One thing I'm really interested in getting is a barrel of water for the attic of the house I'm buying. I need to find instructions on treating and keeping it treated. I know that untreated water becomes a lovely growing medium for all kinds of nasties, but if it's treated and sealed, does it remain sterile?
You can use bottled water. It will stay good as long as it stays sealed and airtight. Also - the water in your hot water tank can be used if you shut off the incoming water flow so no contaminated water from the city water system enters. Make sure you turn off the circuit breaker for the hotwater heater so it doesn't boil dry. That way you have 40 gallons of safe water as long as the heater remains above any floodwater.
Use a plastic container. Wash with a strong chlorine bleach solution and rinse out thoroughly. Fill with water, and add a couple of oz of bleach per 20 gal. or water purifier tablets and seal tightly. Cycle at least yearly. Store water purifier tablets and filter with your supply.
This is a first glance. Water weighs 8.33#/gallon.
Water: Chlorine ~ 6ppm in good old CLEAN H2O. (I can't remember in which water engineering source I found while at Texas A&M) Check Red Cross: I have used ~1-2 teaspoons to 3 tablespoons per gallon depending on source (lake/stream/puddle) which is why your own filtration systems can be valuable. Take time to let muddy/cloudy water settle (sedimentation rates = f(solids' size and organics). Always let shake/stir vigorously over hours up to 2 days for chlorine outgas before you drink "pool" water.
Per 55 gallons (drums)of tap water - 2 tablespoons of unscented household bleech( comprised of sodium, oxygen, hydrogen and chlorine). Six times the concentration of bleach is "muriatic acid) = hydrochloric acid (hydrogen and chlorine). A 55 gal. Drum on 5-wheel dolly with water is nearly 500#; my 75 y.o. mother could easily enough roll my test drum around the house over oak floors and kitchen tiles.
Common Sense ALERT: In attic, with enough insulation, a 55 gallon drum might survive icing during most Southern cold blasts, but think this one through. The larger the mass the less chance of freeze-up, depending...
Storage: bungs sealed tight with black 55 gallon trash bag(s) with elastic strap at bottom of bag to minimize any algae growth - This shoulkd keep for years because the chlorine has no where else to go other than attach to tap water's impurities - it should not go bad, but keep a gallon or four of bleech for when SHTF. Drum pumps cost ~$20-$30.
For drinking, pump/pour into half filled quality jugs to shake vigorously several times over 24 hours to outgas the excess chlorine. To keep the process going, I recommend several Rubber Maid one gallon refer jugs because of their quality and durability. Aeration always makes water taste better than when "flat". Powdered Gator-Aid helps make any off flavors and it contains sugars and "minerals" to improve blood chemistry.
Filtration: I use a 6' 2" copper pipe with reducer fittings to 1/2" with ball valving holding usually ~12" fine pre-washed sand over 6 layers of coffee filters over 4 oz. activated charcoal from any aquarium supply pet store, over 6 layers of coffee filters which drips through double coffees from an old coffee makers grounds cup into a two gallon plastic insulated cooler. The tall the pipe stand, the more pressure to push down through the filtration system. The dirtier the water, the more sand. PVC works just as well and a bit lighter, if not as stout. My next filter will start with a 4" main pipe, but it for a permanent homestead installation.
Research and best of luck and act today.