Silica is becoming the critical limiting factor in the reuse of water. Silica deposit is caused as a result of its polymerization, coprecipitation with other minerals, precipitation with other multivalent ions, and biological activity in the water. Several of these processes may take place concurrently, making it difficult to predict equilibrium solubility. The solubility of amorphous silica is also dependent on many other factors such as, pH , temperature, particle size, particle hydration, and the presence of other ions, such as iron, aluminum, etc.. A hard silica scale is formed when calcium carbonate or other mineral precipitate provides a
crystalline matrix in which silica can be entrapped.