Clean, safe water is one of the three foundational needs in a survival situation. The baseline water requirement is around one-half to one gallon of water per person per day, more or less depending on activity and conditions. The human body can generally survive for up to three days without water. So having enough for fixed or mobile situations becomes a critical need as more time passes.
Both the situation and potential water concerns must be evaluated to support effective planning. Water purification capabilities come in either portable or fixed applications, with portable ranging from straws to pumps. There are five main areas of concern with water quality: clarity, protozoa, bacteria, viruses, and chemicals. These contaminants are removed through one of, or a combination of, three methods: filtration, energetic (boiling, UV lights), or chemical. Not all methods work for all needs. For example, filters come in a range of sizes. A 2.0 micron filter removes most visible particulate for clarity. A 1.0 micron filter will remove 99.9% of protozoa. A 0.1 micron filter will remove most 99.9% of bacteria. A 0.01 micron filter will remove 99.9% of viruses (most personal filters do not go down this far). Chemical contamination requires special methods or equipment.
More detailed information about water needs and purification can be found at: FEMA, Red Cross, CDC (chart), Water Filter Dude, and Outdoor Gear Lab’s backpacking water guide.