How do slow sand filters work and are there any alternatives?
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).
Questions:
- How do slow sand filters work (I'm referring to ones without the Schmutzdecke)?
- Are there any other materials that have the same "water purification" abilities as sand and gravel?
- Is the gravel base optional? Does it help with any of the actual purification?
- 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
- Where will the bacteria come from?
- 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.
This post was sourced from https://outdoors.stackexchange.com/q/15590. It is licensed under CC BY-SA 3.0.
1 answer
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.
This post was sourced from https://outdoors.stackexchange.com/a/15624. It is licensed under CC BY-SA 3.0.
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