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Natural water treatment: the slow sand filter

February 7, 2007

I honestly don’t remember where or when I learned about this topic, but ever since I’ve been looking for an excuse to put it into practice myself. I am talking about what I would call the most elegant water treatment solution ever, the slow sand filter. This is the best example I’ve ever come across that shows what you can accomplish by working with Mother Nature, instead of against her.

Very generally, this is how a slow sand filter works: build a large tank with a tube running from the top to near the bottom, and fill it with progressively finer rocks, gravel, and sand. Fill it with water to approximately 5-10cm [2-4″] above the top level of sand. Now slowly pour (without disturbing the sand) or drip contaminated, dirty, or otherwise unfit-to-drink water into the tank. Over the course of weeks a biological filter medium called the schmutzdecke will form, and will start biologically filtering and deconaminating the water. Depending on the water being filtered, the filter will last for weeks or months. To clean it simply stir up the top 10cm [4″] of sand, remove the resulting muddied water, and let the schmutzdecke redevelop. Later, rinse, repeat.

Isn’t that simply awesome in its simplicity? The only drawbacks to such a system is that it needs to be pretty big to handle large volumes of water, due to the low flow rates required/permitted by the schmutzdecke. While it is very effective at removing conaminants, I’m not certain whether the resulting output still requires disinfection. Regardless, so long as you have fine sand, coarse sand, gravel, and the materials for a tank (concrete, rock, steel, plastic, whatever) you can very effectively filter your own water.

To see how and where slow sand filters are being used, check out, and peruse my other slow sand filter links. The UN promotes its use for obvious reasons, and I’ve even recently come across one of those sponsor-a-child programs that allows you to purchase one for a family. (Sorry, no link as I’ve totally forgotten the program’s name. If I find it, I’ll put it up here.)

Would this not be a great technology (if you can really call it that) to use at a cottage, or camping? Or for that matter, use at home if you’re doing rainwater catchment? I’ve even wondered if it could be used to keep swimming pools clean in place of using all the harsh chemicals that are typically used.

One thing is for sure… as soon as I find an excuse, I’ll be building my own slow sand filter, and then making a T-shirt that says, “Ask me about my schmutzdecke!”

2 Comments leave one →
  1. March 2, 2007 6:53 pm

    Glad to see the Wikipedia article on SSFs being linked, as I did some editing on that, adding info about the alternative method of maintenance (ploughing or stirring the surface, rather than scraping) which gives faster recovery and better performance, potentially reducing the large space requirements (the biggest downside of SSFs). Which makes the filters even cooler.

    Andrew, I’ve found and – is one of these what you’re talking about? Also, how do you see your site in relation to Appropedia, Appropedia is the largest and most active such site that I’m aware of, but we’re keen on collaboration rather than competition.

    Would love to talk about what kind of collaboration is possible – email me or leave a note on my Appropedia talk page.


  2. June 7, 2008 2:25 am

    Have built 2 slow sand filters. They are awesome. The website has all the info. Glad to see others are interested.

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