Solids in water refer to suspended or dissolved materials in drinking, surface and brackish waters, as well as domestic and industrial wastewaters. Solids may affect water and wastewater quality in a number of ways. Waters with high dissolved solids are generally of lower quality and may cause adverse physiological responses in the consumer, so a maximum of 500 mg of solids per liter is desirable for drinking water.

Pure water does not exist in nature due to its high solubility and contains various impurities. Water impurities are classified into three general categories: dissolved solids, suspended and colloidal solids, and gases.