How does water get filtered while passing through sand, charcoal, etc.?
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Particles that are suspended in water get caught up in all the tight spaces that water has to go through when filtering through such materials. Some stuff also sticks directly to the surface of the material (this is called adsorption) or reacts with it; that’s why activated charcoal is made in a way that maximizes its surface area.
The water flows through the filter, whereas the particles that were in the water, are larger than the tiny spaces/gaps that’re in the filter, so they don’t pass through and flare trapped in the filter.
Imagine if I had a sieve full of flour, but I drop some rice in there too. As I shake the sieve, all the flour will pass through the sieve but the rice won’t, because the grains are too big to go through the holes in the sieve.
While most answer have focussed on mechanical filtration: aka sieving and physical stopping of bad particles, i would also like to point out there is a biological component:
Bacteria that stick to the sand/… particles eat certain bad molecules and break them down to less damaging components.
In waste water facilities, they have long settlement ponds/troughs. Anything suspended in the water will settle out as the water slowly moves across the pond/trough. Anything still suspended (fines) will then be filtered out with several gradients of sand, coming out the other end as mostly particulate-free water. The water then gets passed through purification systems with activated charcoal for capturing harmful chemicals, and adding chemicals to kill the various bacterias that are harmful but can’t be removed using charcoal.
At the end of the process, the cleaned water can be either discharged into the local water ways, or sent back through the pumping stations for reuse.
The idea behind water filtration is that water particles are smaller than whatever contaminants are present in it, and we filter it through a series of smaller sieves to trap and separate the contaminants.
Let’s take some muddy water taken from a pond with sticks and leaves, for example:
With your hands, you should be capable of removing large contaminants, like sticks and medium to small sized leaves, but you won’t be able to remove the mud or small leaves and other contaminants, so you use a slightly finer sieve. A layer of gravel would be capable of trapping some mud, larger leaves, bugs and whatever you missed with your hands, but won’t get rid of all the mud or bacteria, so you use a slightly finer sieve. Coarse sand might be able to trap the majority of the mud, but it would not be able to trap other smaller impurities, so you use a finer sieve. Activated charcoal might remove contaminants with affinity to charcoal, but won’t remove bacteria or smaller contaminants, so you use a finer sieve. Fine sand might remove most of the contaminants, but it won’t remove all of the bacteria or contaminants that might go through that sieve. At this point, you could use a finer sieve, but we would be seeing diminishing returns. To kill the bacteria, you can treat it with chlorine, uv radiation, or in a pinch boiling, but this does not guarantee that water is safe to drink because you don’t know what other contaminants are in the water. Finally, you can distill the water to separate it from whatever contaminants are left over.
Liquids are much squigglier than solids. Even very little solids. So we create lots and lots of tiny squiggle points that only water can squiggle good enough to pass. “Filter” is just another word for squiggle.
Two things. First, is that the stuff you’re filtering out is bigger than the water molecules themselves. Like much bigger. So water can get through where the contaminants can’t.
Second is that the filter media (media is the fancy term for the material the filter is made out of, like sand or charcoal) might be sticky for the contaminants (like bacteria) but not the water itself.
Each layer of solid has a different amount of gap. It’s layered with the largest gap on the top so that the larger unwanted particles in the water doesn’t clog up the layers with the smaller gap.
Those things have small holes. Water passes through while nasty stuff like bacteria and dirt get stuck because they can’t fit.
Let’s setup our imaginary workshop!
– Place a strip of the rough side of velcro over the side of a bowl or cup.
– Fill the bowl or cup with water and small particles, such as tiny rocks and dirt.
– Pour the water out slowly, over the velcro.
– Notice all the junk that gets trapped.
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