After the SHTF clean water will be scarce right? Well after you have your self settled in nicely say you want to actually climb the ladder in our radioactive,poisoned ect. world. Water would be like liquid gold. SO if you found yourself a spring (my city has them everywhere) you could start a water processing plant. to sell (trade) the water. First you could draw up the spring water, then filter it , then put it in clear bags and let them sit for a while in the sun (UV kills bacteria), pump it to a solar still or giant boiler then bottle/can it and sell. If you got this set up you would be the richest man in the post apocolyptic world You would have the best water in town.