I have enough food in my pantry to last me weeks in case of some disaster, but only a few gallons worth of bottled water. I was thinking it might be a good idea to keep something around for purifying water in case of some disaster or water-supply problem such as they’re having in Ohio. It’s pointless to have food if I would die of thirst before getting a chance to eat it.
Where I live there is plenty of rain usually, multiple creeks within walking distance and rivers within driving distance. Access to water isn’t a problem, I would just need to make it safe for drinking.
I don’t have room to store large amounts of water. What is the cheapest effective and reasonably space-efficient way to purify water? Filters? Bleach? A solar still? Tablets? Extra gas for the grill, for boiling water?
Sawyer water filters are small and light, popular with backpackers. I have the Sawyer Squeeze – weighs less than three ounces and will filter 1,000,000 gallons of water. They have other options depending on how many people you intend to filter water for.
If you want cheap and small, aquamira is another popular water purification method.
Gotta second the recommendations of the Sawyer (I have the PointOne) for more volume filtering, aquamira or a similar chemical method, and LifeStraw for portable, self-only. Keep bleach anyway, rotate your stock, and replace within 6 months of opening. If there’s a disaster involving sewers/water supply, you may well need bleach to decontaminate your place.
I’d also recommend stashing a big 6-gallon or so jug somewhere for not necessarily drinking water. Some water contamination is bad enough that they don’t want you bathing in it, and you may want a nice big stock to start with in case you have to wash something/yourself off, etc.
Finally, if it’s the kind of disaster you have some forewarning of, they make water bladders designed to be filled from and in your bathtub. Fill 'er up before a big hurricane or something similar.
Given your criteria, without a doubt, bleach. It is dirt cheap. It is effective–recommended by the EPA even. It is space efficient–a small number of drops will purify a gallon of water. And, as a bonus, it can be used to sanitize other things.
ETA: And, as bonus bonus, it is something that is useful in normal everyday life that you are likely to have on hand. It is not an extra expense specific to disaster readiness.
I would like to comment on the use of bleach as some have recommended. Bleach will kill bacteria and viruses. But I’m not sure it is safe to assume it is effective against an unknown toxin. When they had the water problems in Virginia (a while back) and more recently in Ohio, the problem was a toxin. I personally found it quite disturbing that no government agency ever specified (to my knowledge) whether the toxins could be filtered out with readily available filters. Some toxic substances are easily filtered, others are not. There is one effective solution for removal of toxins- distillation. I have a one gallon electric still I purchased for $100, as my insurance for this concern. I don’t believe it is safe to drink substantial amounts of distilled water (this is controversial) and add a mineral supplement product back in to produce water of around 100 parts per milion content. So I recommend keeping a water distiller and a few various filters to address different possibilities.
Of course you are going to need power for a distiller, but a generator is usually an item owned by people who are serious about surviving a disaster.