Stirring and aeration processes are the major operations needed for effective wastewater treatment. Especially in activated sludge processes and the variants of this process stirring and aeration systems need to be chosen carefully since both operations are crucial for the purification power and the energy demand of wwtp's. The aim of the present work is to define the demands on stirring and aeration systems for activated sludge treatment plants, to give an overview about suitable systems currently available on the market including latest developments and to give advice how to choose the optimum system.

A brief review of the literature and the market of stirring and aeration systems leads to a simple classification of the existing numerous systems. In order to choose the optimum system, first one has to decide, about the demands in each process step. In anaerobic and anoxic reactors the use of hyperboloid stirring systems or combined hyperboloid stirring and aeration systems which distribute small air bubbles throughout the entire reactor in the aeration mode (for intermittent aeration) and gently circulate bacteria flocks in the stirring mode is recommended. For aerobic processes like BOD-removal and nitrification highly efficient membrane aerating systems using silicone membranes are recommended.

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