Operation of modern wastewater treatment facilities (in the following: WWTP) are to a very large extent based on different forms of biological treatment. Historically a number of activated sludge models have dominated the market. The model that originally was developed during the second decade of the 20th century is often addressed as a suspended growth system as a contrast to attached growth models, such as trickling filters, rotating biological contactors (RBC:s) and more recently the moving bed biological reactors (MBBR:s). Regardless of the system chosen the biological stage in a modern WWTP represents the major energy consuming stage. The obvious exception for this statement is by convention the anaerobic treatment, especially used when the wastewater is a “high strength” stream, rich in hydrocarbonates. The sharpened demand on biological nutrient removal, especially nitrogen removal has even more highlighted the needs for an efficient process control.