The activated sludge process is the most widely used system for biological treatment of both industrial and domestic. The process in its most conventional form was developed between 1912 and 1914, and involves a system in which microorganisms mix freely within the wastewater where they contact with and biologically degrade nutrients present therein. The "sludge" refers to the slurry of microorganisms mixed with the organic and colloidal matter from the influent wastewater, and is considered "activated" since the microorganisms are all metabolically active and capable of biologically degrading organic matter present within the mixed liquor.
The activated sludge process relies on a dense microbial population being mixed with the wastewater under aerobic conditions. With unlimited food and oxygen, extremely high rates of microbial growth and respiration can be achieved, resulting in the utilisation of the organic matter present either as oxidised end-products (CO2, NO3, SO4, PO4) or the biosynthesis of new micro-organisms. Organics and nutrient removal occurs as a number of successive steps that occur simultaneously as the microbial biomass is mixed with the wastewater.
The system has evolved over the years to accommodate for influent wastewater of varying composition, to select for removal of a target nutrient, or to comply with special limits for specific chemical compounds particularly in the case of industrial wastewater, however its core components have remained unchanged.
The great variety in design is predominantly as a result of the rearrangement, addition or removal of specific tanks, recycles, or extra processes to achieve a specific outcome.