The activated sludge process is a wastewater treatment method in which the carbonaceous organic matter of wastewater provides an energy source for the production of new cells for a mixed population of microorganisms in an aquatic aerobic environment. The microbes convert carbon into cell tissue and oxidized end products that include carbon dioxide and water. In addition, a limited number of microorganisms may exist in activated sludge that obtain energy by oxidizing ammonia nitrogen to nitrate nitrogen in the process known as nitrification. 

Bacteria constitute the majority of microorganisms present in activated sludge. Bacteria that require organic compounds for their supply of carbon and energy (heterotrophic bacteria) predominate, whereas bacteria that use inorganic compounds for cell growth (autotrophic bacteria) occur in proportion to concentrations of carbon and nitrogen. Both aerobic and anaerobic bacteria may exist in the activated sludge, but the preponderance of species are facultative, able to live in either the presence of or lack of dissolved oxygen. 

Fungi, rotifers, and protozoans are also residents of activated sludge. The latter microorganisms are represented largely by ciliated species, but flagellated protozoans and amoebae may also be present. Protozoans serve as indicators of the activated sluge condition, and ciliated species are instrumental in removing Escherichia coli from sewage. Additionally, viruses of human origin may be found in raw sewage influent, but a large percentage appear to be removed by the activated-sludge process. 

The success of the activated-sludge process is dependent upon establishing a mixed community of microorganisms that will remove and consume organic waste material, that will aggregate and adhere in a process known as bioflocculation, and that will settle in such a manner as to produce a concentrated sludge (return activated sludge, or RAS) for recycling. Any of several types of activated sludge solids separations problems indicate an imbalance in the biological component of this process. In the ideal "healthy" system, filamentous organisms grow within a floc (a large aggregate of adherent, or floc-forming, microorganisms, such as bacteria) and give it strength, with few filaments protruding out into the surrounding bulk solution. In such a system, there is no interference with the compaction and settling rates of the activated sludge prior to its recycling.