RO systems can also be used to reduce silica concentration. Although “cellulose acetate” and early “thin film” composite materials only provided moderate silica rejection, newer materials reject silica quite well. The mechanism of removal is probably by “hyper” filtration but is also related to degree of ionization since silica is more completely removed at a high pH. Since silica is concentrated by the membrane in the reject stream, silica solubility can be an important consideration. RO systems are currently unable to achieve as complete removal of "reactive silica" as ion exchange, but are far better at removing various forms of "non-reactive silica".