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This is a great project to have at home for that just in case moment. Bio-filters are a great alternative.
This is a great DIY option to learn about in the event SHTF and you need a clean water filtration system larger than a Lifestraw. As you can see by the picture the water filters through three separate layers, gravel, sand and activated charcoal. The gravel removes the large pieces of debris, the sand removes the smaller particulate matter and finally the activated charcoal removes bacteria and some chemicals. However this system does not completely remove 100% of everything harmful.
To completely kill all pathogens just boil the filtered water. See how to make this awesome long term water filter below: