Occasionally, carbon dioxide is injected into the water for pH control. The carbonic acid formed will lower the pH, but it does not destroy alkalinity. Scale control is therefore affected by increasing the solubility of calcium bicarbonate. A drawback to carbonation is that the carbon dioxide is quickly released from the water. In cooling tower systems, excess carbon dioxide will be released with recirculation over the tower, causing the pH to rise rapidly. The water must then be recarbonated as it is pumped from the sump. The fact that carbon dioxide will not recycle limits its use.