Slight Introduction:

A slow sand filter is a filter used to purify water. The original design is composed of a layer of gravel, then a layer of fine sand. Sometimes, however, there is an extra layer of bacteria on the top doing most of the purification.

This layer of bacteria is called the Schmutzdecke (German for dirt covering).

Slow Sand With Schmutzdecke


  1. How do slow sand filters work (I'm referring to ones without the Schmutzdecke)?
  2. Are there any other materials that have the same "water purification" abilities as sand and gravel?
  3. Is the gravel base optional? Does it help with any of the actual purification?
  4. How do you grow the Schmutzdecke?

I do see that in the picture, the gravel is used as support. But I'm wondering if it'll affect the clarity of the water if removed.

I've seen that to grow the Schmutzdecke, you just have it sit in water for a week, and it should grow. But I'm not sure that's correct, because

  1. Where will the bacteria come from?
  2. The bacteria won't have any food to grow!

And also, setting a bottle with dirty water in it for a week did nothing to the top layer of sand.

  • the bacteria are everywhere. You never really have to worry about where bacteria will come from. If the water you are purifying has any organic component, or if the "dirt" at the top layer does, then the bacteria would have food.
    – That Idiot
    Mar 13, 2017 at 13:11
  • @ThatIdiot So if we just use the filter originally without the bacteria, eventually, the bacteria will settle on the top layer and grow?
    – Frank
    Mar 13, 2017 at 15:22
  • 2
    In theory this is correct. It is a bit like the biological filtration in an aquarium. In these systems water is trickled over spiky balls or some other media with high surface area - the trick is to keep it aerobic - and the bacteria process nutrients like nitrogen and phosphorous. Perhaps a closer analogy from aquariums is the undergravel filter - in which water passes through a sand or gravel bed and is then recirculated to the top of the tank. Here again, once the bacterial colony is established it can do its thing without any intervention. You just don't want it to go anaerobic...
    – That Idiot
    Mar 13, 2017 at 18:42
  • @ThatIdiot Hm... for some reason, my slow sand filter will sometimes work, but if I take all the materials out and put it back in, it won't work. Any ideas? I don't have bacteria in yet.
    – Frank
    Mar 14, 2017 at 2:46

1 Answer 1


Wikipedia has some good info on slow sand filters and this website also has some useful info. Here is an interesting document describing sand filter design. While most of it is quite technical, looking at the engineering side, the first chapter provides a good overview.

The general principle of a sand filter is that the sand forms a porous layer which contaminants (bacteria, organic matter, etc) cannot pass through. Instead, they get stuck to or between sand particles.

The Schmutzdecke is formed primarily from filtered bacteria that then grow in the upper layers of the sand and helps provide additional filtration. The appearance and composition of the Schmutzdecke can vary widely and is dependent on the water being filtered.

As for the gravel, I've been unable to find anything to confirm this but I don't believe it helps with purification as such. I believe it is mainly to help with flow rates and preventing the outflow being blocked with sand.

As for other materials working in the same way, any porous material can have the same effect, but sand is cheap and easily packed into any shape. Ceramic water filters are another common type of filter that work on roughly similar principles.

  • So you're saying that to grow the bacterial layer, first build your filter, then filter water, and wait until the bacteria grows and develops? If so, wouldn't the bacteria be "bad" because it's from the dirty water?
    – Frank
    Mar 15, 2017 at 17:23
  • @Frank That is my understanding yes. The bacteria is kept on top of the filter so should not cause any harm but can provide extra filtration as it grows into a film or mat
    – nivag
    Mar 16, 2017 at 8:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.