Substances that maintain definite size and shape. Solids in water fall into one of the following categories: dissolved, colloidal, and suspended.

  1. Dissolved solids are in solution and pass through a filter. The solution consisting of the dissolved components and water forms a single phase, (a homogenous solution).
  2. Colloidal solids (sols) are uniformly dispersed in solution but they form a solid phase that is distinct from the water phase.
  3. Suspended solids are also a separate phase from the solution. Some suspended solids are classified as settleable solids. Placing a sample in a cylinder and measuring the amount of solids that have settled after a set amount of time determine settleable solids. The size of solids increases going from dissolved solids to suspended solids

Source: Handbook of water and wastewater treatment plant operations