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.

Water treatment wastes are materials that are removed in various treatment processes and contain large amounts of water. These wastes include turbidity and color-producing materials, organic and inorganic solids, algae, bacteria, viruses, and precipitated chemicals. These materials are by-products of chemical coagulation, iron and manganese precipitation, filter washing, hardening, resin regeneration, and microfilters.