I posted this reply about a week ago on a related titled conversation. However, it contains useful information IMHO and bears repeating. So, here goes:
Based on experience I recommend a two-part strategy to obtain drinkable water on the Camino:
1. If you suffer from any chronic health condition, symptom or problem that would be made worse, or which would seriously harm your health, if you drank water that was not fully sterilized and completely potable, I recommend you stick to purchasing bottled water. However, this should include only a relatively low number of pilgrims.
2. For everyone else, including me with a sensitive digestive system and several chronic health problems, Spanish tap water (in hostals, hotels, and cafes), or any outdoor fountain was fully potable. The exception is any outdoor fountain marked "Aqua non potable" or having a sign showing a spigot / tap in a red circle with an international diagonal line through it. On my Camino, and whilst traveling as a tourist around Spain, I used tap water exclusively with no ill effects. In fact, it surprised me that I suffered no GI problems at all in two months... go figure!
The only time I purchased bottles water was to obtain the empty bottles to use as canteens for the rest of my journey. FYI - IMHO the single best water bottle for the Camino is the .5 liter (500 ml) water bottle. It is ubiquitous, inexpensive, light weight, recyclable, easily replaceable, and can be stashed virtually anywhere, and several can be moved about your pack, pockets, belt, and rucksack hip belt or harness to balance weight. Remember, 1 liter of water weighs about one kilo (or just over two pounds). The smaller .5 liter bottle weighs half that much. So, four full bottles would weigh about 2 kilos or 4 pounds.
That is why I prefer to use them, in conjunction with this handy gadget:
http://www.niteize.com/product/Drink-N-Clip.asp (SEE PHOTO)
Using these clips (with large silicone / rubber "O" rings to ensure the clip never pops off - rubber bands work too), I carry four .5 liter plastic water bottles with me. I distribute them around the FRONT of my harness and waist belt to shift up to four pounds (@ 2 kilos) of weight from my back to my front.
As the day progresses these bottles end up empty. Having four bottles also permits me to have plain water in several, while using others to mix my daily required supplements, and or cool instant coffee (buy singles packets in any tienda or supermarket) or a sports rehydration drink made from a single serving powder (brought from home). Using a larger, 1 liter or 1.5 liter bottle would not afford me these conveniences.
Some New Zealand pilgrims showed me just how wonderful this approach was at counterbalancing the overall weight and making each day's walk more comfortable. They were using Aarn rucksacks (
http://www.aarnpacks.com) with specialized front packs to counterbalance the rucksack weight. It was very interesting. The concept is identical.
On my next Camino (Late April - into May/June 2014), I am combining the water bottle solution here with a chest pack I obtained from Zpacks:
http://www.zpacks.com/accessories/backpack_lid.shtml (SEE PHOTO)
I will store other small items that now require that I remove my rucksack in here for rapid access during the day. After the day's walk, the bag unclips to become a city bag. Rebalancing the weight makes a HUGE difference in comfort and convenience. I no longer have to ask others to hand me a water bottle from my rucksack side mesh pockets.
As others have stated here and elsewhere, if you are dubious about the water supply, you can use water purification tablets. Bring them from home. In fact, experiment at home before you come to the Camino. You might have to include a packet of Crystal Light or a similar product to flavor the water to cover the taste of the chemicals. That, in my opinion negates any cost savings. I also used a rehydration powder from Tang called "Tang Sport" to ensure my salt levels were okay on the few hot days I encountered and where I felt water alone was not adequate..
So, the bottom line is, the cost of using bottled water or purification systems or tablets is not worth the expense UNLESS you have a medical condition that specifically precludes taking the risk - however small.
I do hope this helps someone - anyone...