Since the first experiments performed by Arden and Locket (1914; see Section 1.8), many questions have arisen concerning the activated sludge process:

• What is the best type of reactor?

• Which aeration system is most effective?

• What are the real microbial processes which occur during the decrease in organ-ics and the growth of bacteria?

• What parameters influence the rate of these processes?

• How can we determine the right volumes for reactor and settler to reduce the concentration of organics for a given flow rate?

• How can we feasibly optimize the process?

• Is it possible to treat industrial wastewater containing specific synthetic compounds?

In the first 50 years following Arden's and Locket's invention, the technical realization, and particularly research into the fundamentals, were supported only in a few industrialized countries and to a relatively low extent. Only in the past 40 years has biological wastewater treatment been realized to a greater extent. One of the first textbooks having presented fundamentals of the aerobic activated sludge process was written by Metcalf and Eddy 1972. The pollution of our rivers and lakes caused by society's industrialization endangered nature and reduced the quality of drinking water. Only then were the fundamentals of the activated sludge process studied in more detail. The most important aspects are presented in the next sections.

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