I second what Steve said.
(Maybe you should stop at 5 days, just to test this out?)
Keep in mind that it's extremely hard to break a fast the
right way... especially if you weren't eating a very healthy diet beforehand. If it's your first time
ever breaking a long fast, then it can be extremely difficult. You have to be really prepared and experienced to understand how to break it, and to have the emotional maturity & tenacity to handle it all without slipping into the wrong mindset.
(When I was 18, I did a 40-day water fast, and broke it unintentionally with some sunflower seeds and trail mix. I ended up crying in a ball on the floor for the rest of the night, bent over double from the excruciating pain of being unable to digest anything.

Also failed an exam because the pain was so intense that I had to run out of the classroom in tears.

Even eating a salad was very hard for me and really painful. ...Soooo you definitely don't want that to happen to you!

Make sure that you break it the RIGHT way, with light juice or some kind of gentle, fresh fruit.)
And be prepared for a lowered metabolism after the fast is over, because if you're not aware of that, you can pack on the pounds in warp speed time.
I hope that you don't think I'm being discouraging, (because I completely agree that fasting can be beneficial

) -- but I just want you to be sure that you do a lot of research and talk to a lot of experts before embarking on a 30-day fast with nothing but water. Make sure you research the physiology of fasting, too, so you understand exactly what happens in your body, and how certain foods affect it.
(If I were trying to break a food addiction though, I would probably do something a little more gradual... like a 3-day "fast" with nothing but tea, and then a week or two of eating nothing but 2-3 kinds of fruit. Then I would start eating more vegetables, while eliminating all bread, wheat, and flour products to start feeling lighter and healthier. That's usually enough to turn things around and break my little addictions. And it's easier, because your digestive system never really shuts down.)