The homemade system I'm talking about is one that is spoken about on many survivalist sites. The caveat is pollution and nuclear fallout. Homemade filtration systems use various containers to stack charcoal, sand, and gravel to clean the water as much as possible. A thin cloth is used on the bottom with a small exit hole. The cloth holds in the charcoal which then has a sand layer poured on. Gravel is added on top of the sand. After putting the filter system together, you will need to run water through it until the water is clear. This is to clean the system and clean off the charcoal you used. After it runs clear, place a container under it and start collecting water.
This system filters out larger then smaller items as it percolates downward using gravity. The system is great and renewable in most areas. I like to use an equal amount of charcoal to sand to filter out as many chemicals as I can. Not all chemicals will be removed but the majority will be. After filtration the remaining water must be boiled. Boiling is used to kill organisms that are dangerous to the human body. A one minute or more sustained boil is required to kill these organisms. This includes protozao, bacteria, and viruses.
The advantage I find using this system is it is much cheaper than buying the commercial filtration systems. If staying put then this will work well for most people. If you believe your area may not contain safe sand after an event then you can purchase washed asbestos free and silica free sand from a store. These are checked for hazardous particles that may cause cancer and are used for sand boxes for children. Don't use all purpose sand from the store due to its contents.
The bad thing about this type of filtration is it needs to be changed out weekly or biweekly depending on the water source. The gravel and sand can be rinsed and cleaned then spread out on a surface in a thin layer to allow UV light to thoroughly dry and kill pathogens. Allow it to sit in a safe area for several days in direct sunlight. Another option is to heat the sand and gravel over a fire. Place small portions at a time on a designated pan or piece of metal and let it sit over the fire while you do other things. Make sure to dry if for a day first to prevent the gravel from overheating and exploding. Not always a problem just depends on which type you use. With this setup you can have enough materials to switch out each week without needing fresh sand or gravel. They serve mostly to simply remove large particles and trap a small amount of pathogens. The charcoal will need to be replaced each time with fresh charcoal. This means you will need to burn safe wood, i.e. not contaminated, to replenish your charcoal. If its a grid down situation this shouldn't be a problem unless your in the city with no way to gather firewood.
This system is good for bug out situations as long as you have the ability and time to stop and filter fresh water every day or two. This depends on how much you will carry. I like this system also because of the reduced weight load and lack of gear needed to carry. I can either carry a container to put the filter material in or carry extra cloth to make a drip layer system where each cloth is attached to poles, holds the filtration material, then drips to the next level.
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You should carry your cloth with you at all times and can wash it using lye soap made at the fire. With this setup you can travel and gather fresh water without too much concern. Just remember to boil the filter water for one minute or more.
Commercial filtration systems are fine but require you to carry the equipment to use them. Make sure you follow care instructions and understand their limitations. The can be used to filter water without too much setup but most of them require a pre-filter to extend the life of the system. They can be quite expensive for ones that remove viruses without boiling. Commercial water filters are more of a convenience than anything allowing you to keep moving stopping for short periods for "quick" refills. They are good short term but bad long term pieces of equipment.
If your thinking about an end of days scenario then I prefer low tech but if your thinking a few months of hard time then the cheap commercial filter combined with boiling will work. Remember some type of pre-filter like a bandana will extend your filters life in murky water with large grains. For me I would rather spend 10-20 dollars on a proven system than spend 300 on a commercial filtration system that will sit in the closet until I think its time to use it or be left behind if I must leave my bug in location.
Almost forgot, another thing you could do is buy the filters and then build your own unit using food grade buckets, bottles, or cans. You'll need safe gaskets to install the filters. These can be disassembled if need be and using smaller containers they can be made portable. There are many sites that give information on how to do this but since I've not made one using a high quality filter I'll let someone else teach you.
Study hard but don't let it consume you.