Can be purified by a Water Purifier Tablet to give several water rations. To do this, bring a jerry can to your house in your rucksack and "Use" it from the menu. Yields 2-10 rations.
Can also be purified at the Water Purifier to add 5-10 rations directly to the town's well. This is also done in your house.
Or you can kill zombies, with the right attachment.