Flat water VS carbonated water
Flat water and carbonated water, such as seltzer, have different effects on kidney function. Regular water helps the kidneys flush the body of toxins and other excess materials, but carbonated water can complicate the kidney's processes. Carbonated water adds carbon dioxide to the body, which is a gas that the body is constantly trying to remove. Putting carbon dioxide in the body can complicate your bodily functions, since the only place that CO2 is used is in the respiratory process.
The carbon dioxide in carbonated water turns the water much more acidic than regular water. This impacts the kidney's job, because adding acid to the kidneys make the pH level of the body lower, which hurts the body's attempts to balance pH. Flat water is not acidic and therefore does not put any pressure on the kidneys.
Drinking only carbonated water can be very dangerous, as it can prevent kidneys from eliminating wastes. Drinking about eight, 8-oz. glasses of flat, regular water every day, is recommended for keeping your body working properly.
Read more: http://www.livestrong.com/article/114475-carbonated-water-vs.-flat-water/#ixzz1LqZST77K