain barrel.

    4) Clean it out.  If you already have an algae problem its time to do a little cleaning.  Empty all the water, get in there with a pressure washer, sponge, brush, or whatever you have available.  Let it sit totally dry for a few days and then reattach. If you use the above methods to reduce light and nutrients, you algae shouldn't come back.

    5) The nuclear option. Fill it with kerosene and light it on fire.  Just kidding!  As a last resort you can use some bleach to kill everything inside. I have heard 3/4 tablespoon to each gallon of cleaning water. Note, this doesn't mean you need that much per gallon in the rain barrel! Just in the water that you are using to clean it.  If possible, don't let this water end up on your grass or in your storm drain. It is killing the algae which means it isn't so good for you plants and the environment. The whole point is to help our waterways and our gardens so don't dump poison on them.  If you can empty this water into the sanitary sewer (your sink) that would be best.

    Enjoy the rain barrels and good luck with the algae problem!

    UTC 2021-05-26 07:24 PM 0 Comments
  2. J Starr 4395

    You can add a quarter to a half cup of chlorine bleach to the water, stir it around, and let it sit for a few days before using it on plants, but really, that's about it.

    See, the water- usually from the roof via the eaves troughs- is chock full of organic material, and then you close it all together in a nice, cozy barrel, and so it- starts to break down to simpler forms, which then start to grow.  Even without light.  Add light, and you're going to get Swamp Thing Algae- not a pretty picture. 

    You are being so wonderfully "Earth Conscious" and all, but I hope you didn't think that meant it was going to be set-it-and-forget-it living;  being close to nature and off-grid and "sustainable" usually means a lot more work. And so it is with rainwater harvesting. 

    You can try a pre-filter- say, between the eaves troughing and the barrels, but then you'll have to clean the, now well-rotted, crap out of that every so often.  Can't win making this kind of living choice easy and no-fuss.  Sorry.

    If it were me, I'd likely look into those tablets you float in a swimming pool- and I'd bust one up into about 1" square pieces, and put one in each barrel;  again, let it sit for awhile before watering your plants. But that should take care of the problem for a few days- till you get more food-rich rain refilling your barrels, and have to do it again.

    You can also look here and see if any of these ideas appeal:

    UTC 2021-05-26 06:18 PM 0 Comments

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