I would like to mention that there are hundreds of DIY water filters out there on Instructables, Youtube, etc. There are many ways to make a survival or emergency water filter, so I will first say what are the differences with this filter compared to other methods:
1. The real simple innovation in this design is that it increases the flow and rate of filtration by using air pressure from your mouth instead of waiting several minutes for the turbid water to be filtered by gravity as many other filters do. This simple innovation can help you filter lots of liters of turbid water per hour compared to just waiting for gravity to do it's work through the filter. In an emergency situation or disaster, this could be a big difference!
2. It works :) It helps remove dust, particles, chemicals and even metals from turbid water. You can kill viruses and bacteria by adding a few drops of chlorine after the filtering process or placing the filtered water in a clear, clean plastic bottle under the sunlight for UV rays to kill them.
3. It's small and portable, it fits in a 0.5 liter plastic bottle
4. It's low cost, using available materials like a plastic bottle, filter paper or a piece of cloth or kitchen paper towel and cheap activated carbon (I bought 1 kg or 2.2 lb of activated carbon for $5, you will use 1/5th or around 200 gr)
5. It's reusable. You can filter hundreds of gallons of turbid water. Once the rate of filtration becomes slower, you can just take out the activated carbon and replace it with unused carbon. You could use also charcoal, sand and gravel inside the bottle
Step 1: Get a Plastic Bottle and Make Small Holes at the Bottom
The first step is to use any method to make tiny holes in the bottom of a plastic bottle. I used a needle heated with a candle to make the holes through the plastic. You could use also nails or a swiss army knife. The holes should be very small otherwise the activated carbon that you will put inside the bottle will come out of the bottle.
Step 2: Get Bulk Activated Carbon and Fill 1/3 of the Bottle
You can buy granular activated carbon online from different suppliers at ebay or at many department stores like Walmart. It seems that also pet stores sell activated carbon for aquariums and fish tanks. I bought activated carbon from a chemical supply store where I live and payed $5 for 1kg (2.2 pounds), which is very cheap.
If you don't know much about activated carbon I recommend to read more at Wikipedia, activated carbon sounds like a fancy type of carbon or material. I was surprised to know that they use coconut shells for example to make activated carbon. Activated carbon has also very interesting properties, due to its porous characteristics 1 gram of activated carbon has a surface area of 500m2! This is why it is used in many household and industrial applications like filtering drinking water, filtering swimming pools and fish tanks, filtering air, filtering alcohol and other chemicals. If you have a britta or other brand of water filter at home you will notice that these filters use an extruded activated carbon cartridge that you must change from time to time. These cartridges are really expensive compared to what it cost to make them, and that's the business model for most home water filter brands.
One of the most relevant properties is that activated carbon can filter chemicals and metals from water, removing hazardous particles and even bad taste and odors. This is a link to EPA's drinking water treatment page to read more about this.
I used a lot of activated carbon for this filter. The more activated carbon you use, they larger the filtration surface area you will have and thus the more particles you can filter out of turbid water. I tried filling about 1/3rd of the bottle and I got crystal clear water. I am still wanting to test the results with a digital TDS meter that measures the solid particles in water in parts per million, to get more scientific results.
Step 3: Use Filter Paper, a Piece of Cloth or Kitchen Paper Towel to Pre Filter Large Particles
For improved results, I used a small and simple "pre-filter" system before the activated carbon filter to remove dust, stones, leaves or any other large particle from the dirty water. This way your DIY activated carbon filter will work just on removing smaller particles.
I used filter paper that I bought at a chemicals supply store, but you can either buy the filter paper online or use coffee paper filters, a piece of kitchen paper towel or a piece of cloth. I made a cone and used a paper clip at the end to close the hole at the end of the paper cone. Then place the paper cone into your plastic bottle filled with the activated carbon.
You can now pour some dirty water into the cone and it will start filtering the cleaner water into the plastic bottle. Once you have poured all the dirty water into the pre-filter cone you can take the cone out and you will notice the amount of dust and particles that this has already pre-filtered.
Step 4: Final Step: Blow Clean Water Out of the Bottle
Once you pre-filter the dirty water into your bottle filled with activated carbon, you will need to just apply air pressure to the bottle. You can blow air into the bottle using your mouth, this will increase the pressure in the bottle, it will make the dirty water go thru the activated carbon and out of the bottle through the small holes punctured at the bottom of the bottle.
And you are ready, you will end up with crystal clear water!
TIP: This filter will remove most larger particles, some chemicals and even metals as explained on sites like Wikipedia and the EPA, but it won't remove viruses or bacteria. To kill these please follow the guidelines on the EPA's emergency disinfection of drinking water page. Using a few drops of chlorine or bleach will do the job. Using solar disinfection using the UV rays from the sun will help too as explained on Wikipedia.