If you're on the move at all, you can't lug gallons of water around with you. Water is heavy, and when there are zombies around every turn you need to keep your system light. The average person needs at minimum 2 liters of water a day, and more is recommended. Where is that water going to come from?
There are 3 basic types of portable water purification systems: tablets, pumps, and filtered bottles.
Tablets:
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Pros:
- Light
- Easy
- Effective
- Limited amount
- Taste bad
Pumps:
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Pros:
- More uses than tablets
- Still easy to carry
- Easy to use
- Expensive
- Uses filters
Bottles:
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Pros:
- Still portable
- Cheap(ish)
- Long lifespan
- Uses filters
- Limited amount of water at a time
In conclusion, while any of these water purification systems will provide you with clean drinking water, I would recommend a pump or bottle. Unless you have a backpack full of purification tablets, they are going to run out sooner than you realize. A pump or bottle is the best way to obtain clean water when you don't time or aren't able to boil it. They both cost over $100, but you get what you pay for.
If you have any other suggestions, be sure to leave them in the comments.