As of 2016, 1 in 10 people lack access to safe water. Safe water is of the greatest importance to humans on earth.
Tap water is municipal water normally purified with chlorine then flouride added. One of the byproducts from using chlorine is linked to cancer.
This option varies much throughout the world and places your health in the hands of a municipality.
A water purification technology that uses a semipermeable membrane to remove ions, molecules, and larger particles from drinking water. Most commonly known for its use in purification from seawater.
RO water is quite neutral in ph (7). If you wish to raise the ph to make it more alkaline, you can use baking soda (1/8 tbsp to 8 oz water) although it's high in sodium, lemons (the body reacts with the anionic properties of lemons making the water alkaline as you digest it), pH drops, adding calcium bicarbonate and magnesium bicarbonate, or a water ionizer (hydroxide salts).
It is also normally removed of dissolved solids down to 5ppm so there is very little minerals left. Adding minerals back into the water is necessary. They can be added by purchasing a system that adds back the calcium and magnesium, by adding a pinch of himalayan salt / celtic sea salt / pascalite clay / bentonite clay, or by using mineral drops. Re-mineralization of RO water becomes more important if your food is not providing enough of them.
This option normally requires purchasing filters and minerals which can get costly and thus not sustainable.
Rain water is very similar to RO and distilled water where most minerals are no longer present. If collected in pristine environments it can be quite clean but in polluted cities very dirty. It can also be easily contaminated so needs to be handled wisely.
Rain is much cheaper than having to purchase RO filters yet has the same costs in alkaline and minerals.
Deep water wells pulling from aquifers provide water that has been through a natural filtering process like RO without the cost. Minerals from the ground have also been added in the natural process. Only testing is needed to ensure the right pH is maintained and the water is safe.
The downsides to wells are that they can be expensive to drill and require maintenance over their lifetime.
Provide the benefits of wells without the cost. The best option by far but not easily found.