You’ll need a minimum of 1 gallon of water per day/per person. If it is hot outside, or you’re working and sweating a lot, the amount will be greater. Your stored water (bathtubs, bladders, containers, bottled water) will last for a short while, but eventually you’re going to need more fresh water. Fortunately, there are several options:
Toilet tanks and water heaters. These have water reservoirs that you can drink without treatment within the first 30 days. After that, purify it.
Streams, creeks and lakes. Don’t drink water directly from these because it is probably contaminated with bacteria and viruses. You’ll need to process this water to make it safe to consume. First, filter the water through a coffee filter, paper towel or piece of cloth to remove particulate matter, then choose one of the methods below to make it drinkable.
Rain water or snow. Don’t drink rain water or melted snow without processing it. Droplets and flakes absorb contaminants as they fall through the air and you’ll need to remove them. If the event generates smoke from burning chemicals or petroleum products, don’t use the water. In other events not involving smoke, you can use a tarp or gutter downspout to collect water into containers. The water you collect from your home’s downspouts will contain sticks, bird poop, and other debris so PLEASE don’t drink it directly! You’ll need to finish purifying it to make it safe.
Boiling. Before you begin treating your collected water, filter out the particulates as best as you can. Bring the water to a rolling boil and leave it there for at least one minute. If you live above 5000 feet, increase the time to three minutes. Allow the water to cool and store it in covered containers. Boiling water on a propane BBQ grill will consume a lot of fuel.
Chemical treatment. Use unscented, uncolored household bleach. Read the label because it comes in two common concentrations: 6% and 8.25%. Using the 6% you’ll need to add 8 drops per gallon, and with 8.25% only add 6 drops per gallon. If the water looks cloudy or nasty, double these amounts. Stir in the bleach and let it sit for 30 minutes. It’ll probably taste like chlorine but by letting the container sit open for a while the chlorine flavor should mostly dissipate. In my experience, it’ll still taste pretty bad no matter how long you wait, but at least it’s safe.
Filtering. This is the fastest, most efficient way to purify large amounts of water. Modern filters will remove almost 100% of protozoa and bacteria, as well as clarify cloudy water. Filters can be as small as the LifeStraw, but these produce only small quantities of water. For larger amounts consider the larger backpacker models. The two that I’ve are the Katadyn Hiker Pro, and the MSR Sweetwater. These are both priced around $90 and produce about a liter per minute.