Water downstream from an industrial process or a municipal system (particularly considering water used in the process or system) is considered "waste water." Waste water typically contains a complex mixture of solids and dissolved substances, resulting from the use of the water. To reduce the harmful effects that waste water (and its polluting components) can produce, treatment is necessary.

Following a "flow" concept, progressing from initial treatment (possibly a final step of the industrial process) through the usual stages of a waste water treatment plant: Primary Treatment; Secondary Treatment; Advanced Treatment; Sludge Stabilization; Sludge Thickening; and Sludge Dewatering. The latter three topics are associated with treatment of the product of the first three topics. Many aspects of waste water treatment are quite similar to the treatment of raw or natural water.