Safe water is critical and I know I took tap water for granted before I did my 5 day no “tap water test” https://myadventuresinselfreliance.wordpress.com/2012/03/06/water-test-first-2-days-impressions/
But within a few hours of no water you body will start to shut down. You might get a terrible dehydration headache and your body will have a heck of time moderating your core temperature so you will be chilled in cold weather or face heat stroke in hot weather. Unsafe water can lead to diarrhea/dysentery and you will lose fluids even faster.
The consensus for the amount of water to store is 1 gallon per person per day. I think 3-5 gallons, is a more realistic amount if you want to keep your plumbing going and maintain good health. As Sandy has shown things may not get fixed in 3 days. I would not be surprised that many of the sewage problems could be people not flushing because they have little or no water and they don’t have a backup potty system. 3-5 gallons sounds like a lot of water and it is a lot, but the average person uses about 80-100 gallons per day according the the USGS. Using the 5 gallons a day is about 5% of what a normal daily use. You can imagine trying to live on 5% food or heat and many other items in a disaster. WE do waste a lot of water daily because it is always available, but if you have to carry it, and heat it every time you will start conserving. I know after trying to get by with a gallon a day, 3-5 gallons feels absolutely decadent.
The cheapest way I know to build up water storage is soda bottles. I started this way using 2-3 liter pop bottles. Every 4 bottles is just over a gallon and give them a good cleaning and filled with tap water this is a cheap way to start your water storage. The bottles are easy to move especially for disabled or elderly to use compared to bigger water storage containers. The biggest draw back is these bottles use a lot of floor space so they are not ideal for an apartment dweller. Remember water weighs about 8 pounds per gallon so a 5 gallon jug is around 40 pounds maybe to heavy for a person to move easily and that’s only one day’s worth of stored water for one person by my standards!
If you are buying bottled water by the case in 16 oz. bottles. Each case is about 3 gallons total so keep that in mind if you decide that is how you want to store water. It’s a more expensive but you can make the choice. Based on my calculations for 3 days and figuring 3 gallons per day you will need 9 cases per family member. So bottled water will take up a lot of floor space.
Water barrels seem to be the best compromise of cost and the use of floor space. While a 50-60 gallon barrel (400 pounds) could be to big. I like the 15 gallon food safe barrels that hold 15 gallons (120 pounds) 14 inches across by 25 inches high and can fit in a closet easily. I got mine used for $25.00 each and you will need a little siphon hose to draw the water but I think they are super for water storage in a small space. Each family member could have one stashed in the closet and have water for 3-5 days based on what I think you need or for 2 weeks based on a gallon a day. There is a newer product being sold at some sites that is a interlock cube that holds 3 gallons each. I have not used these cubes but they look like a great compromise of weight, floor space and water stored. They also a bit more expensive than the methods I have used. So check out the internet and find a way that you can have your water storage without killing your budget or taking up all your floor space. Don’t forget you can store a lot of items under beds. Maybe you can get by on a gallon a day of water but having extra safe water stored will not hurt you. I’ve done a my test and I was in the Army and being able to have my coffee first thing in the morning and doing my cleanup with warm water instead of cold was a huge morale booster. Some people claim to love cleaning up with cold water, but not me. I never found I needed to practice being miserable. Misery happens enough all on it’s own and never requires practice.
Now that you have a few days of safe water to drink you need to be able to make more safe water. Perhaps you have tap water but there is a boil order to make it safe . You can use a water filter like the big Berkey filter or 6% bleach and iodine will also kill most pathogen in water. This will save your fuel for heating and cooking and not for just boiling water to drink. Never trust any open water source, take the time and treat it or you may deal with dysentery/diarrhea and a host of little bugs that can make you miserable at best and kill you at worse.
That brings us around to the next thing of what to do with the waste from eating and drinking if the sewage system stops working and the trash trucks ain’t coming.