Be careful of the advice to get, trust but verify!!!
I watched a video produced by a preparedness “Expert”. The video showed how to take three five gallon buckets adding washed gravel, clean sand, and a bag of charcoal; you can treat water for drinking. Now I have a master’s degree in environmental quality science. And it is true that you can remove some particulates by running water through gravel, then sand. And you can remove some substances from water using activated charcoal. Lumps of charcoal are not activated charcoal. Activated charcoal is charcoal that has been modified to have a high surface area. It is the high surface area that absorbs substances from water. So the video had some leaf litter in the water and it is run through the three filters and the water comes out clear. The final note is “hopefully your immune system will be able to handle anything that gets through the filter”. My advice is don’t do this at home!!! Water born disease is a major cause of illness around the world. In America we have developed a high level of water quality. During a natural or man-made disaster it is likely that sanitation plants will fail. It is not uncommon for surface water to become contaminated. With the proper treatment you can make a lot of water safe. But you have to have the right treatment. There are many bad microbes that cannot be filtered out of water using a sand filter. It requires a special type of filter to filter out these types of microbes. Don’t waste your money on anything that has filter pores larger than 0.2 microns. If all you have is surface water or water of unknown quality you still have to treat it even after you have used the above described three bucket filter. This will general be done in one of three ways or a combination of the three. First is with special filters that can remove the microbes. The second is chemical treatment that can kill the microbes. The last is heat treatment of the water to kill the microbes or distill the water.
There is a lot of good information on techniques to do this. Just Google Emergency / Survival Water Purification and you will find a lot of filters and chemical systems to purify water. You may have gotten Boil Order which are water orders are issued by Health department to local public water suppliers, who in turn issue advisories to their consumers advising them that they should boil their tap water for drinking and other human-consumption uses like cooking, hand washing, brushing teeth, etc.
So please take the time to verify any preparedness information you gather before you put it into practice. I used to live in Alaska where there were many clear flowing streams untouched by man. I would never think of drinking from any of those streams without treating the water. The reason being was Beaver Fever (Giardiasis) http://en.wikipedia.org/wiki/Giardiasis
If you drank the water you had a very real chance of getting Giardiasis. To make this water safe you could boil your water, treat it chemically, filter it with or without additional chemical treatment, and finally you can use a reverse osmosis filter.
My point is take the time to really prepare by knowing the risk you are trying to deal with and the proper means to really deal with the risk!!!