Antifoams control or eliminate foam in secondary treatment or final effluent areas of a waste treatment plant. Foam is a gas (usually, air) dispersed in a liquid containing some impurity. In secondary treatment systems, bacterial activity is the main cause of foam, although mechanical action (cascading flow, pumps, violent agitation, and so on) and chemical contamination also can cause foam. Reducing foam by using antifoams is desirable for safety, health and aesthetic reasons.