Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Sedimentation is a physical water treatment process used to settle out suspended solids in water under the influence of gravity. Suspended solids (or SS), is the mass of dry solids retained by a filter of a given porosity related to the volume of the water sample. This includes particles of a size not lower than 10 m. Colloids are particles of a size between 0.001 µm and 1 µm depending on the method of quantification. Due to electrostatic forces balancing the gravity, they are not likely to settle naturally.