The activated sludge process is a biological wastewater treatment operation. This means that treatment occurs through many different microorganisms using pollutants as a food source. It is a suspended growth process – since the organisms are suspended in wastewater rather than attached to a medium. The activated sludge process relies on harvesting a population of millions of microorganisms with different characteristics – mostly aerobic, facultative and heterotrophic bacteria suspended in the wastewater – as the wastewater travels through a reactor known as an aeration tank. This suspension, referred to as the mixed liquor or mixed liquor suspended solid, is supplied oxygen and kept mixed by bubbling air through the entire aeration tank. Aeration is used to operate activated sludge process units and is perhaps the most frequently used process to remove biochemical oxygen demand (BOD) from wastewater. Successful BOD removal in an activated sludge process depends on studying and controlling some basics – such as wastewater sources and quantities, wastewater characteristics, and needed execution in any required preliminary and primary treatment.