Sieve sizes used in combinations of four to eight sieves. Typical desiccants consist of silica gel, and molecular sieves. Calcium hydride is a relatively mild desiccant and, compared to molecular sieves, probably inefficient. Molecular sieves are available in different shape and sizes. The oxidation generates water that can be removed by adding molecular sieves. Certain beaded molecular sieves offer lower heat capacity thus lower energy requirements during regeneration. The screening section consists of different types of sieves (screens) and centrifugal cleaning. American standard sieves also known as ASTM sieves conform to ASTM E11 standard. Molecular sieves are often used for this purpose as well. The researchers who invented these types of particles planned to use them as molecular sieves. More modern techniques include the use of molecular sieves or a column purification system. The guano is then placed in sacks and carried to sieves, where impurities are removed. The diameters of the sieves range from 200 to 450 millimeters. Molecular sieves also function in this capacity. A common approach involves the use of molecular sieves.