How does water get filtered while passing through sand, charcoal, etc.?
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With sand filtration, there are two main mechanisms.
1. physical filtration: the spaces between the grains of sand are tiny, and that removes most large sediment particles that make the water cloudy. But the bacteria and viruses are too small to be stopped by this.
2. biological filtration: the upper layers of sand (about the top 5 inches) get colonized by a biofilm of bacteria and other micro-organisms. It’s basically a microscopic ecosystem made of micro-organisms surrounded by a gel that they secrete. This coats all the sand grains. Many of these micro-organisms are predators which feed on bacteria. So the slimy gel of the biofilm traps the viruses and bacteria, which are then consumed by the microbes living in the biofilm. This biofilm is often referred to as the “Schmutzdecke”, which is a german word which literally means “slime layer”.
Imagine you have a cup filled with dirty water. If you pour that water through a layer of sand, the sand will act like a strainer. It has tiny spaces between the grains, which are so small that they can trap or catch some of the impurities in the water. The water can pass through, but the dirt and other particles get stuck in the sand.
Now, let’s move on to charcoal. Charcoal is made by burning wood until it turns black and becomes really lightweight. It has a lot of tiny holes on its surface, kind of like a sponge. These holes can help remove some things from the water that the sand alone can’t get rid of, like chemicals or bad smells.
When we combine sand and charcoal in a filter, the water passes through the sand first. The sand takes out the bigger impurities. Then, the water goes through the charcoal. The charcoal’s tiny holes absorb some of the smaller impurities, making the water even cleaner.
By using both sand and charcoal in a filter, we can make dirty water much cleaner and safer to drink. This process is similar to what happens in some water treatment plants or when people go camping and use a water filter to make sure the water they drink is clean.
Carbon does not actually work like a filter. Instead, organic particles stick to the carbon atoms. This is called ‘adsorption’
Comments are doing this a grave disservice.
Charcoal filters water because it has a lot of surface area which many contaminants stick to. The interaction is actually the same that makes gecko feet sticky. Activated charcoal works many times better than regular charcoal, but regular charcoal can usually filter water in a pinch.
Sand is rarely used to filter water directly except as a way to remove large particles. However, there is such a thing as a biosand filter, where the sand is impregnated with bacteria which eat everything they can digest out of the water. The sand isn’t filtering the water so much as acting as a habitat.
OOH OOH I CAN DO IT
Assuming you’re talking about the kind of filtering caused by gravity pulling water through a mix of filtering material, there’s two types of effects that happen: size-based filtering, and separation based on the polarity of the atoms/molecules.
Start with size, because it’s easier: imagine it like pouring a mixture of pasta, rice, and water into a colander vs a sieve. The colander holes are small enough to catch most of the pasta and a little of the rice, while the sieve catches all of the pasta and, depending on the size of the holes, most or all of the rice. Now imagine the solids in the water are microscopic, and the gaps you’re going through are based not on a metal colander or sieve, but the size of the spaces between grains of sand, or particles of clay. The smaller the particles, the tighter they pack, and the less stuff they let through. Water molecules are among the tiniest molecules, so they can get through almost anything, given enough time. Important to note: the smaller the gaps are, the purer the water will be when it comes out, but the longer it will take.
Now, the polarity thing. Water is a pretty polar molecule (I could do a whole ELI5 on that statement alone, but leave it for now), and there are still materials that are more polar than it, and materials that are less polar than it. The most important thing about polarity: like dissolves like – meaning that the more similar the polarity of two molecules is (across the whole molecule and in each part of it), the better they will dissolve in each other.
Imagine a mixture of gasoline, aspirin, water, and table salt. The oil molecules are not very polar at all, so they don’t dissolve well with water. The aspirin is more polar, but still not quite as polar as water, so it does better than the gasoline, but not perfect. Table salt is even more polar than water, but it’s closer to water than the aspirin is, so it does better than the aspirin. Now, pour all of this mix through a bunch of activated charcoal.
Activated charcoal contains mostly plain carbon, making it even less polar than the gasoline. So, if the gasoline is picking between dissolving in water molecules or sticking to the carbon atoms on the surface of the activated charcoal, it’s gonna pick the carbon. It’s not PERFECT, so if there’s not enough charcoal, a bit of the gas might stay with the water.
The table salt is nothing like the activated charcoal, so it is going to stay in the water wherever it can. Very little salt will come out of the water and stick to the activated charcoal.
The aspirin is the interesting case – it has parts of the molecule that look more like the plain carbon, and parts that look more like water. So, what matters is the amount of each you have. If you have enough carbon, the vast majority of the aspirin will stick to the carbon, while if you don’t have a lot, the aspirin will stick to the water.
Now, activated charcoal is called activated because it’s been burned in such a way that it has a VERY rough surface, with lots and lots of carbon atoms sticking out. In some ways, it’s rough like a sponge is: there are pits and holes for stuff to stick into. This is a way to increase the amount of carbon that stuff can stick to without having to make bigger chunks. We say that the surface area is greater to describe this: the shape has more surface than a block normally would.
The two of these effects are combined in most cases, for example, when rainwater filters into an underground aquifer. You can use them a little differently to do industrial water purification, where you need to use much less “filtering” material than the amount of water you need to purify.
By the way, here’s a couple of fun health facts that come from this polarity separation effect:
1. If you take medications, ingesting activated charcoal might change your dosage without you realizing it. Since charcoal doesn’t get broken down by your body, it makes its way out in your poo, and while it’s inside you, it’s grabbing up everything from vitamins to medicines before your body can absorb them. It’s also why activated charcoal is used for some (but not all) poisonings by ingestion. It’s also why I recommend AGAINST using charcoal scrubs on your teeth or doing charcoal cleanses, especially if you’re taking medications where a change of dose could have significant consequences (like birth control or antidepressants, for example).
2. The dose you need to take for some medicines depends on how much protein or fat you have in your diet. Fats, protein, and water all might dissolve a drug differently, and all of the might help or hurt its progress to get where it needs to go in your body. These are called food effects, and most new drugs have to run studies to determine if food has an effect on dose.
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